GROUP 5 TOPIC: COMPRESSION, TENSION,BENDING,

HARDNESS,INPACT TEST ,FATIGUE AND CREEP TEST

NAME ID

MUHAMMAD AZIM B CHE ZAKARIA MH13049

MUHAMAD ARIF ASHRAF BIN ABD SATAR

MH13041

MOHAMAD ARIF B MUSTAFA MH13042

MUHAMAD ARIFF B ZAKARIA MH13038

LECTURER : Dr. Januar Parlaungan Siregar

ENGINEERING MATERIAL

MECHANICAL TESTING

TYPE OF MECHANICAL TESTING

COMPRESSIONTORSIONBENDINGHARDNESSIMPACT

COMPRESSION

*Many process such as forging, rolling and extrusion subjected to compressive forces.

*Compression test happen where specimen is subjected to compressive load

COMPRESSION TEST

Applies a load that squeezes the ends of a cylindrical specimen between two platens

ENGINERING STRESS IN COMPRESSION

FORMULA :

oe A

F

WHEN THE SPECIMENT IS COMPRERSSED, THE HEIGHT WILL REDUCE AND ITS CROSS SECTIONAL AREA WILL INCREASE

THIS CAN BE SHOWN BY THIS FORMULA

where Ao = original area of the specimen

o

o

hhh

e

*SINCE THE HEIGHT IS REDUCED DURING COMPRESSION, VALUE OF e IS NEGATIVE (THE NEGATIVE SIGN USSUALY IGNORED WHEN EXPERESSING COMPRESSION STRAIN)

ENGINERING STRAIN IN COMPRESSION

FORMULA :

*Shape of plastic region is different from tensile test because cross‑section increases

*Value of engineering stress is higher

*Twisting

*Force acting to turn one end around the longitudinal axis of a rod while the other end remains fixed.

*Torque = P (force) x r (radius), lb-ft

*Torque produces a shear stress and shear deformation both at 90o

and parallel to axis-and separating stress at 45o , which causes

brittle metals to fail (chalk)

TORSION

Typical torsion-test specimen; it is mounted between the two heads of a testing machine and twisted. Note the shear deformation of an element in the reduced section of the specimen

TORSION TEST SPECIMENT

Application of stresses in opposite directions on

either side of a thin

SHEAR STRAIN

SHEAR STRESS

SHEAR PROPERTIES

*SHEAR STRESS AND STRAIN

Shear stress defined as A

F

where F = applied force; and A = area over which deflection occurs.

Shear strain defined as b

where = deflection element; and b = distance over which deflection occurs

shear stress‑strain curve from a torsion test

*Shear Elastic Stress‑Strain Relationship

In the elastic region, the relationship is Gwhere G = shear modulus,

or shear modulus of elasticity

For most materials, G 0.4E, where E =elastic modulus

*Relationship similar to flow curve

*Shear stress at fracture = shear strength S

*Shear strength can be estimated from tensile strength: S 0.7(TS)

*Since cross‑sectional area of test specimen in torsion test does not change as in tensile and compression, engineering stress‑strain curve for shear true stress‑strain curve

SHEAR PLASTIC STRESS‑STRAIN RELATIONSHIP

Two bend-test methods for brittle materials: (a) three-point bending; (b) four-point bending. The areas on the beams represent the bending-

moment diagrams, described in texts on mechanics of solids. Note the region of constant maximum bending moment in (b); by contrast, the maximum bending moment occurs only at the center of the specimen in (a).

