An investigation into the static mechanical performance of composite-metal joints strengthened by surfi-sculpt

Abstract

The aim of this PhD research project was to investigate the static mechanical performance of innovative composite-metal (hybrid) joints strengthened by the presence of protrusions on the metal adherends. This advanced hybrid joining technology uses arrays of macro-scale protrusions (surfi-sculpts) on the surface of the metal part which can penetrate into the composite material during manufacture to achieve a high performance joint in the finished part. This joining technique combines many of the respective advantages of a bonded joint and a fastened joint. Joints formed between an aerospace grade titanium alloy and carbon fibre reinforced composite have been the focus of this study. Firstly, the effects of two composite adherend design parameters on the static mechanical performance of advanced hybrid joints strengthened by surfi-sculpt were investigated experimentally. The design parameters studied were (i) the composite ply orientation and (ii) the composite thickness. Single lap joints were manufactured and tested. The results from this part of the investigation showed that the performance of the hybrid joints was strongly affected by these design parameters. An optimum combination of these parameters was then selected for the next part of the study. Secondly, the effect of the surfi-sculpt protrusion density on joint performance was also investigated experimentally. Single lap joints were again manufactured and tested, and Digital image correlation analysis was used to measure the growth of the debond along the joint overlap during the tests. The results from this part of the investigation showed that the protrusions resist the initial, unstable failure mechanism that was observed in the control joints (without protrusions) and convert it into stable growth. With increasing protrusion density, the debonding propagation rate decreased and the failure mode changed from debonding to intra-laminar failure of the composite and fracture of the metallic protrusions in a mixed shear and bending mode. Increasing the protrusion density was shown to significantly increase the ultimate failure load, joint extension and hence absorbed energy. Thirdly, the effect of inserting disbonds between composite and Titanium adherends during manufacture, extending from either end of the joint overlap, was investigated as a way to study damage tolerance of the hybrid joints. Joints with four different initial debonding ratios (the ratio of disbond area to overlap area) were investigated and for each, two different protrusion densities were studied. Control joints without protrusions were again tested for comparison. The results showed that protrusions were able to maintain the strength and damage tolerance of the hybrid joints. Indeed, by increasing the protrusion density the ultimate failure load of the joints with a disbond was improved. By measuring the propagation of debonding it was shown that increasing protrusion density decreased the debonding rate from both ends of the overlap. Finally, the performance of the adherends and hybrid joints has been studied using finite element models. The models were three-dimensional, implicit, finite element models incorporating interface failure using surface-based behaviour and composite failure using a ‘User Defined Field’ subroutine in Abaqus. Three different joints were modelled: firstly the control (reference joint) without protrusions and two joints with different protrusion densities. The models were able to reproduce the experimental load-displacement traces with good accuracy. The numerical result for the reference joint shows that the resin rich zone in the joint at the end of the composite adherend only affects the first debonding damage location but has negligible effect on the following damage propagation and joint loading capacity. The numerical results for the joints with protrusions reproduce all the critical features of the experimental data: the initial linear behaviour and following softening behaviour; the knee points in the softening stage reflecting the pulling-out of metal protrusions from the composite adherend and finally the transition point (representing joint failure) shown in the load-displacement curve for the joint with the highest protrusion density.