INTRODUCTION - Webs · macroscopic and microscopic behaviour is a central and recurrent theme of...

1 Engineering materials F. J. K. Adzabe (Perro deBosque)

INTRODUCTION

The development of many technologies that make our existence so comfortable has been

intimately associated with the accessibility of suitable materials. Advancement in the

understanding of a material type is often the forerunner to the stepwise progression of a

technology. For example, automobiles would not have been possible without the availability of

inexpensive steel or some other comparable substitute. In our contemporary era, sophisticated

electronic devices rely on components that are made from what are called semiconducting

materials. In everyday life we encounter a remarkable range of engineering materials: metals,

plastics and ceramics are some of the generic terms that we use to describe them. We

acknowledge that these diverse materials are quite literally the stuff of our civilization and have a

determining effect upon its character, just as cast iron did during the Industrial Revolution. The

ways in which we use, or misuse, materials will obviously also influence its future. We should

recognize that the pressing and interrelated global problems of energy utilization and

environmental control each has a substantial and inescapable ‗materials dimension‘.

The engineer is primarily concerned with the function of the component or structure, frequently

with its capacity to transmit working stresses without risk of failure. The secondary task, the

actual choice of a suitable material, requires that the engineer should provide the necessary

design data, synthesize and develop new materials, analyze failures and ultimately produce

material with the desired shape, form and properties at acceptable cost.

Adjectives describing the macroscopic behaviour of materials naturally feature prominently in

any language. We write and speak of materials being hard, strong, brittle, malleable, magnetic,

wear-resistant, etc. Despite their apparent simplicity, such terms have depths of complexity when

subjected to scientific scrutiny, particularly when attempts are made to relate a given property to

the internal structure of a material. In practice, the search for bridges of understanding between

macroscopic and microscopic behaviour is a central and recurrent theme of engineering

materials. In more recent times, the enhancement of analytical techniques for characterizing

structures in fine detail has led to the development and acceptance of polymers and ceramics as

trustworthy engineering materials.

Materials play an important role in the construction and manufacturing of equipment/tools. Right

selection of materials adds to the economy, working and life of machinery. A Chemical Engineer

must be conversant with the properties, uses, availability and costs of materials used for

construction/fabrication to enable him to perform his functions confidently. The subject of

Engineering Materials has been designed to cover the above aspects.

Historical Perspective

Materials are so important in the development of civilization that we associate ages with them. In

the origin of human life on earth, the Stone Age, people used only natural materials, like stone,

clay, skins, and wood. When people found copper and how to make it harder by alloying, the

Bronze Age started about 3000 BC. The use of iron and steel, a stronger material that gave

advantage in wars started at about 1200 BC. The next big step was the discovery of a cheap

process to make steel around 1850, which enabled the railroads and the building of the modern

infrastructure of the industrial world.


Understanding of how materials behave like they do and why they differ in properties was only

possible with the atomistic understanding allowed by quantum mechanics, that first explained

atoms and then solids starting in the 1930s. The combination of physics, chemistry, and the focus

on the relationship between the properties of a material and its microstructure is the domain of

Materials Science. The development of this science allowed designing materials and provided a

knowledge base for the engineering applications.

Why Study Materials Science and Engineering?

• To be able to select a material for a given use based on considerations of cost and performance.

• To understand the limits of materials and the change of their properties with use.

• To be able to create a new material that will have some desirable properties.

All engineering disciplines need to know about materials, whether mechanical, civil, chemical, or

electrical, will at one time or another be exposed to a design problem involving materials.

Examples might include a transmission gear, the superstructure for a building, an oil refinery

component, or an integrated circuit chip.

Many times, engineering materials problem is one of selecting the right material from the many

thousands that are available. There are several criteria on which the final decision is normally

based. First of all, the in-service conditions must be characterized, for these will dictate the

properties required of the material. Only on rare occasions does a material possess the maximum

or ideal combination of properties. Thus, it may be necessary to trade off one characteristic for

another. The classic example involves strength and ductility; normally, a material having a high

strength will have only a limited ductility. In such cases a reasonable compromise between two

or more properties may be necessary.

A second selection consideration is any deterioration of material properties that may occur during

service operation. For example, significant reductions in mechanical strength may result from

exposure to elevated temperatures or corrosive environments.

Finally, probably the overriding consideration is that of economics: What will the finished

product cost? A material may be found that has the ideal set of properties but is prohibitively

expensive. Here again, some compromise is inevitable. The cost of a finished piece also includes

any expense incurred during fabrication to produce the desired shape.

The more familiar an engineer is with the various characteristics and structure–property

relationships, as well as processing techniques of materials, the more proficient and confident he

or she will be to make judicious materials choices based on these criteria.


CHAPTER TWO

Atomic Structure and Atomic Bonding in Engineering Materials

Atomic Structure

Some of the important properties of solid materials depend on geometrical atomic

arrangements, and also the interactions that exist among constituent atoms or

molecules. This chapter, by way of preparation for subsequent discussions,

considers several fundamental and important concepts—namely, atomic structure,

electron configurations in atoms and the various types of primary and secondary

interatomic bonds that hold together the atoms comprising a solid. These topics are

reviewed briefly, under the assumption that some of the material is familiar to the

reader.

Fundamental concepts

Every atom consists of a small nucleus composed of protons and neutrons, which is encircled by

moving electrons in their orbital, i.e. specific energy levels. The top most orbital electrons,

valence electrons, affect most material properties that are of interest to engineers. E.g.: chemical

properties, nature of bonding, size of atom, optical/magnetic/electrical properties. Electrons and

protons are negative and positive charges of the same magnitude being 1.60x10-19

coulombs.

Neutrons are electrically neutral. Protons and neutrons have approximately the same mass,

1.67x10-27

kg, which is larger than that of an electron, 9.11x10-31

kg. The unit of mass is an

atomic mass unit (amu) = 1.66 × 10-27

kg, and equals 1/12 the mass of a carbon atom. Neutrons

and protons have very similar masses, roughly equal to 1 amu. A neutral atom has the same

number of electrons and protons.

Atomic number (Z) - is the number of protons per atoms.

Atomic mass (A) - is the sum of the masses of protons and neutrons within the nucleus.

A ≅ Z+N, where N is number of neutrons.

Isotopes - atoms with same atomic number but different atomic masses.

A mole is the amount of matter that has a mass in grams equal to the atomic mass in amu of the

atoms. Thus, a mole of carbon has a mass of 12 grams. The number of atoms in a mole is called

the Avogadro number, Nav = 6.023 × 1023

. Note that Nav = 1 gram/1 amu. Calculating n, the

number of atoms per cm3 in a piece of material of density δ (g/cm

3).

n = Nav × δ / M

where M is the atomic mass in amu (grams per mol). Thus, for graphite (carbon) with a

density δ = 1.8 g/cm3, M =12, we get 6 × 10

23 atoms/mol × 1.8 g/cm

3 / 12 g/mol) = 9 × 10

22

C/cm3. Most solids have atomic densities around 6 × 10

22 atoms/cm

3. The cube root of that

number gives the number of atoms per centimeter, about 39 million. The mean distance between


atoms is the inverse of that, or 0.25 nm. This is an important number that gives the scale of

atomic structures in solids.

2.2 Atomic bonding in engineering materials

An understanding of many of the physical properties of materials is predicated on knowledge of

the interatomic forces that bind the atoms together. Perhaps the principles of atomic bonding are

best illustrated by considering the interaction between two isolated atoms as they are brought into

close proximity from an infinite separation. At large distances, the interactions are negligible, but

as the atoms approach, each exerts forces on the other. These forces are of two types, attractive

and repulsive, and the magnitude of each is a function of the separation or interatomic distance.

The origin of an attractive force FA depends on the particular type of bonding that exists between

the two atoms. The magnitude of the attractive force varies with the distance, as represented

schematically in Figure 1. Ultimately, the outer electron shells of the two atoms begin to overlap,

and a strong repulsive force FR comes into play. The net force FN between the two atoms is just

the sum of both attractive and repulsive components; that is,

FN = FA + FR

When FA and FR balance, or become equal, there is no net force; that is,

FA + FR = 0

Then a state of equilibrium exists. The centers of the two atoms will remain separated by the

equilibrium spacing r0, as indicated in Figure 1. For many atoms, r0 is approximately 0.3 nm.

Once in this position, the two atoms will counteract any attempt to separate them by an attractive

force, or to push them together by a repulsive action.

Sometimes it is more convenient to work with the potential energies between two atoms instead

of forces. Mathematically, energy (E) and force (F) are related as

E =


for atomic systems

EN =

EN = EA + ER

in which EN, EA, and ER are respectively the net, attractive, and repulsive energies for two

isolated and adjacent atoms.

Although the preceding treatment has dealt with an ideal situation involving only two atoms, a

similar yet more complex condition exists for solid materials because force and energy

interactions among many atoms must be considered. Nevertheless, a bonding energy may be

associated with each atom. The magnitude of this bonding energy and the shape of the energy-

versus interatomic separation curve vary from material to material, and they both depend on the

type of atomic bonding. For example, materials having large bonding energies typically also have

high melting temperatures. At room temperature, solid substances are formed for large bonding

energies, whereas for small energies the gaseous state is favored. Liquids prevail when the

energies are of intermediate magnitude. In addition, the mechanical stiffness (or modulus of

elasticity) of a material is dependent on the shape of its force-versus interatomic separation

curve. Furthermore, how much a material expands upon heating or contracts upon cooling (that

is, its linear coefficient of thermal expansion) is related to the shape of its the energy-versus

interatomic separation curve. A deep and narrow ―trough,‖ which typically occurs for materials

having large bonding energies, normally correlates with a low coefficient of thermal expansion

and relatively small dimensional alterations for changes in temperature.

Bonds are of two kinds – Primary, and Secondary. Secondary or physical forces and energies are

also found in many solid materials; they are weaker than the primary ones, but nonetheless

influence the physical properties of some materials They exist in many substances like water

along with primary bonds. E.g.: Hydrogen and Van der Waals forces. The sections that follow

explain the several kinds of primary and secondary interatomic bonds.

Primary Interatomic Bonds

Three different types of primary or chemical bond are found in solids—ionic, covalent, and

metallic. For each type, the bonding necessarily involves the valence electrons; furthermore, the

nature of the bond depends on the electron structures of the constituent atoms. In general, each of

these three types of bonding arises from the tendency of the atoms to assume stable electron

structures, like those of the inert gases, by completely filling the outermost electron shell.

Primary bonds are relatively stronger. These bonds invariably involve valence electrons. Nature

of bond depends on electron arrangement in respective atoms. Atoms tend to acquire stable

electron arrangement in their valence orbital by transferring (ionic), sharing (covalent and

metallic) valence electrons. They exist in almost all solid materials. E.g.: Ionic, Covalent, and

Metallic bonds. Bond energies are in order of 1000 kJ/mol.


1. Ionic Bonding

Ionic bond is perhaps the easiest to describe and visualize. It is always found in compounds that

are composed of both metallic and nonmetallic elements, elements that are situated at the

horizontal extremities of the periodic table. Atoms of a metallic element easily give up their

valence electrons to the nonmetallic atoms. In the process all the atoms acquire stable or inert gas

configurations and, in addition, an electrical charge; that is, they become ions. Sodium chloride

(NaCl) is the classic ionic material. The attractive bonding forces are coulombic; that is, positive

and negative ions, by virtue of their net electrical charge, attract one another. For two isolated

ions, the attractive energy EA is a function of the interatomic distance according to

Where

A =

8.85 x F/m

= permittivity

Z1 and Z2 are are the valences of the two ion types, and

e is the electronic charge 1.602 x 10-19

C

An analogous equation for the repulsive energy is

In these expressions, A, B, and n are constants whose values depend on the particular ionic

system. The value of n is approximately 8.

Ionic bonding is termed nondirectional; that is, the magnitude of the bond is equal in all

directions around an ion. It follows that for ionic materials to be stable, all positive ions must

have as nearest neighbors negatively charged ions in a three dimensional scheme, and vice versa.

The predominant bonding in ceramic materials is ionic.

Bonding energies, which generally range between 600 and 1500 kJ/mol (3 and 8 eV/atom), are

relatively large, as reflected in high melting temperatures. Ionic materials are characteristically

hard and brittle and, furthermore, electrically and thermally insulative, these properties are a

direct consequence of electron configurations and/or the nature of the ionic bond.

2. Covalent Bonding

In covalent bonding, stable electron configurations are assumed by the sharing of electrons

between adjacent atoms. Two atoms that are covalently bonded will each contribute at least one

electron to the bond, and the shared electrons may be considered to belong to both atoms. The

number of covalent bonds that is possible for a particular atom is determined by the number of


valence electrons. The covalent bond is directional; that is, it is between specific atoms and may

exist only in the direction between one atom and another that participates in the electron sharing.

Many nonmetallic elemental molecules (H2, Cl2, F2, O2) as well as molecules containing

dissimilar atoms, such as CH4, H2O, HNO3 and HF, are covalently bonded. Furthermore, this

type of bonding is found in elemental solids such as diamond (carbon), silicon, and germanium

and other solid compounds composed of elements that are located on the right-hand side of the

periodic table, such as gallium arsenide (GaAs), indium antimonide (InSb), and silicon carbide

(SiC).

Covalent bonds may be very strong, as in diamond, which is very hard and has a very high

melting temperature (about 3550ºC, or they may be very weak, as with bismuth, which melts at

about 270ºC. Polymeric materials typify this bond, the basic molecular structure being a long

chain of carbon atoms that are covalently bonded together with two of their available four bonds

per atom. The remaining two bonds normally are shared with other atoms, which also covalently

bond.

It is possible to have interatomic bonds that are partially ionic and partially covalent, and, in fact,

very few compounds exhibit pure ionic or covalent bonding. For a compound, the degree of

either bond type depends on the relative positions of the constituent atoms in the periodic table or

the difference in their electronegativities. The wider the separation (both horizontally—relative to

Group IVA—and vertically) from the lower left to the upper-right-hand corner (i.e., the greater

the difference in electronegativity), the more ionic the bond. Conversely, the closer the atoms are

together (i.e., the smaller the difference in electronegativity), the greater the degree of covalency.

The percentage ionic character of a bond between elements A and B (A being the most

electronegative) may be approximated by the expression

% ionic character = {1 – exp [- (0.25) (XA – XB) 2]} x 100

Where XA and XB are the electronegativities for the respective elements.

3. Metallic Bonding

Metallic bonding is found in metals and their alloys. Metallic materials have one, two, or at most,

three valence electrons. These valence electrons are not bound to any particular atom in the solid

and are more or less free to drift throughout the entire metal. They may be thought of as

belonging to the metal as a whole, or forming a ―sea of electrons‖ or an ―electron cloud.‖ The

remaining nonvalence electrons and atomic nuclei form what are called ion cores, which possess

a net positive charge equal in magnitude to the total valence electron charge per atom. The free

electrons shield the positively charged ion cores from mutually repulsive electrostatic forces,

which they would otherwise exert upon one another; consequently the metallic bond is

nondirectional in character. In addition, these free electrons act as a ―glue‖ to hold the ion cores

together. Bonding may be weak or strong; energies range from 68 kJ/mol (0.7 eV/atom) for

mercury to 850 kJ/mol (8.8 eV/atom) for tungsten. Their respective melting temperatures are -39

and 3410 ºC.

Furthermore, we note that at room temperature, most metals and their alloys fail in a ductile

manner; that is, fracture occurs after the materials have experienced significant degrees of


permanent deformation. This behavior is explained in terms of deformation mechanism, which is

implicitly related to the characteristics of the metallic bond. Conversely, at room temperature

ionically bonded materials are intrinsically brittle as a consequence of the electrically charged

nature of their component ions.

Secondary Bonding (Van der Waals)

Secondary, van der Waals, or physical bonds are weak in comparison to the primary or chemical

ones; bonding energies are typically on the order of only 10 kJ/mol (0.1 eV/atom). Secondary

bonding exists between virtually all atoms or molecules, but its presence may be obscured if any

of the three primary bonding types is present. Secondary bonding is evidenced for the inert gases,

which have stable electron structures, and, in addition, between molecules in molecular structures

that are covalently bonded.

Secondary bonding forces arise from atomic or molecular dipoles. In essence, an electric dipole

exists whenever there is some separation of positive and negative portions of an atom or

molecule. The bonding results from the coulombic attraction between the positive end of one

dipole and the negative region of an adjacent one. Dipole interactions occur between induced

dipoles, between induced dipoles and polar molecules (which have permanent dipoles), and

between polar molecules. Hydrogen bonding, a special type of secondary bonding, is found to

exist between some molecules that have hydrogen as one of the constituents.

