New hybrid cork-STF (Shear thickening fluid) polymeric composites to enhance head safety in micro-mobility accidents

Cork is a natural material that presents excellent properties for applications concerning impact resistance, and its performance can be further enhanced by combining it with other materials into composites. This study tests several combinations of hybrid composites in layered structures involving cork laminates, different types of polymers such as polyvinyl chloride (PVC) and polyurethane (PU) containing shear thickening fluid (STF) in their composition, fabric impregnated with STF, and the fluid in bulk used as an interlaying agent. The aim is to evaluate, through a series of drop impact tests, the robustness of the composites applied in head protection devices used in micro-mobility. During the study, various composite structures were subject to impact tests of 20 and 100 J, deliberately lower and higher energy levels than the 69 J established by the European standard for bicycle helmets – EN1078. The authors directly compared composites of the same thickness and dimensions, determining the influence of the material supplementary to cork in the impact performance. Results are promising for two different types of hybrid composites: a) a solution with encapsulated STF, showing a reduction of up to 8.5 % in peak acceleration and smoother deacceleration through higher deformation levels for a given amount of energy absorbed per unit volume; b) a solution with impregnated fabric, which is the lightest and less dense amongst all, displaying very good compromise between density and good impact resistance. These aspects are desirable features for helmets used in micro-mobility.