Effect of base steels on mechanical behavior of adhesive joints with dissimilar steel substrates

The hybrid steel materials (e.g., advanced high strength steels combined with mild steels) have been increasingly applied in vehicle components to achieve the compromise among mechanical performance, lightweight and material cost. A significant issue related to the bonding of dissimilar steel materials is that the differences in substrate physical properties often lead to bond separation at strength levels far less than the bond strength established by the adhesive manufacturer for the joint with similar substrates. This research studied several important factors influencing the strengths of adhesive-bonded lap shear (LS) and coach-peel (CP) joints. Four types of steel substrates were used to fabricate the unbalanced adhesive joints (dissimilar steels or same steels with different thicknesses). The quasi-static tensile tests of the unbalanced joints were performed. A finite-element model was proposed to characterize the fracture behavior of joints. The effects of substrate properties such as bending stiffness and load-carrying capability of substrates on the joint strength (peak force) and fracture modes were investigated. It was observed that the interfacial fracture is prone to occur on the substrate with the relatively weak yield strength. A cohesive fracture can be produced when the load-carrying capabilities of the two substrates are similar. In order to optimize the load-carrying capability of the unbalanced joint, a design guideline was put forward.