Study of displacement and residual displacement field of an interface crack by moire interferometry

Abstract

Moire interferometry was used to study the deformation field and the residual deformation field around a crack laying along the interface of the adhesive layer and the adherend of graphite epoxy composite. Two steps of the experiment were carried on. First, the specimen was loaded in four levels and the fringe patterns are recorded for analyzing the in-plane displacements under each load level. Then a bigger load was applied until the crack propagation occurred. The specimen was then put in front of the interferometry mirrors so that the residual deformation could be recorded. A crack tip stress field dominated by shear was produced by the three points bending configuration with which the crack lied along the neutral surface of the beam. The complex stress intensity factor K was determined from the crack face displacement jump. The v component of the crack face displacement jump was zero for a certain segment under a limited load. It resulted in the phase angle of Krie did not change with the distance r from the crack tip. After the crack propagated, the whole crack opened completely when the load was removed. The crack kinked initially and propagated along the interface when the kinking reached the adherend. The kinking of the crack suggested that the original crack tip stress field was dominated by shear. Large difference of the stiffness of the adherend and adhesive made the adhesive sustain the shear strain about four times as large as the adherend do. Accurate measurements of surface displacement and residual displacement fields of the specimen were made by moire interferometry. Some useful information were hidden in the zone with large out-of-plane displacement resulting in the blurring of fringes. Analysis was based on a 2D assumption, although 3D deformation at the crack tip was quite evident.