Effect of damage-related microstructural parameters on plate tearing at steady state

The tearing of ductile metal plates can take place in three distinctly different modes: cup-cup, cup-cone, and crack slanting, but they are often observed in combination. It is well established that all tearing modes are governed by nucleation, growth, and coalescence of voids at the micron scale. What controls the shift between different tearing modes is, however, unclear and a micro-mechanics-based investigation is launched here to shed light on the issue. The present work takes as a starting point the hypothesis that the volume fraction, (average) size, and distribution of second phase particles, which act as void nucleation sites, are the key microstructural parameters that determine the tearing mode. In accordance with this hypothesis, the plates are modeled here by embedding randomly distributed void nucleation sites in a homogeneous matrix material. A parameter study is performed, and by adjusting the number, size and distribution of the nucleation sites in the fracture process zone, a shift in the tearing mode is achieved: a low number of small (relative to the plate thickness) randomly distributed particles link up in a void-by-void-type failure, whereas bigger particles, or a large number of small particles, can facilitate multiple void interactions. The present work also demonstrates that, for plates with intermediate or low volume fraction of nucleation sites, the localization of deformation in a macroscopic band precedes the microscopic localization that eventually links the voids. However, the two modes of plastic flow localization occur simultaneously for large volume fractions.