AN EDGE CRACK TORSION METHOD FOR MODE-III DELAMINATION FRACTURE TESTING

A new fracture toughness test, the edge crack torsion method, has been developed for characterizing the mode III delamination behavior of co mposites. The test is based on a laminate specimen subjected to torsio n to propagate an edge delamination crack in its midplane. The crack g rowth mode of the specimen has been deduced to be mode III from fractu re mechanics principles. The torsional behavior and the corresponding fracture parameter G(IIIC) have been analyzed on the basis of plate to rsion and laminate theory. Edge crack torsion tests were performed to measure G(IIIC) of several carbon fiber/epoxy composite systems. Lamin ate layups were optimized to yield linear elastic fracture behavior of the specimens. The specimens were also sufficiently compliant to allo w G(IIIC) to be readily obtained by using the compliance calibration m ethod. The deformation characteristics of the specimens were found to follow the laminate torsion description. SEM observations showed fract ure surface morphology varying with resin microstructures and, for the case of an untoughened matrix, were consistent with what was reported in the literature for other proposed mode (III) fracture tests.