Experimental and numerical analysis of the ballistic response of agglomerated cork and its bio-based sandwich structures

Considering the susceptibility of sandwich structures to impact events and the increasing environmental awareness due to pollution, the present work provides a thorough understanding of the ballistic impact behavior of agglomerated cork and of the resulting green sandwich structures produced with polypropylene (PP) skins reinforced with a flax/basalt intraply hybrid fabric. The effect of density on the agglomerated cork response was evaluated (NL10 ρ = 0.14 g/cm3, NL20 ρ = 0.20 g/cm3 and NL25 ρ = 0.25 g/cm3) and a comparison with commercial polyvinyl(chloride) foams was provided (HP130 ρ = 0.13 g/cm3, HP200 ρ = 0.20 g/cm3 and HP250 ρ = 0.25 g/cm3). The effect of a maleic anhydride coupling agent on the mechanical properties of the skins and of the overall sandwich structures was also investigated. The results highlighted a compromising effect of the weak interface between cork granules and polymeric binder on the impact resistance of the agglomerated cork, but a clear improvement of its performance was observed when embedded as core material between the two skins. Indeed, the two classes of sandwich structures produced with neat PP skins and with the two cores with the same density, i.e. agglomerated cork NL10 and PVC foam HP130, displayed the same ballistic limit of 171 m/s confirming that cork integration in the overall structures allows to approach PVC foam performance. The high-velocity impact response of one agglomerated cork (NL25) and one PVC foam (HP130) was also subjected to finite element analysis employing the CRUSHABLE FOAM model available in ABAQUS obtaining a good fitting with the experimental data.