Crack nucleation from a notch in a ductile material under shear dominant loading

We examine the nucleation of a crack from a notch under a dominant shear loading in Al 6061-T6. The specimen is loaded in nominally pure shear over the gage section in an Arcan specimen configuration. The evolution of deformation is monitored using optical and scanning electron microscopy. Quantitative measurements of strain are made using the 2nd phase particles as Lagrangian markers which enable identification of the true (logarithmic) strains to levels in the range of two. Electron microscopy reveals further that the 2nd phase particles do not act as nucleation sites for damage in the regions of pure shear deformation. The initial notch is shown to “straighten out”, forming a new, sharper notch and triggering failure at the newly formed notch. Numerical simulations of the experiment, using the conventional Johnson–Cook model and a modified version based on grain level calibration of the failure strains, reveal that it is necessary to account for large local strain levels prior to the nucleation of a crack in order to capture the large deformations observed in the experiment.