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Most of the currently available methods for predicting fatigue crack growth in structures do not consider effects due to mixed-mode loading. This is partly due to the limited information available concerning the fatigue crack growth under mixed mode loading. Previous investigations of mixed mode effects on fatigue crack growth have shown that under combined opening and in-plane shear loading, the cracks turn abruptly so that the in-plane shear mode is eliminated [1–3]. Very little data are generated before the crack curvature eliminates the mixed mode loading. The changing orientation of the crack during the test makes analysis of the problem difficult and may cloud the intrinsic fatigue crack growth rate behavior under mixed mode loading. Situations can exist in engineering structures in which cracks may grow in a self-similar manner even under the influence of mixed mode loading. Examples of such situations are radial cracks in bearing races  and transverse cracks in rotating shafts under shear and bending loads. The cracks may not change orientation because the in-plane shear component is fully reversed. It is necessary to understand the effects of mixed mode loading on fatigue crack growth in order to properly predict the behavior of these components.
This paper presents the results of an investigation to measure the fatigue crack growth rate of a high-strength steel under various ratios of mixed mode loading while maintaining self-similar crack growth. A primary objective of this investigation was to generate data for relatively large amounts of crack growth under a wide range of mixed mode ratios. Several models for predicting the mixed mode fatigue crack growth rate from opening mode data are evaluated based on the data generated in this investigation.
fatigue crack growth, mixed mode fatigue, predictive models
Senior mechanical engineer, David Taylor Research Center, Annapolis, MD