BENDING

*Bending of a rectangular cross‑section results in both tensile and compressive

stresses in the material: (1) initial loading; (2) highly stressed and strained

specimen; and (3) bent part

*Testing of Brittle Materials

*Hard brittle materials (e.g., ceramics) possess elasticity but little or no plasticity

*Often tested by a bending test (also called flexure test)

*Specimen of rectangular cross‑section is positioned between two supports, and a load is applied at its center

*Testing of Brittle Materials

*Brittle materials do not flex

*They deform elastically until fracture

*Failure occurs because tensile strength of outer fibers of specimen are exceeded

*Failure type: cleavage - common with ceramics and metals at low temperatures, in which separation rather than slip occurs along certain crystallographic planes

*Transverse Rupture Strength

The strength value derived from the bending test:

2

51

bt

FLTRS

.

where TRS = transverse rupture strength; F = applied load at fracture; L = length of specimen between supports; and b and t are dimensions of cross-section

HardnessResistance to permanent indentation Good hardness generally means material is

resistant to scratching and wear Most tooling used in manufacturing must be

hard for scratch and wear resistance measure of a material’s resistance to

penetration 2 most common stationary hardness

testsBrinell - stress testRockwell - strain test

*Hardness and Hardness Testing

*General procedure:

Press the indenter thatis harder than the metal

Into metal surface.

Withdraw the indenter

Measure hardness by measuring depth or

width of indentation.

Rockwell hardnesstester

*Hardness Tests *Commonly used for assessing material

properties because they are quick and convenient

*Variety of testing methods are appropriate due to differences in hardness among different materials

*Vickers (HV) and Knoop (HK)- similar to Brinell (stress tests), but microhardness

*Other test methods are also available, such as Scleroscope, and durometer

*Scleroscope - rebound of a tup or hammer.

*Mohs - scratch test

Hardness Tests

Figure 2.12 General characteristics of hardness-testing methods and formulas for calculating hardness. The quantity P is the load applied. Source: H. W. Hayden, et al., The Structure and Properties of Materials, Vol. III (John Wiley & Sons, 1965).

Widely used for testing metals and nonmetals of low to medium hardness

*A hard ball is pressed into specimen surface with a load of 500, 1500, or 3000 kg

Figure 3.14 ‑ Hardness testing methods: (a) Brinell

Brinell Hardness

Brinell Testing

(c)

Indentation geometry in Brinell testing; (a) annealed metal; (b) work-hardened metal; (c) deformation of mild steel under a spherical indenter. Note that the depth of the permanently deformed zone is about one order of magnitude larger than the depth of indentation. For a hardness test to be valid, this zone should be fully developed in the material. Source: M. C. Shaw and C. T. Yang.

*Brinell Hardness Number

Load divided into indentation area = Brinell Hardness Number (BHN)

)( 22

2

ibbb DDDD

FHB

where HB = Brinell Hardness Number (BHN), F = indentation load, kg; Db = diameter of ball, mm, and Di = diameter of indentation, mm

*Rockwell Hardness Test

*Another widely used test

*A cone shaped indenter is pressed into specimen using a minor load of 10 kg, thus seating indenter in material

*Then, a major load of 150 kg is applied, causing indenter to penetrate beyond its initial position

*Additional penetration distance d is converted into a Rockwell hardness reading by the testing machine

Figure 3.14 ‑ Hardness testing methods: (b) Rockwell:

(1) initial minor load and (2) major load

Hardness Conversion Chart

Chart for converting various hardness scales. Note the limited range of most scales. Because of the many factors involved, these conversions are approximate.

*Toughness● Measure the amount of energy a material can

absorb before fracturing● Ability of metal can withstand an impact load without fracturing● Experiment Impact test Izod & Charpy test

Impact Test Specimens

Impact test specimens: (a) Charpy; (b) Izod.

Impact test

Transition Temperature

Schematic illustration of transition temperature in metals.E.g. Titanic hull steel transition temperature at -2 C

Carbon contents

CHAPTER 3

•Fatigue of Metals

•Creep of Metals

•Fracture of Metals

Fatigue Failure

• Metal parts are often design under an assumption of a single static load with a factor of safety (e.g. 0.5 of yield stress).

• Metal parts in service are often subjected to repetitive loading.

• Failure occurs after repetitious or cyclic loading.• Examples: shafts, connecting rods and gears.• Fatigue crack surface:• Refer to Figure 6.19 (3rd Edition)

Fatigue Failure

• Example: Fatigue failure of keyed steel shaft.• A fatigue failure at a point of stress concentration (e.g.

sharp corners or notch).• Figure 6.19 (At the root of the keyway)• Stages of fatigue fracture are:

1. 1.Nucleation.

