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Study of the flat to slant crack transition in ductile thin sheet material : simulations and experiments

Flat to slant crack transition can typically be observed in ductile thin sheet materials. The crack initiates perpendicularly to the loading direction from the notch and then turns to 45° with respect to the loading direction during crack propagation. This phenomenon is, however, still not well understood and, so far, attempts to simulate the transition in three dimensions often fail to predict macroscopic loads correctly. In this study an initial attempt has been made to reproduce the flat to slant transition performing an implicit 3D Finite Element simulation via adapting a Gurson-type model. A second void nucleation term for deformation under shear was introduced.The Lode parameter was used here to identify shear deformation. Using this modification the flat to slant transition has been reproduced successfully at loads similar to the experimental results. Further experimental investigations of void growth in the flat and slant crack propagation regime have been carried out. Cracks in Kahn tear test specimens have been arrested in the three regimes and subsequently been observed via Synchrotron Radiation Tomography of the crack tips 3D quantitative void growth analyses ahead of the crack tip in the flat and slant regimes have confirmed the change in fracture mechanisms: void growth in the flat region is substantially higher as compared to the slant crack propagation region.