Role of large-scale slip in mode II fracture of bimaterial interface produced by diffusion bonding
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I. INTRODUCTION
MIXED-mode loading has been studied extensively for bimaterial interfaces using linear elastic fracture mechanics.[1–6] Increases in interface fracture toughness over mode I toughness have been observed in mixed-mode and mode II loading of model laminates.[1,2,3] Increases are attributed, at least in part, to a frictional sliding and locking mechanism that occurs in nonplanar interfaces.[1,2,4] Crack deviation from planarity, or kinking, is typical for bimaterial interface fracture, producing changes to the local mode mixity on the crack tip, which may make crack propagation easier or harder, depending on the direction of the deviation.[3,4] Crack propagation in tough interfaces can be associated with large-scale slip, particularly in cases with large mode II loading components. Typically, large-scale slip has been associated with a crack-tip blunting process, which increases the crack opening displacement (COD) prior to crack propagation.[7,8] An increase in fracture energy is associated with large-scale slip, since energy is consumed in dislocation nucleation, emission, and eventual formation of pileups and microcracks. In the previous studies, the role of large-scale slip in interface crack initiation was described for model laminates loaded in mode II.[9,10] A model for crack initiation associated with dislocation motion, the Stroh model,[11] was presented and is shown in Figure 1. In that model, a large stress concentration forms along the interface at the head of a crystallographic slip plane where slip is initiated, eventually leading to crack initiation at the interface. The model shows how realistic opening stresses could be reached at the interface in cases where continuum fracture mechanics would M.R. FOX is with the Metallurgy Lab National Transportation Safety Board, Washington, DC. A.K. GHOSH, Professor, is with the Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109-2136. Manuscript submitted September 30, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
predict low or negative opening stresses. The model was presented as a mechanism for interface crack initiation in NiAl/Mo laminates.[9,10] Bimaterial interfaces present in diffusion-bonded (and insitu) composites are often not flat interfaces. The uneveness of the interface can result not only from interface reaction products but also from long-range waviness associated with the surfaces of the component phases bonded together. Experimental studies aimed at determining interface mechanical properties generally ignore the departure in the local stress state due to waviness and assume a theoretically flat interface. Furthermore, the commonly used testing methods involving superimposed tension often renders the interface so extremely brittle that if microplastic effects were present it becomes impossible to perceive them. In this study, details of the role of large-scale slip in crack propagation are presented. In-situ observation during steady-state crack growth showed the presence of large-
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