Protective performance of auxetic TPU pad for helmet: An investigation into design improvements for blunt impact protection

Traditional helmet foam pads have limited energy absorption for blunt impacts, unable to meet protective needs in complex ballistic scenarios such as fragments and bullets. Auxetic (negative Poisson's ratio) materials have been tested for helmet pads, but existing studies focus mainly on low-velocity impact protection. Thus, optimizing auxetic pad structures for high-velocity impacts is essential. In this study, lightweight expanded thermoplastic polyurethane (TPU-LW) was used as the base material, with 3D printing to fabricate pad samples. First, TPU-LW's material constitutive model was established via uniaxial tensile tests. Simulations later revealed a key issue: a single auxetic pad caused excessive skull peak stress. To solve this, an innovative "auxetic + foam" composite pad was designed, verified by 9 mm pistol bullet and 1.1 g fragment tests. The composite pad outperformed single auxetic and foam pads in key head blunt impact indicators. Simulations showed that under high-velocity fragment impact, the helmet's maximum backface deformation (BFD) dropped to 14.50 mm, and skull peak stress was 22.7 % lower than the foam pad. Experiments indicated that under 714 m/s fragment impact, peak head pressure was only 25 kPa - far below the foam pad's 165 kPa. This study fills the biomechanical data gap of auxetic TPU-LW in ballistic protection. The proposed composite structure provides a theoretical basis and technical solution for upgrading helmet pads from "single-material" to "composite energy-absorbing structure," applicable to various protective helmet research and development.