Fracture behavior and strength enhancement of 3D-printed stepped-lap bonded joints using geometrically graded interfaces: experimental validation and damage modelling

Single lap joints (SLJs) are prone to peel stress and have a limited load capacity, whereas stepped lap joints (SPLJs) improve load distribution and strength in high-performance applications. Stepped lap joints reduce peeling stress, although interface geometry affects their efficiency. A finite element (FE)-based failure analysis showcased the potential of enhancing lap shear strength by delaying crack initiation and propagation through a geometrically graded bond-layer interface. Building on this analysis, a design-driven approach is developed by introducing a novel tooth-shaped joint interface. This approach integrates mechanical interlocking and adhesive bonding, inducing a mixed-mode failure mechanism. Positive and negative interlocking features generate local tensile and compressive stress, which helps delay damage onset. Prolonged crack nucleation along the bond layer extends the fracture process, slowing crack propagation and ultimate failure. Computational studies coupled with ductile damage for adherends and cohesive zone modelling for adhesives to gather failure insights and forecast strength. Material characterization of polylactic acid (PLA) is done to predict damage models. Adherends are manufactured via fused deposition modeling (FDM) and bonded with structural adhesives for experimental validation. A FE-based parametric study determined the optimal tooth geometry, balancing interlocking efficiency and bonding strength. The optimized joint witnessed a significant improvement in strength of 263% and 23% failure strain over baseline joints. The observed failure load for the optimized joint was 4423 ± 143 N, while the baseline joint recorded a 1216 ± 65 N failure load. The proposed design enhances fracture toughness, requires no structural redesign, and is flexible to other manufacturing means.