Multiscale characterisation of a novel multi-material hybrid joining technique

The growing usage of carbon fibre composites within the automotive industry has led to an increased interest in using adhesive joining as the primary structural joining method. The present work presents a novel interlocking joining concept, focused on joining metals and carbon fibre composites. This concept incorporates interlocking features on the faying surface of the adherends with a male feature on the metal surface and corresponding female feature on the composite adherend. The interlocking features create a mechanically interlocked structure after the adherends are bonded creating a bonded/interlocking joint (BIJ). The efficacy of the interlocking features was evaluated through the use of single lap shear testing at two different scales.

Microscale testing allowed for the examination of single features within a shear dominated environment. Testing at this scale, with input from the numerical models developed in a parallel study, investigated the effects of a single feature on mechanical performance while also establishing the most favourable interlocking feature geometry. In aluminium joints, all feature types were found to significantly increase the apparent shear strength of the joints. Wide and short features were found to be the optimum geometry due to their further ability to inhibit crack initiation and propagation leading to large increases in work to failure. BIJs were also found to improve apparent shear strength in aluminium/CFRP joints. However, this was not observed for all feature geometries due to the failure behaviour of the CFRP adherends. Failure was particularly prevalent in wide and short features leading to features with long and narrow or cruciform geometries performing best. Although most features increased work to failure, the features were not as effective as crack inhibitors in the alumini-um/CFRP joints.

Standard sized joints which incorporated multi-feature arrays were then investigated. The individual feature geometries were based on the most suitable geometry as determined by the microscale testing. The interlocking features had no discernible effect on the performance of aluminium joints bonded with a stiff adhesive. When bonded with a more ductile adhesive, the BIJ delivered improvements of up to 21% in ultimate joint strength. The BIJs can not significantly inhibit crack initiation or propagation and, hence, did not improve the work to failure under either test condition. When incorporated into aluminium/CFRP joints, the features do significantly inhibit crack growth in the bondline. While this does not lead to improvements in joint strength, the work to failure of BIJs improved by up to 481%. Thus, the BIJs can be designed to deliver a stronger or more durable adhesive joint depending on the adhesive and adherend materials used.