Variable amplitude fatigue behavior and life predictions of case-hardened steels

Fatigue behavior of case-hardened steel specimens were investigated under variable amplitude loading conditions including two-level load tests, periodic overload tests, and service load history tests. Fatigue life predictions were made using a two-layer model based on case and core material fatigue properties and compared to the experimental results. Both standard strain-life curve based on constant amplitude data as well as effective strain-life curve based on periodic overload data were used for life predictions. The effective strain-life curve showed that periodic overloads have a strong effect on fatigue life of case-hardened specimens. In service load history tests with plastic deformation, the fatigue damage caused by the small cycles in the load history is larger than the damage calculated from the standard strain-life curve. For periodic overload or variable amplitude service load history, the linear damage rule (LDR) resulted in reasonably good life predictions, while for high–low step loading tests the damage curve approach (DCA) resulted in better predictions.