Evaluation of Coefficient of Friction in Oblique Helmet Impacts: An Experimental and Numerical Study

Abstract: The focus of this thesis was to increase the knowledge in the area of "dynamic friction" during oblique helmet impacts, both experimentally and numerically. Physical experiments have been performed with multiple helmets, different angles of the anvil and different surface materials, with results of impact forces from the anvil and accelerations of the head. By the use of the Coulomb friction model the friction, over time, during the impact of the head and helmet has been calculated. Finite element method has then been used in LS-DYNA to try and replicate the physical results with the goal of creating an accurate friction model. There has been previous work done where the commonly used abrasive paper has been compared to asphalt as ground material in the drop tests. A similar study has been performed in this project which has been compared to the previous work. The results when comparing asphalt to abrasive paper and stainless steel shows that abrasive paper has a higher friction and rotational acceleration of the head compared to asphalt. Stainless steel however displays similar characteristics as asphalt in both friction and accelerations. The load cell used during the experimental testing has been examined carefully since the value of the friction coefficient has differed depending on the angle and the impact location on the anvil. There are still uncertainties surrounding the reliability of the results from the load cell. LS-DYNA has two different ways of modelling friction, one where LS-DYNA uses a modified equation of the Coulomb friction model and another one where the user can insert a table of values on coefficient of friction and relative velocity. Both methods have been used, however the latter method has proven to be more suitable for the kind of tests used in this project.