Page 447 - Mechanical Behavior of Materials

Section 9.6 Trends in S-N Curves 447

Figure 9.30 Influence of grain size and cold work on rotating bending S-N curves for
70Cu-30Zn brass. (From [Sinclair 52]; used with permission.)

member, so the procedure is useful only if the bending in service is expected to be primarily in one
direction, as for leaf springs in ground vehicle suspensions. Various combinations of presetting and
shot peening affect the S-N curves of steel leaf springs under zero-to-maximum bending, as shown
in Fig. 9.31. The effects correlate as expected with measured residual stresses.
Smoother surfaces that result from more careful machining in general improve resistance to
fatigue, although some machining procedures are harmful, as they introduce tensile residual stresses.
Various surface treatments, such as carburizing or nitriding of steels, may alter the microstructure,
chemical composition, or residual stress of the surface and therefore affect the fatigue resistance.
Plating, such as nickel or chromium plating of steel, generally introduces tensile residual stresses
and is therefore often harmful. Also, the deposited material itself may have poorer resistance to
fatigue than the base material, so cracks easily start there and then grow into the base material. Shot
peening after plating can help by changing the residual stress to compression.
Welding results in geometries that involve stress raisers, and residual stresses often occur as a
result of uneven cooling from the molten state. Unusual microstructure may exist, as well as porosity
or other small flaws. Hence, the presence of welds generally reduces fatigue strength and requires
special attention.

9.6.5 Fatigue Limit Behavior

Many steels and some other materials appear to exhibit a distinct fatigue limit —that is, a safe
stress below which fatigue failure appears to never occur, as in Fig. 9.5. For other materials, such