Material and adhesive effect in adhesively-bonded composite stepped-lap joints

Adhesive bonding is a predominant bonding technique in the aeronautical and automotive industries. Cohesive zone models, used together with the finite element method, are a viable tool to predict the strength of adhesive joints. The main objective of this study is to evaluate experimentally and numerically (by cohesive zone model) the mechanical performance of carbon-fiber reinforced polymer stepped-lap bonded joints submitted to tensile loads, for different overlap lengths (L_O) and adhesives. The failure mode analysis showed a predominant failure type for all adhesives and good correspondence with the numerical predictions. Normalized peel (σ_y) and shear (τ_xy) stresses in the adhesive highly increased with L_O, which then reflected on different maximum load (P_m) evolution with L_O, depending on the adhesive's ductility. The damage variable SDEG (stiffness degradation) was also evaluated and emphasized on the smaller damage zone at P_m for the brittle adhesive. A significant geometry and material effect were found on P_m of the stepped-lap joints, with benefit for large L_O. In this regard, cohesive zone model revealed to be a suitable tool in determining the behavior of different joints. Comparison with joints with aluminum showed that, provided that no carbon-fiber reinforced polymer delamination occurs, stepped-lap joints between carbon-fiber reinforced polymer adherends give better results due to the higher composite stiffness.