Chap 6 Learn Obj

CHAPTER 7 MECHANICAL PROPERTIES

LEARNING OBJECTIVES

1. List three factors that should be considered in designing laboratory tests to assess the mechanical characteristics of materials for service use.

2. Given the tensile load on a specimen and its original and instantaneous cross- sectional dimensions, be able to compute the engineering stress and true stress.

3. Given the original and instantaneous lengths of a specimen that is being loaded in tension, be able to compute the engineering strain and true strain.

4. Given the magnitude of a tensile stress that is applied parallel to the specimen axis, compute the magnitudes of normal and shear stresses on a plane that is oriented at some specified angle relative to the specimen end- face.

5. Distinguish between elastic and plastic deformations, both by definition, and in terms of behavior on a stress-strain plot.

6. Compute the elastic modulus from a stress-strain diagram.

7. Given the elastic modulus and either elastic engineering stress or strain, be able to compute the other (strain or stress). 8. For a material that exhibits nonlinear elastic behavior, be able to compute tangent and secant moduli from its stress-strain diagram.

9. State what is occurring on an atomic level as a material is elastically deformed.

10. Briefly explain how the shape of a material's force versus interatomic separation curve influences its modulus of elasticity.

11. Given the cross-sectional area of a specimen over which a shear forced of specified magnitude acts, and, in addition, the resulting shear strain, be able to compute the shear modulus.

12. Define anelasticity.

13. Given Poisson's ratio and the elastic strain in the direction of the applied load (or axial strain), be able to compute the elastic strain in the lateral (or perpendicular) direction.

14. Cite the typical value range of Poisson's ratio for metallic materials.

15. Given values of modulus of elasticity and Poisson's ratio for an isotropic material, estimate the value of its shear modulus.

16. Given an engineering stress-strain diagram for a metallic material, determine (a) the proportional limit, (b) the yield strength (0.002 strain offset), and (c) the tensile strength, and (d) estimate the percent elongation. 17. Schematically sketch the stress-strain behavior for a metal that displays distinct upper and lower yield points, and then explain how the yield strength is determined.

18. Given the stress-strain behavior for two metals, be able to distinguish which is the stronger.

19. For the tensile deformation of a ductile cylindrical specimen, describe changes in specimen profile to the point of fracture.

20. Explain why engineering stress decreases with increasing engineering strain past the tensile strength point.

21. Cite typical yield and tensile strength ranges for metal alloys.

22. Give a brief definition of ductility, and schematically sketch the engineering stress-strain behaviors for both ductile and brittle metals.

23. Given the original and fracture dimensions of a specimen deformed in tension, be able to determine its ductility in terms of both percent elongation and percent reduction of area.

24. For metallic materials cite which tensile parameters are sensitive (and also insensitive) to any prior deformation, the presence of impurities, and/or any heat treatment.

25. For metallic materials cite which tensile parameters are sensitive (and also insensitive) to any prior deformation, the presence of impurities, and/or any heat treatment. 26. For metallic materials cite how elastic modulus, tensile and yield strengths, and ductility change with increasing temperature.

27. Give brief definitions of and the units for modulus of resilience and toughness (static).

28. Given yield strength and modulus of elasticity values for some material, compute its modulus of resilience.

29. Given the stress-strain behavior for two metals, determine which is the most resilient and which is the toughest.

30. Given the values of the constants K and n in the equation relating plastic true stress and true strain, be able to compute the true stress necessary to produce some specified true strain.

31. Schematically plot both the tensile engineering stress-strain and true stress- strain behaviors for the same material and explain the difference between the two curves.

32. Describe the phenomenon of elastic recovery using a stress-strain plot.

33. Determine the elastic strain recovered for some material, given its stress- strain plot and the total strain to which a specimen has been subjected.

34. Cite the major differences in room temperature mechanical characteristics for metals and ceramics. 35. Note the reason why ceramic materials are stronger in compression than in tension.

36. Give three reasons why the stress-strain characteristics of ceramic materials are determined using transverse bending tests rather than tensile tests.

37. Given the cross-sectional dimensions of a rectangular ceramic rod bent to fracture using a three-point loading technique, as well as the distance between support points, and the fracture load, compute the flexural strength.

38. Given the radius of a cylindrical ceramic rod that is bent to fracture using a three-point loading technique, as well as the distance between support points, and the fracture load, compute the flexural strength.

39. Briefly describe the mechanism by which plastic deformation occurs for each of crystalline and noncrystalline ceramic materials.

40. On the basis of slip considerations, explain why crystalline ceramic materials are normally brittle.

41. Briefly define viscosity and cite the units in which it is expressed.

42. For a porous ceramic, do the following: (b) Given the modulus of elasticity for the nonporous material, compute E for a specified volume fraction of porosity.

(c) Given values of the experimental 0 and n constants, calculate the flexural strength at some given P. 43. (a) Schematically plot the three different characteristic types of stress-strain behavior for polymeric materials. (b) Now note which type(s) of polymer(s) display(s) each of these behaviors.

44. Make a comparison of the general mechanical properties (i.e., modulus of elasticity, tensile strength, and ductility) of plastics and elastomeric materials with metals and ceramics.

45. Cite three affects on the mechanical characteristics of a polymer as its temperature is increased, or as the deformation strain rate is decreased.

46. (a) Describe the macroscopic tensile deformation of a cylindrical dog-bone specimen (i.e., specimen profile) of a typical ductile plastic to fracture. (b) Correlate this behavior with the stress-strain plot.

47. Define viscoelasticity.

48. (a) Describe the manner in which stress relaxation measurements are conducted. (b) Using the results of a stress relaxation test, briefly explain how the relaxation modulus is determined.

49. (a) On a graph of the logarithm of relaxation modulus versus temperature plot schematic curves for semicrystalline, amorphous, and crosslinked polymers. (b) On this plot note melting and glass transition temperatures. (c) In addition, indicate on this same plot glass, leathery, rubbery, and viscous flow regions. 50. Define hardness in a one- or two-sentence statement.

51. Cite three reasons why hardness tests are performed more frequently than any other mechanical test on metals.

52. Name the two most common hardness-testing techniques that are used in the U.S., and give two differences between them.

53. Name and briefly describe the two different microindentation hardness testing techniques. Now cite situations for which these techniques are generally used.

54. Cite three precautions that should be taken when performing hardness tests in order to insure accurate readings.

55. Schematically diagram tensile strength versus hardness for a typical metal.

56. Cite five factors that can lead to scatter in measured data.

57. Given a series of data values that have been collected, be able to compute both the average and the standard deviation.

58. Given the yield strength of a ductile material, be able to compute the working stress.