Fluctuating Induced Dipole Bonds

A dipole may be created or induced in an atom or molecule that is normally electrically

symmetric; that is, the overall spatial distribution of the electrons is symmetric with respect to the

positively charged nucleus. All the atoms experience constant vibrational motion that can cause

instantaneous and short-lived distortions of this electrical symmetry for some of the atoms or

molecules, and hence the creation of small electric dipoles. One of these dipoles can in turn

produce a displacement of the electron distribution of an adjacent molecule or atom, which

induces the second one also to become a dipole that is then weakly, attracted or bonded to the

first; this is one type of van der Waals bonding. These attractive forces may exist between large

numbers of atoms or molecules, which forces are temporary and fluctuate with time.

The liquefaction and, in some cases, the solidification of the inert gases and other electrically

neutral and symmetric molecules such as H2 and Cl2 are realized because of this type of bonding.

Melting and boiling temperatures are extremely low in materials for which induced dipole

bonding predominates; of all possible intermolecular bonds, these are the weakest.

Polar Molecule-Induced Dipole Bonds

Permanent dipole moments exist in some molecules by virtue of an asymmetrical arrangement of

positively and negatively charged regions; such molecules are termed polar molecules. Polar

molecules can also induce dipoles in adjacent nonpolar molecules, and a bond will form as a

result of attractive forces between the two molecules. Furthermore, the magnitude of this bond

will be greater than for fluctuating induced dipoles.


Permanent Dipole Bonds

Van der Waals forces will also exist between adjacent polar molecules. The associated bonding

energies are significantly greater than for bonds involving induced dipoles. The strongest

secondary bonding type, the hydrogen bond, is a special case of polar molecule bonding. It

occurs between molecules in which hydrogen is covalently bonded to fluorine (as in HF), oxygen

(as in H2O), and nitrogen (as in NH3). For each H—F, H—O, or H—N bond, the single hydrogen

electron is shared with the other atom. Thus, the hydrogen end of the bond is essentially a

positively charged bare proton that is unscreened by any electrons. This highly positively charged

end of the molecule is capable of a strong attractive force with the negative end of an adjacent

molecule. In essence, this single proton forms a bridge between two negatively charged atoms.

The magnitude of the hydrogen bond is generally greater than that of the other types of secondary

bonds and may be as high as 51 kJ/mol (0.52 eV/molecule). Melting and boiling temperatures for

hydrogen fluoride and water are abnormally high in light of their low molecular weights, as a

consequence of hydrogen bonding.

Crystal Systems

Since there are many different possible crystal structures, it is sometimes convenient to divide

them into groups according to unit cell configurations and/or atomic arrangements. One such

scheme is based on the unit cell geometry, that is, the shape of the appropriate unit cell

parallelepiped without regard to the atomic positions in the cell. Within this framework, an x, y, z

coordinate system is established with its origin at one of the unit cell corners; each of the x, y,

and z axes coincides with one of the three parallelepiped edges that extend from this corner. The

unit cell geometry is completely defined in terms of six parameters: the three edge lengths a, b,

and c, and the three interaxial angles α, β, and γ. These are indicated in Figure 2.4, and are

sometimes termed the lattice parameters of a crystal structure.

On this basis there are seven different possible combinations of a, b, and c, and α, β and γ, each

of which represents a distinct crystal system. These seven crystal systems are cubic, tetragonal,

hexagonal, orthorhombic, rhombohedral, monoclinic, and triclinic. The lattice parameter

relationships and unit cell sketches for each are represented in Table 3.2.The cubic system has

the greatest degree of symmetry. Least symmetry is displayed by the triclinic system.

From the discussion of metallic crystal structures, it should be apparent that both FCC and BCC

structures belong to the cubic crystal system, whereas HCP falls within hexagonal.

Crystal Structures

Solid materials may be classified according to the regularity with which atoms or ions are

arranged with respect to one another. A crystalline material is one in which the atoms are situated

in a repeating or periodic array over large atomic distances; that is, long-range order exists, such

that upon solidification, the atoms will position themselves in a repetitive three-dimensional

pattern, in which each atom is bonded to its nearest-neighbor atoms. All metals, many ceramic

materials, and certain polymers form crystalline structures under normal solidification conditions.

For those that do not crystallize (noncrystalline or amorphous), this long-range atomic order is

absent

Some of the properties of crystalline solids depend on the crystal structure of the material, the

manner in which atoms, ions, or molecules are spatially arranged. There is an extremely large


number of different crystal structures all having long range atomic order; these vary from

relatively simple structures for metals to exceedingly complex ones, as displayed by some of the

ceramic and polymeric materials.

When describing crystalline structures, atoms (or ions) are thought of as being solid spheres

having well-defined diameters. This is termed the atomic hard sphere model in which spheres

representing nearest-neighbor atoms touch one another. Sometimes the term lattice is used in the

context of crystal structures; in this sense ―lattice‖ means a three-dimensional array of points

coinciding with atom positions (or sphere centers).

Unit Cells

The atomic order in crystalline solids indicates that small groups of atoms form a repetitive

pattern. Thus, in describing crystal structures, it is often convenient to subdivide the structure

into small repeat entities called unit cells. Unit cells for most crystal structures are

parallelepipeds or prisms having three sets of parallel faces; one is drawn within the aggregate of

spheres. A unit cell is chosen to represent the symmetry of the crystal structure, wherein all the

atom positions in the crystal may be generated by translations of the unit cell integral distances

along each of its edges. Thus, the unit cell is the basic structural unit or building block of the

crystal structure and defines the crystal structure by virtue of its geometry and the atom positions

within. Convenience usually dictates that parallelepiped corners coincide with centers of the hard

sphere atoms. Furthermore, more than a single unit cell may be chosen for a particular crystal

structure; however, the unit cell having the highest level of geometrical symmetry is generally

used.

Metallic Crystal Structures

The atomic bonding in this group of materials is metallic and thus nondirectional in nature.

Consequently, there are minimal restrictions as to the number and position of nearest-neighbor

atoms; this leads to relatively large numbers of nearest neighbors and dense atomic packings for

most metallic crystal structures. Also, for metals, using the hard sphere model for the crystal

structure, each sphere represents an ion core. Three relatively simple crystal structures are found

for most of the common metals: face centered cubic, FCC, body-centered cubic, BCC and

hexagonal close-packed, HCP.

The Face-Centered Cubic Crystal Structure

The crystal structure found for many metals has a unit cell of cubic geometry, with atoms located

at each of the corners and the centers of all the cube faces. It is aptly called the face-centered

cubic (FCC) crystal structure. Some of the familiar metals having this crystal structure are

copper, aluminum, silver, and gold.


Figure 2: A hard sphere model for the FCC unit cell

The atom centers are represented by small circles to provide a better perspective of atom

positions. The aggregate of atoms in Figure 2 represents a section of crystal consisting of many

FCC unit cells. These spheres or ion cores touch one another across a face diagonal; the cube

edge length a and the atomic radius R are related through

a = 2R

For the FCC crystal structure, each corner atom is shared among eight unit cells, whereas a face-

centered atom belongs to only two. Therefore, one-eighth of each of the eight corner atoms and

one-half of each of the six face atoms, or a total of four whole atoms, may be assigned to a given

unit cell.

Two other important characteristics of a crystal structure are the coordination number and the

atomic packing factor (APF). For metals, each atom has the same number of nearest-neighbor

or touching atoms, which is the coordination number. For face-centered cubics, the coordination

number is 12.

The APF is the sum of the sphere volumes of all atoms within a unit cell (assuming the atomic

hard sphere model) divided by the unit cell volume—that is

APF =

For the FCC structure, the atomic packing factor is 0.74, which is the maximum packing possible

for spheres all having the same diameter. Metals typically have relatively large atomic packing

factors to maximize the shielding provided by the free electron cloud.

The Body-Centered Cubic Crystal Structure

Another common metallic crystal structure also has a cubic unit cell with atoms located at all

eight corners and a single atom at the cube center. This is called a body-centered cubic (BCC)

crystal structure. Center and corner atoms touch one another along cube diagonals, and unit cell

length a and atomic radius R are related through

Chromium, iron, tungsten, as well as several other metals exhibit a BCC structure.

Figure 3: A hard sphere model for the BCC unit cell


Two atoms are associated with each BCC unit cell: the equivalent of one atom from the eight

corners, each of which is shared among eight unit cells, and the single center atom, which is

wholly contained within its cell. In addition, corner and center atom positions are equivalent. The

coordination number for the BCC crystal structure is 8; each center atom has as nearest neighbors

its eight corner atoms. Since the coordination number is less for BCC than FCC, so also is the

atomic packing factor for BCC lower—0.68.

The Hexagonal Close-Packed Crystal Structure

Not all metals have unit cells with cubic symmetry; the final common metallic crystal structure to

be discussed has a unit cell that is hexagonal. Figure 4 a reduced-sphere unit cell for this

structure, which is termed hexagonal close packed (HCP). The top and bottom faces of the unit

cell consist of six atoms that form regular hexagons and surround a single atom in the center.

Another plane that provides three additional atoms to the unit cell is situated between the top and

bottom planes. The atoms in this mid plane have as nearest neighbors atoms in both of the

adjacent two planes. The equivalent of six atoms is contained in each unit cell; one-sixth of each

of the 12 top and bottom face corner atoms, one-half of each of the 2 center face atoms, and all 3

mid plane interior atoms. If a and c represent, respectively, the short and long unit cell

dimensions of Figure 4, the c/a ratio should be 1.633; however, for some HCP metals this ratio

deviates from the ideal value. The coordination number and the atomic packing factor for the

HCP crystal structure are the same as for FCC: 12 and 0.74, respectively. The HCP metals

include cadmium, magnesium, titanium, and zinc.

Figure 2.3: A hard sphere model for the HCP unit cell

EXAMPLE PROBLEM

Determination of FCC Unit Cell Volume

Calculate the volume of an FCC unit cell in terms of the atomic radius R.


EXAMPLE PROBLEM 2.2

Computation of the Atomic Packing Factor for FCC

Show that the atomic packing factor for the FCC crystal structure is 0.74.

DENSITY

COMPUTATIONS

Knowledge of the crystal structure of a metallic solid permits computation of its theoretical

density through the relationship


. where

n = number of atoms associated with each unit cell

A = atomic weight

= Avogadro‘s number (6.023 × 1023

atoms/mol)

= volume of the unit cell

EXAMPLE PROBLEM 3.3

Theoretical Density Computation for Copper

Copper has an atomic radius of 0.128 nm, an FCC crystal structure, and an atomic weight of 63.5

g/mol. Compute its theoretical density and compare the answer with its measured density.

Polymorphism And Allotropy

Some metals, as well as nonmetals, may have more than one crystal structure, a phenomenon

known as polymorphism. When found in elemental solids, the condition is often termed

allotropy. The prevailing crystal structure depends on both the temperature and the external

pressure. One familiar example is found in carbon: graphite is the stable polymorph at ambient

conditions, whereas diamond is formed at extremely high pressures. Also, pure iron has a BCC

crystal structure at room temperature, which changes to FCC iron at 912 ºC (167 °F). Most often

a modification of the density and other physical properties accompanies a polymorphic

transformation.

Point Coordinates, Crystallographic Directions and Crystallographic Planes

Crystallographic points, directions, and planes are specified in terms of indexing schemes. The

basis for the determination of each index is a coordinate axis system defined by the unit cell for

the particular crystal structure. The location of a point within a unit cell is specified using

coordinates that are fractional multiples of the cell edge lengths. Directional indices are

computed in terms of the vector projection on each of the coordinate axes, whereas planar indices

are determined from the reciprocals of axial intercepts. For hexagonal unit cells, a four-index

scheme for both directions and planes is found to be more convenient.

Linear and Planar Densities

Crystallographic directional and planar equivalencies are related to atomic linear and planar

densities, respectively. The atomic packing (i.e., planar density) of spheres in a crystallographic

plane depends on the indices of the plane as well as the crystal structure. For a given crystal

structure, planes having identical atomic packing yet different Miller indices belong to the same

family.


SINGLE CRYSTALS

For a crystalline solid, when the periodic and repeated arrangement of atoms is perfect or extends

throughout the entirety of the specimen without interruption, the result is a single crystal. All

unit cells interlock in the same way and have the same orientation. Single crystals exist in nature,

but they may also be produced artificially. Within the past few years, single crystals have become

extremely important in many of our modern technologies, in particular electronic microcircuits,

which employ single crystals of silicon and other semiconductors.

POLYCRYSTALLINE MATERIALS

Most crystalline solids are composed of a collection of many small crystals or grains; such

materials are termed polycrystalline.

X-Ray Diffraction: Determination of Crystal Structures

X-ray diffractometry is used for crystal structure and interplanar spacing determinations.

A beam of x-rays directed on a crystalline material may experience diffraction (constructive

interference) as a result of its interaction with a series of parallel atomic planes according to

Bragg‘s law. Interplanar spacing is a function of the Miller indices and lattice parameter(s) as

well as the crystal structure.

NONCRYSTALLINE SOLIDS

It has been mentioned that noncrystalline solids lack a systematic and regular arrangement of

atoms over relatively large atomic distances. Sometimes such materials are also called

amorphous (meaning literally without form), or supercooled liquids, inasmuch as their atomic

structure resembles that of a liquid.

An amorphous condition may be illustrated by comparison of the crystalline and noncrystalline

structures of the ceramic compound silicon dioxide (SiO2). Even though each silicon ion bonds to

three oxygen ions for both states, beyond this, the structure is much more disordered and

irregular for the noncrystalline structure.

Whether a crystalline or amorphous solid forms depends on the ease with which a random atomic

structure in the liquid can transform to an ordered state during solidification. Amorphous

materials, therefore, are characterized by atomic or molecular structures that are relatively

complex and become ordered only with some difficulty. Furthermore, rapidly cooling through the

freezing temperature favors the formation of a noncrystalline solid, since little time is allowed for

the ordering process.


Metals normally form crystalline solids, but some ceramic materials are crystalline, whereas

others, the inorganic glasses, are amorphous. Polymers may be completely noncrystalline and

semicrystalline consisting of varying degrees of crystallinity.

QUESTIONS AND PROBLEMS

Fundamental Concepts

2.1 (a) Cite the difference between atomic mass and atomic weight.

2.2 Silicon has three naturally-occurring isotopes: 92.23% of 28

Si, with an atomic weight of

27.9769 amu, 4.68% of 29

Si, with an atomic weight of 28.9765 amu, and 3.09% of 30

Si, with an

atomic weight of 29.9738 amu. On the basis of these data, confirm that the average atomic

weight of Si is 28.0854 amu.

2.3 (a) How many grams are there in one amu of a material?

(b) Mole, in the context of this book, is taken in units of gram-mole. On this basis, how many

atoms are there in a pound-mole of a substance?

2.4 (a) Cite two important quantum-mechanical concepts associated with the Bohr model of the

atom.

(b) Cite two important additional refinements that resulted from the wave-mechanical atomic

model.

2.5 Relative to electrons and electron states, what does each of the four quantum numbers

specify?

2.6 Give the electron configurations for the following ions: P5+

, P3-

, Sn4+

, Se2-

, I-, and Ni

2+

2.7 Potassium iodide (KI) exhibits predominantly ionic bonding. The K+ and I

- ions have electron

structures that are identical to which two inert gases?

2.1 What is the difference between atomic structure and crystal structure?

Bonding Forces and Energies

2.13 Calculate the force of attraction between a Ca2+

and an O2-

ion the centers of which are

separated by a distance of 1.25 nm.

2.14 For a Na+ - Cl

- ion pair, attractive and repulsive energies EA and ER respectively, depend on

the distance between the ions r, according to

=

For these expressions, energies are expressed in electron volts per Na+ - Cl

- pair, and r is the

distance in nanometers. The net energy is just the sum of the two expressions above.

(a) Superimpose on a single plot versus r up to 1.0 nm.


(b) On the basis of this plot, determine (i) the equilibrium spacing between the Na+ and Cl

-

ions, and (ii) the magnitude of the bonding energy E0 between the two ions.

2.17 The net potential energy between two adjacent ions is sometimes represented by the

expression

in which r is the interionic separation and C, D, and ρ are constants whose values depend on the

specific material.

(a) Derive an expression for the bonding energy in terms of the equilibrium interionic

separation and the constants D and ρ using the following procedure:

1. Differentiate with respect to r and set the resulting expression equal to zero.

2. Solve for C in terms of D, ρ, and

3. Determine the expression for by substitution for C in the equation above.

(b) Derive another expression for in terms of ,

C and ρ using a procedure analogous to the one outlined in part (a).

Primary Interatomic Bonds

2.18 (a) Briefly cite the main differences between ionic, covalent, and metallic bonding.

(b) State the Pauli Exclusion Principle.

2.19 Compute the percentage ionic character of the interatomic bond for each of the following

compounds: MgO, GaP, CsF, CdS, and FeO.

2.22 What type(s) of bonding would be expected for each of the following materials: solid xenon,

calcium fluoride (CaF2), bronze, cadmium telluride (CdTe), rubber, and tungsten?

