Deformation mechanisms and energy absorption characteristics of 3D-printed negative poisson's ratio sandwich structures subjected to underwater impulsive loading

Negative Poisson's ratio (NPR) structures are known for their superior mechanical properties and energy absorption capabilities, yet research on their resilience against underwater explosions remains limited. In this study, two types of NPR sandwich structures were designed: a coaxial re-entrant honeycomb sandwich structure (S-CRHS) and a non-coaxial re-entrant honeycomb structure (S-NRHS). Specimens, both heat-treated and non-heat-treated, were fabricated using 3D printing technology. The deformation mechanisms and energy absorption capacities of these structures under various intensities of underwater impact loads were investigated using experimental and numerical approaches. The underwater shock wave, which decays exponentially, was generated by a fly plate striking a diffusion-type fluid–solid interaction experimental apparatus. High-speed cameras, integrated with three-dimensional digital image correlation technology (3D-DIC), recorded the real-time deformation of the back panels. The impact process was also simulated using the coupled Eulerian–Lagrangian method in Abaqus/Explicit, with the numerical results showing good agreement with experimental data. The findings revealed that S-CRHS displayed symmetric deformation patterns and a linear relationship between dimensionless impulse and displacement, indicative of stability. In contrast, the asymmetrical design of S-NRHS resulted in varied deformation modes and energy absorption characteristics, with a notable NPR effect leading to a nonlinear relationship between dimensionless impulse and dimensionless displacement, characterized by three phases and two critical strength transition points. When compared to S-CRHS, the S-NRHS core demonstrated enhanced energy absorption capabilities, showing superior impact resistance when the non-dimensional impulse was below 0.0503. Additionally, heat treatment significantly improved the toughness of the 3D-printed materials and reduced cell wall fractures. This research provides valuable insights into the potential applications of NPR structures in underwater protection scenarios.