The role of plasticity in bimaterial fracture with ductile interlayers

Evaluation of the plasticity effects in fracture along ductile/brittle interfaces requires appropriate models for plastic dissipation in a ductile component. For thin ductile films, constitutive properties appropriate to the small volumes involved are essential for adequate modeling. Here, yield stress is of primary importance. With nanoindentation, one can obtain both a large strain flow stress as well as the far field yield stress representing the small strain elastic-plastic boundary. Using these to estimate an appropriate plastic strain energy density, the crack tip plastic energy dissipation rates associated with the interfacial crack extension can be estimated for a ductile film. With the preceding analysis, plasticity effects on the interfacial toughness have been evaluated for external measures of strain energy release rates as obtained from indentation tests using the axisymmetric bilayer theory. Comparison involved RF sputtered 200- to 2000-nm-thick Cu interlayers between oxidized silicon and sputtered tungsten. Experimental values for the Cu/SiO₂ interface increased with Cu film thickness from 1 to 15 J/m². This was in qualitative agreement with the theoretical predictions for plastic energy dissipation rates. In contrast, first-order estimates suggest that the observed interfacial toughness increases cannot be attributed to either mode mixity effects or increased intrinsic interfacial fracture energies. As such, crack tip plasticity is identified as the dominant mechanism for increasing interfacial toughness. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.