Fatigue Damage under Variable Amplitude Loads

Abstract

A truck passing over a bridge induces loads which vary with time. To test the ability of available analytical techniques to estimate fatigue life under such loading, crack growth rate tests are conducted with eleven compact‐type specimens using constant and variable amplitude load‐time histories. One variable amplitude load‐time history used in these tests is recorded from an in‐service bridge; other histories used are constructed to isolate minor cycle amplitude and minor cycle mean relative to major cycle mean as test variables. The data show that the mean level of minor cycles within a complex cycle significantly affects the damage caused by the complex cycle, causing the standard rainflow counting‐Miner's rule fatigue life estimating technique to be unconservative. Furthermore, cycles below the constant amplitude fatigue stress range threshold are shown to be damaging when applied as part of a variable amplitude load‐time history. It is proposed that complex load‐time histories be converted to single equivalent cycles through a damage index.