Abstract: In this paper, numerical simulations combined with experimental investigations were carried out to study the strength and failure modes of the bonded-bolted double-lap sandwich composite-steel joints under tensile loads. The influences of the joint width, end distance and hole spacing on the strength of the joint were studied respectively, and single factor optimization design was conducted. The results show that the overall trend and failure modes of the numerical and experimental load-displacement curves are in good agreement. The initial failure occurs in the adhesive layer at the end of the bonding area. As the tensile displacement increases, the cracks of the adhesive layer propagate, and the bolt gradually bears the load together with the adhesive layer with compressive failure of the skin occurring at the screw hole. The final failure of the structure occurs when the adhesive layer completely fails and the sandwich panel and steel plate are completely detached. The single factor optimization design shows that the joint stiffness and ultimate load increase continuously with the increase of joint width, but the failure displacement and connection efficiency decrease. The results also show that the stiffness of the joint is not affected by the end distance and hole spacing, whereas the ultimate load and connection efficiency increase with the increase of end distance and hole spacing. The optimal combination of the design factors is as follows: the width-to-aperture ratio is 3.5, the end distance-to-aperture ratio is 3.0, and the hole spacing-to-aperture ratio is 3.5.

Key words: sandwich composite, bonded-bolted hybrid joint, numerical simulation, optimization