CHAPTER 9
FAILURE
LEARNING OBJECTIVES
1. Cite the three usual causes of failure.
2. (a) Cite the two modes of fracture and the differences between them.
(b) Note which type of fracture is preferred, and give two reasons why.
3. Describe the mechanism of crack propagation for both ductile and brittle modes of fracture.
4. Describe the two different types of fracture surfaces for ductile metals, and, for each, cite the general mechanical characteristics of the material.
5. Briefly describe the mechanism of crack formation and growth in moderately ductile materials.
6. Briefly describe the macroscopic fracture profile for a material that has failed in a brittle manner.
7. Name and briefly describe the two crack propagation paths for polycrystalline brittle materials.
8. Explain why the strengths of brittle materials are much lower than predicted by theoretical calculations.
9. Given the magnitude of an applied tensile stress, and the length and tip radius of a small crack which axis is perpendicular to the direction of the applied stress, compute the maximum stress that may exist at the crack tip.
10. Cite the conditions that must be met in order for a brittle material to experience fracture.
11. Briefly state why sharp corners should be avoided in designing structures that are subjected to stresses.
12. For some material, given values of the modulus of elasticity and specific surface energy, and the length of an internal crack, be able to compute the critical stress for propagation of this crack.
13. Define critical strain energy release rate, and cite an equation for its determination.
14. Describe/illustrate the three different crack displacement modes.
15. Describe the condition of plane strain.
16. Define fracture toughness in terms of (a) a brief statement, and (b) an equation (define all parameters in this equation). (c) Specify the units for fracture toughness.
17. Make a distinction between fracture toughness and plane strain fracture toughness
18. Given the plane-strain fracture toughness of a material, the length of the longest surface crack, and the value of Y, compute the critical (or design) stress.
19. Determine whether or not a flaw of critical length is subject to detection given the resolution limit of the detection apparatus, the maximum applied tensile stress, the plane strain fracture toughness of the material, as well as a value for the scale parameter (Y).
20. Briefly explain why there is normally significant scatter in the fracture strength for identical specimens of the same ceramic material.
21. Note the reason why ceramic materials are stronger in compression than in tension.
22. Name and describe (or sketch) the typical features that are observed on the fracture surface of a brittle ceramic material.
23. (a) Briefly describe the phenomenon of crazing.
(b) Note for which type of polymers and under what experimental/service conditions is crazing observed.
(c) Cite experimental/service conditions that produce crazing in polymeric materials.
24. Name three factors that are critical relative to a metal experiencing a transition from ductile to brittle fracture.
25. Name and briefly describe the two techniques that are used to measure impact energy (or notch toughness) of a material.
26. Make a schematic plot of the dependence of impact energy on temperature for a metal that experiences a ductile-to-brittle transition.
27. Note which types of materials do, and also those which do not, experience a ductile-to-brittle transition with decreasing temperature.
28. Cite two measures that may be taken to lower the ductile-to-brittle transition temperature in steels.
29. Define fatigue and specify the conditions under which it occurs.
30. Name and describe the three different stress-versus-time cycle modes that lead to fatigue failure.
31. Given a sinusoidal stress-versus-time curve, be able to determine the stress amplitude and mean stress.
32. (a) Briefly describe the manner in which tests are performed to generate a plot of fatigue stress versus the logarithm of the number of cycles.
(b) Note which three in-service conditions should be replicated in a fatigue test.
33. Schematically plot the fatigue stress as a function of the logarithm of the number of cycles to failure for both materials that do and that do not exhibit a fatigue limit. For the former, label the fatigue limit.
34. From a fatigue plot for some material determine:
(b) the fatigue lifetime (at a specified stress level), and
(c) the fatigue strength (at a specified number of cycles).
35. Briefly describe the two stages of crack propagation in polycrystalline materials that may ultimately lead to fatigue failure.
36. Describe the two differently types of fatigue surface features, and cite the conditions under which they occur.
37. Cite five measures that may be taken to improve the fatigue resistance of a metal.
38. Describe thermal fatigue failure, and note how it may be prevented.
39. Describe corrosion fatigue, and then cite five measures that may be taken to prevent it.
40. Define creep and specify the conditions under which it occurs.
41. Make a schematic sketch of a typical creep curve, and then note on this curve the three different creep stages.
42. Given a creep plot for some material, determine (a) the steady-state creep rate, and (b) the rupture lifetime.
43. Given the absolute melting temperature of a metal, estimate the temperature at which creep becomes important.
44. Schematically sketch how the creep behavior of a material changes with increasing temperature and increasing load (or stress).
45. Make schematic plots showing how the rupture lifetime and steady-state creep rate for a material are represented as functions of stress and temperature.
46. Cite the generalized mathematical expression for the dependence of steady-state creep rate on both applied stress and temperature.
47. Given a Larson-Miller master plot of creep data for some material, determine the rupture lifetime at a given temperature and stress level.