Void nucleation in constrained silver interlayers
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INTRODUCTION
THE process of fracture in ductile metals is primarily governed by the mechanisms of void nucleation, growth, and coalescence. The influence of triaxial tension on increasing void growth rate has been investigated through experimental, 1~-5]theoretical, t6'7'8] and computational [9-~2t treatments. Similarly, theoretical and experimental investigations have addressed the role of triaxial tension in promoting void nucleation at inclusions./~3-L71 These effects have traditionally been studied with circumferentially notched tensile specimens which produce only a limited range of triaxial stresses. ~81 In many structural materials, geometrical constraints can result in triaxial tensile stress exceeding that which occurs in the circumferentially notched specimen. An example is the state of triaxial tension which develops in thin brazed joints. In typical brazing applications, the filler material is much softer than the components being joined. When the brazed assembly is stressed normal to the joint, the filler material is prevented from deforming plastically by the surrounding rigid material. The magnitude of the triaxial tension in the constrained interlayer is a function of its thickness, T, and diameter, D. Orowan et al/191 have approximated the variation in radial stress with radial position, r, in an axisymmetric interlayer as
Integration of Eq. [1] results in a linear relationship between the interlayer failure stress, o"I, and D/T for a rigidperfectly plastic interlayer. This linear relationship has been confirmed experimentally for numerous types of interlayers over a range of interlayer thicknesses] 2°-261 with deviations being reported only for very thin interlayers. [24,25,261 Finite element calculations of the distribution of stresses across metal interlayers joining ceramic pieces have indicated that the triaxial tension in the interlayer, prior to fracture, can exceed 6 times O'y~e~aof the interlayer material.t J2.27 29j The constrained interlayer is, therefore, a useful model system for studying the effects of triaxial tension on the mechanisms of fracture. This article presents the results of an experimental investigation which combined mechanical testing and metallographic examination to study the initiation of fracture, through void nucleation, in a brazed steel/Ag/steel interlayer system. The role of triaxial stress was investigated by tensile testing several thicknesses of Ag interlayers. The role of inclusions was investigated with interlayers made by two brazing treatments. The experimental tests were simulated with a series of finite element models which determined the general stress field in the interlayers as well as the local stresses near inclusions. II.
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R.J. KLASSEN, formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto, is Research Scientist, Atomic Energy of Canada Ltd., Chalk River, ON, Canada K0J l J0. G.C. WEATHERLY, Professor, Iormerly with the Department of Metallurgy and Materials Science, University of T
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