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I.

INTRODUCTION

THE mechanisms of void growth and coalescence have long been recognized as being influential in the fracture toughness of ductile metals. Experimental measurements of the growth of voids prior to fracture in tensile and notched steel specimens have shown that increased triaxial tension enhances the transverse growth rate of voids. E1'2'31Finite element analyses of the growth of voids subjected to various stress conditions have also indicated that the void growth rate increases dramatically with increased triaxial tension, t4-71 The effect of triaxial stress on the transverse growth rate of voids has also been treated theoretically, t8,9,~°~The theory of Rice and Tracey I91 for the growth rate of an isolated elliptical void in a nonhardening material subjected to a triaxial stress field has been applied to describe the growth of voids prior to fracture in spheriodized steel. ~1 In many instances, the processes of void growth and coalescence occur spontaneously at the final stages of deformation. In these situations, the process of void coalescence can be described by a local plastic instability criterion. The role of stress triaxiality in promoting void coalescence by localized plastic instability has been analyzed by Thomason. ~2,~31 Many structural components contain microstructures consisting of hard and soft phases. In these situations, the harder material constrains the deformation of the softer material, resulting in large triaxial stresses in the ductile phase. Triaxial tensile stresses, in the ductile material, of 4.5 to 10 times ~ryie~dhave been reported in systems where that phase is very thin and highly constrained, l~4'~51In these cases, the transverse void growth R.J. KLASSEN, formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto, is with Atomic Energy of Canada Ltd., Chalk River, ON, Canada K0J 1J0. G.C. WEATHERLY, Professor, formerly with the Department of Metallurgy and Materials Science, University of Toronto, is with the Department of Materials Science, McMaster University, Hamilton, ON, Canada L8S 4MI. B. RAMASWAMI, Professor, is with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada M5S 1A4. Manuscript submitted August 8, 1991. METALLURGICAL TRANSACTIONS A

rates are greatly enhanced and can adversely affect the toughness of the component. A common example of this occurrence is in thin metal interlayers produced during brazing. Typically, the filler metal is more ductile than the pieces it is joining. During tensile loading, the rigid surrounding material constrains the deformation of the interlayer and produces in it a state of triaxial tension. The stress conditions in the brazed interlayers have been studied experimentallyt~5-2°] as well as with finite element modeling, tT,2~-24j This article reports the results of an investigation of void growth and coalescence in constrained Ag interlayers formed by brazing between two pieces of steel. The analysis was performed by testing the interlayer