Secondary Bonding or van der Waals Bonding

2.23 Explain why hydrogen fluoride (HF) has a higher boiling temperature than hydrogen

chloride (HCl), even though HF has a lower molecular weight.

Unit Cells

Metallic Crystal Structures

2.2 If the atomic radius of lead is 0.175 nm, calculate the volume of its unit cell in cubic meters.

2.3 Show for the body-centered cubic crystal structure that the unit cell edge length a and the

atomic radius R are related through

2.4 For the HCP crystal structure shows that the ideal c/a ratio is 1.633.

2.5 Show that the atomic packing factor for BCC is 0.68.


2.6 Show that the atomic packing factor for HCP is 0.74.

Density Computations

2.7 Molybdenum has a BCC crystal structure, an atomic radius of 0.1363 nm, and an atomic

weight of 95.94 g/mol. Compute and compare its theoretical density with the experimental value.

2.8 Calculate the radius of a palladium atom, given that Pd has an FCC crystal structure, a

density of 12.0 g/cm3, and an atomic weight of 106.4 g/mol.

2.9 Calculate the radius of a tantalum atom, given that Ta has a BCC crystal structure, a density

of 16.6 g/cm3, and an atomic weight of 180.9 g/mol.

2.10 Titanium has an HCP crystal structure and a density of 4.51 g/cm3.

(a) What is the volume of its unit cell in cubic meters?

(b) If the c/a ratio is 1.58, compute the values of c and a.

2.11 Using atomic weight, crystal structure, and atomic radius data, compute the theoretical

densities of aluminum, nickel, magnesium, and tungsten, and then compare these values with the

measured densities. The c/a ratio for magnesium is 1.624.

2.12 Niobium has an atomic radius of 0.1430 nm and a density of 8.57 g/cm3. Determine

whether it has an FCC or BCC crystal structure.

2.13 The unit cell for uranium has orthorhombic symmetry, with a, b, and c lattice parameters of

0.286, 0.587, and 0.495 nm, respectively. If its density, atomic weight, and atomic radius are

19.05 g/cm3, 238.03 g/mol, and 0.1385 nm, respectively, compute the atomic packing factor.

2.14 Indium has a tetragonal unit cell for which the a and c lattice parameters are 0.459 and

0.495 nm, respectively.

(a) If the atomic packing factor and atomic radius are 0.693 and 0.1625 nm, respectively,

determine the number of atoms in each unit cell.

(b) The atomic weight of indium is 114.82 g/mol; compute its theoretical density.

2.15 Beryllium has an HCP unit cell for which the ratio of the lattice parameters c/a is 1.568. If

the radius of the Be atom is 0.1143 nm, (a) determine the unit cell volume, and (b) calculate the

theoretical density of Be and compare it with the literature value.

2.16 Magnesium has an HCP crystal structure, a c/a ratio of 1.624, and a density of 1.74 g/cm3.

Compute the atomic radius for Mg.

2.17 Cobalt has an HCP crystal structure, an atomic radius of 0.1253 nm, and a c/a ratio of 1.623.

Compute the volume of the unit cell for Co.

Polycrystalline Materials

3.56 Explain why the properties of polycrystalline materials are most often isotropic.


Noncrystalline Solids

3.66 Would you expect a material in which the atomic bonding is predominantly ionic in nature

to be more or less likely to form a noncrystalline solid upon solidification than a covalent

material? Why?

APPLICATIONS

1. What is meant by the design-limiting properties of a material in a given application?

2. There have been many attempts to manufacture and market plastic bicycles. All have

been too flexible. Which design-limiting property is insufficiently large?

3. What, in your judgement, are the design-limiting properties for the material for the blade

of a knife that will be used to gut fish?

4. What, in your judgement, are the design-limiting properties for the material of an oven

glove?

5. What, in your judgement, are the design-limiting properties for the material of an electric

lamp filament?

6. A material is needed for a tube to carry fuel from the fuel tank to the carburetor of a

motor mower. The design requires that the tube can bend and that the fuel be visible. List

what you would think to be the design-limiting properties.

7. A material is required as the magnet for a magnetic soap holder. Soap is mildly alkaline.

List what you would judge to be the design-limiting properties.

8. The cases in which most CDs are sold have an irritating way of cracking and breaking.

Which design-limiting property has been neglected in selecting the material of which they

are made?

9. List three applications that, in your judgement, need high stiffness and low weight.

10. List three applications that, in your judgement, need optical quality glass.

11. Would you expect MgO or magnesium to have the higher modulus of elasticity?

12. Would you expect Al2O3 or aluminum to have the higher coefficient of thermal

expansion? Explain.


CHAPTER 3

THE ELASTIC MODULI

INTRODUCTION

Stress causes strain. If you are human, the ability to cope with stress without undue strain is

called resilience. If you are a material, it is called elastic modulus. Stress is something that is

applied to a material by loading it. Strain depends on the magnitude of the stress and the way it is

applied—the mode of loading. Ties carry tension—often, they are cables. Columns carry

compression—tubes are more efficient as columns than solid rods because they don‘t buckle as

easily. Beams carry bending moments, like the wing spar of the plane or the horizontal roof

beams of the airport. Shafts carry torsion, as in the drive shaft of cars or the propeller shaft of the

plane. Pressure vessels contain a pressure, as in the tires of the plane. Often they are shells:

curved, thin-walled structures.

Stiffness is the resistance to change of shape that is elastic, meaning that the material returns to its

original shape when the stress is removed.

Strength is its resistance to permanent distortion or total failure. Stress and strain are not material

properties; they describe a stimulus and a response. Stiffness (measured by the elastic modulus

E) and strength (measured by the elastic limit or tensile strength) are material properties.

Stiffness and strength are central to mechanical design, often in combination with the density, ρ.

This chapter introduces stress and strain and the elastic moduli that relate them. Density and

elastic moduli reflect the mass of the atoms, the way they are packed in a material and the

stiffness of the bonds that hold them together.

There is not much you can do to change any of these, so the density and moduli of pure materials

cannot be manipulated at all. If you want to control these properties you can either mix materials

together, making composites, or disperse space within them, making foams. Property charts are a

good way to show how this works.

Concepts Of Stress And Strain

Stress is a defined quantity that cannot be directly observed or measured but it is the cause of

most failures in manufactured products. Materials respond to stress by straining. Under a given

stress, a stiff material (like steel) strains only slightly. Unlike stress, strain is a measurable

quantity. When the size or shape of a component is altered, the deformation in any dimension can

be characterized by the deformation per unit length or strain.

If a load is static or changes relatively slowly with time and is applied uniformly over a cross

section or surface of a material, the mechanical behavior may be ascertained by a simple stress–

strain test. There are three principal ways in which a load may be applied: namely, tension,

compression, and shear. In engineering practice many loads are torsional rather than pure shear.

Load

If a cylindrical bar is subjected to a direct pull or push along its axis, then it is said to be

subjected to tension or compression. Typical examples of tension are the forces present in

towing ropes or lifting hoists, whilst compression occurs in the legs of your chair as you sit on it

or in the support pillars of buildings.


In the SI system of units load is measured in newtons, although In most engineering applications,

loads appear in SI multiples, i.e. kilonewtons (kN) or meganewtons (MN).

There are a number of different ways in which load can be applied to a material. Typical loading

types are:

(a) Static or dead loads, i.e. non-fluctuating loads, generally caused by gravity effects.

(b) Liue loads, as produced by, for example, lorries crossing a bridge.

(c) Impact or shock loads caused by sudden blows.

(d) Fatigue fluctuating or alternating loads, the magnitude and sign of the load changing with

time.

Modes of loading

Most engineering components carry loads. Their elastic response depends on the way the loads

are applied. The different modes of loading are tension, compression, bending, torsion and

internal pressure. Usually one mode dominates, and the component can be idealized as one of the

simply loaded cases e.g. ties carry simple axial tension, columns do the same in simple

compression. Bending of a beam creates simple axial tension in elements on one side the neutral

axis (the center-line, for a beam with a symmetric cross-section) and simple compression in those

on the other. Shafts carry twisting or torsion, which generates shear rather than axial load.

Pressure difference applied to a shell, like a cylindrical tube generates bi-axial tension or

compression.

Direct or normal stress (a)

If a bar is subjected to a uniform tension or compression, i.e. a direct force, which is uniformly or

equally applied across the cross-section, then the internal forces set up are also distributed

uniformly and the bar is said to be subjected to a uniform direct or normal stress, the stress being

defined as

Stress,

Stress may thus be compressive or tensile depending on the nature of the load and will be

measured in units of newtons per square metre (N/m2) or multiples of this.


Direct strain (Ԑ)

If a bar is subjected to a direct load, and hence a stress, the bar will change in length. If the bar

has an original length L and changes in length by an amount L, the strain produced is defined as

follows:

Strain is thus a measure of the deformation of the material and is non-dimensional, i.e. it has no

units; it is simply a ratio of two quantities with the same unit.

Since, in practice, the extensions of materials under load are very small, it is often convenient to

eexpressed strain as a percentage.

Sign convention for direct stress and strain

Tensile stresses and strains are considered POSITIVE in sense producing an increase in length.

Compressive stresses and strains are considered NEGATIVE in sense producing a decrease in

length.

Elastic materials - Hooke’s law

A material is said to be elastic if it returns to its original, unloaded dimensions when load is

removed. A particular form of elasticity which applies to a large range of engineering materials,

at least over part of their load range, produces deformations which are proportional to the loads

producing them. Since loads are proportional to the stresses they produce and deformations are

proportional to the strains, this also implies that, whilst materials are elastic, stress is proportional

to strain. Hooke‘s law, in its simplest form, therefore states that

stress ( ) directly proportional to strain ( )

A material which has a uniform structure throughout without any flaws or discontinuities is

termed a homogeneous material. Non-homogeneous or inhomogeneous materials such as

concrete and poor-quality cast iron will thus have a structure which varies from point to point

depending on its constituents and the presence of casting flaws or impurities.


If a material exhibits uniform properties throughout in all directions it is said to be isotropic;

conversely one which does not exhibit this uniform behaviour is said to be nonisotropic or

anisotropic.

An orthotropic material is one which has different properties in different planes. A typical

example of such a material is wood, although some composites which contain systematically

orientated ―inhomogeneities‖ may also be considered to fall into this category.

Modulus of elasticity - Young’s modulus

Within the elastic limits of materials, i.e. within the limits in which Hooke‘s law applies, it has

been shown that

This constant is given the symbol E and termed the modulus of elasticity or Young‘s modulus.

Young‘s modulus E is generally assumed to be the same in tension or compression and for most

engineering materials has a high numerical value. Typically, E = 200 x l09 N/m

2 for steel, so that

it will be observed that strains are normally very small. In most common engineering applications

strains do not often exceed 0.003 or 0.3 %. The actual value of Young‘s modulus for any material

is normally determined by carrying out a standard tensile test on a specimen of the material.

Tensile test

In order to compare the strengths of various materials it is necessary to carry out some standard

form of test to establish their relative properties. One such test is the standard tensile test in

which a circular bar of uniform cross-section is subjected to a gradually increasing tensile load

until failure occurs. Measurements of the change in length of a selected gauge length of the bar

are recorded throughout the loading operation by means of extensometers and a graph of load

against extension or stress against strain is produced as shown.

The specimen is mounted by its ends into the holding grips of the testing apparatus. The tensile

testing machine is designed to elongate the specimen at a constant rate, and to continuously and

simultaneously measure the instantaneous applied load (with a load cell) and the resulting

elongations (using an extensometer).A stress–strain test typically takes several minutes to

perform and is destructive; that is, the test specimen is permanently deformed and usually

fractured.


The output of such a tensile test is recorded (usually on a computer) as load or force versus

elongation. These load–deformation characteristics are dependent on the specimen size. For

example, it will require twice the load to produce the same elongation if the cross-sectional area

of the specimen is doubled. To minimize these geometrical factors, load and elongation are

normalized to the respective parameters of engineering stress and engineering strain

Compression Test

Compression stress–strain tests may be conducted if in-service forces are of this type. A

compression test is conducted in a manner similar to the tensile test, except that the force is

compressive and the specimen contracts along the direction of the stress. By convention, a

compressive force is taken to be negative, which yields a negative stress. Tensile tests are more

common because they are easier to perform; also, for most materials used in structural

applications, very little additional information is obtained from compressive tests.

Ductile materials

The capacity of a material to allow large extensions, i.e. the ability to be drawn out plastically, is

termed its ductility. Materials with high ductility are termed ductile materials; materials with low

ductility are termed brittle materials. A quantitative value of the ductility is obtained by

measurements of the percentage elongation or percentage reduction in area.

The latter value, being independent of any selected gauge length, is generally taken to be the

more useful measure of ductility for reference purposes.

A property closely related to ductility is malleability, which defines a material's ability to be

hammered out into thin sheets. A typical example of a malleable material is lead. This is used

extensively in the plumbing trade where it is hammered or beaten into corners or joints to provide

a weatherproof seal. Malleability thus represents the ability of a material to allow permanent

extensions in all lateral directions under compressive loadings.

Brittle materials

A brittle material is one which exhibits relatively small extensions to fracture so that the partially

plastic region of the tensile test graph is much reduced. There is little or no necking at fracture

for brittle materials.


Poisson’s ratio

Consider a bar subjected to a tensile load. Under the action of this load the bar will increase in

length by an amount δL giving a longitudinal strain in the bar

The bar will also exhibit, however, a reduction in dimensions laterally, i.e. its breadth and depth

will both reduce. The associated lateral strains will both be equal, will be of opposite sense to the

longitudinal strain, and will be given by

Provided the load on the material is retained within the elastic range the ratio of the lateral and

longitudinal strains will always be constant. This ratio is termed Poisson‘s ratio.

The negative sign of the lateral strain is normally ignored to leave Poisson‘s ratio simply as a

ratio of strain magnitudes. It must be remembered, however, that the longitudinal strain induces a

lateral strain of opposite sign, e.g. tensile longitudinal strain induces compressive lateral strain.

For most engineering materials the value of v lies between 0.25 and 0.33.

Since

Hence

Shear stress

Consider a block or portion of material subjected to a set of equal and opposite forces Q. (Such a

system could be realized in a bicycle brake block when contacted with the wheel.) There is then a

tendency for one layer of the material to slide over another to produce the form of failure shown

in the figure below. If this failure is restricted, then a shear stress is set up, defined as follows

This shear stress will always be tangential to the area on which it acts; direct stresses, however,

are always normal to the area on which they act.


Shear strain

If one again considers the block of Fig. 1.12a to be a bicycle brake block it is clear that the

rectangular shape of the block will not be retained as the brake is applied and the shear forces

introduced. The block will in fact change shape or ―strain‖ into the form shown with the dotted

line.

Shear strain,

The angle is the angle of deformation.

Shear strain is measured in radians and hence is non-dimensional, i.e. it has no units.

Stress–strain curves and moduli

The figure below shows a typical tensile stress–strain curves for a ceramic, a metal and a

polymer The initial part, up to the elastic limit, is approximately linear (Hooke‘s law), and it is

elastic, meaning that the strain is recoverable—the material returns to its original shape when the

stress is removed. Stresses above the elastic limit cause permanent deformation (ductile

behavior) or brittle fracture.

Within the linear elastic regime, strain is proportional to stress. The tensile strain is proportional

to the tensile stress:

and the same is true in compression. The constant of proportionality, E, is called Young‘s

modulus. Similarly, the shear strain is proportional to the

shear stress :

and the dilatation is proportional to the pressure p:

where G is the shear modulus and K the bulk modulus, as illustrated. All three of these moduli

have the same dimensions as stress, force per unit area (N/m2 or Pa). As with stress it is

convenient to use a larger unit, gigapascals, or GPa.


Young‘s modulus, the shear modulus and the bulk modulus are related to Poisson‘s ratio. In an

isotropic material (one for which the moduli do not depend on the direction in which the load is

applied) the moduli are related in the following ways:

Commonly when

and

Elastomers are exceptional. For these when

and

This means that rubber (an elastomer) is easy to stretch in tension (low E), but if constrained

from changing shape, or loaded hydrostatically, it is very stiff (large K)—a feature designers of

shoes have to allow for.

Elastic energy

If you stretch an elastic band, energy is stored in it. The energy can be considerable: catapults can

kill people. The super-weapon of the Roman arsenal at one time was a wind-up mechanism that

stored enough elastic energy to hurl a 10 kg stone projectile 100 yards or more.

How do you calculate this energy? A force F acting through a displacement dL does work F dL.

A stress acting through a strain increment does work per unit volume

with units of J/m3. If the stress is acting on an elastic material, this work is stored as elastic

energy. The work done per unit volume as the stress is raised from zero to a final value is the

area under the stress–strain curve:

This is the energy that is stored, per unit volume, in an elastically strained material. The energy is

released when the stress is relaxed.

