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

Abstract

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 R_a of the polymeric component surface on the critical value of strain energy release rate G_Ic. 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 G_Ic, using concepts of beam theory and equivalent crack length. The procedure was numerically validated for each roughness value R_a tested experimentally (i.e., 0.45, 1.12 and 4.50 μm) 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 R_a for which the critical strain energy release rate attains its maximum under mode I loading before it slightly decreases.