Experimental and numerical failure analysis of SLJ and SSLJ CFRP joints using the LaRC05 criterion within a combined XFEM-CZM

Abstract In this study, we present an experimental and numerical analysis of single-stepped-lap joint (SSLJ) and single-lap joint (SLJ) assemblies made from carbon/epoxy composites (CFRP). The study aims to evaluate the impact of joint geometry and substrate thickness (2.0 mm and 4.0 mm) on the mechanical behavior and structural integrity of the joints. CFRP composite plates were machined using CNC milling and then bonded using a structural epoxy adhesive before being subjected to tensile testing. A three-dimensional finite-element model in Abaqus, incorporating cohesive zone modeling (CZM) for the adhesive and the LaRC05 criterion coupled with the extended finite element method (XFEM) for the composite substrate, was developed. The numerical predictions show very good agreement with the experimental data. The results show that the stepped joint (SSLJ) consistently outperforms the conventional SLJ joint, increasing the ultimate load at failure by 11.15% for a thickness of 2.0 mm (reaching 7,584 N compared to 6,823 N for the SLJ). This improvement is due to better alignment of the load line and a reduction in the secondary bending moment (Goland and Reissner effect). Conversely, increasing the substrate thickness from 2.0 mm to 4.0 mm results in a gradual reduction in ultimate strength for both configurations (a decrease of 4.13% for the stepped joint and 6.36% for the SLJ). Although a thicker substrate increases the overall flexural stiffness of the plate outside the joint, the increased eccentricity of the load accentuates the local out-of-plane bending moment and intensifies the combined stresses at the ends of the adhesive joint. The LaRC05/XFEM model accurately captured the transition from cohesive failure of the adhesive to localized delamination of the substrate. This work demonstrates that the stepped configuration, combined with an optimal substrate thickness of 2.0 mm, offers the best mechanical performance.