Measurement of Young’s modulus

In reality, moduli measured as slopes of stress–strain curves are inaccurate, often by a factor of 2

or more, because of contributions to the strain from material creep or deflection of the test

machine. Accurate moduli are measured dynamically: by measuring the frequency of natural

vibrations of a beam or wire, or by measuring the velocity of sound waves in the material. Both

depend on , so if you know the density you can calculate E.


Stress-free strain

Stress is not the only stimulus that causes strain. Certain materials respond to a magnetic field by

undergoing strain—an effect known as magneto-striction. Others respond to an electrostatic field

in the same way—they are known as piezo-electric materials. In each case a material property

relates the magnitude of the strain to the intensity of the stimulus. The strains are small but can

be controlled with great accuracy and, in the case of magneto-striction and piezo-electric strain,

can be changed with a very high frequency. This is exploited in precision positioning devices,

acoustic generators and sensors.

Modulus of rigidity

For materials within the elastic range the shear strain is proportional to the shear stress producing

it,

Double shear

Consider the simple riveted lap joint shown in Fig. 1.14a. When load is applied to the plates the

rivet is subjected to shear forces tending to shear it on one plane as indicated. In the butt joint

with two cover plates of Fig. 1.14b, however, each rivet is subjected to possible shearing on two

faces, i.e. double shear. In such cases twice the area of metal is resisting the applied forces so that

the shear stress set up is given by:

Allowable working stress-factor of safety

The most suitable strength or stiffness criterion for any structural element or component is

normally some maximum stress or deformation which must not be exceeded. In the case of

stresses the value is generally known as the maximum allowable working stress. Because of

uncertainties of loading conditions, design procedures, production methods, etc., designers

generally introduce a factor of safety into their designs, defined as follows


However, in view of the fact that plastic deformations are seldom accepted this definition is

sometimes modified to:

In the absence of any information as to which definition has been used for any quoted value of

safety factor the former definition must be assumed. In this case a factor of safety of 3 implies

that the design is capable of carrying three times the maximum stress to which it is expected the

structure will be subjected in any normal loading condition. There is seldom any realistic basis

for the selection of a particular safety factor and values vary significantly from one branch of

engineering to another. Values are normally selected on the basis of a consideration of the social,

human safety and economic consequences of failure. Typical values range from 2.5 (for

relatively low consequence, static load cases) to 10 (for shock load and high safety risk

applications)

Load factor

In some loading cases, e.g. buckling of struts, neither the yield stress nor the ultimate strength is

a realistic criterion for failure of components. In such cases it is convenient to replace the safety

factor, based on stresses, with a different factor based on loads. The load factor is therefore

defined as:

This is particularly useful in applications of the so-called plastic limit design procedures.

Temperature stresses

When the temperature of a component is increased or decreased the material respectively

expands or contracts. If this expansion or contraction is not resisted in any way then the

processes take place free of stress. If, however, the changes in dimensions are restricted then

stresses termed temperature stresses will be set up within the material.

Consider a bar of material with a linear coefficient of expansion . Let the original length of the

bar be L and let the temperature increase be t. If the bar is free to expand the change in length

would be given by

and the new length

If this extension were totally prevented, then a compressive stress would be set up equal to that

produced when a bar of length L ( 1 + t) is compressed through a distance of L t. In this case

the bar experiences a compressive strain

In most cases is very small compared with unity so that


But

Stress concentrations- stress concentration factor

If a bar of uniform cross-section is subjected to an axial tensile or compressive load the stress is

assumed to be uniform across the section. However, in the presence of any sudden change of

section, hole, sharp corner, notch, keyway, material flaw, etc., the local stress will rise

significantly. The ratio of this stress to the nominal stress at the section in the absence of any of

these so-called stress concentrations is termed the stress concentration factor.

Toughness

Toughness is defined as the ability of a material to withstand cracks, i.e. to prevent the transfer or

propagation of cracks across its section hence causing failure. Two distinct types of toughness

mechanism exist and in each case it is appropriate to consider the crack as a very high local stress

concentration.

The first type of mechanism relates particularly to ductile materials which are generally regarded

as tough. This arises because the very high stresses at the end of the crack produce local yielding

of the material and local plastic flow at the crack tip. This has the action of blunting the sharp tip

of the crack and hence reduces its stress concentration effect considerably.

The second mechanism refers to fibrous, reinforced or resin-based materials which have weak

interfaces. Typical examples are glass-fibre reinforced materials and wood. It can be shown that a

region of local tensile stress always exists at the front of a propagating crack and provided that

the adhesive strength of the fibre/resin interface is relatively low (one-fifth the cohesive strength

of the complete material) this tensile stress opens up the interface and produces a crack sink, i.e.

it blunts the crack by effectively increasing the radius at the crack tip, thereby reducing the

stress-concentration effect.

This principle is used on occasions to stop, or at least delay, crack propagation in engineering

components when a temporary "repair" is carried out by drilling a hole at the end of a crack,

again reducing its stress-concentration effect.


Plastic Deformation

For most metallic materials, elastic deformation persists only to strains of about 0.005. As the

material is deformed beyond this point, the stress is no longer proportional to strain, and

permanent, nonrecoverable, or plastic deformation occurs. The figure below plots schematically

the tensile stress–strain behavior into the plastic region for a typical metal. The transition from

elastic to plastic is a gradual one for most metals; some curvature results at the onset of plastic

deformation, which increases more rapidly with rising stress.

Tensile Properties

Yielding and Yield Strength

Most structures are designed to ensure that only elastic deformation will result when a stress is

applied. A structure or component that has plastically deformed, or experienced a permanent

change in shape, may not be capable of functioning as intended.

It is therefore desirable to know the stress level at which plastic deformation begins, or where the

phenomenon of yielding occurs. For metals that experience this gradual elastic–plastic transition,

the point of yielding may be determined as the initial departure from linearity of the stress–strain

curve; this is sometimes called the proportional limit, as indicated by point P in the figure above.

In such cases the position of this point may not be determined precisely. As a consequence, a

convention has been established wherein a straight line is constructed parallel to the elastic

portion of the stress–strain curve at some specified strain offset, usually 0.002.

The stress corresponding to the intersection of this line and the stress–strain curve as it bends

over in the plastic region is defined as the yield strength. The magnitude of the yield strength for

a metal is a measure of its resistance to plastic deformation.

Tensile Strength

After yielding, the stress necessary to continue plastic deformation in metals increases to a

maximum, point M, and then decreases to the eventual fracture. The tensile strength TS (MPa or

psi) is the stress at the maximum on the engineering stress–strain curve. This corresponds to the

maximum stress that can be sustained by a structure in tension; if this stress is applied and

maintained, fracture will result. All deformation up to this point is uniform throughout the narrow

region of the tensile specimen. However, at this maximum stress, a small constriction or neck

begins to form at some point, and all subsequent deformation is confined at this neck. This


phenomenon is termed ―necking,‖ and fracture ultimately occurs at the neck. The fracture

strength corresponds to the stress at fracture.

Tensile strengths may vary anywhere from 50 MPa (7000 psi) for an aluminum to as high as

3000 MPa (450,000 psi) for the high-strength steels. Ordinarily, when the strength of a metal is

cited for design purposes, the yield strength is used. This is because by the time a stress

corresponding to the tensile strength has been applied, often a structure has experienced so much

plastic deformation that it is useless. Furthermore, fracture strengths are not normally specified

for engineering design purposes.

Hardness

Hardness is a measure of the resistance to localized plastic deformation. In several popular

hardness-testing techniques a small indenter is forced into the surface of the material, and an

index number is determined on the basis of the size or depth of the resulting indentation. For

many metals, hardness and tensile strength are approximately proportional to each other.


Beams

A beam is commonly regarded as a structural member that is much longer than its cross-sectional

dimensions and is subjected to loads applied transverse to its longitudinal axis. Beams are

classified with respect to the type of support applied to the beam and they include:

Simple beam with pinned ends-rotation of the ends is allowed but translation is restrained

such that vertical and horizontal reactions are developed;

Simple beam with rollers-rotation of ends is allowed and only vertical reactions are

developed;

Simple beam with overhang-beam overhangs supports either at one or both ends;

Cantilever beam-one end is built into a wall preventing rotation and transverse motion

such that a moment and reactions are developed.

Continuous beam – a beam that has more than 2 simple supports.

Beams are said to be statically determinant when the re actions at the supports can be determined

by use of the equations of static equilibrium. If the number of reactions exceeds the number of

equations of static equilibrium, the beam is statically indeterminate and additional equations

based upon the deformations of the beam must be used to solve for the reactions.


Table 1: Data for Young‘s modulus, E (Ashby and Jones, Engineering Materials 1, 2nd

Edition,

Butterworth-Heinemann, 1996)


Pressure Vessels

A pressure vessel is a closed container that holds gases or liquids at a pressure different from

atmospheric. The pressure differential is potentially dangerous and many fatal accidents have

occurred in its development and operation. The design and operation of these vessels (mainly

cylindrical with hemispherical ends) are strictly regulated. Maximum safe operating conditions

are essential in avoiding accidents.

There are several examples of pressure vessels: industrial compressed air receivers, hot water

storage tanks, boilers, gas storage tanks, diving cylinders, distillation towers, nuclear

reactor vessels, habitat of a space ship or a submarine, rail/road vehicle airbrake reservoir,

vessels for storage of liquified gases such as ammonia, chlorine, propane, butane and LPG.

Generally, almost any material with good tensile properties that is chemically stable in the

chosen application can be employed. Pressure vessels are mainly made of steel with high impact

and

PROBLEMS

1. Identify which of the modes of loading is dominant in the following components:

• Fizzy drinks container.

• Overhead electric cable.

• Shoe soles.

• Wind turbine blade.

• Climbing rope.

• Bicycle forks.

• Aircraft fuselage.

Can you think of another example for each mode of loading?

1. The cable of a hoist has a cross-section of 80mm2. The hoist is used to lift a crate

weighing 500 kg. What is the stress in the cable? The free length of the cable is 3 m. How

much will it extend if it is made of steel (modulus 200 GPa)? How much if it is made of

polypropylene, PP (modulus 1.2 GPa)?

2. Water has a density of 1000 kg/m3. What is the hydrostatic pressure at a depth of 100 m?

3. A catapult has two rubber arms, each with a square cross-section with a width 4 mm and

length 300 mm. In use its arms are stretched to three times their original length before

release. Assume the modulus of rubber is 10-3

GPa and that it does not change when the

rubber is stretched. How much energy is stored in the catapult just before release?

4. What is meant by:

• A crystalline solid?

• An amorphous solid?

5. The stiffness S of an atomic bond in a particular material is 50 N/m and its center-to-

center atom spacing is 0.3 nm. What, approximately, is its elastic modulus?

6. Explain why the difference between engineering strain and true strain becomes larger as

strain increases. Is this phenomenon true for both tensile and compressive strains?

Explain.

7. A specimen of copper having a rectangular cross section 15.2 mm 19.1 mm (0.60 in.


8. 0.75 in.) is pulled in tension with 44,500 N (10,000 lb ) force, producing only elastic

deformation. Calculate the resulting strain.

9. A cylindrical specimen of a nickel alloy having an elastic modulus of 207 GPa and an

original diameter of 10.2 mm (0.40 in.) will experience only elastic deformation when a

tensile load of 8900 N (2000 lb ) is applied. Compute the maximum length of the

specimen before deformation if the maximum allowable elongation is 0.25 mm (0.010

in.).

10. An aluminum bar 125 mm (5.0 in.) long and having a square cross section 16.5 m (0.65

in.) on an edge is pulled in tension with a load of 66,700 N (15,000 lb ), and experiences

an elongation of 0.43 mm ( in.). Assuming that the deformation is entirely elastic,

calculate the modulus of elasticity of the aluminum.

11. Consider a cylindrical nickel wire 2.0 mm (0.08 in.) in diameter and mm (1200 in.) long.

Calculate its elongation when a load of 300 N (67 lb ) is applied. Assume that the

deformation is totally elastic.

12. For a brass alloy, the stress at which plastic deformation begins is 345 MPa (50,000 psi),

and the modulus of elasticity is 103 GPa

(a) What is the maximum load that may be applied to a specimen with a cross-sectional

area of 130 mm2 (0.2 in.

2) without plastic deformation?

(b) If the original specimen length is 76 mm (3.0 in.), what is the maximum length to

which it may be stretched without causing plastic deformation?

13. A cylindrical specimen of a hypothetical metal alloy is stressed in compression. If its

original and final diameters are 30.00 and 30.04 mm, respectively, and its final length is

105.20 mm, compute its original length if the deformation is totally elastic. The elastic

and shear moduli for this alloy are 65.5 and 25.4 GPa, respectively.

14. For some metal alloy, a true stress of 345 MPa (50,000 psi) produces a plastic true strain

of 0.02. How much will a specimen of this material elongate when a true stress of 415

MPa (60,000 psi) is applied if the original length is 500 mm (20 in.)? Assume a value of

0.22 for the strain-hardening exponent, n.

15. (a) A 10-mm-diameter Brinell hardness indenter produced an indentation 2.50 mm in

diameter in a steel alloy when a load of 1000 kg was used. Compute the HB of this

material.

(b) What will be the diameter of an indentation to yield a hardness of 300 HB when a

500-kg load is used?

16. A large tower is to be supported by a series of steel wires; it is estimated that the load on

each wire will be 13,300 N (3000 lbf). Determine the minimum required wire diameter,

assuming a factor of safety of 2 and a yield strength of 860 MPa (125,000 psi) for the

steel.

(a) A test piece is cut from a brass bar and subjected to a tensile test. With a load of 6.4

kN the test piece, of diameter 11.28 mm, extends by 0.04 mm over a gauge length of 50

mm. Determine:

(i) the stress, (ii) the strain, (hi) the modulus of elasticity.

(b) A spacer is turned from the same bar. The spacer has a diameter of 28 mm and a

length of 250mm. both measurements being made at 20°C. The temperature of the spacer

is then increased to 100°C, the natural expansion being entirely prevented. Taking the


coefficient of linear expansion to be 18 x 10-6/‖C determine: (i) the stress in the spacer,

(ii) the compressive load on the spacer.

17. An I-seetion girder is constructed from two 80mm x 12 mm flanges joined by an 80mm x

12mm web. Four such girders are mounted vertically one at each corner of a horizontal

platform which the girders support. The platform is 4 m above ground level and weighs

10 kN. Assuming that each girder supports an equal share of the load, determine the

maximum compressive stress set up in the material of each girder when the platform

supports an additional load of 15 kN. The weight of the girders may not be neglected. The

density of the cast iron from which the girders are constructed is 7470 kg/m3.

18. A simple turnbuckle arrangement is constructed from a 40 mm outside diameter tube

threaded internally at each end to take two rods of 25 mm outside diameter with threaded

ends. What will be the nominal stresses set up in the tube and the rods, ignoring thread

depth, when the turnbuckle cames an axial load of 30 kN? Assuming a sufficient strength

of thread, what maximum load can be transmitted by the turnbuckle if the maximum

stress is limited to 180 MN/m2?


CHAPTER 4

OVERVIEW OF ENGINEERING MATERIALS

The expertise of a chemical engineer is needed in all industries in which matter is treated to

change its state, energy or composition. Ghana, like all nations, consumes steel, concrete and

wood in construction: steel and aluminium in general engineering, copper in electrical

conductors, polymers in appliances, among others. Among metals, steel is used in the greatest

quantities, with about 90% of all the metal produced in the world being steel. The non-metals

wood and concrete are used in even greater volumes.

A small number of engineering materials, such as magnesium, are synthesised from compounds

found in the earth‘s oceans and atmosphere. Most, however, are produced by mining their ore

from the earth‗s crust, and concentrating it sufficiently to allow the material to be extracted or

synthesised from it. Copper, silver, tungsten, tin and mercury are rare, and their deposits are

comparatively small. Not all metals are thinly spread: iron and aluminium are relatively

abundant. Table 1 gives the occurrence of elements in the earth‘s crust, the oceans, and the

atmosphere.

Table 2: Abundance of elements/weight percent

The most abundant solid materials available are silicates and alumino-silicates. Carbon is the

backbone of almost all polymers, including wood. The second ingredient of most polymers -

hydrogen - is also one of the most plentiful of elements. Overall, oxygen and its compounds are

overwhelmingly plentiful – oxide-ceramics or the raw materials used to make them are abundant.

Potential shortages of materials in future can be addressed by:

Relying on material-efficient design;


Substitution – sometimes a more available material can replace the scarce one, for

example, the substitution of stone and wood by steel and concrete in construction, the

replacement of copper by polythene in plumbing, the change from copper to aluminium

in electrical wiring, and the change from wood and metals to polymers in domestic

goods; exceptions may include platinum as catalyst, liquid helium as refrigerant, and

silver on electrical contact areas; and

Recycling – include reuse of scrap metal and plastics; it is labour intensive and is usually

less economic, and therefore relies mostly on governmental interventions.

