1. Lecture Summary Lecture Topic: Lecture Topic: Materials – Fundamentals of Material Properties, Part 1 Materials – Fundamentals of Material Properties, Part 1 Lecture Notes by: Lecture Notes by: Darrell Wallace Darrell Wallace Allocated Time: Allocated Time: 1 hr 15 minutes 1 hr 15 minutes Teaching Objectives: (At the conclusion of this lecture, students should know ____) Teaching Objectives: (At the conclusion of this lecture, students should know ____) Importance of material properties and selection for meeting both design and manufacturing requirements Importance of material properties and selection for meeting both design and manufacturing requirements Most common descriptive material properties (definitions, implications) Most common descriptive material properties (definitions, implications) Stress, strain, ductility, toughness, Stress, strain, ductility, toughness, Required Materials / Preparation / Props Required Materials / Preparation / Props LCD Projector and PC or overheads LCD Projector and PC or overheads Large tension spring Large tension spring 2 copies of OSU Lantern (one notched) for notch-sensitivity demonstration 2 copies of OSU Lantern (one notched) for notch-sensitivity demonstration 1

2. Darrell Wallace Youngstown State University Department of Mechanical and Industrial Engineering Fundamentals of Material Properties - Part 1- 2

3. What is the importance of understanding material properties? Design Design Must meet required product characteristics Must meet required product characteristics Manufacturing Manufacturing Selection of material determines applicable processes Selection of material determines applicable processes Processing affects material properties Processing affects material properties Costs Costs Processing Processing Manufacturing Processes Manufacturing Processes End-of-Service (Life Cycle) End-of-Service (Life Cycle) 3

4. Dimensional and Surface Characteristics Size Size Shape Shape Surface Roughness Surface Roughness 4

5. Intrinsic Material Properties Thermal properties Thermal properties Optical characteristics Optical characteristics Conductivity Conductivity Chemical reactivity Chemical reactivity 5

6. Functional Material Properties Strength Strength Toughness Toughness Hardness Hardness Durability (Fatigue) Durability (Fatigue) Formability Formability Thermal Properties Thermal Properties 6

7. Tensile Test 7 Simple, low-cost test Provides a wide variety of information about material characteristics Heavily standardized under ASTM

8. Conducting a Tensile Test Prior to the test, the cross- section of the test specimen is carefully measured so that the initial area is known. Prior to the test, the cross- section of the test specimen is carefully measured so that the initial area is known. During the test cycle, an increasing load is applied to the test specimen. During the test cycle, an increasing load is applied to the test specimen. The change in length of the test region is measured throughout the test, usually using an instrument called an extensometer. The change in length of the test region is measured throughout the test, usually using an instrument called an extensometer. 8

9. Results of a Tensile Test: Load-Elongation Curve The raw output of a tensile test is a Load-Elongation curve. The raw output of a tensile test is a Load-Elongation curve. These data are used to calculate stresses and strains which are more useful for making comparisons between materials. These data are used to calculate stresses and strains which are more useful for making comparisons between materials. 9 Force (lbf) Elongation (in)

10. Engineering Stress and Strain In the first step of the tensile test analysis we evaluate the relationship between the force applied and the deformation of the material based on its initial state. These calculations involve significant simplification of the problem which will be discussed later. 10

11. Engineering Stress 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 A 0 : initial cross sectional area in in² s: engineering stress in psi A FF

12. Engineering Strain Ratio of change in length to original length: Ratio of change in length to original length: e=  L/L 0 =(L-L 0 )/L 0 12 L0L0 LL L Calculation is always based on the original length, L 0, 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.

13. Engineering Stress-Strain Curve The engineering stress-strain curve looks very similar to the load-elongation curve. The engineering stress-strain curve looks very similar to the load-elongation curve. 13 s (psi) e (in/in)

14. Observable Features on the Engineering Stress Strain Curve 14 s (psi) e (in/in) Test Start Elastic Region Plastic Deformation Begins Onset of Necking Fracture

15. “True” Stress and Strain Engineering Stress and Strain are based on a critical simplifying assumption: they neglect the changes that occur in the length and cross-section of the specimen as it deforms. Engineering Stress and Strain are based on a critical simplifying assumption: they neglect the changes that occur in the length and cross-section of the specimen as it deforms. True Stress and True Strain are instantaneous values that eliminate this simplification. True Stress and True Strain are instantaneous values that eliminate this simplification. 15

16. True Strain 16 L i =L i-1 +  L L 0 =gage L 1 =L 0 +  L L 2 =L 1 +  L=L 0 +2  L... L n = L 0 +n  L The incremental strain, therefore Is found to be: Strain =  L / L n-1 The true strain is the sum of the Incremental strains as  L  0. Thus:  =ln(1+e)

