Time- and temperature-dependent failures of a metal-to-composites bonded joint with PMMA adhesive material

A metal-composite bonded joint with a PMMA adhesive material was fabricated and tested under various loading conditions to help a short-term and long-term design of the bonded-jointed structures. The bonded joint is composed of a cast-iron rod and a GFRP composite pipe as adherends, and ductile PMMA as adhesive material (Plexus AO425). Tubular-lap bonded joints were fabricated conveniently and consistently with a devised mold and a curing condition to yield a reliable test data set. Experiment was performed to measure creep compliances of the PMMA material, and constant strain rate (CSR) and fatigue strengths of the bonded joint under various loading and temperature conditions. We observed the CSR and fatigue strengths significantly depended on time and temperature, but little on direction of loadings and size of joints. The bonded joints failed under consistent damage mechanism in the combination of the cohesive and interfacial failure modes, which mainly occurred in the adhesive material and at the interface between the cast-iron rod and the adhesive material, respectively. In order to further substantiate the utility of the time-temperature superposition principle that has been extended to describe destructive behavior of composite materials, we made the first attempt to apply the superposition principle to failure behavior of the bonded joints. The superpostion principle yielded smooth master curves for the creep compliance of the adhesive material as well as the CSR and fatigue strengths of the bonded joint. Time-temperature shift factors of the non-destructive creep compliance of the adhesive material were closely related to destructive CSR strength of the bonded joint. The master curves that were created by shifting fatigue curves with shift factors of the CSR strength yielded an extended range of the long-term life prediction at any temperatures, frequencies and stress ratios. It was shown that the life prediction of the bonded joints by using the master curves made a good agreement with the experimental results.