Metal Z-Pin Reinforcement for Improved Tensile Strength in Thin Stepped Composite Joints

This study investigates the effect of selective z-pinning on the tensile strength of single step-lap composite joints through a combined experimental and numerical investigation. Uniaxial tensile tests were performed on unpinned and z-pinned samples to assess the effect of z-pins on the strength and failure of joints. Unpinned and pinned joints exhibited an initial linear load–strain response, followed by stiffness degradation due to the onset and propagation of interfacial delamination. However, z-pinned joints exhibit increased strength and ultimate strain, indicating enhanced energy absorption and damage tolerance without compromising joint stiffness. The damage of single step-lap joints was mainly governed by matrix cracking and delamination. In order to identify and investigate the toughening mechanisms specifically induced by z-pins, a three-dimensional finite element (FE) model was developed in ABAQUS/Explicit to simulate the mechanical response of the joints. Both the bonding and the pin-laminate interface were modeled as a cohesive contact interface modeled with a bilinear traction–separation law, using a quadratic nominal stress criterion to initiate damage and the Benzeggagh–Kenane criterion to control damage evolution. The numerical results exhibited a relatively good agreement with the experimental data in terms of load-bearing capacity, ultimate strain, and damage evolution.