Residual Stresses and Resulting Damage Within Fibers Intersecting A Free Surface
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RESIDUAL STRESSES AND RESULTING DAMAGE WITHIN FIBERS INTERSECTING A FREE SURFACE J. M. GALBRAITH, M. N. KALLAS, D. A. KOSS, AND J. R. HELLMANN Center for Advanced Materials, The Pennsylvania State University, University Park, PA 16802
ABSTRACT Finite element as well as indentation fracture mechanics modeling have been used to analyze the evolution of fiber damage that was observed at the ends of fibers intersecting a free surface in sapphire-reinforced TiAl matrix composites. Experimental observations indicate that, under certain conditions, surface cracks introduced during cutting will propagate along the fiber axis due to thermally-induced residual stresses. Finite element computations predict that significant thermally-induced residual tensile stresses exist near the ends of sapphire fibers which are embedded within TiAI-based matrices and are oriented normal to a free surface. Crack growth behavior induced by microhardness indentations is used to experimentally verify the FEM predictions. The results indicate that a biaxial tensile residual stress state exists near the fiber ends due to a thermal expansion mismatch. The magnitude of the residual stresses are a sensitive function of interfacial bond strength and elastic/plastic properties of the interfacial region and may be sufficient to propagate pre-existing cracks.
INTRODUCTION Since most intermetallic matrix composites (IMCs) are reinforced with brittle ceramic fibers, it is important to understand the implications of exposing the fiber ends as a result of cutting and machining operations which may induce damage. It is well known that, due to the large mismatch in coefficients of thermal expansion (CTE) between constituents and the high stressfree temperature in these systems, significant thermally-induced residual stresses develop within IMCs during post-consolidation cool-down. The purpose of this study is to illustrate that the combination of brittle fibers, significant tensile residual stress, and machining damage can result in longitudinal cracks (i.e., fiber splitting) when a fiber intersects a free surface. Assuming a well-bonded interface, finite element modeling (FEM) calculations of the residual thermoelastic stress distribution within a sapphire fiber in a TiAl matrix are presented for the case of a fiber intersecting a free surface and with its axis normal to the surface. The computations predict a gradient of tensile stresses within the fiber such that, given the relatively low fracture toughness of single crystal sapphire, fiber splitting will occur along the fiber axis as matrix material is removed. Due to the size of the sapphire fibers and the distribution of stresses within the composites, experimental verification of the predicted residual stress distribution by conventional experimental methods is extremely difficult at best. However, it has been shown in several studies of brittle materials that indentation cracks can be used to estimate the magnitude of pre-existing residual stresses [1-4]. This method offers the advantages of being
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