Microstructural effects on length scales for plastic blunting of stage II fatigue cracks in metallic materials

A length scale has been recently proposed for plastic blunting of stage II fatigue cracks in metals that is proportional to the area integral of the opening strain ahead of the crack tip. This quantity was measured via in-situ loading experiments in compact tension specimens of Al 2024-T351, with cracks running either parallel or perpendicular to the rolling direction at chosen values of AK and load ratio. The strain fields ahead of the crack tip were quantified in-situ using Digital Image Correlation (DIC) and the microstructure ahead of the crack tip was characterized using Electron Backscattering Diffraction (EBSD). Results indicate that strain localized along deformation bands, and that the strain field was affected by the orientation of the crack with respect to the rolling direction. In addition, the integrated strain over the whole area imaged in these samples correlated well with the maximum applied loads used, but the strain was more uniformly distributed ahead of the sample with a lower crack growth rate. When the integrated strain was calculated over a smaller area closer to the crack tip, differences were consistent with the trends in crack growth rates observed experimentally. This indicates that the microstructure can influence the fraction of inelastic deformation involved in crack growth via plastic blunting.