A shear-lag approach to the early stage of interfacial failure in the fiber direction in notched two-dimensional unidirectional composites

When the interface between fiber and matrix is not strong, interfacial failure (debonding) occurs in the fiber direction in notched unidirectional composites. To simulate the early stage of such longitudinal debonding, a shear-lag approach has been applied to a two-dimensional, double-edge-notched composite composed of elastic fiber and elastic matrix. The shear stress and energy release rate criteria are used in the analysis. These assume that debonding occurs when the shear stress exerted at the interface and the energy release rate exceed critical values. The main results are summarized as follows. (I) The shear stress and energy release rate to cause debonding depend on the number of cut elements (fiber and matrix) and on the species of the final cut element in the notch. Debonding tends to occur at a lower applied stress when the number of cut elements is large and the final cut element is a fiber in both criteria. (2) For the growth of debonding, three different types of behaviour appear in both criteria. In type (a), debonding grows unstably upon initiation. This type tends to occur when the final cut element in the notch is matrix and the frictional shear stress acting at the interface after debonding is very low. In type (b), debonding grows to some extent upon initiation, but then stops and starts to grow again with increasing applied stress. This type tends to occur when the final cut element in the notch is matrix and the frictional shear stress is low. In type (c), debonding grows stably with increasing applied stress. This type tends to occur when the final cut element in the notch is matrix and the frictional shear stress is high and when the final cut element is a fiber for any frictional stress. (3) Both shear stress and energy release rate criteria give similar tendencies for initiation and growth of debonding. (C) 1997 Elsevier Science Limited.