17. True Stress The engineering stress calculation is based on the assumption that the cross-sectional area remains unchanged. This violates volume constancy. The engineering stress calculation is based on the assumption that the cross-sectional area remains unchanged. This violates volume constancy. The change in cross-sectional area is a function of strain, thus the “true stress” (flow stress) of the material is calculated as: The change in cross-sectional area is a function of strain, thus the “true stress” (flow stress) of the material is calculated as:  =s(1+e) where e is the corresponding value of engineering strain for each stress/strain data pair. 17

18. True Stress-Strain Curve Notice that the true stress-strain curve does not reach a peak value and then decrease. As the area decreases, the true stress continues to increase. Notice that the true stress-strain curve does not reach a peak value and then decrease. As the area decreases, the true stress continues to increase. 18  (psi)  (in/in)

19. Interpreting the Tensile Test Results We can extract a lot of information from a tensile test. Let’s now consider some of the material characteristics that will be important for design and manufacturing and gather information about those characteristics from the stress-strain curves. We can extract a lot of information from a tensile test. Let’s now consider some of the material characteristics that will be important for design and manufacturing and gather information about those characteristics from the stress-strain curves. 19

20. Stress-Strain Characteristics – Perfectly Elastic 20  

21. Stress-Strain Characteristics – Elastic Perfectly Plastic 21  

22. Stress-Strain Characteristics – Elastic Strain Hardening 22  

23. Elasticity Elasticity is the tendency of a material to return to its original size and shape after deformation. Most materials, particularly metals, exhibit a region of elastic deformation Elasticity is the tendency of a material to return to its original size and shape after deformation. Most materials, particularly metals, exhibit a region of elastic deformation In this region, the material behaves much like a spring. Any strain that is created in the part will be restored when the forces are released. In this region, the material behaves much like a spring. Any strain that is created in the part will be restored when the forces are released. The behavior of the material is virtually identical for both engineering and true stresses and strains in the elastic region. The behavior of the material is virtually identical for both engineering and true stresses and strains in the elastic region. 23

24. Elasticity – Hooke’s Law and Young’s Modulus Hooke’s Law:  =E  Young’s Modulus: E=  24  (psi)  (in/in) } Elastic Region Slope=E

25. Elasticity Considerations For Design: In many applications stiffness, rather than strength, determines the suitability of a material. (e.g. fishing pole) For Design: In many applications stiffness, rather than strength, determines the suitability of a material. (e.g. fishing pole) For Manufacturing: The more elastic a material is, the more deformation you must apply before you are actually deforming the material. This leads to significant “springback” considerations. For Manufacturing: The more elastic a material is, the more deformation you must apply before you are actually deforming the material. This leads to significant “springback” considerations. 25

26. Strength “Strength” has several interpretations, depending on our particular concern. We can ask: “Strength” has several interpretations, depending on our particular concern. We can ask: How much stress can this material sustain before it deforms? (Yield Strength) How much stress can this material sustain before it deforms? (Yield Strength) How much stress can this material sustain before it fails? (Ultimate Strength) How much stress can this material sustain before it fails? (Ultimate Strength) Though we have shown the approximations of engineering stress and strain, by convention the values of Yield Strength and Ultimate Tensile Strength are based on the engineering values. Though we have shown the approximations of engineering stress and strain, by convention the values of Yield Strength and Ultimate Tensile Strength are based on the engineering values. 26

27. Determining Yield Strength and UTS 27

28. Other Important Features of the Stress-Strain Curves We can observe some other important aspects of the stress-strain curves: We can observe some other important aspects of the stress-strain curves: True stress-strain curve for most strain- hardening metals can be modeled as an exponential curve True stress-strain curve for most strain- hardening metals can be modeled as an exponential curve Onset of necking can be observed in the engineering stress-strain curve Onset of necking can be observed in the engineering stress-strain curve 28

29. Strength Considerations For Design: For Design: UTS will determine point of catastrophic failure UTS will determine point of catastrophic failure Yield will determine loading under which permanent deformation occurs Yield will determine loading under which permanent deformation occurs For Manufacturing: For Manufacturing: Combination of material and manufacturing processes must achieve required strength characteristics (work hardening, annealing) Combination of material and manufacturing processes must achieve required strength characteristics (work hardening, annealing) Forces required for forming processes will depend on yield strength Forces required for forming processes will depend on yield strength 29

