Local crack-tip strain behavior during fatigue crack initiation under mixed mode loading

Generally fatigue cracks initiated in machine parts and structural members such as automobiles, aircrafts and ships have been investigated under opening-mode loadings. However fatigue crack initiation must be evaluated under mixed-mode loadings because actual fatigue crack are caused by combinations of tension, compression, in-plane shear and out-plane shear. In this study, local strain at the notch root during crack-initiation process and local strain at the crack tip during crack-propagation process are measured in real time in polycarbonate specimens with crack orientations of 0,15,30,45 and 60deg subjected to low-cycle fatigue tests under mixed-mode loadings by the fine-grid method. As a result, the linear damage accumulation law based on local-strain range is found to be effective for estimating fatigue-crack-initiation life under mixed-mode loadings.