Darrell WallaceDarrell Wallace

Youngstown State UniversityYoungstown State UniversityDepartment of Mechanical and Department of Mechanical and

Industrial EngineeringIndustrial Engineering

Fundamentals of Material Properties

- Part 1-

2

What is the importance of understanding material

properties? DesignDesign

Must meet required product characteristicsMust meet required product characteristics

ManufacturingManufacturing Selection of material determines applicable Selection of material determines applicable

processesprocesses Processing affects material propertiesProcessing affects material properties

CostsCosts ProcessingProcessing Manufacturing ProcessesManufacturing Processes End-of-Service (Life Cycle)End-of-Service (Life Cycle)

3

Dimensional and Surface Characteristics

SizeSizeShapeShapeSurface RoughnessSurface Roughness

4

Intrinsic Material PropertiesThermal propertiesThermal propertiesOptical characteristicsOptical characteristicsConductivityConductivityChemical reactivityChemical reactivity

5

Functional Material PropertiesStrengthStrengthToughnessToughnessHardnessHardnessDurability (Fatigue)Durability (Fatigue)FormabilityFormabilityThermal PropertiesThermal Properties

6

Tensile Test

7

•Simple, low-cost test

•Provides a wide variety of information about material characteristics

•Heavily standardized under ASTM

Conducting a Tensile Test

Prior to the test, the cross-Prior to the test, the cross-section of the test specimen section of the test specimen is carefully measured so is carefully measured so that the initial area is known.that the initial area is known.

During the test cycle, an During the test cycle, an increasing load is applied to increasing load is applied to the test specimen.the test specimen.

The change in length of the The change in length of the test region is measured test region is measured throughout the test, usually throughout the test, usually using an instrument called using an instrument called an extensometer.an extensometer.

8

Results of a Tensile Test: Load-Elongation Curve

The raw output of a tensile test is a Load-Elongation The raw output of a tensile test is a Load-Elongation curve.curve.

These data are used to calculate stresses and strains These data are used to calculate stresses and strains which are more useful for making comparisons between which are more useful for making comparisons between materials.materials.

9

Fo

rce

(lb

f)

Elongation (in)

Engineering Stress and Strain

In the first step of the tensile test analysis In the first step of the tensile test analysis we evaluate the relationship between the we evaluate the relationship between the force applied and the deformation of the force applied and the deformation of the material based on its initial state. These material based on its initial state. These calculations involve significant calculations involve significant simplification of the problem which will be simplification of the problem which will be discussed later.discussed later.

10

Engineering Stress

0A

Fs

11

• Stress : force per unit Area•Engineering stress is always calculated based on the initial area of the test specimen.

F : load applied in pounds

A0 : initial cross sectional area in in²

s: engineering stress in psiA

F F

Engineering Strain

Ratio of change in length to original length:Ratio of change in length to original length:

e=e=L/LL/L00=(L-L=(L-L00)/L)/L00

12

L0 L

L•Calculation is always based on the original length, L0, regardless of the size of L•The engineering strain does not consider that the incremental change in length is now being spread over a longer distance.

Engineering Stress-Strain Curve

The engineering stress-strain curve looks The engineering stress-strain curve looks very similar to the load-elongation curve.very similar to the load-elongation curve.

13

s (p

si)

e (in/in)

Observable Features on the Engineering Stress Strain Curve

14

s (p

si)

e (in/in)

Test Start

Elastic Region

Plastic Deformation Begins

Onset of Necking

Fracture

“True” Stress and StrainEngineering Stress and Strain are based Engineering Stress and Strain are based

on a critical simplifying assumption: they on a critical simplifying assumption: they neglect the changes that occur in the neglect the changes that occur in the length and cross-section of the specimen length and cross-section of the specimen as it deforms.as it deforms.

True Stress and True Strain are True Stress and True Strain are instantaneous values that eliminate this instantaneous values that eliminate this simplification.simplification.

15

True Strain

16

Li=Li-1+L

L0=gage

L1 =L0+L

L2=L1+L=L0+2L

...

L n = L0+nL

The incremental strain, thereforeIs found to be:

Strain = L / Ln-1

The true strain is the sum of theIncremental strains as L 0. Thus:

=ln(1+e)

True Stress

The engineering stress calculation is based on The engineering stress calculation is based on the assumption that the cross-sectional area the assumption that the cross-sectional area remains unchanged. This violates volume remains unchanged. This violates volume constancy.constancy.

