Analysis of aircraft adhesive joints under combined thermal and mechanical cyclic loadings

effect of thermal aging on the static and fatigue behavior of adhesively bonded aircraft joints has been investigated. The aging cycle consisted of high and low temperatures at different levels of humidity in accordance with ASTM D1183-70 test procedure C for exterior land and air conditions. Single lap joints of aluminum 7075-T6 and 3M structural adhesive prepreg were used. The static loading behavior showed no effect of thermal cycling on the load-carrying capacity of the joints. However, the joints' static deformation increased with increasing number of aging cycles. The specific energy of damage concept was used to extract parameters characteristic of the joint resistance to fatigue loading. These parameters are the specific energy of damage, gamma', and the energy dissipation coefficient, beta'. It was found that the specific energy of damage, gamma', was reduced from 35.40 kJ/m(3) for joints without thermal cycling to 28.9, 27.1, and 25.9 kJ/m(3) for joints subjected to 2, 4, and 6 thermal aging cycles, respectively. However, the energy dissipation coefficient, beta', increased from 23 x 10(-5) forjoints without thermal aging to 26.5 x 10(-5), 27.6 x 10(-5), and 28.8 x 10(-5) for joints with 2, 4, and 6 aging cycles, respectively. The decrease in the value of gamma' and the increase in beta' indicate a loss of resistance to fatigue crack propagation. The greatest loss of fatigue resistance was encountered after the first two thermal aging cycles. While the static results showed an enhancement in the joints' fracture toughness, based on the area under the load-displacement curve, the fatigue crack propagation data showed a considerable loss in resistance due to aging. Hence this paper emphasizes that it is important to evaluate the resistance of adhesive joints to combined thermal and mechanical cycling if the joints are expected to serve in such conditions.