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 methods for estimating the normal strength of the fiber-matrix interface in unidirectional composites. In order to remove the influence of the free surfaces from tests on traditional straight-sided exposed fiber specimens, two alternate geometries, namely, embedded fiber and cruciform specimen, were considered. These specimens, based on a model transparent material combination, were incrementally loaded in tension to failure, while strain and acoustic emission activity were continuously monitored for damage initiation. The cruciform geometry provides the largest value of failure stress, the embedded fiber geometry yields intermediate value, whereas the exposed fiber specimen results in lowest failure stress values. Analytical and computational modelling, which combines an asymptotic analysis based on Mellin transform at the local scale with a finite element procedure at the global scale, was then utilized to rationalize the different strength estimates. Results demonstrate that stress field singularities control damage initiation in straight-sided samples and emphasize the success of cross-shaped design in forcing debond initiation in the interior of the sample (region free of initial stress singularities). (C) 1999 Published by Elsevier Science Ltd. All rights reserved.