Mode mixity effect on the damage of a constrained ductile layer

A partially fractured thin metal foil bonded between ceramic blocks subject to mode I loading was studied in a previous paper. The constrained plastic flow easily elevates the hydrostatic stress in the metal to more than 5 times the yield stress. The maximum stress occurs at a distance of several foil thicknesses ahead of the crack tip, and can trigger cavitation or debonding. The large spacing between the cavities and the crack tip allows the intact metal patches to bridge the crack, leading to rising crack growth resistance curves. The maximum foil thickness which would allow the above bridging mechanism to develop has been identified. In this paper mode mixity effects are investigated. As the mode II component increases, the plastic zone becomes longer while the stress triaxiality becomes smaller. The loss of constraint is detailed in this paper. Under mixed-mode loading, high tensile stresses also occur in the ceramics. These stresses are large enough to cause ceramic cleavage. Using a full-field large-deformation elastic-plastic analysis, the multi-mechanism competition is assessed. The dependence of nominal fracture toughness on failure mechanisms and on geometric and material parameters is also discussed.