Void nucleation in constrained silver interlayers

Abstract

The process of void nucleation during fracture in thin brazed Ag interlayers has been investigated. Tensile tests were performed on interlayers of five thicknesses. The tensile strength increased rapidly with the ratio of interlayer diameter to thickness(D/T) for the thick interlayers(D/T < approximately 40) while the rate decreased significantly in the thin interlayers. All specimens fractured in the Ag interlayer along a plane near, and parallel to, the steel interface. The fracture surfaces showed silicon oxide inclusions at the bottom of many of the dimples. A series of finite element models were constructed to determine the general stress state in the interlayers and to determine the development of local stresses around inclusions in the interlayer. The finite ele- ment calculations indicated that the distribution of triaxial tension radially across the interlayer varied withD/T. Triaxial stresses, at failure, up to 10 times the uniaxial yield stress of the Ag were predicted from the model. The local stresses around a rigid inclusion in the interlayer developed more quickly, with applied stress, in the thicker interlayers as a result of the increased plastic deformation. The development of local stresses also increased with the proximity of the inclusion to the steel interface. By assuming a critical stress criterion for void nucleation at an inclusion interface, the finite element model was able to predict the experimentally observed nonlinear relationship between the interlayer failure stress andD/T.