A Novel Joint between Dissimilar Materials Inspired by the Mechanics of Trees
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A Novel Joint between Dissimilar Materials Inspired by the Mechanics of Trees L. Roy Xu, Huacheng Kuai and Sreeparna Sengupta Department of Civil and Environmental Engineering & Material Science Program VU Station B 351831, 2301 Vanderbilt Place Vanderbilt University, Nashville, TN 37235, USA ABSTRACT An integrated experimental and numerical investigation was conducted for removing the free-edge stress concentrations in dissimilar material joints. A convex interface/joint design, inspired by the shape and mechanics of trees, allows for least stress concentrations at bi-material corners for most engineering material combinations. In-situ photoelasticity experiments on convex polycarbonate-aluminum joints showed that the free-edge stress concentration was successfully removed. As a result, the new design not only improves the static load transfer capacity of dissimilar material joints, but also yields more reasonable interfacial tensile strength evaluation. For convex polycarbonate-aluminum and PMMA-aluminum joint specimens, the ultimate tensile load increased up to 81% while the total material volume reduced by at least 15% over that of traditional butt-joint specimens with severe free-edge stress concentrations. INTRODUCTION Dissimilar material interfaces/joints can be found in numerous modern engineering and science fields, for example, adhesive bonded interfaces of two dissimilar materials. One major research effort in interface studies has been the interfacial strength evaluation of dissimilar materials [1-3]. Meanwhile, numerous studies have shown that failure often occurs along the interface/joint between two kinds of materials with high property mismatch (e.g., free-edge delamination in composite laminates and debonding between thin films/substrates), and that improving the interfacial properties (especially reducing the interfacial stress level) can modify overall material/structural behavior [4, 5]. So the first important step for measuring intrinsic interfacial strength is the elimination of stress concentrations for the creation of a uniform interfacial stress state. The objectives of this investigation are to propose a novel specimen design to remove the stress concentration, and therefore to provide reasonable interfacial strength measurements and suppress edge debonding of dissimilar material joints. We shall review the origin of stress concentrations, and propose a biologically inspired design based on the mechanics of formation of trees [6]. Typical metal/polymer joints will be selected for demonstration of the proposed new design through in-situ photoelasticity experiments. THEORETICAL BACKGROUND Free-edge stress concentrations in dissimilar material interfaces/joints As illustrated in Fig. 1(a), a butt-joint specimen was used to demonstrate the free-edge stress concentration in steel 4340 and Plexiglas (PMMA) joints [7]. Significant stress concentrations
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were found at the bi-material corners using the Coherent Gradient Sensing (CGS) technique [8] for full-field mechanical-optical m
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