2. 2.Propagation (clamshell marks)

3. 3.Fracture (Area under load are too small to support further load).

• 4.Surface appearance: (1) smooth striations (clamshell marks) and (2) rough surface formed by fracture

crack origin

Adapted fromFig. 8.21, Callister & Rethwisch 8e. (Fig. 8.21 is from D.J. Wulpi, Understanding How Components Fail, American Society for Metals, Materials Park, OH, 1985.)

Fatigue Failure

Fatigue Test

R.R. Moore reversed-bending fatigue test machine

Fatigue Test

• E.g. R.R. Moore reversed-bending fatigue test machine.• The specimen is under bending load

from the applied weights.• Top surface is compressed and

bottom part is stretched.• Rotation caused the bending

stress to be alternated between

the top and bottom surface, i.e. cyclic loading• A revolution counter records when the fatigue failure

occurs in numbers of cycle (e.g. 106 cycles)• Test is repeated using a number of different loads to

cause different stress level.

S-N Curves

Typical S-N curves for two metals. Note that, unlike steel, aluminium does not have an endurance limit.

S-N Curves

• Data from the test are plotted as Stress (S) versus Number of cycles (N)• Figure 6.23

• There is a general decrease of the level of stress to cause failure as the number cycles is increased.

• For carbon steel, there is levelling off in the decrease.• The maximum stress that cause fatigue failure, regardless of the

number of cycles.

• Endurance limit of fatigue limit• Carbon steels have an endurance limit of 0.4-0.5 time its UTS

• A component can have an infinite life if design with stress level below the endurance limit

Endurance Limit/Tensile Strength versus Tensile Strength

●Infinite life design is not always possible.●E.g. Aluminium does not have an endurance limit.●Small cyclic stress can cause fatigue failure●Components need to be very light.

●Other methods are available for fatigue life calculation (Advance course!, Section 6.4).

●Aircraft parts are design to have finite life and need to be change at a specified intervals.

Factors that Affect the Fatigue Strength of Metals

• Stress concentration. Avoid sharp corners.

• Surface roughness. Ensure smooth surface finish.

• Surface conditions. Surface harden to avoid crack nucleation.

• Environment. Chemical attack on surface initiates cracks.

*CREEP OF METALS

*What is creep?Creep is a permanent deformation, under a constant static load over a period of time.

Creep may also be defined as a time-dependent deformation at elevated temperature and constant stress.

It is a process of slow plastic deformation. Creep is more severe in materials that are subjected to heat for long periods.

The effect is particularly important if the temperature of stressing is above the recrystallization temperature of the metal.

Creep is important in high temperature applications, e.g. gas turbines, steam lines, nuclear reactors.

*CREEPA creep test involves in subjecting a specimen to a

constant tensile load at a certain temperature.Measurements are taken for changes in length (strain)

at various time increments.The creep test results are presented as Creep strain

vs. time, at a constant stress and temperature.Three stage of creep:

1. Primary

2. Secondary

3. Tertiary

Begins with an instantaneous rapid elongation as the load is applied.

Creep rate dƐ/dt, slope of the creep curve.Primary creep is where the creep rate progressively decreases with time.

Secondary creep is where the creep rate is constant (steady state creep)

Tertiary creep is where creep rate rapidly increase with time to the strain at fracture.

*CREEP

*CREEPShape of creep curve depends strongly on the applied load (stress) and temperature.

The secondary creep is a constant creep rate and referred to as the minimum creep rate.

For design data purposes, a specimen is subjected to a minimum creep rate of 10-5 percent/hour.

This data is used for design purposes.To be learned in advanced mechanical engineering courses.

*FRACTURE OF METALS

Failure of Liberty Ship during services in World War II

Collapse of Point Pleasant suspension bride, West Virginia, 1967.

Failure of structures leads to lost of properties and sometimes lost human lives unfortunately.