Types of Engineering Materials

The materials available to engineers can be grouped into three wide families namely:

Metal and alloys;

Ceramics and glasses; and

Polymers and elastomers.

A fourth family may be formed by combining materials from two or more of the aforementioned

families, namely, composite materials. This is shown in Figure 9 and Table 3.

Figure 4.1: The classes of engineering materials from which articles are made

Table 3: Classes of materials

Note: Ceramics are crystalline, inorganic non-metals; Glasses are non-crystalline (or

amorphous) solids. Most engineering glasses are non-metals, but a range of metallic glasses with

useful properties are available.

Note:

CFRP – Carbon fibre

reinforced polymers;

GFRP – Glass fibre

reinforced polymers


More engineering components are made of metals and alloys than of any other class of solid.

However, polymers are replacing metals because they offer a combination of properties that are

attractive. Moreover ceramics are an emerging class of engineering material which may permit

more efficient heat engines, sharper knives, and bearings with lower friction. The engineer

(designer) can combine the best properties of these materials to make composites (e.g. fibreglass)

which offer attractive packages of properties. The range of composites is a large and growing

one.

Metals

Table 4 gives a list of the generic iron-based metals. Another important group of alloys are those

based on copper (Table 5). Nickel and its alloys form another important class of non-ferrous

metals (Table 6). Nickel has good mechanical properties and is easily worked; the alloys of

nickel are usually preferred to the pure metal for most applications

Table 4: Generic iron-based metals

Table 5: Generic copper-based metals

Table 6: Generic nickel-based metals


Monel, a nickel-copper alloy, is widely used in the chemical industry. It is easily worked and has

good mechanical properties up to 500 °C. It is more expensive than stainless steel and has good

resistance to dilute mineral acids and can be used in reducing conditions, where the stainless

steels would be unsuitable. Monel may be used for equipment handling, alkalies, organic acids

and salts, and sea water.

Pure aluminium lacks mechanical strength but has higher resistance to corrosion than its alloys.

Aluminium-based metals (Table 7) are light, corrosion-resistant and non-toxic. Because

aluminium is lighter than most other metals it is the choice for transportation: aircraft, high-speed

trains, cars, etc.

Table 7: Generic aluminium-based metals

Aluminium is attacked by mineral acids, and by alkalies; but is suitable for concentrated nitric

acid, greater than 80 %. It is widely used in the textile and food industries, where the use of mild

steel would cause contamination. It is also used for the storage and distribution of deionized

water.

Titanium-based metals (Table 8) are capable of resisting corrosion, stress corrosion, and

corrosion fatigue; and are used in replacing bony parts of the human body. Titanium is used quite

widely in the chemical industry, mainly for its resistance to chloride solutions, including sea

water and wet chlorine. Titanium is being increasingly used for heat exchangers, for both shell

and tube, and plate exchangers; replacing cupro-nickel for use with sea water.

Table 8: Generic titanium-based metals


Tantalum (metallic glass) has similar corrosion resistance as glass. It is very expensive and is

used for applications where glass would not be suitable. Tantalum plugs are used to repair glass-

lined equipment.

Zirconium and its alloys have found application in the nuclear industry. In the chemical industry

it is used in places that are subjected to boiling acids: nitric, sulphuric, and hydrochloric. It is less

expensive compared to tantalum.

Silver linings are used for vessels and equipment handling hydrofluoric acid. It is used in food

and other industries where contamination is of prime importance.

Gold is rarely used for construction due to its high cost. It resists attack by dilute nitric acid and

hot concentrated sulphuric acid, but is dissolved by aqua regia (a mixture of concentrated nitric

and sulphuric acids). It is attacked by chlorine and bromine, and forms an amalgam with

mercury. It is sometimes used as thin plating on condenser tubes and other surfaces.

Platinum resist oxidation at high temperatures. One of its main uses has been, in the form of an

alloy with copper, in the manufacture of the spinnerets used in synthetic textile spinning

processes.

Property Data for Metals

A Chemical Engineer must select appropriate materials for his design buy relying on data on

material properties. Data for the main generic metals, shown in Table 9, are usually used during

the first phase of a design work. Detailed data compilations should then be used after the initial

selection. Usually, detailed material specifications should be obtained from the supplier before

final decisions are taken. And if the component is a critical one (meaning that its failure could

precipitate a catastrophe) the engineer/designer must test it himself.

Material properties are usually categorized into two – structure-insensitive properties and

structure-sensitive properties. Structure-insensitive properties depend very little on

microstructure: for e.g. the density, modulus, thermal expansion, and specific heat of any steel

are quite close to those listed in the table. On the other hand, structure-sensitive properties vary

greatly with the heat treatment and mechanical treatment, and the detailed alloy composition;

they include yield and tensile strength, ductility, fracture toughness, and creep and fatigue

strength. Note that they cannot be guessed from data for other alloys, even when differences in

composition are minute. For these it is essential to consult manufacturers‘ data sheets listing the

properties of the alloy one intends to use, with the same mechanical and heat treatment.


Table 9: Properties of the generic metal

The Light Alloys

About 14 metals, including titanium, aluminium and magnesium, have densities ≤ 4.5 Mg/m3.

The aforementioned metals are in common use as structural materials. Table gives a list of the

light alloys.


Beryllium is difficult to work with and is toxic, but it is used in moderate quantities for heat

shields and structural members in rockets. Lithium is used as an alloying element in aluminium

to lower its density and save weight on airframes1. Yttrium has an excellent set of properties and,

although scarce, has high prospects of being used in nuclear-powered aircrafts.

Even though majority of the light alloys are not suited for structural applications due to their low

melting points and reactive nature, they are widely used in non-structural applications: for

example, liquid sodium is used for cooling nuclear reactors, beryllium is used in windows for X-

ray tubes, magnesium is used as catalyst in organic reactions; and calcium, caesium, and lithium

are used as residual gas scavengers in vacuum systems.

Table 10: The light alloys

Table 11: Mechanical properties of structural light alloys

Alloys based on aluminium, magnesium and titanium are corrosion resistant and non-toxic.

Titanium has good creep2 properties. Aluminium alloys are used extensively in building and

construction: panels, roofs, and frames; in container and packaging industry, in transportation

systems, and in electrical conductors. Magnesium is lighter than aluminium but more expensive.

1 The framework and covering of an aeroplane or rocket (excluding the engines) 2 Creep is the process by which plastic flow occurs when a constant stress is applied to a metal for a prolonged

period of time.


Titanium alloys are mostly used in aerospace applications where the temperatures are too high

for aluminium or magnesium; but its extreme corrosion resistance makes it attractive in chemical

engineering, food processing and bio-engineering.

The growth in the use of these alloys is rapid, and is only surpassed by polymers.

Steels – Carbon Steels

Iron is one of the oldest known metals and is widely used in the production of steel. A range of

alloy steels are listed below:

low alloy steels (containing up to 6 % of chromium, nickel, etc.);

stainless steels (containing, typically, 18 % chromium and 8 % nickel); and

tool steels (heavily alloyed with chromium, molybdenum, tungsten, vanadium and

cobalt).

Carbon is the cheapest and most effective alloying element for hardening iron. Carbon is added

to iron in quantities ranging from 0.04 - 4 wt % to make low, medium and high carbon steels,

and cast iron3. Mild steel or low carbon steel (contains less than 0.15 % carbon) is the most

commonly used engineering material; it is cheap, available in a wide range of standard forms and

sizes, and can easily be welded. The mechanical properties are strongly dependent on both the

carbon content and on the type of heat treatment. Steels and cast iron can therefore be used in a

very wide range of applications.

Carbon lowers the melting point of iron: a medium-carbon steel must be heated to about 1500 °C

to melt it, whereas a 4 % cast iron is molten at only 1160 °C. This is why cast iron is called cast

iron: it can be melted with crude furnaces and can be cast into intricate shapes using primitive

sand casting technology.

Most cast irons are brittle and should not be used where they are subjected to shock loading or

high tensile stresses.

Steels – Alloy Steels

The alloy steels are grouped into three:

low-alloy steels;

high-alloy ―stainless‖ steels; and

tool steels.

Alloying elements are added to steels in order to:

to improve the hardenability4 of the steel;

to give solution strengthening5 and precipitation hardening;

to give corrosion resistance6;

to stabilise austenite, giving a steel that is austenitic at room temperature.

3 Alloys of iron containing more than 1.7 wt % carbon

4 The ability to form martensite in thick sections when quenched 5 The alloying elements in the low-alloy steels dissolve in the ferrite to form a substitutional solid solution. This

solution strengthens the steel and gives useful additional strength. 6 Plain carbon steels rust in wet environments and oxidise if heated in air. But if chromium is added to steel, a hard,

compact film of Cr2O3 will form on the surface and this will help to protect the underlying metal.


The stainless steels are the most widely used corrosion resistant materials in the chemical

industry.

To impart corrosion resistance the chromium content must be above 12 %, and the higher the

chromium content, the more resistant is the alloy to corrosion in oxidising condit ions. Nickel is

added to improve the corrosion resistance in non-oxidising environments. Three main types of

stainless steels, based on their microstructure, are:

Ferritic: 13-20 % Cr, < 0.1 % C, with no nickel

Austenitic: 18-20 % Cr, > 7 % Ni

Martensitic: 12-10 % Cr, 0.2 to 0.4 % C, up to 2 % Ni

The term ceramic covers an extremely

broad range of inorganic materials; they

contain non-metallic and metallic elements

and are produced by a wide variety of

manufacturing techniques. Ceramic

structures are larger and have a long

history; ancient structures such as the

Parthenon (Figure 10), the Forum, the Great

Wall of China, and the Pyramids of Egypt,

and old buildings such as Elmina castle in

Ghana, were all made from ceramic

materials.

Ceramics and Glasses

Even though ceramics are less tough than metals, they show more resistance to corrosion, wear,

decay and corruption, than metals. Most modern structures are constructed with cement and

concrete, a replacement of stone. High-performing and advanced ceramic materials have been

developed and have replaced metals in many challenging engineering applications. Wear-

resistant engineering materials are used to clad the leading edges of agricultural machinery such

as harrows, increasing the life span considerably.

In addition, due to their biocompatibility, some ceramic materials are used for making artificial

joints and other implants. They can stand much higher temperatures than metals, and are

Using chemical composition as a basis, it is possible

to classify ceramics into five main categories:

1. Oxides — alumina, Al2O3 (spark plug insulators,

grinding wheel grits), magnesia, MgO (refractory

linings of furnaces, crucibles), zirconia, ZrO2

(piston caps, refractory lining of glass tank furnaces), zirconia/alumina (grinding media),

spinels,

2. Carbides — silicon carbide, SiC (chemical plant,

crucibles, ceramic armour), silicon nitride, Si3N4

(spouts for molten aluminium, high-temperature bearings), boron nitride, BN (crucibles, grinding

wheels for high-strength steels).

3. Silicates — porcelain (electrical components), steatites (insulators), mullite (refractories).

4. Sialons — based on Si–Al–O–N and M–Si–Al–O–N where M D Li, Be, Mg, Ca, Sc, Y, rare earths

(tool inserts for high-speed cutting, extrusion

dies, turbine blades).

5. Glass-ceramics — Pyroceram, Cercor, Pyrosil

(recuperator discs for heat exchangers).


increasingly being used to replace metals in reciprocating engines, turbines, and turbochargers7.

Cutting tools made of sialons8 or of dense alumina can cut faster and last longer than the best

metal tools. Modern body-armour is made of boron carbide or alumina, weaved into a fabric vest.

Five classes of ceramic materials of particular interest to engineers are:

Glasses, all of them based on silica (SiO2), with additions to reduce the melting point, or

give other special properties;

The traditional vitreous ceramics, or clay products, used in vast quantities for plates and

cups, sanitary ware, tiles, bricks, and so forth;

The new high-performance ceramics, now finding application for cutting tools, dies,

engine parts and wear-resistant parts;

Cement and concrete: a complex ceramic with many phases, and one of three essential

bulk materials of civil engineering; and

Rocks and minerals, including ice.

Like metals, the number of different ceramics is vast. It is important for a chemical engineer to

be aware of the generic ceramics since they embody the important features. The properties of

individual ceramic materials differ widely. Common features of all ceramics include inherent

brittleness, a feature that dictates the way in which they can be used. Most ceramics are

compounds of oxygen, carbon or nitrogen with metals like aluminium or silicon; all five are

among the most plentiful and widespread elements in the Earth‘s crust.

They are cheap. The processing costs may be high, but the ingredients are almost as cheap as dirt,

which is a ceramic itself.

Glasses

Glasses are used in ginormous quantities. Large surface areas of modern office buildings in

Ghana are usually made of glass. Glass is used in a load-bearing capacity in car windows,

containers, diving bells and vacuum equipment. All important glasses are based on silica (SiO2).

Two are of primary interest: common window glass, and the temperature-resisting borosilicate

glasses. Table 12 gives details.

Table 12: Generic glasses

7 Supercharger (compressor) driven by an exhaust gas turbine

8 Sialon ceramics are a specialist class of high temperature refractory materials, with high strength (including at temperature), good thermal shock resistance and exceptional resistance to wetting or corrosion by molten non-ferrous metals, compared to other refractory materials such as, for example, alumina.


Vitreous ceramics

Potters are craftsmen who shape pottery on a potter‘s wheel and bake them in a kiln. In Ghana,

Pottery has been practised extensively by Kwahu women since the days of yore. Pottery,

porcelain, tiles, and structural and refractory bricks are made by simple processes.

All are made from clays, which are formed in the wet, plastic state and then dried and fired. Afte

firing, they consist of crystalline phases (mostly silicates) held together by a glassy phase based

on silica (SiO2). The glassy phase forms and melts when the clay is fired, and spreads around the

surface of the inert, but strong, crystalline phases, bonding them together.

Table 13: Generic vitreous ceramics

Refractory bricks and cements are needed for equipment operating at elevated temperatures, such

as fired heaters, kilns, high-temperature reactors and boilers. The refractory bricks in common

use are composed of mixtures of silica (SiO2) and alumina (Al2 O3).

High-performance engineering ceramics

Diamond is the ultimate engineering ceramic, and it is used for cutting tools, dies, rock drills,

and as an abrasive. Diamond is expensive.

The strength of a ceramic is largely determined by two characteristics: its toughness, and the size

distribution of microcracks it contains. A new class of relatively cheap, fully dense, high-strength

ceramics has being developed, which combine a higher toughness with a much narrower

distribution

of smaller microcracks, giving properties which make them competitive with metals, cermets9,

and even with diamond, for cutting tools, dies, implants and engine parts.

Cement and concrete

Cement and concrete are used in construction on an enormous scale, equalled only by structural

steel, brick and wood. Cement is a mixture of a combination of lime (CaO), silica (SiO2) and

alumina (Al2O3), which sets when mixed with water. Concrete is sand and stones (aggregate)

held together by cement. Table 15 summarises the most important facts.

9 A cermet is a composite material composed of ceramic (cer) and metallic (met) materials. A cermet is

ideally designed to have the optimal properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation.


Table 14: Generic high-performance ceramics

Table 15: Generic cements and concretes

Natural ceramics

Stone is the oldest of all construction materials and the most perdurable. The Pyramids of Egypt

are 5000 years old; the Parthenon 2200. Stone used in a load-bearing capacity behaves like any

other ceramic.

Ice forms on the Earth‘s surface in gargantuan volumes: the Antarctic ice cap, for instance, is up

to 3 km thick and almost 3000 km across. The mechanical properties are important in

engineering problems such as ice breaking, and the construction of offshore oil rigs in the Arctic.

Table 16 lists the important natural ceramics

Table 16: Natural Ceramics

Ceramic composites

Ceramic composites involve the amalgamation of ceramics and polymers or metals. The

composite will then possess the stiffness and hardness of ceramics, and the toughness of metals

or polymers.

Glass- and carbon-fibre reinforced plastics are examples.

Cermets are another example: particles of hard tungsten carbide bonded by metallic cobalt, much

as gravel is bonded with tar to give a hard-wearing road surface (another ceramic-composite).

Bone is a natural ceramic-composite: particles of hydroxyapatite (the ceramic) bonded together

by collagen (a polymer). Synthetic ceramic–ceramic composites (like glass fibres in cement, or

silicon carbide fibres in silicon carbide) are being developed.


Table 17: Ceramic Composites

Data for Ceramics

Ceramics are hard and brittle. In designing with metals, failure by plastic collapse and by fatigue

are the primary considerations; while for ceramics, plastic collapse and fatigue are seldom

problems, and rather the brittle failure, caused by direct loading or by thermal stresses is the

overriding consideration. The properties of ceramics are more variable than those of metals: the

same material, from two different suppliers, could have significant differences in toughness and

strength.

Data presented in Table 18 are approximate, suitable for the first phase of design. When the

choice has narrowed sufficiently, it is important to consult more exhaustive data compilations

and then to obtain detailed specifications from the supplier of the material you intend to use.

