Effect of the T-stress in the adhesively bonded joints on the locus of failure in rubber toughened epoxy system

Abstract

Summary form only given. This study addresses T-stress effects on the locus of failure in adhesively bonded joints. Double cantilever beam (DCB) specimens were made using Al 6061-T6 adherends and DER 331 epoxy resin with rubber concentrations from 0% to 8%. In order to obtain different stress levels, specimens were loaded uniaxially to plastically deform the adherends. T-stress increased with the plastic deformation left in the specimen upon unloading. Under mode I loading, unstable or alternating crack propagation was seen when the T-stress in the specimen increased. After this stability transition occurred, failure occurred at or very close to the interfaces. However, the T-stress level where the stability transition occurred increased with adhesive rubber concentration. Post-failure XPS and SEM analysis showed that as rubber concentrations in the adhesive increased, failure tended to be more cohesive. FEA modeling using Franc2DI was conducted to predict crack propagation behavior. Results showed that when the crack is perturbed by an air bubble or flaw ahead of the crack tip, the crack continued to deviate from its original path if the T-stress was tensile. However, if the T-stress was compressive, a deviated crack would converge back to its original path. The convergence rate depended on the T-stress value. The FEA also predicted that under mode I loading, T-stress in specimens with thinner adherends was higher than that in specimens with thicker adherends, and failure tended to occur closer to the interfaces. This prediction was verified using DER 331 epoxy resin with 8.1% rubber and Al 6061-T6 adherends of varying thickness.