Predicting the service-life of adhesively-bonded joints

A fracture-mechanics approach has been used to predict the cyclic-fatigue p erformance of the adhesively bonded single-lap joint and a typical bonded c omponent, represented by an adhesively-bonded 'top-hat' box-beam joint. The joints were tested under cyclic-fatigue loading in either a 'wet' or 'dry' environment, respectively. Several steps were needed to predict the: cycli c-fatigue lifetime of these joints. Firstly, fracture-mechanics tests were used to obtain the relationship between the rate of fatigue crack growth pe r cycle, da/dN, and the maximum strain-energy release-rate, G(max), applied during the Fatigue cycle for the adhesive/substrate system under investiga tion, in both a 'dry' and a 'wet' test environment. Secondly, analytical an d finite-element theoretical models were developed to describe the variatio n of the strain-energy release-rate with crack length, as a function of the applied fatigue loads, for the single-lap joint and the 'top-hat' box-beam joint. Thirdly, the experimental results from the short-term fracture-mech anics tests, obtained under similar test conditions and in the same environ ment as were used for the single-lap or bonded box-beam joints, were combin ed with the modelling results from the theoretical studies. This enabled th e cyclic-fatigue performance of the single-lap or bonded box-beam joints to be predicted over relatively long time-periods. Finally, the agreement bet ween the theoretical predictions and the experimentally-measured cyclic-fat igue behaviour for the joints was found to be very good.