Seminars

Abstract: Rate-dependent fracture has been observed for many polymer-based interfaces, where both the interfacial strength and adhesion energy (or fracture toughness) often increase with increasing separation rates, while the opposite trend typically defines the behavior of the bulk polymer. This dissertation mainly addresses the following two aspects: the characterization of the rate-dependent fracture for a silicon/epoxy interface under mixed-mode loading conditions, and the development of a multiscale, mechanism-based model for simulating rate-dependent fracture at interfaces.

The rate-dependent fracture of a silicon/epoxy interface was examined under mixed-mode loading conditions. A dual-actuator loading device was designed and developed to achieve a full range of the mode-mix with double cantilever beam specimens, where the two displacements at the loading end can be controlled independently. For each mode mix, the ratio of the end displacements between the upper and lower beams was kept a constant, while the rate effect was examined by varying the displacement rates proportionally. The mixed-mode interfacial fracture was found to be rate dependent as both normal and shear components of the interfacial strength and toughness increased with increasing displacement rates. To model the rate-dependent fracture, a multiscale mechanism-based approach was proposed to include the entropic effects of polymer chains and a nonlinear energy barrier for bond rupture. A rate-dependent cohesive zone model was developed from the bottom up at four levels: the bond level, the chain level, the interface level, and the specimen level. To compare with the fracture experiments at the specimen level, the interface model was implemented via a user-defined surface interaction subroutine in the finite element package ABAQUS for numerical simulations. With a few parameters extracted for the molecular structures of the interface, the model was able to reproduce the rate-dependent fracture of the silicon/epoxy interface under mode-I conditions.