Volume 1, Issue 3 (March 2004)
Application of Moire Interferometry for Mode II Testing of Stitched Composites
In this paper, two experimental approaches were used to quantify the Mode II fracture toughness of stitched composites. In the End Notched Flexure (ENF) test, which is the traditional Mode II test, the delamination does not open, but slides. This makes it difficult to locate the crack-tip as the crack propagates and hence to estimate the length of crack propagation. This uncertainty introduces significant errors in the estimated value of Mode II fracture toughness, especially of stitched composite laminates. In the present study high sensitive moiré interferometry was used to obtain the full-field displacements as well as the crack tip location in ENF specimens. By using linear elastic fracture mechanics for orthotropic materials, the stress intensity factor can be determined from the relative crack surface displacements behind the crack tip. The results from the displacement matching method are compared with the conventional area method for calculating fracture toughness. It is found that stitching can increase the Mode II fracture toughness of laminated composites by a factor of 2 to 3 depending on the stitch density.