A Finite Element and Experimental Evaluation of Boron-Epoxy Doublers Bonded to an Aluminum Substrate

A multi-faceted study was performed to investigate the durability of a boron-epoxy doubler (patch) adhesively bonded to an aluminum substrate. Double cantilever beam (DCB) specimens were tested to determine the fracture toughness and fatigue characteristics of the adhesive bond line. A finite element analysis using ABAQUS was performed to determine the levels of Mode I and Mode II strain energy release rate present at a crack tip within the adhesive. These tests and analyses were used to determine the fatigue threshold in terms of the total strain energy release rate. To evaluate the utility of these results for the analysis of a more realistic geometry, independent fatigue tests of a boron-epoxy doubler bonded to a cracked aluminum sheet were also examined. No debonding of the doubler occurred during 300 000 cycles. A finite element analysis of the independent study revealed that strain energy release rate levels were below the experimentally determined threshold. Thus, the lack of debonding was consistent with previous results. This study demonstrates the utility of using fracture mechanics to evaluate the fatigue and durability characteristics of adhesively bonded joints.