Impact resistance of an enhanced auxetic steel system under high-velocity impact loading

Abstract Vehicle-borne improvised devices (VBIDs) represent a severe threat in terrorist attacks due to their combined impact and explosive effects. Protection systems may lose effectiveness when first subjected to high-impact forces from VBIDs or high-velocity projectiles before subsequent blasts. Thus, protection systems must endure severe impacts while maintaining structural integrity for blast resistance. Auxetic structures offer potential solutions due to their unique densification behaviour, which enhances impact resistance. However, conventional auxetics exhibit limited energy absorption under high-velocity impacts. This study introduces and validates an enhanced auxetic steel system, namely, Enhanced Re-entrant with Straight and Curved members (ERSAC), designed to improve specific energy absorption (SEA) through multiple sequential interlocking points in the densification zone, leading to increased mean crushing force (MCF), while reducing peak crushing force (PCF). To verify the proposed design mechanism, 24 ERSAC and conventional re-entrant auxetic cell specimens with varying thicknesses and gap sizes are experimentally tested under quasi-static out-of-plane loading. Results reveal that ERSAC cells achieve average increases of 46% in SEA and 40% in MCF, while reducing PCF by 15% compared to conventional cells. Additionally, increasing cell thickness improves SEA and MCF, whereas increasing gap size reduces both. Validated numerical ANSYS/AUTODYN models are then used to compare the impact performance of solid, re-entrant-core, and ERSAC-core sandwich panels under two impact scenarios involving solid spheres and VBIDs. ERSAC panels can resist impacts up to 140 m/s for spheres and 220 km/h for VBIDs, outperforming solid panels (60 m/s, 120 km/h) and re-entrant panels (90 m/s, 140 km/h).