Fundamentals of Material Properties - Part 1-

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

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

3. Dimensional and Surface Characteristics • Size • Shape • Surface Roughness

4. Intrinsic Material Properties • Thermal properties • Optical characteristics • Conductivity • Chemical reactivity

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

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

7. 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. • 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.

8. Results of a Tensile Test: 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. Force (lbf) Elongation (in)

9. 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. Engineering Stress • 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 psi A F F

11. Engineering Strain • Ratio of change in length to original length: e=DL/L0=(L-L0)/L0 DL L0 L • Calculation is always based on the original length, L0, regardless of the size of DL • The engineering strain does not consider that the incremental change in length is now being spread over a longer distance.

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

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

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

15. True Strain The incremental strain, therefore Is found to be: Strain = DL / Ln-1 The true strain is the sum of the Incremental strains as DL  0. Thus: e=ln(1+e) Li=Li-1+DL L0=gage L1 =L0+DL L2=L1+DL=L0+2DL ... L n = L0+nDL

16. True Stress • 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: s=s(1+e) where e is the corresponding value of engineering strain for each stress/strain data pair.

17. s (psi) e (in/in) 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.

18. 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.

19. Stress-Strain Characteristics –Perfectly Elastic s e

20. Stress-Strain Characteristics –Elastic Perfectly Plastic s e

21. Stress-Strain Characteristics –Elastic Strain Hardening s e

22. 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 • 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.

23. s (psi) e (in/in) Elasticity – Hooke’s Law and Young’s Modulus Hooke’s Law: s=Ee Young’s Modulus: E=s/e Slope=E }Elastic Region

24. Elasticity Considerations • 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.

25. Strength • “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 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.

26. Determining Yield Strength and UTS

27. Other Important Features 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 • Onset of necking can be observed in the engineering stress-strain curve

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

29. Exponential Approximation forStrain-Hardening Materials

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

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

32. Ductility Considerations • For Design: • Ductile materials tend to be able to absorb energy • 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. • For Manufacturing: • Ductile materials tend to be easy to form (particularly in forging) • 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

33. Toughness • “Ability to absorb Energy” • Area under the stress-strain curve • Can be measured by impact tests • May be sensitive to a wide variety of factors • Material purity (internal defects) • Surface characteristics (notch sensitivity) • Rate of deformation (strain rate sensitivity) • Temperature sensitivity (ductile to brittle temp)

34. Charpy Impact Test

35. Charpy Impact Test – Ductile Material

36. Charpy Impact Test – Brittle Material

37. 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.

38. 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.

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

40. Hardness – Other Tests • 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 • Durometer – indentation test specifically for polymers and elastomers

41. Fatigue Testing • Under cyclic loading, most materials exhibit some degradation of strength characteristics. • 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

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

43. Yield-Point Elongation