Finally, if the component is a critical one, the engineer must conduct his own tests.

There are many more ceramics available than those listed in the table: alumina is available in

many densities, silicon carbide in many qualities. Note that the structure-insensitive properties

(density, modulus and melting point) depend little on quality, while the structure-sensitive

properties (fracture toughness, modulus of rupture and some thermal properties including

expansion) are much more variable.

Table 18: Properties of ceramics

Note:

The modulus of rupture is the maximum surface stress when a beam breaks in

bending and the thermal shock resistance is the ability of the solid to withstand

abrupt changes in temperature. These, rather than the yield strength, tend to be

the critical properties in any design exercise.



Polymers

Most biological systems are built of polymers which not only perform mechanical functions (like

wood, bone, cartilage, leather) but also contain and regulate chemical reactions (leaf, veins,

cells).

Natural polymers have been used for thousands of years.

Wood and bone are composites: they are really made up of

stiff fibres or particles, embedded in a matrix of simple

polymer.

Man-made polymers have been used since its invention in

the last century. The industries that make high-performance

composites such as glass, carbon, or Kevlar-fibre reinforced

polymers (GFRP, CFRP, and KFRP) enjoy a relatively

faster growth rate. Composites are stiff and strong. But they

are often anisotropic, and because they are bound by

polymers, their properties can change radically with a small

change in temperature.

Proper design with polymers requires a good understanding

of their properties and the source of the polymer. Polymers

are made up of long molecules with a covalently bonded

backbone of carbon atoms. These long molecules are bonded together by weak Van der Waals

and hydrogen (―secondary‖) bonds, or by these plus covalent cross-links. The melting point of

the weak bonds is low, not far from room temperature. Most polymers are made from oil; the

technology needed to make them from coal is still poorly developed.

Important groupings of engineering polymers include:

Thermoplastics such as polyethylene, which soften on heating;

Thermosets or resins such as epoxy which harden when two components (a resin and a

hardener) are heated together;

Elastomers or rubbers; and

Natural polymers such as cellulose, lignin and protein, which provide the mechanical

basis of most plant and animal life.

The biggest use of plastics is for piping; sheets are also used for lining vessels and for fabricated

ducting and fan casings. Mouldings are used for small items; such as, pump impellers, valve parts

and pipe fittings.

The generic polymers

Thermoplastics

Thermoplastic materials soften with increasing temperatures and are readily deformed, but on

cooling they assume their original low-temperature properties and retain the shape into which

they were formed. Polyethylene is the commonest of the thermoplastics. They are linear

polymers, meaning the chains are not cross-linked (though they may branch occasionally). This

explains why they soften if heated: the secondary bonds which bind the molecules to each other

melt so that it flows like a viscous liquid, allowing it to be formed. The molecules in linear

No designer can afford to

neglect the opportunities

now offered by polymers

and composites.

But it is a mistake to imagine

that metal components can

simply be replaced by

components of these newer

materials without rethinking

the design. Polymers are less

stiff, less strong and less

tough than most metals, so

the new component requires

careful redesign.


polymers have a range of molecular weights, and they pack together in a variety of

configurations. Some, like polystyrene, are amorphous; others, like polyethylene, are partly

crystalline. Thermoplastics do not have a sharp melting point.

Instead, their viscosity falls over a range of temperature. Thermoplastics are made by adding

together sub-units (―monomers‖) to form long chains. Many of them are made of the unit

repeated several times. The radical R may simply be hydrogen (polyethylene), or —CH3

(polypropylene) or —Cl (polyvinylchloride). Nylon is more complicated. PVC is greatly used in

the chemical industry. Rigid PVC is resistant to most inorganic acids, except strong sulphuric and

nitric, and inorganic salt solutions. It is unsuitable, due to swelling, for use with most organic

solvents. The maximum operating temperature for PVC is 60 °C.

Polytetrafluroethylene (PTFE), also known as Teflon, is resistant to all chemicals except molten

alaklies , and can be used at temperatures up to 250 °C. It is a relatively weak material and is

used extensively for gaskets, gland packing, and as liner for vessels. It is difficult to fabricate and

is therefore expensive

Thermosets (Resins)

Thermosets are made by mixing two components (a resin and a hardener) which react and

harden, either at room temperature or on heating. The resulting polymer is usually heavily cross-

linked; thermosets are often described as network polymers. The cross-links form during the

polymerisation of the liquid resin and hardener, so the structure is almost always amorphous. On

reheating, the additional secondary bonds melt, and the modulus of the polymer drops; but the

cross-links prevent true melting or viscous flow so the polymer cannot be hot-worked (it turns

into a rubber). Further heating just causes it to decompose.

Epoxy, popularly used as an adhesive and as the matrix of fibre-glass, is a thermoset. Polyesters

are also widely used as matrix materials for fibre-reinforced polymers. Formaldehyde plastics are

used for moulding and hard surfacing. Urea-formaldehyde is used for electrical fittings and

melamine-formaldehyde is used for tableware10

. The main types of thermosets are shown below:

10

Articles for use at the table (dishes and silverware and glassware)


Table 19: Generic Thermoplastics

Elastomers (Rubbers)

Elastomers or rubbers are almost-linear polymers with occasional cross-links. The cross-links

ensure that the material returns to its original shape on unloading. The common rubbers are all

based on the single structure with the position R occupied by H, CH3 or Cl.

Table 20: Generic Thermosets


Natural polymers

Polyisoprene, a rubber, is a natural polymer. Other natural rubbers are cellulose and lignin, the

main components of wood and straw; and so are proteins like wool or silk. Cellulose is used in

gigantic quantities as paper. Celluloid11

and cellophane12

can also be made out of cellulose by

treating it with nitric acid. Note that the vast surplus of lignin left from wood processing, or

available in straw, cannot be processed to give a useful polymer.

Table 21: Generic Elastomers

Table 22: Generic natural polymers

11 Highly flammable substance made from cellulose nitrate and camphor; used in e.g. motion-picture and X-ray film; its use has decreased with the development of nonflammable thermoplastics 12

A transparent paper-like product that is impervious to moisture and used to wrap candy or cigarettes etc.


Data on Polymers

Selecting and using data for the properties of polymers requires caution. Specifications for metals

and alloys from two different manufacturers will not differ much, but not for polymers. For

example, polyethylene made by one manufacturer may be very different from that of another. It

is partly because all polymers contain a spectrum of molecular lengths; slight changes in

processing change this spectrum. For accurate data you must use the manufacturers‘ data sheets,

or conduct your own tests.

Table 23 Data no polymers

Many polymers contain additives – plasticisers, fillers, colourants – which change the mechanical

properties. Manufacturers will identify the polymers they sell, but will rarely disclose their

additives.


Composites

The composite industry is relatively new but has grown rapidly with the development of fibrous

composites: glass-fibre reinforced polymers (GFRP or fibreglass) and carbon-fibre reinforced

polymers (CFRP). Their use in boats, and their increasing replacement of metals in aircraft and

ground transport systems, is a revolution in material usage which is still accelerating.

Paper and concrete are composites. And almost all natural materials which must bear load –

wood, bone, muscle – are composites. Composites need not be made of fibres. Plywood is a

lamellar composite.

Particulate composites are the cheapest of all composites. Aggregate plus cement gives

concrete, and the composite is cheaper (per unit volume) than the cement itself. Polymers can be

filled with sand, silica flour, or glass particles, increasing the stiffness and wear-resistance, and

often reducing the price.

One particulate composite, tungsten carbide particles in cobalt (known as ―cemented carbide‖ or

―hard metal‖), is the basis of the heavy-duty cutting tool industry. Particulate composites are

made by blending silica flour, glass beads, and sand into a polymer during processing. There is a

small gain in stiffness, and sometimes in strength and toughness, but it is far less than in a fibrous

composite. Their attraction lies more in their low cost and in the good wear resistance that a hard

filler can give. Road surfaces are a good example: they are either macadam (a particulate

composite of gravel in bitumen, a polymer) or concrete (a composite of gravel in cement).

High stiffness is not always what is needed; cushions, packaging and crash-padding require

materials with moduli that are lower than those of any solid. This can be done by using foams –

composites of a solid and a gas – which have properties which can be tailored, with great

precision, to match the engineering need.

Cellular solids (foams)

Many natural materials, including wood, bone, cork, and coral, are cellular; cellular materials

permit an optimisation of stiffness, or strength, or of energy absorption, for a given weight of

material. These natural foams are widely used by people (wood for structures, cork for thermal

insulation), and synthetic foams are common too: cushions, padding, packaging, insulation, are

all functions filled by cellular polymers. Foams give a way of making solids which are very light

and, if combined with stiff skins to make sandwich panels, they give structures which are

exceptionally stiff and light.

Most polymers can be foamed by simple mechanical stirring or by blowing a gas under pressure

into the molten polymer. But by far the most useful method is to mix a chemical blowing agent

with the granules of polymer before processing: it releases CO2 during the heating cycle,

generating gas bubbles in the final moulding. The properties of a foam are determined by the

properties of the polymer.


CHAPTER 5

PROPERTIES OF MATERIALS

Properties of materials are highlighted in Table 24 together with other common classes of

property that the designer must consider when selecting a material. Natural materials, including

wood and leather, have properties that man-made materials are hard to beak.

Table 24: General Properties of Materials

Note: The modulus of a material

measures the resistance of the

material to elastic deflection or

bending.

Engineering design involves many

considerations, as shown in Figure

11. The selection of a material

must meet certain criteria on bulk

and surface properties (e.g.

strength and corrosion resistance).

It must also be easy to fabricate

and must appeal to potential

consumers. Furthermore, it must

compete economically with other

alternative materials.


Figure 1: How the properties of engineering materials affect the way in which products are

designed

Material Property Chart

A material property chart gives a broad overview of materials to the engineer; they usually show

relationships between two selected properties. Figure 12 shows a Young‘s modulus – density

chart for engineering materials.

From the chart, data on a given family of materials are clustered together. Each class of materials

have similar characteristics even though there are usually variations within each family due to

variations in structure with regards to:

Atomic arrangement (crystal structure);

Microstructure – size and arrangement of the crystals; and

Molecular structure


Molecular Structure of Organic Polymers and Glasses

Although their properties differ widely, all polymers are made up of long molecules. In organic

polymers the covalent chain is of carbon atoms, but in other polymers the chain could be of

oxygen or silicon atoms, for example.

The configuration of the polymer molecule is the arrangements of atoms which cannot be altered

except by breaking primary chemical bonds. The simplest structure of this kind to consider is the

linear chain of polyethylene, which is the material used in plastic shopping bags, for example.

The molecule of ethylene, C2H4, which is the monomer in this case, consists of the tetravalent

carbon atoms forming strong covalent bonds with two hydrogen atoms, leaving a double bond

between the carbon atoms, i.e.

CH2 ==CH2.

Polymerization breaks the double bond, allowing it to link with other activated ethylene

monomers forming a long chain or macromolecule. The ends of the chain either link with other

macromolecules

or end with a terminator, such as an —OH group.


In order to form solids with useful mechanical properties the polymer chains must be long: such

high polymers may contain between 103 and 10

5 monomer units in a molecule, this number being

known as the degree of polymerization.

The simple linear chain of polyethylene may have its chemical constitution modified in order to

produce materials with different properties. By replacing one or two H atoms of the monomer by

a side-group or radical, the vinyl group of polymers is formed: — (CH2—CXY)n —. If X is H

and Y is Cl, polyvinyl chloride is produced, CH3 substitution for Y produces polypropylene and

C6H5 gives polystyrene. If X is CH3 and Y is COOCH3, polymethyl methacrylate (PMMA) is

produced.

These substitutions make the monomer molecule asymmetrical so the polymer chain now can be

formed in several ways: isotactic – all side group are found on the same side, syndiotactic – side

groups alternate regularly on either side of the chain, and atactic – groups randomly alternates.

Figure 2: Schematic representation of the arrangement of side groups in linear polymers with carbon chain

C—C: (a) isotactic, (b) syndiotactic and (c) atactic.

Many apparently linear polymers are actually branched as a result of subsidiary reactions

occurring during polymerization. Polyethylene is available with a wide range of structures and

hence properties. The low density (LDPE) types are extensively branched, while high density

polyethylene (HDPE) is essentially linear. Medium density (MDPE) types fall between these

extremes and grades. They are used for in several applications, ranging from packaging for the

flexible lower density types to semi-structural for the stiffer high-density polyethylenes

Thermosets are made by mixing two components (a resin and a hardener) which react and

harden. During polymerization, chemical bonds are formed which cross-link the polymer chains.

These strong covalent bonds between adjacent chains produce polymers of greater rigidity than

the thermoplastics, which cannot be re-softened by heating once the network of cross-linking

primary bonds has been established. A common example is epoxy, commonly used as an

adhesive.

Elastomers may be classified as linear thermoset polymers with occasional cross-links which,

after removal of the load, enable the material to return to its original shape. The common

elastomers are based on the structure:


If ‗R‘is replaced by CH3 natural rubber is obtained; if replaced by H, synthetic rubber is

produced, or Cl as in neoprene which is used for making seals due to its oil-resistance.

Organic polymers are summarily classified into thermoplastics (which may in turn be either

amorphous or partially crystalline) and thermosets (which may be either highly or lightly cross-

linked).

Inorganic glasses

Glasses are the most common inorganic polymers. The main difference between them and the

long-chain organic materials described above is that the molecular chains in glass consist of more

complex units based on the SiO4 tetrahedral unit.

Mechanical Properties of Materials

Tensile strength (Tensile stress or Ultimate Tensile Stress)

The tensile strength of a material is a measure of the basic strength of the material. It represents

the maximum stress that the material can withstand, measured by a standard tensile test. The

design stress for a material, the value used in any design calculations, is based on the tensile

strength, or on the yield or proof stress.

Stiffness

Stiffness is the ability to resist bending and buckling. It is a affected by the elastic modulus of the

material and the shape of the cross-section of the member

Toughness

Toughness is a measure of the material's resistance to crack propagation. The crystal structure of

ductile materials, such as steel, aluminium and copper, is such that they stop the propagation of a

crack by local yielding at the crack tip. In other materials, such as the cast irons and glass, the

structure is such that local yielding does not occur and the materials are brittle.

Brittle materials are weak in tension but strong in compression. Under compression any incipient

cracks present are closed up. Various techniques have been developed to allow the use of brittle

materials in situations where tensile stress would normally occur. For example, the use of

pressurised concrete, and glass-fibre-reinforced plastics in pressure vessels construction.

Hardness

The surface hardness indicates the ability of a material to withstand wear. Hardness is usually

significant if the designer is selecting a material to handle abrasive solids or liquids containing

suspended solids which are likely to cause erosion.

Fatigue

Fatigue failure is likely to occur in equipment subject to cyclic loading; for example, rotating

equipment, such as pumps and compressors, and equipment subjected to pressure cycling.


Creep

Creep is the gradual extension of a material under a steady tensile stress, over a prolonged period

of time. It is usually only important at high temperatures; for instance, with steam and gas turbine

blades. For a few materials, notably lead, the rate of creep is significant at moderate

temperatures. Lead will creep under its own weight at room temperature and lead linings must be

supported at frequent intervals. The creep strength of a material is usually reported as the stress

to cause rupture in 100,000 hours, at the test temperature

Physical Properties of Engineering Materials

Density

Density is defined as the mass per unit volume of a material. It increases regularly with

increasing atomic numbers in each sub-group. The reciprocal of the density is the specific

volume v, and the product of specific volume and the relative atomic mass W is known as the

atomic volume Ω. The density is usually determined by the ‘immersion’ method or by the use of

X-rays.

The density depends on the mass of the atoms, their size and the way they are packed. Metals

have high densities because they have heavy atoms and are closely packed; ceramics have lower

densities than metals because they contain light atoms, C, N or O; and polymers have low

densities because they consist of light atoms in chains.

Thermal Properties

Thermal Expansion

The change in volume with temperature is important in many metallurgical operations such as

casting, welding and heat treatment.

Most metals increase their volume by about 3% on melting, although those metals which have

crystal structures of lower coordination, such as bismuth, antimony or gallium, contract on

melting. For the simple metals the latent heat of melting, which is merely the work done in

separating the atoms from the close-packed structure of the solid to the more open liquid

structure, is only about one thirtieth of the latent heat of evaporation.

Specific Heat Capacity

The specific heat is another thermal property important in the processing operations of casting or

heat treatment, since it determines the amount of heat required in the process. The specific heat

of a metal is due almost entirely to the vibrational motion of the ions.

Other thermal properties of engineering materials include Specific Heat Curve and Free Energy

of Transformation.


Figure 3: Material Property Chart for density

Electrical Properties

Electrical conductivity

One of the most important electronic properties of metals is the electrical conductivity, and the

reciprocal of the conductivity, resistivity, is defined by the relation

R = ρl/A;

where R is the resistance of the specimen, l is the length and A is thecross-sectional area.

