Page 446 - Mechanical Behavior of Materials

446 Chapter 9 Fatigue of Materials: Introduction and Stress-Based Approach

Figure 9.29 Temperature and frequency effects on axial S-N curves for the nickel-base alloy
Inconel. (Illustration from [Gohn 64] of data in [Carlson 59]; used with permission.)

of cold work by drawing increases the dislocation density and hence the fatigue strength. Larger
grain sizes are obtained by more thorough annealing, thus lowering the fatigue strength.
Microstructures of materials often vary with direction, such as the elongation of grains and
inclusions in the rolling direction of metal plates. Fatigue resistance may be lower in directions
where the stress is normal to the long direction of such an elongated or layered grain structure.
Similar effects are especially pronounced in fibrous composite materials, where the properties and
structure are highly dependent on direction. Fatigue resistance is higher where larger numbers of
fibers are parallel to the applied stress, and especially low for stresses normal to the plane of a
laminated structure.

9.6.4 Residual Stress and Other Surface Effects
Internal stresses in the material, called residual stresses, have an effect similar to an applied mean
stress. Hence, compressive residual stresses are beneficial. These can be introduced by permanently
stretching a thin surface layer, yielding it in tension. The underlying material then attempts to
recover its original size by elastic deformation, forcing the surface layer into compression.
One means of doing this is by bombarding the surface with small steel or glass shot, which is
called shot peening. Another is by sufficient bending to yield a thin surface layer, which is called
presetting. However, the latter has an opposite (hence harmful) effect on the other side of a bending