30. Exponential Approximation for Strain-Hardening Materials 30

31. Formability This is an ambiguous term that has a variety of meanings depending on the operation(s) to be performed. Some factors: This is an ambiguous term that has a variety of meanings depending on the operation(s) to be performed. Some factors: Strength Strength % cold work % cold work Strain hardening (n) Strain hardening (n) Anisotropy Anisotropy Alloying Alloying 31

32. Ductility Directly for Tensile Test: Directly for Tensile Test: Uniform Elongation Uniform Elongation Elongation at Failure Elongation at Failure Secondary Measurements: Secondary Measurements: % Area Reduction at failure % Area Reduction at failure 32

33. Ductility Considerations For Design: For Design: Ductile materials tend to be able to absorb energy Ductile materials tend to be able to absorb energy These materials will tend not to crack or fail catastrophically under many impact conditions These materials will tend not to crack or fail catastrophically under many impact conditions If the design implementation subjects the part to loads that exceed the yield strength, permanent deformation will occur. If the design implementation subjects the part to loads that exceed the yield strength, permanent deformation will occur. For Manufacturing: For Manufacturing: Ductile materials tend to be easy to form (particularly in forging) Ductile materials tend to be easy to form (particularly in forging) Formability in sheet will depend on strain hardening exponent Formability in sheet will depend on strain hardening exponent Very ductile materials tend to be “gummy” and may cause difficulties in machining or extrusion operations Very ductile materials tend to be “gummy” and may cause difficulties in machining or extrusion operations 33

34. Toughness “Ability to absorb Energy” “Ability to absorb Energy” Area under the stress-strain curve Area under the stress-strain curve Can be measured by impact tests Can be measured by impact tests May be sensitive to a wide variety of factors May be sensitive to a wide variety of factors Material 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

35. Charpy Impact Test 35

36. Charpy Impact Test – Ductile Material 36

37. Charpy Impact Test – Brittle Material 37

38. Compression Testing Some materials exhibit different flow- stress characteristics in compression than in tension. Compression tests are particularly relevant (from a process standpoint) for predicting forming behavior in forging. Some materials exhibit different flow- stress characteristics in compression than in tension. Compression tests are particularly relevant (from a process standpoint) for predicting forming behavior in forging. 38

39. Hardness Tests Hardness is defined as a material’s ability to resist indentation. A variety of tests exist depending on the hardness of the material and the circumstances under which it can be measured. Hardness is defined as a material’s ability to resist indentation. A variety of tests exist depending on the hardness of the material and the circumstances under which it can be measured. 39

40. Hardness – Indentor Tests Brinell (HB, BHN) – round indentor, widely used, correlates very well to strength: Brinell (HB, BHN) – round indentor, widely used, correlates very well to strength: Approximation: TS(psi)=500 * HB Approximation: TS(psi)=500 * HB Vickers (HV, VHN) – pyramidal indentor Vickers (HV, VHN) – pyramidal indentor Knoop (HK) – for checking localized hardness Knoop (HK) – for checking localized hardness 40

41. Hardness – Other Tests Scleroscope – measures hardness based on coefficient of restitution (bouncing) Scleroscope – measures hardness based on coefficient of restitution (bouncing) Scratch Test – relative hardness measure, most commonly used for very hard materials such as minerals and ceramics Scratch Test – relative hardness measure, most commonly used for very hard materials such as minerals and ceramics Durometer – indentation test specifically for polymers and elastomers Durometer – indentation test specifically for polymers and elastomers 41

42. Fatigue Testing Under cyclic loading, most materials exhibit some degradation of strength characteristics. Under cyclic loading, most materials exhibit some degradation of strength characteristics. Some materials, such as steel, approach some fatigue limit Some materials, such as steel, approach some fatigue limit Other materials, such as Aluminum, have no fatigue limit and will continue to fatigue until failure Other materials, such as Aluminum, have no fatigue limit and will continue to fatigue until failure 42

43. Creep Some materials will continue to undergo strain over time at a given load Some materials will continue to undergo strain over time at a given load This behavior is often temperature sensitive This behavior is often temperature sensitive Very common in polymers and elastomers Very common in polymers and elastomers 43

44. Yield-Point Elongation 44

45. Suggested Problems: Kalpakjian, Chapter 2 Kalpakjian, Chapter 2 In-Chapter Example Problems In-Chapter Example Problems Problems 2.31, 2.32, 2.38, 2.39, 2.42, 2.43, 2.44 Problems 2.31, 2.32, 2.38, 2.39, 2.42, 2.43, 2.44 Example provided in supplemental notes on stress-strain Example provided in supplemental notes on stress-strain 45