Enhancing interfacial shear debonding resistance by mechanical mismatch

Abstract

Layered systems of similar/dissimilar soft materials are widely used in advanced device applications, including bioelectronics, soft robotics, and energy generators. Device designers strive to minimize the mechanical mismatch between the soft material layers, claiming that the mismatch could be associated with low interfacial debonding resistance and then cause device malfunction. Although strong interfacial bonding has been repeatedly achieved based on the principle of minimizing-mechanical-mismatch, the specific question of whether mechanical matching is the optimal solution for high debonding resistance has never been solidly answered, leaving ambiguity in the further optimization of the layered soft material systems. In this paper, the influence of the stiffness ratio between the soft material layers on the interfacial shear debonding resistance is systematically analyzed through theoretical modeling and finite element simulation approaches. Monotonic improvement of the shear debonding resistance with increasing the loading layer stiffness, even when exceeding that of the substrate layer, is analytically derived by establishing a shear debonding theory of a hyperelastic bilayer. This mechanical-mismatch induced improvement is due to the effective consumption of the external work by the strain energy storage of the loading layer and the large increase in the shear lag length. The theoretical predictions of the debonding resistance agree well with the finite element simulation results, which implies the local non-uniform deformation in the loading and substrate layers near the interfacial crack front causes insignificant influence on the shear debonding. Increasing the modulus or thickness of the loading layer or adding a stiff backing can equivalently improve the shear debonding resistance, which can be quantitatively predicted by the established shear debonding theory. Reasonable applications of the proposed debonding resistance enhancement strategy are also discussed. The findings provide good guidance for the safe service and further optimization of the layered soft material systems to improve the device reliability.