Surface Toughening - A Concept to Decrease Stress Peaks in Bonded Joints

Abstract

With the use of fiber-reinforced plastics as a material for lightweight structures, adhesive bonding becomes a more and more 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 stress and the peel stress distribution have a peak at the free edge and a lower loaded part in the middle, as we know from Volkersen and Goland and Reissner. To decrease the stress concentrations and thereby to increase the joint strength, local joint modifications have to be done. Known for these modifications are adherend chamfering, different adhesive spew fillet geometries or mixed adhesive joints as da Silva et al. investigates. All techniques are based on a reduction of the stiffness at the outer joints edge and so on a transfer of load from the outer to the inner joint area. For joints with thin epoxy film adhesives which cure under elevated temperature, the known principles are difficult to use. On the one hand, there is no flexible epoxy film adhesive, and on the other hand, there is not enough adhesive bleed out to build a defined spew geometry. Finite element simulations show a possibility to decrease the stress peaks near the outer joint edges by toughening the surface of the adherends in this area. This investigation shows the comparison of the known stress reducing techniques from the literature with the Surface Toughening concept. Also, the results of this study are compared to tested specimens. Therefore, high resolution digital image correlation is used to visualise the strain in bondline.