Predicting sequence effects and cumulative damage in fatigue using a unified model for crack initiation and growth

A new unified fatigue model which covers both crack initiation and crack growth has been used to predict load sequence effects in fatigue failure of 7075-T6 aluminum subjected to two-level loading histories. Dubbed ''FATIGMOD,'' the model simulates fatigue as the successive failure of elements within, and on the surface of, a material; failure of each element occurs when the local variables at that element reach any one of the three failure criteria, which are formulated in terms of local accumulated plastic work density, local tensile stress, and local tensile strain. For ''smooth'' (i.e., unnotched and non-precracked) specimens, FATIGMOD predictions indicate that a ''high-low'' load sequence results in a cycle fraction sum at failure which is smaller than unity, while a ''low-high'' load history produces the reverse behavior. In contrast, the cycle fraction sum at failure for a precracked specimen is predicted to exceed unity in a stress reduction test but is less than one in a test with stress increases. These predictions are in good agreement with experimental results. The effects of load sequence on fatigue damage are explained in terms of the curves of crack length vs. number of cycles generated by the unified model. The predicted curves for fatigue and the resulting cumulative damage behavior are compared with those observed in stress corrosion cracking and creep rupture