Fracture characterisation of a bi-material adhesive joint under mode I loading: Effect of roughness at the bonding interface

This work addresses the mode I fracture characterisation of a common bi-material adhesive joint used in the automotive industry. The joint is composed of a copolymer of polycarbonate and acrylonitrile butadiene styrene (PC + ABS) bonded to a woven glass fibre reinforced with epoxy matrix composite (PCB) using urethane polymer adhesive cured with UV light and suitable humidity. The bi-material adhesive joint was prepared to analyse the effect of the arithmetic average roughness Ra of the polymeric component surface on the critical value of strain energy release rate GIc. Due to significant differences in the elastic properties of the PC + ABS copolymer and the PCB, the asymmetric double-cantilever beam (ADCB) test was chosen, with flexural stiffness appropriately balanced to induce predominant mode I loading. Since the crack length could not be accurately tracked during the loading process, the compliance-based beam method was employed as a data reduction scheme to assess GIc, using concepts of beam theory and equivalent crack length. The procedure was numerically validated for each roughness value Ra tested experimentally (i.e., 0.45, 1.12 and 4.50 mu m) using a cohesive zone model, with a trapezoidal-linear cohesive law, to simulate damage initiation and growth. Apart from the cohesive laws further determined by a developed inverse method, the experimental work allowed to identify a value of arithmetic average roughness Ra for which the critical strain energy release rate attains its maximum under mode I loading before it slightly decreases.