Experimental Assessment of Human and Crash Dummy Skin to Vehicle Air Bag Fabric Coefficients of Friction

Abstract

Oblique motor vehicle crashes can cause serious head or brain injuries due to contact with interior vehicle structures even with the deployment of air bags, as they are not yet completely successful in preventing traumatic brain injury. Rotational head velocity is strongly correlated to the risk of brain injury, and this head motion is potentially related to the tangential friction force developed during contact between the head and air bags. Although crash test dummy head skins are designed with appropriate mass properties and anthropometry to simulate the normal direction impact response of the human head, it is not known whether they accurately represent the frictional properties of human skin during air bag interaction. This study experimentally characterized the dynamic friction coefficient between human/dummy skins and air bag fabrics using a pin-on-disc tribometer. Human skin samples were harvested from five locations (left and right forehead, left and right cheek, and chin) from male and female postmortem human subjects (PMHSs); some samples had previously been frozen and some were fresh. Crash dummy head skin samples were obtained from Hybrid III, ES-2re, and THOR-50M 50th-percentile male anthropomorphic test devices (ATDs) and were characterized in both chalked and unchalked conditions. Fabric samples were obtained from five different air bags spanning various vehicle manufacturers and interior mounting locations. Neither sex, linear speed, nor the harvested skin location on the head played a significant role on the dynamic friction between PMHS skin samples and air bag fabrics, while PMHS skin samples that had not been previously frozen had a higher coefficient of friction than those that had. Further, increasing normal load reduced the dynamic friction coefficient between PMHS skin samples and air bag fabrics. Unchalked ATD head skins exhibited significantly higher dynamic friction coefficients than PMHS skins for the air bag fabrics tested. The presence of a thin chalk layer on ATD skins reduced friction and produced dynamic friction coefficients with air bag fabrics that were not significantly different from those of PMHS skins; however, neither unchalked nor chalked ATD head skins differentiated the air bag fabric dynamic friction coefficients in the same pattern as the PMHS skin samples.