*Failure Mode

-High energy is absorbed bymicrovoid coalescence duringductile failure (high energyfracture mode)

Less catastrophic

-Low energy is absorbed duringtransgranular cleavage fracture(low energy fracture mode)

More catastrophic

*FRACTURE

*Static load ( Ultimate Tensile Strength, although static load are design to a factor of safety to yield strength)

*Impact load (Design to absorb impact load without fracturing, Fracture Toughness)

*Cyclic load (Failure happens without overloading, Fatigue Limit/advance method)

*Creep load (Creep failure for at high temperature for metal, design with constant creep rate in mind/advance method)

Failure in metallic

materials can be divided

into two main categories

Failure in metallic

materials can be divided into

two main categories:

Ductile failure

Brittle failure

occurs after extensive plastic deformation and is characterised by slow crack propagation

happens with almost no plastic deformations. Occurs in which the separation rather than slip occurs along certain crystallographic planes with rapid crack propagation:- common with ceramics and metals (BCC) at low temperatures and high strain rate

Theory of brittle failure

*Ductile Fracture of a Tensile-Test

specimen

o Surface of ductile fracture in low-carbon steel, showing dimples.

o Fracture is usually initiated at impurities, inclusions, or preexisting voids (microporosity) in the metal

o Source: K.-H. Habig and D. Klaffke. Photo by BAM Berlin/Germany.

Ductile Fracture

* Deformation of Soft and Hard Inclusion in Ductile fracture

Stage of Ductile Fracturea) Small void begin to form within the necked region.b) Void coalesce, producing an internal crack.

* Failure and Fracture of Materials in Tension

Schematic illustration of the types of fracture in tension: (a) brittle fracture in polycrystalline metals; (b) shear fracture in ductile single crystals(c) ductile cup-and-cone fracture in polycrystalline metals;(d) complete ductile fracture in polycrystalline metals, with 100% reduction of area.

*Brittle Fracture

Brittle fracture in polycrystalline metals propagates along the matrix of the grains.

Brittle fracture can also happens along grain boundaries if the grain boundaries are weaken by brittle film or segregation.

Figure 6.13 (3rd Edition): Brittle cleavage fracture of ferritic ductile iron)

Stages of Brittle fractures:1. Plastic deformation concentrates dislocation along slip

planes at obstacles.

2. Shear stress build up at the blocked area, micro cracks nucleated separating slip planes.

3. Further stress and stored elastic energy quickly propagates cracks into fracture.

*Torque produces a shear stress and shear deformation both at 90 and normal to axis of shaft

*Brittle fracture of metals fail at 45°

*Fracture Toughness

Concept

• Impact load i.e. high strain rate and low temperature favours brittle factures.

• Impact tests (Izod/Charpy) do not provide date for design purposes for material/components which already contains internal flaws or cracks.

• Test specimens are intentionally made with cracks.• Introduction of stress intensity factors (KIC), to be learned in

advance classes.

*Mechanical Properties in Design and Manufacturing

*Mechanical properties determine a material’s behavior when subjected to mechanical stresses

*Properties include elastic modulus, ductility, hardness, and various measures of strength

*Dilemma: mechanical properties desirable to the designer, such as high strength, usually make manufacturing more difficult

*The manufacturing engineer should appreciate the design viewpoint and the designer should be aware of the manufacturing viewpoint

* Why Mechanical Properties Important?

Parameters Applications

Hardness Components subject to wear

Fracture toughness

Sudden highly stressed or safety critical components

Fatique life Repeated cyclic loading conditions

Impact properties

Components exposed to sudden stress, especially at low temperatures

Creep stiffness srtengh

High temperature operationTo retain accuracy of positionsStatic load carrying capabilities

*Stiffness-Weight Design Consideration

-EXAMPLE

*Design (safety) Factor

• Design uncertainties mean we do not push the limit.• Factor of safety, N

Often N isbetween1.2 and 4

• Ex: Calculate a diameter, d, to ensure that yield does not occur in the 1045 carbon steel rod below. Use a factor of safety of 5.

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