Study of Possible Use of Strain-rate Dependent Materials in Protective Helmets

The probability of brain injury during an accident is dependent on the force history acting on brain tissue. The mostly used criterion for the injury probability assessment, the Head Injury Criterion (HIC), is based on the time integral of force (acceleration) history curve. HIC is used in the work for the assessment of helmet performance. Because of limited weight of the helmet, current designs prefer to use foams of very low density, where the strain-rate dependency is negligible. The possibility of developing a material consuming more energy at high strain-rates without significant increase of density is challenging. The work is focused on a parametric study of a helmet with new design: the liner is made of a denser strainrate dependent material. Because of increased weight of the helmet, the outer shell is designed thinner than those used in conventional helmets nowadays. The analysis if performed using numerical simulation software LS-DYNA. Advanced foam material models with strain-rate dependency have been used. The structure has been loaded using a falling dummy headform producing different strain-rates (up to 280/s). The thickness of the liner layer and its material properties have been varied. Different designs of the helmet liner have been compared in terms of protective performance expressed using the HIC values. The material properties of the liner improving the performance have been determined.