Influence of free edge and corner singularities on interfacial normal strength: application in model unidirectional composites

The present study was aimed at designing and evaluating some simple test me thods for estimating the normal strength of the fiber-matrix interface in u nidirectional composites. In order to remove the influence of the free surf aces from tests on traditional straight-sided exposed fiber specimens, two alternate geometries, namely, embedded fiber and cruciform specimen, were c onsidered. These specimens, based on a model transparent material combinati on, were incrementally loaded in tension to failure, while strain and acous tic emission activity were continuously monitored for damage initiation. Th e cruciform geometry provides the largest value of failure stress, the embe dded fiber geometry yields intermediate value, whereas the exposed fiber sp ecimen results in lowest failure stress values. Analytical and computationa l modelling, which combines an asymptotic analysis based on Mellin transfor m at the local scale with a finite element procedure at the global scale, w as then utilized to rationalize the different strength estimates. Results d emonstrate that stress field singularities control damage initiation in str aight-sided samples and emphasize the success of cross-shaped design in for cing debond initiation in the interior of the sample (region free of initia l stress singularities). (C) 1999 Published by Elsevier Science Ltd. All ri ghts reserved.