Some aspects of fatigue thresholds under mode III and mixed mode III and I loadings

Abstract Fatigue threshold behaviour under mode III and in-phase mixed mode III and I loading conditions has been studied in a 3.5% NiCrMoV steel, using V-notched and precracked cylindrical bar specimens at load ratios R = 0.1 and −1. Comparisons have been made between the results from V-notched, precracked specimens and the results from earlier work using slit specimens. While little influence of R -ratio was found for slit specimens, R -ratio appears to be a decisive component in defining mixed mode fatigue thresholds in precracked specimens. Ambiguities arise as to the best strategy for mixed mode fatigue tests. Crack surface interference appears to be the predominant force to modify fatigue crack growth behaviour under mixed mode loading conditions. In such circumstances the measured mixed mode fatigue thresholds are generally geometry dependent and failure analyses based on LEFM can not be applied directly.