8/3/2019 1 Introduction Before Formal Discussion

1/17

ENGINEERINGMATERIALS

8/3/2019 1 Introduction Before Formal Discussion

2/17

What is your idea about engineering materials?

Can you cite different types of engineered materials?

Why do you think it is called as engineering or engineered

materials?

What do you think is the importance of understanding thesubject?

What could be its application in your future career?

8/3/2019 1 Introduction Before Formal Discussion

3/17

INTRODUCTION

As we begin the 21st century, advances in materials research andtechnology offer great promise.

Materials Science forms the foundation for engineers in productdevelopment because the structures, components, and devices that

engineers design are limited by the properties of the materials thatare available and the techniques that can be used for fabrication.

Selecting the "best" material is usually a difficult task, requiringtradeoffs between different material properties including: General Physical Properties

Mechanical Properties Thermal Properties

8/3/2019 1 Introduction Before Formal Discussion

4/17

GENERAL PHYSICAL PROPERTIES

Density

Density is one of the most fundamental physical properties of anymaterial.

It is defined as the ratio of an objects mass to its volume.

Because most designs are limited by either size and or weightdensity is an important consideration in many calculations.

Density is a function of the mass of the atoms making up thematerials and the distance between them.

Massive, closely packed atoms characterize high densitymaterials such as Tungsten or Neptunium.

8/3/2019 1 Introduction Before Formal Discussion

5/17

In contrast light, relatively distant atoms compose low densitymaterials such as Beryllium or Aluminum.

8/3/2019 1 Introduction Before Formal Discussion

6/17

MECHANICAL PROPERTIES

The mechanical properties of a material describe how it will reactto physical forces.

Mechanical properties occur as a result of the physical propertiesinherent to each material, and are determined through a series of

standardized mechanical tests.

8/3/2019 1 Introduction Before Formal Discussion

7/17

Strength

Strength has several definitions depending on the material typeand application.

Before choosing a material based on its published or measured

strength it is important to understand the manner in whichstrength is defined and how it is measured.

When designing for strength, material class and mode of loadingare important considerations.

8/3/2019 1 Introduction Before Formal Discussion

8/17

For metals the most common measure of strength is the yieldstrength.

For most polymers it is more convenient to measure the failurestrength, the stress at the point where the stress strain curve

becomes obviously non-linear. Strength, for ceramics however, is more difficult to define. Failure

in ceramics is highly dependent on the mode of loading.

The typical failure strength in compression is fifteen times thefailure strength in tension.

The more common reported value is the compressive failurestrength.

8/3/2019 1 Introduction Before Formal Discussion

9/17

Elastic limit

The elastic limit is the highest stress at which all deformationstrains are fully recoverable.

For most materials and applications this can be considered the

practical limit to the maximum stress a component can withstandand still function as designed.

Beyond the elastic limit permanent strains are likely to deform thematerial to the point where its function is impaired.

8/3/2019 1 Introduction Before Formal Discussion

10/17

Proportional limit

The proportional limit is the highest stress at which stress islinearly proportional to strain.

This is the same as the elastic limit for most materials. Some

materials may show a slight deviation from proportionality whilestill under recoverable strain.

In these cases the proportional limit is preferred as a maximumstress level because deformation becomes less predictable aboveit.

8/3/2019 1 Introduction Before Formal Discussion

11/17

Yield Strength

The yield strength is the minimum stress which producespermanent plastic deformation.

This is perhaps the most common material property reported for

structural materials because of the ease and relative accuracy ofits measurement.

The yield strength is usually defined at a specific amount ofplastic strain, or offset, which may vary by material and orspecification.

The offset is the amount that the stress-strain curve deviates fromthe linear elastic line. The most common offset for structuralmetals is 0.2%.

8/3/2019 1 Introduction Before Formal Discussion

12/17

Ultimate Tensile Strength

The ultimate tensile strength is an engineering value calculated bydividing the maximum load on a material experienced during atensile test by the initial cross section of the test sample.

When viewed in light of the other tensile test data the ultimatetensile strength helps to provide a good indication of a material'stoughness but is not by itself a useful design limit.

Conversely this can be construed as the minimum stress that isnecessary to ensure the failure of a material.

8/3/2019 1 Introduction Before Formal Discussion

13/17

True Fracture Strength

The true fracture strength is the load at fracture divided by thecross sectional area of the sample.

Like the ultimate tensile strength the true fracture strength can

help an engineer to predict the behavior of the material but is notitself a practical strength limit.

Because the tensile test seeks to standardize variables such asspecimen geometry, strain rate and uniformity of stress it can beconsidered a kind of best case scenario of failure.

8/3/2019 1 Introduction Before Formal Discussion

14/17

Ductility

Ductility is a measure of how much deformation or strain amaterial can withstand before breaking.

The most common measure of ductility is the percentage of

change in length of a tensile sample after breaking. This is generally reported as % El or percent elongation.

The R.A. or reduction of area of the sample also gives someindication of ductility.

8/3/2019 1 Introduction Before Formal Discussion

15/17

Toughness

Toughness describes a material's resistance to fracture.

It is often expressed in terms of the amount of energy a materialcan absorb before fracture.

Tough materials can absorb a considerable amount of energybefore fracture while brittle materials absorb very little.

Neither strong materials such as glass or very ductile materialssuch as taffy can absorb large amounts of energy before failure.

Toughness is not a single property but rather a combination ofstrength and ductility.

8/3/2019 1 Introduction Before Formal Discussion

16/17

The toughness of a material can be related to the total area underits stress-strain curve.

A comparison of the relative magnitudes of the yield strength,ultimate tensile strength and percent elongation of different

material will give a good indication of their relative toughness. Materials with high yield strength and high ductility have high

toughness. Integrated stress-strain data is not readily available formost materials so other test methods have been devised to helpquantify toughness.

The most common test for toughness is the Charpy impact test.

8/3/2019 1 Introduction Before Formal Discussion

17/17

Fatigue ratio

The dimensionless fatigue ratio f is the ratio of the stress requiredto cause failure after a specific number of cycles to the yieldstress of a material.

Fatigue tests are generally run through 107 or 108 cycles. A highfatigue ratio indicates materials which are more susceptible tocrack growth during cyclic loading.