Axial Impact Response of Flax, Carbon and Hybrid Composite Thin-Walled Tubes: Numerical and Experimental Study on Circular and Square Geometries

The replacement of conventional metals with composite materials has been widely investigated in engineering, motivated by their superior specific mechanical properties, durability, and design flexibility. Composite thin-walled structures are widely used as energy absorbers under axial loading. This study investigates the effect of different cross-sections, i.e., circular and square, on tubular structures under axial crushing. Three composite material configurations are explored:carbon/epoxy, flax/epoxy, and hybrid carbon-flax/epoxy. Flax fibres are proposed as sustainable alternatives to carbon counterparts. However, owing to their inferior mechanical properties, interply hybridisation is implemented as a compromise between performance and sustainability. The experimental activities investigate both quasi-static and dynamic loading conditions, but instabilities are observed in the quasi-static tests—particularly for flax—requiring further investigation. The numerical analysis therefore focuses only on dynamic crushing conditions. A finite-element stacked-shell model is developed in LS-DYNA to simulate crushing. Numerical simulations are validated against test results, showing good agreement with relative errors below 6%. The tested carbon-flax hybrid tubes retain energy absorption efficiency close to that of carbon configurations, with average specific energy absorption (SEA) reductions below 15% for both circular and square tubes, supporting their use as a sustainable solution for crashworthy component design.