Mechanics driven non-uniform Z-pin layout: Topology optimization based anti-impact performance design of foam sandwich structures

Traditional X-COR sandwich structures with Z-pins arranged uniformly or empirically suffer from low material utilization and unreasonable reinforcement between high and low stress regions. This paper proposes a reverse design method for non-uniform Z-pin layout combining topological optimization and cluster constraints. Taking Z-pins as the design domain, a model is established to maximize structural stiffness under volume fraction constraints, so as to obtain the non-uniform reinforcement layout matching the impact stress field. Finite element simulations and low-velocity impact tests are conducted to compare the mechanical response, energy absorption and delamination evolution of specimens without Z-pins, with uniformly arranged Z-pins and topologically optimized Z-pins. The results indicate that the optimized cross-shaped Z-pin distribution well conforms to the principal stress paths under impact. Compared with the specimens without Z-pins, the peak load increases by 28%, the peak displacement decreases by 33%, and the delamination area is reduced from 350 mm² to 231 mm². Z-pins restrain face-core delamination and dissipate impact energy via bridging, buckling, debonding and fracture. The optimized layout cuts Z-pin consumption by 50% with no obvious performance loss and lower data dispersion. CAI tests prove that Z-pin reinforcement improves the post-impact compressive strength, and the optimized group outperforms the uniformly arranged group. The proposed method enhances material utilization while maintaining impact resistance, which provides theoretical and technical support for the reverse optimal design of Z-pins in composite sandwich structures.