Measurements of Size Scale Effects in Layered Structures
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Measurements of Size Scale Effects in Layered Structures Ashraf Bastawros and Antonia Antoniou Dept. of Aerospace Engineering and Mechanics, Iowa State University, Ames, IA 50011 ABSTRACT A novel experimental configuration is devised to measure the evolution of the deformation field and the corresponding hardening evolution within soft metallic films constrained by hard layers. The experimental configuration provides pure shear state within the constrained film. The material system utilized comprised ductile layer of tin based solder, encapsulated within relatively hard copper shoulders. Different tin-lead compositions are tested with grain size approaching the film thickness. The in-plane strain distribution within the film layer is measured by a microscopic digital image correlation system. The hardening evolution within such highly gradient deformation field is monitored qualitatively through a 2D surface scan with a nanoindentor. The measurements showed a highly inhomogeneous deformation field within the film with discreet shear bands of concentrated strain. The localized shear bands showed long-range correlations of the order of 3-4 the grain size. A size-dependent macroscopic response on the layer thickness is observed. However, the corresponding film thickness is approximately 100-1000 times larger than those predicted by non-local continuum theories and discrete dislocation. INTRODUCTION Recent trends for a range of high-technology applications focus on utilizing smaller microstructure features with improved structural reliability. These include films, coatings, multilayers, interpenetrating networks, and composites, applied in a wide range of thermal, mechanical, electronics, bioengineering technologies, and microelectromechanical systems (MEMS) [1]. The common feature among these problems rests in sharing a highly localized plastic deformation at a scale that is comparable to the microstructure length scale, either within the bulk at reinforced particles or at the macroscopic layer interface. Soft metallic layers such as those utilized in solder joints of microelectronic packaging, will be subjected to a highly nonuniform deformation field [2]. Size-dependent hardening is likely to play a significant role in the mechanics of these joints. Such dependence will manifest itself as an apparent strength improvement of the joint. The improved joint strength is known in literature as Friction-Hill phenomenon [3, 4], due to the triaxial stress build up when the joint is loaded normal to the interface. However, under shear loading, such strength enhancement is not known except for solder joints that posses thickness to width ratio, t/Lo>0.68 [5]. Moreover, when the joint thickness becomes of the same order of the grain size, the dislocation mobility within individual grains will be severely constrained by the adjacent boundary [6]. This work is focused on understanding the thickness dependent response of constrained ductile layer, and the associated joint strength and hardness. The main focus will be to und
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