Crack growth analysis of adhesively-bonded stepped joints in aluminium structures

Adhesive bonding has become a promising joining technique, by satisfying the needs of diverse industrial fields. Several joint configurations are available for the designer, and the most common and exhaustively studied are single-lap, doublelap and scarf joints (Petrie in Handbook of adhesives and sealants. McGraw-Hill, New York, 2000; Moreira and Campilho in Eng Struct 101:99-110, 2015). Stepped-lap joints consist of a finite number of steps in the bonded region, and they are characterized by having stress concentrations at the end of each step rather than only at the bonded length edges, which leads to a higher load transfer through the interior part of the overlap compared to lap joints and, consequently, to a higher performance. The present work experimentally and numerically evaluates the static tensile strength of stepped-lap joints bonded with three adhesives, Araldite (R) AV138 (brittle), Araldite (R) 2015 (moderately ductile) and Sikaforce (R) 7752 (ductile), considering different overlap lengths (L-O). A finite element analysis was undertaken to compare through-thickness normal (sigma(y)) and shear (tau(xy)) stresses in the adhesive layer between joint configurations. Additionally, a cohesive zone model (CZM) analysis was considered to study the failure modes and the CZM damage variable during failure and to evaluate the CZM technique's accuracy. It was possible to conclude that the recommended adhesive depends on the joint geometry, since strong and brittle adhesives are better for small L-O, while for higher L-O ductile adhesives with less strength can give better results. The CZM technique was accurate in the strength prediction, although with slight under-predictions for highly ductile adhesives.