Micromechanics of shear deformations in cracked bonded joints

Abstract

Experiments and analytical analysis were carried out to elucidate the process of crack propagation in adhesively bonded joints loaded in mode II. The adhesive used was a toughened epoxy resin, with the bond thickness varying from a few micrometers to 0.6mm. The development of a plastic deformation zone at the crack tip was monitored in real-time using a high-magnification video camera. Within the plastic zone the adhesive shear strain, determined from scratch marks applied to the specimen edge, was uniform across the bond except for several bond thicknesses long region just ahead of the crack tip where, depending on bond thickness, noticeable strain gradients may develop. The experimental results suggest that the critical shear strain at the crack tip is a viable fracture criterion. A simplified analysis for the cracked bond which is based on the technical theory of beams/plates and which considers nonlinear adhesive behavior was developed. The model prediction for the increase in the plastic deformation zone with load and the distribution of shear strain within the zone agreed well with the experimental results. An expression for the energy dissipated by the advancing crack was derived which accounted for the nonlinearity in the load vs. deflection curve observed in the fracture experiments and allowed Gnc to be calculated from easily measurable test parameters. Considerable work has been done to determine the evolution of damage at the crack tip in engineering materials, and the parameters controlling crack propagation. The majority of these works employed monolithic specimens loaded in mode I. Unfortunately, such test configuration gives rise to a number of complex phenomena, including surface or free-edge effects (i.e. the observed surface deformations may greatly depart from the deformations in the specimen's interior), a three-dimensional stress field within the damage zone, and large strain gradients at the crack tip which may well exceed the resolution capability of the detecting apparatus. To date, no universally accepted criterion seems to have emerged for predicting the material resistance to crack propagation. In this work, a fracture approach based on testing the material under severe spatial constraint and a pure shear stress state was developed in order to remove some of the aforementioned difficulties. The End-Notch Flexure (ENF) adhesive bond specimen shown in Fig. 1 was used here to monitor the deformation process at the crack tip. This test, originally developed for studying