Local strain based approaches have been used frequently for the prediction of crack initiation life of notched components. Limitations of the transferability of smooth specimen fatigue data to the notch root as well as cumulative damage problems have been handled by use of fatigue notch factors and consideration of some damage contribution below the fatigue limit. This type of a conventional approach has been applied as a reference case for the present study.
Based on the mechanics of local strain approaches, a model for crack initiation life prediction is outlined. It takes explicitly into account the phenomena related to transferability (size, multiaxiality) as well as a load-sequence dependent damage accumulation. It is based on the closure behavior of short cracks that gives rise to load sequence dependent effective load ranges and continuously decreasing fatigue limits under spectrum loading.
Prediction results are compared to experimental data from notched aluminum 7075-T7351 specimens of various shapes and sizes under both Gaussian and MINITWIST loading. The conventional approach provides unconservative estimates with a broad scatter of life ratios. The fracture mechanics based model gives a relatively low scatter of life ratios around a mean value of unity.