Page 441 - Mechanical Behavior of Materials

Section 9.6 Trends in S-N Curves 441

Figure 9.22 Fatigue striations spaced approximately 0.12 μm apart, from a fracture surface of
a Ni-Cr-Mo-V steel. (Photo courtesy of A. Madeyski, Westinghouse Science and Technology Ctr.,
Pittsburgh, PA. Published in [Madeyski 78]; copyright c ASTM; reprinted with permission.)

accelerated, which may occur due to an altered stress level, temperature, or chemical environment.
Beach marks may also be caused by discoloration due to greater amounts of corrosion on older
portions of the fracture surface.
After the crack has reached a sufficient size, a final failure occurs that may be ductile, involving
considerable deformation, or brittle and involving little deformation. The final fracture area is
usually rough in texture, and in ductile materials forms a shear lip, inclined at approximately
◦
45 to the applied stress. These features can be seen in Figs. 9.16, 9.20, and 9.21. Microscopic
examination of fatigue fracture surfaces in ductile materials often reveals the presence of marks left
by the progress of the crack on each cycle. These are called striations and can be seen in Fig. 9.22.

9.6 TRENDS IN S-N CURVES

S-N curves vary widely for different classes of materials, and they are affected by a variety of factors.
Any processing that changes the static mechanical properties or microstructure is also likely to affect
the S-N curve. Additional factors of importance include mean stress, member geometry, chemical
environment, temperature, cyclic frequency, and residual stress. Some typical S-N curves for metals
have already been presented, and curves for several polymers are shown in Fig. 9.23.

9.6.1 Trends with Ultimate Strength, Mean Stress, and Geometry
Smooth specimen fatigue limits of steels are often about half of the ultimate tensile strength, which
is illustrated in Fig. 9.24. Values drop below σ e ≈ 0.5σ u at high-strength levels where most steels
have limited ductility. This indicates that a reasonable degree of ductility is helpful in providing
resistance to cyclic loading. Lack of appreciation for this fact can lead to fatigue failures in situations
where fatigue was not previously a problem, as in substituting high-strength materials to save weight
in vehicles. Similar correlations exist for other metals, but the fatigue limits are generally lower than
half the ultimate. This is illustrated for wrought aluminum alloys by Fig. 9.25.