Page 307 - Materials Science and Engineering An Introduction

8.10 Factors that Affect Fatigue Life • 279

to a discussion of these factors and to measures that may be taken to improve the fatigue
resistance of structural components.
Mean Stress
The dependence of fatigue life on stress amplitude is represented on the S–N plot. Such
data are taken for a constant mean stress s m , often for the reversed cycle situation (s m 0).
Mean stress, however, also affects fatigue life; this influence may be represented by a series
of S–N curves, each measured at a different s m , as depicted schematically in Figure 8.25. As
may be noted, increasing the mean stress level leads to a decrease in fatigue life.

Surface Effects
For many common loading situations, the maximum stress within a component or
structure occurs at its surface. Consequently, most cracks leading to fatigue failure
originate at surface positions, specifically at stress amplification sites. Therefore, it has
been observed that fatigue life is especially sensitive to the condition and configuration
of the component surface. Numerous factors influence fatigue resistance, the proper
management of which will lead to an improvement in fatigue life. These include design
criteria as well as various surface treatments.

Design Factors
The design of a component can have a significant influence on its fatigue character-
istics. Any notch or geometrical discontinuity can act as a stress raiser and fatigue crack
initiation site; these design features include grooves, holes, keyways, threads, and so on.
The sharper the discontinuity (i.e., the smaller the radius of curvature), the more severe the
stress concentration. The probability of fatigue failure may be reduced by avoiding (when
possible) these structural irregularities or by making design modifications by which sudden
contour changes leading to sharp corners are eliminated—for example, calling for rounded
fillets with large radii of curvature at the point where there is a change in diameter for a
rotating shaft (Figure 8.26).
Surface Treatments
During machining operations, small scratches and grooves are invariably intro-
duced into the workpiece surface by cutting-tool action. These surface markings can
limit the fatigue life. It has been observed that improving the surface finish by polishing
enhances fatigue life significantly.
One of the most effective methods of increasing fatigue performance is by imposing
residual compressive stresses within a thin outer surface layer. Thus, a surface tensile

> m >
m m
3 2 1
Stress amplitude, a m 1 Fillet

m
m 2
3
(a) (b)
Figure 8.26 Demonstration of how design can reduce
Cycles to failure, N stress amplification. (a) Poor design: sharp corner. (b) Good
(logarithmic scale) design: fatigue lifetime is improved by incorporating a rounded
Figure 8.25 Demonstration of the influence fillet into a rotating shaft at the point where there is a change
of mean stress s m on S–N fatigue behavior. in diameter.