Page 450 - Mechanical Behavior of Materials

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

800 Man-Ten steel

600
σ a , Stress Amplitude, MPa 400 No overstrain σ e
σ = 557 MPa
u
300

200

100 Periodic overstrain
10 2 10 3 10 4 10 5 10 6 10 7
N , Cycles to Failure
f

Figure 9.33 Stress–life data for a low-strength steel tested under constant amplitude cycling
5
with zero mean stress. Periodic overstrain tests included severe cycles applied every 10 cycles,
but with their N/N f not exceeding a few percent. (Data from [Brose 74].)

Figure 9.34 Scatter in rotating bending S-N data for an unnotched aluminum alloy.
(Adapted from [Grover 66] p. 44.)

due to sample-to-sample variation in materials properties, internal defect sizes, and surface
roughness, as well as imperfect control of test variables, such as humidity and specimen alignment.
If the statistical scatter in cycles to failure N f is considered, a skewed distribution usually occurs,