The effects of residual macrostresses and microstresses on fatigue crack initiation

The combined techniques of X-ray diffraction and Finite Element (FE) analysis have been used to analyze and predict fatigue crack initiation in the presence of residual stresses. These stresses were introduced into polished double-edge notched 1080 steel specimens by pre-straining and press-fitting operations, and crack initiation was monitored during high-cycle fatigue tests using surface replicas. The local stress–strain behavior in the vicinity of the initiation sites was tracked using FE analysis. Microbeam X-ray diffraction measurements were also made in each phase of the steel to determine both residual macrostresses, which compared well with the FE computations, and microstresses that were employed along with mechanical tests to determine input parameters for the FE analysis. Microstresses were observed to fade rapidly during fatigue, while macrostresses relaxed less rapidly and were observed to strongly affect crack initiation behavior. The fatigue behavior was correlated with the observed stresses via a stress–life relationship that incorporates mean stress effects.