Metals have high electrical conductivities due to the ease with which the electrons can migrate

through the lattice. The high thermal conduction of metals also has a similar explanation. Since

conductivity arises from the motion of conduction electrons through the lattice, resistance must

be caused by the scattering of electron waves by any kind of irregularity in the lattice

arrangement. Irregularities can arise from any one of several sources, such as temperature,

alloying, deformation or nuclear irradiation, since all will disturb, to some extent, the periodicity

of the lattice.

The resistance increases linearly with temperature above about 100 K up to the melting point. On

melting, the resistance increases markedly because of the exceptional disorder of the liquid state.


However, for some metals such as bismuth, the resistance actually decreases, owing to the fact

that the special zone structure which makes bismuth a poor conductor in the solid state is

destroyed on melting.

In most metals the resistance approaches zero at absolute zero, but in some (e.g. lead, tin and

mercury) the resistance suddenly drops to zero at some finite critical temperature above 0 K.

Such metals are called superconductors.

Magnetic Properties

Magnetic properties of materials include diamagnetism, paramagnetism and ferromagnetism

Dielectric Materials

Pyroelectric and Ferroelectric materials

Some materials, associated with low crystal symmetry, are observed to acquire an electric charge

when heated; this is known as pyroelectricity. Pyroelectric materials are used as detectors of

electromagnetic radiation in a wide band from ultraviolet to microwave, in radiometers and in

thermometers sensitive to changes of very low temperatures. Pyroelectric TV camera tubes have

also been developed for long-wavelength infrared imaging and are useful in providing visibility

through smoke. A typical material is strontium barium niobate.

Optical Properties

The optical properties of a material are related to the interaction of the material with

electromagnetic radiation, especially visible light. Photons incident on a material may be

reflected, absorbed or transmitted. Metals are opaque to all electromagnetic radiation except

high-energy X-rays and γ-rays. Much of the absorbed radiation is reemitted as radiation of the

same wavelength (i.e. reflected). Metals are both opaque and reflective and it is the wavelength

distribution of the reflected light, which we see, that determines the colour of the metal. Thus

copper and gold reflect only a certain range of wavelengths and absorb the remaining photons,

i.e. copper reflects the longer-wavelength red light and absorbs the shorter-wavelength blue.

Aluminium and silver are highly reflective over the complete range of the visible spectrum and

appear silvery.

High-purity ceramics and polymers are transparent to visible light. Semiconductors are opaque to

short wavelengths and transparent to long ones. Glasses and polymers may be transparent in the

amorphous state but opaque when crystalline. High-purity non-metallics such as glasses,

diamond, or sapphire (Al2O3) are colourless but are changed by impurities.

Lasers

A laser (Light Amplification by Stimulated Emission of Radiation) is a powerful source of

coherent light (i.e. monochromatic and all in phase). The original laser material, still used, is a

single crystal rod of ruby, i.e. Al2O3 containing dopant13

Cr3+

ions in solid solution. Lasers can be

solid, liquid or

gaseous materials and ceramics, glasses and semiconductors.

13

Something added in trace amounts to a semiconductor to produce the desired electrical properties


CHAPTER 6

OXIDATION AND CORROSION

Corrosion involves the deterioration of a material as it reacts with its environment. Corrosion is the

primary means by which metals deteriorate. Corrosion involves two chemical processes - oxidation

and reduction; oxidation is the process of stripping electrons from an atom and reduction occurs

when an electron is added to an atom. Corrosion reduces the load carrying capability of materials and

causes stress concentrations. It is a major part of maintenance cost and its prevention is vital in many

engineering applications. It is a natural process that commonly occurs because unstable materials,

such as refined metals want to return to a more stable compound. For example, some metals, such as

gold and silver, can be found in the earth in their natural, metallic state and they have little tendency

to corrode.

Surface problems such as corrosion and wear, and fatigue-cracking of metals are very important to

engineers. For instance, with regard to corrosion, metal surfaces commonly oxidize in air at ambient

temperatures to form a very thin oxide film (tarnish). This ‗dry‘ corrosion is limited, destroys little of

the metallic substrate and is not normally a serious problem.

However, at elevated temperatures, nearly all metals and alloys react with their environment at an

appreciable rate to form a thick nonprotective oxide layer (scale). ‗Wet‘ or aqueous corrosion, in

which electrochemical attack proceeds in the presence of water, can also destroy metallic surfaces

and is responsible for a wide variety of difficult problems in all branches of industry. Requirements

of high temperature materials such as turbine blades or superheater tubes include the ability to resist

attack by high temperature gases, and to resist corrosion.

Ceramics are abundant; the earth's crust is almost entirely made of oxides, silicates, aluminates and

other compounds of oxygen; and being oxides already, they are completely stable. Alkali halides -

NaC1, KC1, NaBr – are also stable and are widely found in nature.

By contrast, metals are unstable: only gold is found in 'native' form under normal circumstances. All

the others will oxidise in contact with air. Polymers are not stable either and most will burn if

ignited, meaning that they oxidise readily. Coal and oil (the raw materials for polymers), are sealed

from all contact with air.

A few polymers, among them PTFE (a polymer based on -CF2-), are so stable that they survive long

periods at high temperatures.

The Energy of Oxidation

The tendency of a material to react with oxygen can be determined by laboratory tests which

measure the energy needed for the reaction:

Material + Oxygen + Energy → Oxide of material

If this energy is positive, the material is stable; if negative, it will oxidise. Figure 18 and Table 25

give the energies of oxide formation for four groups of materials, and numerical values, respectively.


Kinetics of Oxidation

During oxidation the first oxygen molecules to be absorbed on the metal surface dissociate into their

component atoms before bonding chemically to the surface atoms of the metal. This process,

involving dissociation and ionization, is known as chemisorption14

. After the build-up of a few

adsorbed layers the oxide is nucleated epitaxially15

on the grains of the base metal at favourable sites,

such as dislocations and impurity atoms. Each nucleated region grows, impinging on one another

until the oxide film forms over the whole surface.

If the oxide film initially produced is porous the oxygen is able to pass through and continue to react

at the oxide–metal interface. Usually, however, the film is not porous and continued oxidation

involves diffusion through the oxide layer. If oxidation takes place at the oxygen–oxide surface, then

metal ions and electrons have to diffuse through from the underlying metal. When the oxidation

reaction occurs at the metal–oxide interface, oxygen ions have to diffuse through the oxide and

electrons migrate in the opposite direction to complete the reaction. The rate at which the oxide film

thickens depends on the temperature and the material.

Figure 4: Energies of formation of oxides at 273 K/kJmol-1

of oxygen O2

14 Adsorption (especially when irreversible) by means of chemical instead of physical forces 15 Related to the growth of a crystal layer of one mineral on the crystal base of another mineral in such a manner that its crystalline orientation is the same as that of the substrate


Dry Oxidation

Most materials that are unstable in oxygen tend to oxidise. Engineers are concerned with the loss of

material at high temperatures, in dry environments, and under dry conditions. Oxidation is usually

controlled by the diffusion of ions or the conduction of electrons through oxide films that formed on

the material surface.

Since diffusion and reaction processes increase with temperature, the rate of oxidation is much

greater at high temperatures than at low, even though at room temperature very thin films of oxide do

form on all unstable metals. This minute amount of oxidation is essential: it protects and prevents

further attack, it causes tarnishing; it makes joining difficult; and it helps keep sliding surfaces apart

and thus influences the coefficient of friction.

Table 25: Energies of formation of oxides at 273 K

Figure 5: Dry oxidation


Making Stainless Alloys

Mild steel is an excellent structural material - cheap, easily formed and strong mechanically. But at

low temperatures it rusts, and at high, it oxidises rapidly. A range of stainless

irons and steels has been developed to meet the demand for those products.

When mild steel is exposed to hot air, it oxidises quickly to form FeO (or higher

oxides). But if one of the elements near the top of Table 25 with a large energy

of oxidation is dissolved in the steel, then this element oxidises preferentially

(because it is much more stable than FeO), forming a layer of its oxide on the

surface. And if this oxide is a protective one, like Cr2O3, A1203, SiO, or BeO, it

stifles further growth, and protects the steel.

A considerable quantity of this foreign element is needed to give adequate

protection. The best is chromium which gives a very protective oxide film.

Other elements, when dissolved in steel, cut down the rate of oxidation. A1203

and SiO2 both form in preference to FeO and form protective films.

Thus 5 % A1 dissolved in steel decreases the oxidation rate by 30 times, and 5%

Si by 20 times. Even silver can be prevented from tarnishing (reaction with

sulphur) by alloying it with aluminium or silicon, giving protective A1203 or

Si02 surface films.

Ceramics such as silicon carbide, SiC, and silicon nitride, Si3N4 both have large negative energies of

oxidation (meaning that they oxidise easily). But when they do, the silicon in them turns to SiO,

which quickly forms a protective skin and prevents further attack. This protection-by-alloying has

one great advantage over protection by a surface coating (like chromium plating or gold plating): it

repairs itself when damaged. If the protective film is scored or abraded, fresh metal is exposed, and

the chromium (or aluminium or silicon) it contains immediately oxidises, healing the break in the

film.

Protection of Turbine Blades

The materials at present used for turbine blades consist chiefly of nickel, with various foreign

elements added to get favourable creep properties. The alloys used for turbine blades contain large

amounts of chromium, dissolved in solid solution in the nickel matrix.

Wet Corrosion of Materials

Under wet conditions, a different picture of oxidation is realized. When mild steel is exposed to

oxygen and water at room temperature, it rusts rapidly and the loss of metal quickly becomes

appreciable. Unless special precautions are taken, the life of most structures, from bicycles to

bridges, from buckets to battleships, is limited by wet corrosion.

Schematic of Wet Corrosion

Consider iron immersed in aerated water (Figure 20).

Note:

The excess electrons generated in the

electrolyte either

reduce hydrogen ions (particularly in acid

solutions) according

to

222 HeH

so that gas is evolved

from the metal.


Figure 6: Wet corrosion

Iron atoms go into solution in the water as Fe++

, leaving behind two electrons each (the anodic

reaction):

eFeFe 22

These are conducted through the metal to a place where the 'oxygen reduction' reaction can take

place to consume the electrons (the cathodic reaction):

OHOHeO 424 22

This reaction forms OH- ions which then amalgamates with the Fe

++ ions to form a hydrated iron

oxide Fe(OH)2 (really FeO, H2O); but instead of forming on the surface where it might give some

protection, it often forms as a precipitate in the water itself. The reaction can be summarised by

Material + Oxygen → (Hydrated) Material Oxide

Note that the formation and solution of Fe2+

is analogous to the formation and diffusion of M2+

in an

oxide film under dry oxidation; and the formation of OH- is closely similar to the reduction of

oxygen on the surface of an oxide film.

The much faster attack found in wet corrosion is due to the following:

a. The Fe(OH)2 either deposits away from the corroding material; or, if it deposits on the

surface, it does so as a loose deposit, giving little or no protection.

b. Consequently M++

and OH- usually diffuse in the liquid state, and therefore do so very

quickly;

c. In conducting materials, the electrons can move very easily as well.

The result is that the oxidation of iron in aerated water (rusting) goes on at a rate which is millions of

times faster than that in dry air. Because of the importance of (c), wet oxidation is a particular

problem with metals.

Voltage differences as a driving force for wet oxidation

In dry oxidation, the tendency for a material to oxidize is depended on the energy needed, in kJ/mol

of O2. Because wet oxidation involves electron flow in conductors, which is easier to measure, the

tendency of a metal to oxidise in solution is determined by using a voltage scale rather than an

energy one. The Figure below shows the voltage differences that would just stop various metals

oxidising in aerated water.


The information in Figure 21 is similar to the energies of oxidation, even though there are some

differences in ranking as a result of differences between the detailed reactions that go on in the dry

and wet oxidation.

Rates of wet oxidation

The rates of wet oxidation found in practice bear little relationship to the voltage driving forces for

wet oxidation. Figure 22 shows the approximate surface losses of some metals in mm per year in

clean water, which is almost the reverse of the order expected in terms of the voltage driving forces

for wet oxidation.

Figure 7: Corrosion rates of some metals in clean water


The slow rate of wet oxidation for Al, for example, arises because it is very difficult to prevent a

thin, dry oxidation film of A12O3 forming on the metal surface. In brine, on the other hand, A1

corrodes very rapidly because the chloride ions tend to break down the protective A12O3 film.

Because of the effect of 'foreign' ions like this in most practical environments, corrosion rates vary

very widely for most materials.

Localized Attack – Corrosion Cracking

Often wet corrosion attacks metals selectively as well as (or instead of) uniformly, and this can lead

to component failure much more rapidly and insidiously. Stress and corrosion acting together can be

particularly bad, giving cracks which propagate rapidly and unexpectedly. Four types of corrosion

cracking commonly lead to unplanned failures, and they include:

a. Stress corrosion cracking

In some materials and environments, cracks grow steadily under a constant stress intensity. This is

dangerous since a structure which is safe when built can become unsafe with time. Examples are

brass in ammonia, mild steel in caustic soda, and some A1 and Ti alloys in salt water.

Figure 8: Stress corrosion cracking

a. Corrosion fatigue

Corrosion increases the rate of growth of fatigue cracks in most metals and alloys. The crack growth

rate is usually larger than the sum of the rates of corrosion and fatigue, each acting alone.

Figure 9: Corrosion fatigue.

a. Intergranular attack

Grain boundaries have different corrosion properties from the grain and may corrode preferentially,

giving cracks that then propagate by stress corrosion or corrosion fatigue.

Figure 10: Intergranular attack


a. Pitting

Pitting is the formation of small pits in a surface as a consequence of corrosion. Preferential attack

can also occur at breaks in the oxide film (caused by abrasion), or at precipitated compounds in

certain alloys.

Figure 11: Pitting corrosion

Case Studies in Wet Corrosion

a. Protection of Underground Pipes

Steel pipelines are laid under, or in contact with, the ground for long-distance transport of oil, natural

gas, etc. Corrosion is a problem if the ground is damp and if the depth of soil is not so great that

oxygen is effectively excluded. Then the oxygen reduction reaction

O2 + 2H20 + 4e = 40H-

And the metal-corroding reaction

Fe = Fe++

+ 2e

can take place, causing the pipe to corrode. Because of the capital cost of pipelines, their

inaccessibility if buried, the disruption to supplies caused by renewal, and the potentially catastrophic

consequences of undetected corrosion failure, it is very important to make sure that pipelines do not

corrode.

One way of protecting the pipe is by covering it with some unreactive material to keep water and

oxygen out: thick polyethylene sheet stuck in position with butyl glue, for example. However, such

coverings rarely provide complete protection - rough handling on site frequently leads to breakages

of the film, and careless wrapping of welds leaves metal exposed.

Engineers usually resort to sacrificial protection to arrest the problem. If the pipe is connected to a

slab of material which has a more negative corrosion voltage, then the couple forms an electrolytic

cell, and the more electronegative material becomes the anode (and dissolves), and the pipe becomes

the cathode (and is protected).


12: Sacrificial protection of pipelines; typical materials used are Mg (with 6% AI, 3% Zn, 0.2% Mn),

AI (with 5% Zn) and Zn

Pipelines are protected from corrosion by being wired to anodes in just this way, as shown in Figure

27. Magnesium alloy is often used because its corrosion voltage is very low (much lower than that of

zinc) and this attracts Fe++

to the steel very strongly; aluminium alloys and zinc are also used. The

alloying additions help prevent the formation of a protective oxide on the anode - which might make

it become cathodic.

Naturally, because the protection depends on the dissolution of the anodes, these require replacement

from time to time (hence the term 'sacrificial' anodes). In order to minimise the loss of anode metal, it

is important to have as good a barrier layer around the pipe as possible, even though the pipe would

still be protected with no barrier layer at all.

a. Protection by imposing a potential

In this scenario, scrap steel is buried near the pipe and connected to it through a battery or d.c. power

supply, which maintains a sufficient potential difference between them to make sure that the scrap is

always the anode and the pipe the cathode. This alone will protect the pipe, but unless the pipe is

coated, a large current will be needed to maintain this potential difference.

Figure 13: Protection of pipelines by imposed potential


b. Alternative materials

Most alternative materials for long-distance pipe lines are not used because of cost; it is much

cheaper to build and protect a mild steel pipe than to use stainless steel instead – even though no

protection is then needed. The only competing material is a polymer, which is completely immune to

wet corrosion of this kind. For large diameter transmission lines, the mechanical strength of steel

makes it the preferred choice.

Materials for a Lightweight Factory Roof

Galvanized steel is usually preferred by many people because it is strong, light, cheap, and easy to

install. Even though new galvanized steel is rust free, rusting sets in after about 20 years of use,

thereby causing roofing failure.

The galvanising process leaves a thin layer of zinc on the surface of the steel. This acts as a barrier

between the steel and the atmosphere; and although the driving voltage for the corrosion of zinc is

greater than that for steel in fact zinc corrodes quite slowly in a normal urban atmosphere because of

the barrier effect of its oxide film.