The change in cross-sectional area is a function The change in cross-sectional area is a function of strain, thus the “true stress” (flow stress) of the of strain, thus the “true stress” (flow stress) of the material is calculated as:material is calculated as:

=s(1+e)=s(1+e)where e is the corresponding value of engineering where e is the corresponding value of engineering strain for each stress/strain data pair.strain for each stress/strain data pair. 17

True Stress-Strain Curve

Notice that the true stress-strain curve does not reach a Notice that the true stress-strain curve does not reach a peak value and then decrease. As the area decreases, peak value and then decrease. As the area decreases, the true stress continues to increase.the true stress continues to increase.

18

(p

si)

(in/in)

Interpreting the Tensile Test Results

We can extract a lot of information from a We can extract a lot of information from a tensile test. Let’s now consider some of tensile test. Let’s now consider some of the material characteristics that will be the material characteristics that will be important for design and manufacturing important for design and manufacturing and gather information about those and gather information about those characteristics from the stress-strain characteristics from the stress-strain curves.curves.

19

Stress-Strain Characteristics –Perfectly Elastic

20

Stress-Strain Characteristics –Elastic Perfectly Plastic

21

Stress-Strain Characteristics –Elastic Strain Hardening

22

Elasticity Elasticity is the tendency of a material to return to Elasticity is the tendency of a material to return to

its original size and shape after deformation. Most its original size and shape after deformation. Most materials, particularly metals, exhibit a region of materials, particularly metals, exhibit a region of elastic deformationelastic deformation

In this region, the material behaves much like a In this region, the material behaves much like a spring. Any strain that is created in the part will be spring. Any strain that is created in the part will be restored when the forces are released.restored when the forces are released.

The behavior of the material is virtually identical The behavior of the material is virtually identical for both engineering and true stresses and strains for both engineering and true stresses and strains in the elastic region.in the elastic region.

23

Elasticity – Hooke’s Law and Young’s Modulus

Hooke’s Law:Hooke’s Law:

=E=EYoung’s Modulus:Young’s Modulus:

E=E=

24

(p

si)

(in/in)

}Elastic Region

Slope=E

Elasticity ConsiderationsFor Design: For Design: In many applications stiffness, In many applications stiffness,

rather than strength, determines the rather than strength, determines the suitability of a material. (e.g. fishing pole)suitability of a material. (e.g. fishing pole)

For Manufacturing:For Manufacturing: The more elastic a The more elastic a material is, the more deformation you must material is, the more deformation you must apply before you are actually deforming apply before you are actually deforming the material. This leads to significant the material. This leads to significant “springback” considerations.“springback” considerations.

25

Strength

““Strength” has several interpretations, depending Strength” has several interpretations, depending on our particular concern. We can ask:on our particular concern. We can ask: How much stress can this material sustain before it How much stress can this material sustain before it

deforms? (Yield Strength)deforms? (Yield Strength) How much stress can this material sustain before it fails? How much stress can this material sustain before it fails?

(Ultimate Strength)(Ultimate Strength)

Though we have shown the approximations of Though we have shown the approximations of engineering stress and strain, by convention the engineering stress and strain, by convention the values of Yield Strength and Ultimate Tensile values of Yield Strength and Ultimate Tensile Strength are based on the engineering values.Strength are based on the engineering values.

26

Determining Yield Strength and UTS

27

Other Important Features of the Stress-Strain Curves

We can observe some other important We can observe some other important aspects of the stress-strain curves:aspects of the stress-strain curves:True stress-strain curve for most strain-True stress-strain curve for most strain-

hardening metals can be modeled as an hardening metals can be modeled as an exponential curveexponential curve

Onset of necking can be observed in the Onset of necking can be observed in the engineering stress-strain curveengineering stress-strain curve

28

Strength Considerations For Design:For Design:

UTS will determine point of catastrophic failureUTS will determine point of catastrophic failure Yield will determine loading under which permanent Yield will determine loading under which permanent

deformation occursdeformation occurs

For Manufacturing:For Manufacturing: Combination of material and manufacturing processes Combination of material and manufacturing processes

must achieve required strength characteristics (work must achieve required strength characteristics (work hardening, annealing)hardening, annealing)

Forces required for forming processes will depend on yield Forces required for forming processes will depend on yield strengthstrength

