Page 317 - Materials Science and Engineering An Introduction

Summary • 289

• For other metals (e.g., ferrous and titanium alloys), at some point, stress ceases to de-
crease with, and becomes independent of, the number of cycles; the fatigue behavior
of these materials is expressed in terms of fatigue limit (Figure 8.19a).

Crack Initiation and • Fatigue cracks normally nucleate on the surface of a component at some point of
Propagation stress concentration.
• Two characteristic fatigue surface features are beachmarks and striations.
Beachmarks form on components that experience applied stress interruptions;
they normally may be observed with the naked eye.
Fatigue striations are of microscopic dimensions, and each is thought to represent
the crack tip advance distance over a single load cycle.

Factors That Affect • Measures that may be taken to extend fatigue life include the following:
Fatigue Life Reducing the mean stress level
Eliminating sharp surface discontinuities
Improving the surface finish by polishing
Imposing surface residual compressive stresses by shot peening
Case hardening by using a carburizing or nitriding process
Environmental • Thermal stresses may be induced in components that are exposed to elevated tem-
Effects perature fluctuations and when thermal expansion and/or contraction is restrained;
fatigue for these conditions is termed thermal fatigue.
• The presence of a chemically active environment may lead to a reduction in fatigue
life for corrosion fatigue. Measures that may be taken to prevent this type of fatigue
include the following:
Application of a surface coating
Use of a more corrosion-resistant material
Reducing the corrosiveness of the environment
Reducing the applied tensile stress level
Imposing residual compressive stresses on the surface of the specimen
Generalized Creep • The time-dependent plastic deformation of metals subjected to a constant load (or
Behavior stress) and at temperatures greater than about 0.4T is termed creep.
m
• A typical creep curve (strain versus time) normally exhibits three distinct regions
(Figure 8.29): transient (or primary), steady-state (or secondary), and tertiary.
• Important design parameters available from such a plot include the steady-state creep
rate (slope of the linear region) and rupture lifetime (Figure 8.29).

Stress and • Both temperature and applied stress level influence creep behavior. Increasing either
Temperature Effects of these parameters produces the following effects:
An increase in the instantaneous initial deformation
An increase in the steady-state creep rate
A decrease in the rupture lifetime
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• An analytical expression was presented that relates P s to both temperature and
stress—see Equation 8.25.
Data Extrapolation • Extrapolation of creep test data to lower-temperature/longer-time regimes is possible
Methods using a plot of logarithm of stress versus the Larson–Miller parameter for the particular
alloy (Figure 8.33).

Alloys for High- • Metal alloys that are especially resistant to creep have high elastic moduli and melting
Temperature Use temperatures; these include the superalloys, the stainless steels, and the refractory
metals. Various processing techniques are employed to improve the creep properties
of these materials.