Friction Modeling in FE Simulation : Identification of Friction Model Parameters in Airbag and Crash Dummy Head Contact through Simulation and Experimental Data Response Correlation

Sammanfattning: Motor vehicle-traffic accidents are a common cause of traumatic brain injuries, resulting in severe and sustaining disabilities, or even fatality. In an effort to mitigate injuries related to vehicle crashes, various safety systems such as the occupant airbag has been implemented. In angled impacts, occupant interaction with the airbags can lead to head rotation, and during recent years head rotation has been emphasized as an important contributor to head injury risk. Therefore, for prediction of head injury risk in crash simulations it is important to correctly model the friction force which arises in the contact between occupants and the car interior. The aim of this thesis is therefore to study the friction within such a system. More specifically, the analysis is focusing on dummy head to airbag interaction and to correlate a three parameter friction model for this contact pair, as well as a one parameter model currently used by Volvo Car Group, with measured laboratory test data in the software LS-DYNA.A preliminary study in LS-DYNA was conducted to determine the configuration of the laboratory setup consisting of a statically inflated customized driver airbag and a crash dummy head being launched to impact the airbag. The laboratory test data was analyzed using linear regression and Students T-test to identify the influence of parameters on the measured responses. The simulation model was then modified to represent the laboratory setup, prior to an optimization study performed to correlate simulation and laboratory test data responses. Lastly, an evaluation study was made to test whether or not the proposed friction model could improve occupant crash simulations.It was found in the thesis study that the friction force had a large effect on the rotation of the head around the vertical axis (z−axis in the anatomical coordinate system of the head). The experimental data showed that the internal pressure of the airbag had little effect on the response. This was likely due to the studied pressures being large enough for the airbag to be so stiff that no plowing effect of the dummy head moving through the airbag fabric could be seen. Furthermore, results from the optimization study indicated that the model correlation was improved when a three parameter friction model with velocity dependence was used. This implies that the friction coefficient is dependent on the velocity. It was also shown that material properties affecting friction behavior vary between different crash dummy heads, as well as different surface coating. Both dummy T-shirt fabric and grease paint resulted in significantly lower surface friction.Due to the difference in friction for different dummy heads, a single set of friction model parameter values that describes the friction behavior of all crash dummy heads does not exist. The study finds that when sliding is present in a contact, a three parameter model for describing the friction improves the correlation, as it can account for the velocity dependence of the friction in the contact. In contrast, when sliding is not present the one parameter and the three parameter model give similar results.Keywords: friction, velocity dependent friction coefficient, finite element analysis, car crash simulation, Volvo Cars, crash test dummy head, driver airbag, LS-DYNA, laboratory testing, optimization study.