29

Exponential Approximation forStrain-Hardening Materials

30

FormabilityThis is an ambiguous term that has a This is an ambiguous term that has a

variety of meanings depending on the variety of meanings depending on the operation(s) to be performed. Some operation(s) to be performed. Some factors:factors:StrengthStrength% cold work% cold workStrain hardening (n)Strain hardening (n)AnisotropyAnisotropyAlloyingAlloying

31

DuctilityDirectly for Tensile Test:Directly for Tensile Test:

Uniform ElongationUniform ElongationElongation at FailureElongation at Failure

Secondary Measurements:Secondary Measurements:% Area Reduction at failure% Area Reduction at failure

32

Ductility Considerations For Design:For Design:

Ductile materials tend to be able to absorb energyDuctile materials tend to be able to absorb energy These materials will tend not to crack or fail catastrophically These materials will tend not to crack or fail catastrophically

under many impact conditionsunder many impact conditions If the design implementation subjects the part to loads that If the design implementation subjects the part to loads that

exceed the yield strength, permanent deformation will occur.exceed the yield strength, permanent deformation will occur. For Manufacturing:For Manufacturing:

Ductile materials tend to be easy to form (particularly in Ductile materials tend to be easy to form (particularly in forging) forging)

Formability in sheet will depend on strain hardening exponentFormability in sheet will depend on strain hardening exponent Very ductile materials tend to be “gummy” and may cause Very ductile materials tend to be “gummy” and may cause

difficulties in machining or extrusion operationsdifficulties in machining or extrusion operations

33

Toughness ““Ability to absorb Energy”Ability to absorb Energy”

Area under the stress-strain curveArea under the stress-strain curveCan be measured by impact testsCan be measured by impact tests

May be sensitive to a wide variety of May be sensitive to a wide variety of factorsfactorsMaterial purity (internal defects)Material purity (internal defects)Surface characteristics (notch sensitivity)Surface characteristics (notch sensitivity)Rate of deformation (strain rate sensitivity)Rate of deformation (strain rate sensitivity)Temperature sensitivity (ductile to brittle temp)Temperature sensitivity (ductile to brittle temp)

34

Charpy Impact Test

35

Charpy Impact Test – Ductile Material

36

Charpy Impact Test – Brittle Material

37

Compression TestingSome materials exhibit different flow-Some materials exhibit different flow-

stress characteristics in compression than stress characteristics in compression than in tension. Compression tests are in tension. Compression tests are particularly relevant (from a process particularly relevant (from a process standpoint) for predicting forming behavior standpoint) for predicting forming behavior in forging.in forging.

38

Hardness TestsHardness is defined as a material’s ability Hardness is defined as a material’s ability

to resist indentation. A variety of tests to resist indentation. A variety of tests exist depending on the hardness of the exist depending on the hardness of the material and the circumstances under material and the circumstances under which it can be measured.which it can be measured.

39

Hardness – Indentor TestsBrinell (HB, BHN) – round indentor, widely Brinell (HB, BHN) – round indentor, widely

used, correlates very well to strength:used, correlates very well to strength:Approximation: TS(psi)=500 * HBApproximation: TS(psi)=500 * HB

Vickers (HV, VHN) – pyramidal indentorVickers (HV, VHN) – pyramidal indentorKnoop (HK) – for checking localized Knoop (HK) – for checking localized

hardnesshardness

40

Hardness – Other TestsScleroscopeScleroscope – measures hardness based – measures hardness based

on coefficient of restitution (bouncing)on coefficient of restitution (bouncing)

Scratch TestScratch Test – relative hardness – relative hardness measure, most commonly used for very measure, most commonly used for very hard materials such as minerals and hard materials such as minerals and ceramicsceramics

Durometer Durometer – indentation test specifically – indentation test specifically for polymers and elastomersfor polymers and elastomers

41

Fatigue TestingUnder cyclic loading, most materials Under cyclic loading, most materials

exhibit some degradation of strength exhibit some degradation of strength characteristics.characteristics.Some materials, such as steel, approach Some materials, such as steel, approach

some fatigue limitsome fatigue limitOther materials, such as Aluminum, have no Other materials, such as Aluminum, have no

fatigue limit and will continue to fatigue until fatigue limit and will continue to fatigue until failurefailure

42

CreepSome materials will continue to undergo Some materials will continue to undergo

strain over time at a given loadstrain over time at a given loadThis behavior is often temperature This behavior is often temperature

sensitivesensitiveVery common in polymers and elastomersVery common in polymers and elastomers

43

Yield-Point Elongation

44