Influence of adhesive spew geometry and load eccentricity angle on metal-composite bonded joints tested at quasi-static and dynamic loading rates

Joining techniques for multi-material structures are critical for increased use of lightweight materials in the automotive industry. This paper investigates the combined effect of spew geometry and load eccentricity angle on the mechanical performance of bonded single-lap joints (SLJ), employing carbon-fibre thermoplastic composite and aluminium adherends. Spew geometries – half-rounded and flat, and eccentricity angles – 0.48°, 0.62°, 0.78°, and 0.91°, considered here, are relevant to typical joints in thin-walled automotive structures. Of the geometries investigated, in the first the excess adhesive was allowed to take its “natural” half-rounded shape, while the second involved a simple additional step, suitable for high-volume production. Moreover, considering the use of dissimilar adherends and crash-durable epoxy adhesives, and limited accessibility to both adherend free ends in practical joints, the flat-spew geometry was formed at the free end of the composite adherend, resulting in SLJs with asymmetrical spew geometries. The SLJs were tested at dynamic crash loading rates (0.5 m/s and 3 m/s) as well as in-service quasi-static loading rates. The load eccentricity was found to significantly affect the half-rounded spew joint performance, particularly at the dynamic loading rates. The asymmetric spew shape significantly minimised the detrimental eccentricity effects, at all loading rates. The SLJs with flat-spew shows 92%−120% higher energy absorption under dynamic loading, relative to half-rounded spew joints. The joint deformation mechanisms and post-failure surfaces are examined. Further, finite element analysis was performed to understand the influence of the investigated spew geometries on the adhesive stress distribution. The proposed flat spew geometry achieved the lowest peel stresses and the highest shear to peel stress ratio, in addition to having low sensitivity to changes in geometry.