A New Approach to Predicting Fatigue Crack Propagation

A new approach was developed for the prediction of fatigue crack growth. Based upon the elastic-plastic stress analysis of a crack component and the application of a general multiaxial fatigue criterion, fatigue crack propagation was modeled by using the cyclic plasticity material properties and fatigue constants obtained for crack initiation. The cyclic elastic-plastic stress-strain field near the crack tip was analyzed using the finite element method with the implementation of a robust cyclic plasticity theory. A multiaxial fatigue criterion was employed to determine the fatigue damage. With an incremental form, the multiaxial fatigue criterion can be directly used for assessing fatigue damage near the crack tip. A straightforward method was developed to determine the fatigue crack growth rate. Crack propagation behavior of a material was obtained without any additional assumptions or fitting. Benchmark Mode I fatigue crack growth experiments were conducted using 1070 steel at room temperature. The approach was able to quantitatively capture all the important fatigue crack propagation behaviors including the overload and the R-ratio effects on crack propagation and threshold. The models provide a new perspective for the R-ratio effects. The success of the approach confirms that the fatigue crack initiation and propagation behaviors are governed by the same fatigue damage mechanisms. Crack growth can be treated as a process of continuous crack nucleation. In addition, the sensitivity of the predicted fatigue growth rate to the element size in the finite element model was discussed.