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. / Fokuset för denna rapport var att öka förståelsen inom området ”dynamisk friktion” under sneda hjälmislag, både experimentellt och numeriskt. Fysiska experiment har genomförts med flertalet hjälmar och olika vinklar samt underlag på städet, med resultat på både krafter från städet samt accelerationer från huvudet. Genom användandet av Coulombs friktionsmodell har friktionen, över tid, under islag med en hjälm beräknats. Finita elementmetoden har använts i LS-DYNA för att försöka efterlikna resultaten från de fysiska experimenten med målet att kunna skapa en friktionsmodell. Det har gjorts liknande arbete tidigare där sandpapper har jämförts mot asfalt som underlagsmaterial i falltesterna. En liknande studie har genomförts i detta projekt vilket har jämförts med den tidigare gjorda studien. Resultaten när asfalt jämförs mot sandpapper och rostfritt stål visar att sandpapper har en högre friktion samt rotations acceleration av huvudet jämfört med asfalt. Rostfritt stål uppvisar dock liknande egenskaper som asfalt när det kommer till både friktion och acceleration. Lastcellen som har använts har undersökts noggrant, då värdet på friktionskoefficienten har förändrats beroende på vinkeln och träffpunkten på städet. Osäkerheter kring trovärdigheten på resultatet från lastcellen kvarstår. LS-DYNA har två olika sätt att modellera friktionen på, antingen så används en modifierad ekvation av Coulombs friktionsmodell eller så skapar användaren en tabell med värden på friktionskoefficienten och den relativa hastigheten. Båda metoderna har använts men det har visat sig att den senare metoden är bättre anpassad för de test som har utförts i detta projekt.

The objective of this research was to develop an optimal design for a polymeric American football helmet liner for concussion prevention utilizing experiments and high performance. Along with well-established injury criteria (HIC, SI, and Peak acceleration), localized brain injury mechanisms were explored by employing Finite Element simulations and experimental validation. Varying strain rate experiments (monotonic and hysteresis) were conducted on modern football helmet (Rush, Rawlings, Riddell, Schutt, and Xenith) liners and new possible polymeric foam liner materials. These experiments were used to characterize each material at low strain rates (0.1/sec; Instron), intermediate strain rates (100-120/sec; NOCSAE drop tower) and high strain rates (600-1000/sec; Split Hopkinson Pressure Bar). Experimental design optimization was performed on a football helmet liner by utilizing an exploratory Design of Experiments by National Operating Committee on Standards for Athletic Equipment (NOCSAE) drop tests. FEA simulations of drop impact tests were conducted on a helmeted NOCSAE headform model and a helmeted human head model. Correlations were made between both models to relate localized brain response to the global acceleration and the dynamic-based injury criteria HIC, SI, and Peak acceleration). FEA simulations were experimentally validated by twin-wire drop tests of the NOCSAE headform using correlations for validation of the human head model. The helmeted human head simulations were used to explore a Mild Traumatic Brain Injury (MTBI) limits based localized brain response (e.g. pressure and impulse). Based on these limits, future FEA simulations will be used to explore these limits as helmet liner design criteria.