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.
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