High toughness fibrillating metal-elastomer interfaces: On the role of discrete fibrils within the fracture process zone

Fibrillating metal-elastomer interfacial systems, typically used in stretchable electronics applications, can exhibit remarkably high values for the interface fracture toughness. Consequently, a huge gap exists between the low adhesion energy at the microscopic scale and the measured macroscopic work of separation. This contribution aims to close this energy gap by unravelling the underlying dissipative mechanisms through a multi-scale approach. The first scale transition was established in earlier work, and concerned the formation and deformation of a single fibril at the copper-rubber interface up to failure. It was shown that the obtained work of separation was significantly larger than the small-scale interface adhesion, yet a decade too small with respect to the experimental values. In order to close the energy gap, in this contribution, the second scale transition is achieved by considering a finite number of elongating discrete hyperelastic fibrils within the fracture process zone. It is shown that the dynamic release of the stored elastic energy by fibril fracture that results from the spatial discreteness of multiple fibrils, the interaction with the adjacent deforming bulk elastomer material and the highly nonlinear behavior of the elastomer provides an explanation for the high work of separation values. In addition, an intrinsic shortcoming of cohesive zone formulations at the macroscopic scale is revealed. The results provide a mechanistic understanding of the physics involved with interface delamination through fibrillation in metal-elastomer interfaces.