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CHAPTER 8
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. Describe/illustrate the three different crack displacement modes.
14. Describe the condition of plane strain.
15. 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.
16. Make a distinction between fracture toughness and plane strain fracture
toughness.
17. 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.
18. 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).
19.
Name three factors that are critical relative to a metal experiencing a
transition from ductile to brittle fracture.
20. Name and describe the two techniques that are used to measure impact energy
(or notch toughness) of a material.
21. Make a schematic plot of the dependence of impact energy on temperature for
a metal that experiences a ductile-to-brittle transition.
22. Note which types of materials do, and also those which do not, experience a
ductile-to-brittle transition with decreasing temperature.
23. Cite two measures that may be taken to lower the ductile-to-brittle transition
temperature in steels.
24. Define fatigue and specify the conditions under which it occurs.
25. Name and describe the three different stress-versus-time cycle modes that
lead to fatigue failure.
26. Given a sinusoidal stress-versus-time curve, be able to determine the stress
amplitude and mean stress.
27. (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.
28. Schematically plot the fatigue stress as a function of the logarithm of the
number of cycles to failure for both materials which do and which do not
exhibit a fatigue limit. For the former, label the fatigue limit.
29. From a fatigue plot for some material determine:
(a) the fatigue lifetime (at a specified stress level), and
(b) the fatigue strength (at a specified number of cycles).
30. Describe the two different types of fatigue surface features, and cite the
conditions under which they occur.
31. Cite five measures that may be taken to improve the fatigue resistance of a
metal.
32. Describe thermal fatigue failure, and note how it may be prevented.
33. Describe corrosion fatigue, and then cite five measures that may be taken to
prevent it.
34. Define creep and specify the conditions under which it occurs.
35. Make a schematic sketch of a typical creep curve, and then note on this curve
the three different creep stages.
36. Given a creep plot for some material, determine (a) the steady-state creep
rate, and (b) the rupture lifetime.
37. Given the absolute melting temperature of a metal, estimate the temperature
at which creep becomes important.
38. Schematically sketch how the creep behavior of a material changes with
increasing temperature and increasing load (or stress).
39. Make schematic plots showing how the rupture lifetime and steady-state
creep rate for a material are represented as functions of stress and
temperature.
40. Cite the generalized mathematical expression for the dependence of steadystate creep rate on both applied stress and temperature.
41. Given a Larson-Miller master plot of creep data for some material, determine
the rupture lifetime at a given temperature and stress level.