Auxetic dimensionality governs impact resilience in composite laminates

Auxetic layered composites are promoted for impact tolerance, but their performance is withdrawn by a trade-off in auxetic dimensionality. Here, we introduce the auxetic dimensionality framework by isolating in-plane from out-of-plane auxeticity and link this distinction to the cracking–delamination energy dissipation mechanisms. We compare in-plane auxetic and out-of-plane auxetic laminates with their non-auxetic (NA) counterparts under low-velocity impact at 5, 10, and 25 J, followed by compression-after-impact (CAI). This dimensionality-based framework yields two directional damage modes that convert mixed auxetic impact results into a practical laminate-level design rule. In-plane auxeticity biases deformation toward transverse strain amplification due to in-plane negative Poisson’s ratio, producing a crack-intensive pathway and a more distributed delamination network relative to its NA counterpart. In contrast, out-of-plane auxeticity shifts deformation toward a localized drawing-in response that suppresses interior delamination growth and limits matrix cracking relative to its NA counterpart. This distinct behavior is quantified by establishing an energy-normalized index r , defined the cost of crack generation per unit absorbed impact energy. The in-plane auxetic laminate shows a higher r index than its NA counterpart, while the out-of-plane auxetic laminate shows the opposite trend indicating suppressed interior delamination growth and limited cracking than its NA counterpart. This directional distinction dictates post-impact survivability, where the out-of-plane auxetic laminate retains 66.8% CAI versus 45.9% of its NA counterpart, while the in-plane auxetic laminate retains 66.0% versus 70.0%. This dimensionality-based perspective provides a practical laminate-level design rule for selecting in-plane versus out-of-plane auxeticity to control crack- versus delamination-dominated impact damage.