Fiber Stress Concentrations in Kevlar/Epoxy Monolayers

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Abstract
A stress-based finite element analysis, formulated within the context of a shear-lag theory, is used to analyze a Kevlar 49/epoxy monolayer containing a single, preexisting fiber break. The highest and most localized stress concentration occurs when the epoxy matrix is idealized as linearly elastic and perfectly bonded to the fibers. This calculation suggests, however, that the fiber-matrix bond strength will be exceeded at relatively low load levels and fiber-matrix debonding will occur. Calculations which permit debond ing and also consider a residual frictional interface stress after the primary bond fails are presented. When there is no residual frictional stress, the fiber stress concentration is fully relieved and debonding occurs along the entire fiber length at loads only slightly higher (∼ 10%) than that needed to initiate bond failure. A residual frictional stress stabilizes debond growth. The fiber stress concentration factor decreases as the magnitude of the residual frictional stress decreases, while debond length and the length of fiber subject to the overstress increase.