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This paper investigates the effects of plasticity on the effective fracturetoughness. A layered material is considered as a modelling system.  An elastic-plastic phase-field model and a surfing boundary condition areused to study how the crack propagates throughout the material and theevolution of the effective toughness as a function of the layer angle. We firststudy three idealized situations, where only one property among fracturetoughness, Young's modulus and yield strength is heterogeneous whereas theothers are uniform. We observe that in the case of toughness and strengthheterogeneity, the material exhibits anomalous isotropy: the effectivetoughness is equal to the largest of the point-wise values for any layer angleexcept when the layers are parallel to the macroscopic direction ofpropagation. As the layer angle decreases, the crack propagates along thebrittle-to-tough interfaces, whereas it goes straight when the layers havedifferent yield strength but uniform toughness. We find that smooth deflectionsin the crack path do not induce any overall toughening and that the effectivetoughness is not proportional to either the cumulated fracture energy or thecumulated plastic work. In the case of elastic heterogeneity, the material isanisotropic in the sense of the effective toughness, as the latter varies as afunction of the layer angle. Four toughening mechanisms are active: stressfluctuations, crack renucleation, plastic dissipation and plastic blunting.Finally, we consider a layered medium comprised of compliant-tough-weak andstiff-brittle-strong phases, as it is the case for many structural composites.We observe a transition from an interface-dominated to a plasticity-dominatedfailure regime, as the phase constituents become more ductile. The material isanisotropic in the sense of the effective toughness.

Effects of plasticity on the anisotropy of the effective fracture toughness