Fatigue-Limit Effect on Variable-Amplitude Fatigue of Stiffeners

The effect of the constant-amplitude fatigue limit on the variable-amplitude fatigue life was determined experimentally. Tensile specimens with transverse stiffeners welded either automatically or manually were tested in such a way that, from one set of tests to another, an increasing number of cycles fell below the fatigue limit. Data were developed from 500,000 cycles—60,000,000 cycles of loading. The root-mean-cube stress range was found to be a valuable transfer function between constant and variable-amplitude fatigue life data when its value exceeded the fatigue limit. When it was lower, the modification suggested by Yamada and the first writer—based on the assumption that those cycles in the histogram that fall below the fatigue limit do not drive the crack—led to good predictions of variable amplitude fatigue life.