On the mode I toughness of adhesive bonds exhibiting strain-softening and re-hardening

The mode I fracture toughness of an interface between an adhesive material exhibiting strain-softening behavior and an elastic substrate is investigated by finite element modeling. The model is based on an asymptotic K -field formulation relying on cohesive zone elements to simulate the fracture process. A parametric phenomenological hardening law is used to cover the plastic response behavior of a wide range of amorphous glassy polymers, involving a first hardening peak followed by a softening and a re-hardening response. The effect of the peak hardening stress and of the softening stress on the joint toughness is investigated for different elastic mismatch and interface strength. The joint toughness is found to significantly increase with increasing amplitude of the softening, and, this, at constant yield stress and peak hardening stress. This is related to larger plastic zone sizes and larger plastic strains, hence to more energy dissipation. Increasing the interface work of fracture magnifies this effect. There is an interesting coupling between the elastic mismatch and softening, related to their effect on the fracture process zone length. One outcome of these findings is the potential impact of ageing which is known to modify the amplitude of the peak hardening and softening, hence leading to an evolution of the joint toughness with time