If scratches and breaks occur in the zinc layers by accidental damage - which is certain to occur

when the sheets are erected - then the zinc will cathodically protect the iron in exactly the way that

pipelines are protected using zinc anodes. This explains the long postponement of rusting. But the

coating is only about 0.15 mm thick, so after about 30 years most of the zinc has gone, rusting

suddenly becomes prevalent, and the roof fails.

Figure 14: Galvanised steel is protected by a sacrificial layer of zinc

Alternative materials

A relatively recent innovation has been the architectural use of anodised aluminium. Although the

driving force for the wet oxidation of aluminium is very large, aluminium corrodes very slowly in

fresh-water environments because it carries a very adherent film of the poorly conducting A12O3. In

anodised aluminium, the A12O3 film is artificially thickened in order to make this barrier to corrosion

extremely effective.

Also, corrugated plastic sheet is commonly used for roofing small sheds, car ports and similar

buildings; but although polymers do not generally corrode - they are often used in special

environments like chemical plant - they are prone to damage by the ultraviolet wavelengths of the

sun's radiation.

Note:


Fixing a galvanized or aluminium roof with nails or screws of a different metal, such as Cu or brass,

should always be avoided. Cu acts as cathode, and the zinc or aluminium corrodes away rapidly near

to the fastening.

Automobile Exhaust Systems

The lifetime of a conventional exhaust system on an average family car is about 2 years. Mild steel is

the usual material for making exhaust system. Mild steel corrodes easily. The interior of the exhaust

system is not painted and begins to corrode immediately in the damp exhaust gases from the engine.

A single coat of cheap cosmetic paint soon fades and rusting sets in, under the aegis of road salt.

The lifetime of the exhaust system could be improved by galvanising the steel. But there are

problems in using platings where steel has to be joined by welding. Zinc, for example, melts at

420 °C and would be burnt off the welds; and breaks would still occur if plating metals of higher

melting point (e.g. Ni, 1455 °C) were used.

Alternative materials

The most successful way of combating exhaust-system corrosion is the use of stainless steel. This is

a good example of how - just as with dry oxidation - the addition of foreign atoms to a metal can

produce stable oxide films that act as barriers to corrosion.

Rates of oxidation

It is always important to know the quickness of oxidation when designing with oxidation-prone

materials. Oxidation rates follow Arrheniu‘s law, that is, the kinetic constants kL and kp increase

exponentially with temperature:

RTQe

pA

pk

andRT

LQ

eL

AL

k

/

;/

Uniform Corrosion

In uniform corrosion, there is uniform wastage of material, and there is no pitting or other forms of

local attack. If the corrosion of a material can be considered to be uniform the life of the material in

service can be predicted from experimentally determined corrosion rates.

Corrosion rates are expressed as a penetration rate in inches per year (ipy), or mills16

per year (mpy).

They are also expressed as a weight loss in milligrams per square decimetre17

per day (mdd). In

corrosion testing, the corrosion rate is measured by the reduction in weight of a specimen of known

area over a fixed period of time.

tA

wipy

12

where w = mass loss in time t, Ib,

t = time, years,

16 1 mill = 10-3 inches 17 1 metre = 10 decimetres


A = surface area, ft2,

p = density of material, lb/ft3,

Note: 1 ipy = 25 mm per year.

For ferrous metals 100 mdd = 0.02 ipy.

An acceptable rate of attack will depend on the cost of the material, the safety and the economic life

of the plant. For inexpensive materials such as the carbon and low alloy steels, a guide to what is

considered acceptable is given in the table below. For the more expensive alloys, such as the high

alloy steels, the brasses and aluminium, the figures given in the table should be multiplied by 0.5.


CHAPTER 7

BIOMATERIALS

Biomaterials are materials used in medicine and dentistry that are intended to come in contact with

living tissue. Biomaterials are used in tooth filling and used for implants such as joint replacements,

particularly hips, and cardiovascular repairs.

Biomaterial implants improve the quality of life for an increasing number of people globally, for

both old and young people with heart problems, injuries or inherited diseases. First-generation

biomaterials largely depended on being inert with minimal tissue reaction. For these materials a

minimal fibrous layer forms between the biomaterials and the body when the material is not totally

accepted by the body. The success of this type of implant depends largely on the selection of

materials for their manufacture.

Standard hip replacement initially used a multi-component assembly made with austenitic stainless

steel for the stem, PMMA for fixation and polyethylene for the acetabular cup (Figure 30).

Figure 15: Schematic diagram of a replacement hip joint

While continuing with improved bioinert materials, development has focused on bioactive materials

which influence the biological response in a positive way, e.g. encourage bonding to surrounding

tissue with stimulation of new bone growth. With this bio-active approach the interface between the

body cells and the implant is critical and the materials science of the biomaterials surface is

extremely important.

Requirements for biomaterials

Biomaterial applications make use of all classes of material, metals, ceramics, polymers and

composites, divided roughly into three user-types, namely:

inert or relatively inert with minimal host response;

bioactive which actually stimulates bonding to the surrounding tissue; and

biodegradable which reabsorbed in the body over a period of time.


Metals are generally chosen for their inert qualities whereas ceramics and polymers may offer

bioactivity or resorption. The most common metallic materials used are austenitic stainless steels,

cobalt–chromium alloys or titanium, and titanium alloys.

Of the ceramics, aluminium oxide, calcium phosphate, apatite, carbon/graphite and bioglass are in

use mainly for their inertness, good wear characteristics, high compressive strength and in some

cases bioactivity. Their poor tensile properties and fracture toughness are design limitations.

Polymers are widely used, both alone and in combination with ceramics or metals. These include;

polymethyl methacrylate (PMMA) for cement and lenses; polyethylene for orthopaedics;

polyurethane as blood contact material, e.g. vascular tubing, cardiovascular devices, catheters;

polysiloxanes in plastic surgery, maxillofacial and cardiovascular surgery; polyesters and polyamides

in wound closure management.

Composites such as ultrahigh-molecular-weight polyethylene reinforced with either carbon fibres or

the ceramic hydroxyapatite are increasingly being considered for applications involving high contact

stress and wear resistance.

In dentistry, dental amalgam is made by mixing silver, tin, copper alloy powder with mercury and

this mixture is packed into the cavity where it hardens to produce a strong, corrosion-resistant,

biocompatible filling. Cavities in front teeth are usually filled with glass cements to match the colour

and translucency of the enamel.

Missing teeth may be replaced by artificial teeth in a number of different ways. For a group of

missing teeth, removing partial dentures (RPDs) may be the answer; it consist of a cast metal

framework of Co–Cr or Ni–Cr alloy carrying the artificial teeth and having end clasps to retain it to

good natural teeth nearby.


CHAPTER 8

SELECTION OF MATERIALS

Materials selection is an integral part of chemical engineering design. Innovation in design work is

usually made possible by the use of new materials. Designers have at their disposal a whole range of

materials – metals, ceramics, polymers, and composites. Each class of material has its own strengths

and limitations, which the designer must be fully aware of.

Many factors have to be considered when selecting engineering materials; for chemical process plant

the predominant consideration is usually the ability of the material to resist corrosion. The process

designer will be responsible for recommending materials that will be suitable for the process

conditions. He must also consider the requirements of the mechanical design. In other words, the

material selected must have sufficient strength and be easily worked.

The most economical material that satisfies both process and mechanical requirements should be

selected; this will be the material that gives the lowest cost over the working life of the plant,

allowing for maintenance and replacement. Other factors, such as product contamination and process

safety, must also be considered.

The mechanical properties that are important in the selection of materials of construction include:

Strength - tensile strength;

Stiffness - elastic modulus (Young's modulus);

Toughness - ability to resist fracture;

Hardness - ability to resist wear;

Fatigue resistance; and

Creep resistance

Note: The effect of temperature on the aforementioned properties is equally important

Other important characteristics include:

Resistance to corrosion;

Special properties that may be required such as, thermal conductivity, electrical resistance,

magnetic properties, optical characteristics, etc

Ease of fabrication-forming, welding, casting;

Availability in standard sizes-plates, sections, tubes; and

Cost


Selection of Materials for Corrosion Resistance

Before the designer selects a material for a specific application, he needs to identify the process

environment to which the material will be exposed. Additional to the main corrosive chemicals

present, the following factors must be considered:

Temperature – affects corrosion rate and mechanical properties;

Pressure;

pH;

Presence of trace impurities - stress corrosion;

The amount of aeration - differential oxidation cells;

Stream velocity and agitation-erosion-corrosion; and

Heat-transfer rates - differential temperatures.

The conditions that may arise during abnormal operation, such as at start-up and shutdown, must be

considered, in addition to normal, steady state, operation. Handbooks of corrosion gives detailed

information on the selection of materials for corrosion prone environment.

Cost of Materials

The cost of materials is one of the most important factors engineers consider in selecting materials.

Surface finish

Surface finish is very important in some industries such as food, textile, biochemical, and

pharmaceutical. Avoidance of contamination is of prime interest. Stainless steel is widely used and

the surfaces are usually polished for hygienic reasons – to aid cleaning and to prevent bacteria

growth.

Effect of Temperature on the Properties of Materials

The elastic modulus and the tensile strength of metals decrease with increasing temperature. At high

temperatures materials that retain their strength must always be selected for used in equipment

working under such conditions. Stainless steels are superior for such purposes.

In conditions where there are high stresses in addition to very high temperatures (e.g. furnace tubes),

creep resistance is common and materials and special alloys such as Inconel (International Nickel

Co.) are suited for such purposes. In low temperature applications, including cryogenic plants and

liquefied gas storage, austenitic stainless steel or aluminium alloys (hex) should be specified.


PROBLEMS

1. Explain what is meant by the following terms:

structure-sensitive property;

structure-insensitive property.

a. List five different structure-sensitive properties.

b. List four different structure-insensitive properties.

2. What are the five main generic classes of metals?

For each generic class:

(a) give one example of a specific component made from that class;

(b) briefly explain why that class was selected for the component.

3. What are the four main generic classes of polymers?



(b) indicate why that class was selected for the component.

4. How do the unique characteristics of polymers influence the way in which they are used?

5. Why are non-heat treatable alloys used for can manufacture?

6. What are the five main generic classes of ceramics and glasses?



(b) indicate why that class was selected for the component.

7. How do the unique characteristics of ceramics and glasses influence the way in which these

materials are used?

8. Select a suitable material to be used for constructing a pipeline for carrying each of the

following fluids at an approximate pressure of 2 bar:

a. 98 per cent w/w sulphuric acid at 70 °C.

b. 5 per cent w/w sulphuric acid at 30 °C.

c. 30 per cent w/w hydrochloric acid at 50 °C.

d. 5 per cent aqueous sodium hydroxide solution at 30 °C.

e. Concentrated aqueous sodium hydroxide solution at 50 oC.

f. 5 per cent w/w nitric acid at 30 °C.

g. Boiling concentrated nitric acid.

h. 10 per cent w/w sodium chloride solution.

i. A 5 per cent w/w solution of cuprous chloride in hydrochloric acid.

j. 10 per cent w/w hydrofluoric acid.

9. Suggest suitable materials of construction for the following applications:

i. A 10,000 m3 storage tank for toluene.

ii. A 5.0 m3 tank for storing a 30 % w/w aqueous solution of sodium chloride.

iii. A 2 m diameter, 20 m high distillation column, distilling acrylonitrile.

iv. A 100 m3 storage tank for strong nitric acid.


v. A 500 m3 aqueous waste hold-up tank. The wastewater pH can vary from 1 to

12, and may contain traces of organic material.

vi. A packed absorption column 0.5 m diameter, 3 m high, absorbing gaseous

hydrochloric acid into water. The column will operate at essentially atmospheric

pressure.

10. A pipeline constructed of carbon steel failed after 3 years operation. On examination it was

found that the wall thickness had been reduced by corrosion to about half the original value.

The pipeline was constructed of nominal 100 mm (4 in) schedule 40, pipe, inside diameter

102.3 mm (4.026 in), outside diameter 114.3 mm (4.5 in). Estimate the rate of corrosion in

ipy and in

mm per year.

11 The pipeline described above was used to carry waste effluent to a tank. The effluent is not

hazardous. A decision has to be made on what material to use to replace the pipe. Three

suggestions have been proposed:

Replace with the same schedule carbon steel pipe and accept renewal at 3-year

intervals;

Replace with a thicker pipe, schedule 80, outside diameter 114.3 mm (4.5 In), inside

diameter 97.2 mm (3.826 in); and

Use stainless steel pipe, which will not corrode.

The estimated cost of the pipes, per unit length is given below:

schedule 40 carbon steel GH¢ 5

schedule 80 carbon steel GH¢ 8.3

stainless steel (304) schedule GH¢ 24.8

Installation and fittings for all the materials add GH¢ 16.5 per unit length. The downtime

required to replace the pipe does not result in a loss of production. If the expected future life

of the plant is 7 years, recommend which pipe to use.

12. A slurry of acrylic polymer particles in water is held in storage tanks prior to filtering and

drying. Plain carbon steel would be a suitable material for the tanks, but it is essential that

the polymer does not become contaminated with iron in storage. Suggest some alternative

materials of construction for the tanks.

13. In the manufacture of aniline by the hydrogenation of nitrobenzene, the off-gases from the

reactor are cooled and the products and unreacted nitrobenzene condensed in a shell and

tube exchanger. A typical composition of the condensate is, kmol/h: aniline 950, cyclo-

hexylamine 10, water 1920, nitrobenzene 40. The gases enter the condenser at 230 °C and

leave at 50 °C. The cooling water enters the tubes at 20 oC and leaves at 50 °C. Suggest

suitable materials of construction for the shell and the tubes.


14. Aniline is manufactured by the hydrogenation of nitrobenzene in a fluidised bed reactor. The

reactor operates at 250 °C and 20 bar. The reactor vessel is approximately 3 m diameter and

9 m high. Suggest suitable materials of construction for this reactor.

15. Methyl ethyl ketone (MEK) is manufactured by the dehydrogenation of 2-butanol using a

shell and tube type reactor. Flue gases are used for heating and pass though the tubes. The

flue gases will contain traces of sulphur dioxide. The reaction products include hydrogen.

The reaction takes place in the shell at a pressure of 3 bar and temperature of 500 oC. Select

suitable materials for the tubes and shell.

16. Discuss ways of conserving engineering materials, and the technical and social problems

involved in implementing them.

17.Define Poisson's ratio, and the dilatation in the straining of an elastic solid.

(b) Calculate the dilatation in the uniaxial elastic extension of a bar of material, assuming

strains are small, in terms of v and the tensile strain. Hence find the value of v for which the

volume change during elastic deformation is zero.

(c) Poisson's ratio for most metals is about 0.3. For cork it is close to zero; for rubber it is

close to 0.5. What are the approximate volume changes in each of these materials during an

elastic tensile strain?

18.


19. Indicate, giving specific examples, why some composite materials are particularly

attractive in materials applications.

a. Define a high polymer; list three engineering polymers.

b. Define a thermoplastic and a thermoset.

c. Distinguish between a glassy polymer, a crystalline polymer and a rubber.

d. Distinguish between a cross-linked and a non-cross-linked polymer.

e. What is a co-polymer?

f. List the monomers of polyethylene (PE) and polyvinyl chloride (PVC)

g. What is the glass transition temperature, TG?

h. Explain the change of moduli of polymers at the glass transition temperature.

i. What is the order of magnitude of the number of carbon atoms in a single

j. molecule of a high polymer?

k. What is the range of temperature in which TG lies for most engineering polymers?

l. How would you increase the modulus of a polymer?

20. Explain the following observations, using diagrams to illustrate your answer wherever you

can.

(a) A reaction vessel for a chemical plant was fabricated by welding together stainless-steel

plates (containing 18% chromium, 8% nickel and 0.1% carbon by weight). During

service the vessel corroded badly at the grain boundaries near the welds.

(b) Mild-steel radiators in a central-heating system were found to have undergone little

corrosion after several years' service.

(c) In order to prevent the corrosion of a mild-steel structure immersed in sea water, a newly

qualified engineer suggested the attachment of titanium plates in the expectation of

powerful cathodic action. He later found to his chagrin that the structure had corroded

badly.

21. Explain the following observations, using diagrams to illustrate your answer wherever

possible:

a. Diffusion of aluminium into the surface of a nickel super-alloy turbine blade reduced

the rate of high-temperature oxidation.

b. Steel nails used to hold copper roofing sheet in position failed rapidly by wet

corrosion.

c. The corrosion of an underground steel pipeline was greatly reduced when the

pipeline was connected to a buried bar of magnesium alloy.

Date post:	27-Mar-2020
Category:	Documents
Upload:	others
View:	1 times
Download:	0 times

INTRODUCTION - Webs · macroscopic and microscopic behaviour is a central and recurrent theme of...

Documents