Surface toughening – a concept to decrease stress peaks in bonded joints

By using fiber-reinforced plastics as the material for lightweight structures, adhesive bonding becomes an increasingly focused solution for joining these components. In comparison to bolted joints, adhesive bonding has a lot of known benefits, e.g. sealing, no weakening of adherends and a homogeneous load transfer. However, the stress distributions in single-lap joints are not homogeneous all over. Both the shear and peel stress distribution have a peak at the overlap ends of the joint and a lower loaded part in the middle, as shown by Volkersen and Goland and Reissner. In order to reduce these stress concentrations, there are known concepts for the modification of the joint. In this paper, adherend chamfering, different adhesive spew fillet geometries, and the mixed adhesive joint are investigated and compared to a novel local adherend surface toughening concept by using a thermoplastic PVDF layer. A high-resolution digital image correlation is used to visualize the deformation in the bondline. While the state-of-the-art used concepts do not show a high increase of joint strength for single lap joints, the joint strength for the surface toughening specimens can be increased by up to 84%.