Ageing of adhesively bonded joints—fracture and failure analysis using video imaging techniques

An investigation into the durability of adhesively bonded joints has been undertaken to help improve the prediction of joint lifetimes. Polymethylmethacrylate (PMMA) substrates have been bonded with a two-part acrylic adhesive to make single lap-shear joints. Joints have been aged in a hot/wet environment (40°C and 95% humidity) with no applied stress for up to 4000 h and were tested in tension. The novel aspect of the research has been the development of a video imaging analysis technique which allows damage initiation and propagation within the joint to be detected as load is applied to the joint. Images of fracture initiation and damage propagation have been correlated with stress/displacement data for joints under tensile loading. The data from aged samples is compared with data from un-aged samples. Both the stresses at which damage is seen to initiate and the final failure stress of the joints decrease as the ageing time increases. The failure mode changes from cohesive failure within the PMMA substrate to failure within the adhesive, near the PMMA/acrylic adhesive interface.