Bio-inspired optimisation design strategy of graded hemp core for hybrid carbon/hemp sandwich laminates

This paper explores the production and mechanical analysis of hemp bi-grid cores designed for use in sandwich structures, using an ad hoc continuous manufacturing process. The proposed customised manufacturing process allows for the production of cores with varying density values (ranging from 0.37 to 0.63 g/cm3). Both graded cores, with varying fibre content through the thickness, and non-graded cores were produced. The mechanical properties of these cores were evaluated under flexural and compressive testing with specific flexural strength reaching approximately 80 MPa·cm3/g and specific compressive elastic modulus around 340 MPa·cm3/g. Additionally, to assess their impact performance in sandwich panels, hybrid panels with CFRP skins were fabricated and subjected to Low Velocity Impact tests at 25 J, 45 J and 50 J. The results revealed that optimizing core density had increasingly beneficial effects as the impact energy increased. These findings suggest that the possibility of customising failure modes and tailor energy absorption capabilities of the proposed structures open the way for new, exciting perspectives for the development of innovative sandwich structures, broadening the application of natural fibres in various industrial sectors.