A New Methodology for Investigating Airbag Induced Skin Abrasions

Although airbags have been shown to reduce the incidence of life threatening injuries, they have increased the risk of minor injuries such as those to the skin. Based on the distribution of injuries that can be directly attributed to the airbag itself, it is believed that shear loading exists as a mechanism for these skin injuries. The purpose of this study was to develop a new methodology designed to assess the injury potential from different types of airbags with respect to shear loading. This new methodology utilized a high-speed impactor to accelerate the airbag fabric past a sample of skin. Contact normal forces were monitored by the use of pressure sensors, and fabric velocity was determined from high-speed video. The abraded skin samples were analyzed using light microscopic analysis and ultraviolet light source photography. A new abrasion rating method was developed called the Total Abrasion Score (TAS), which allows for quantifiable differentiation between the abrasions caused by different airbag fabric and seam types. INTRODUCTION lthough airbags have been shown to reduce the incidence of life threatening injuries, they have increased the risk of minor injuries such as those to the skin (Dalmotas et al., 1995, Foley and Mallory, 1995, and Foley and Helm, 2000). A study of the National Automobile Sampling System (NASS) found that 66% of front seat occupants exposed to an airbag deployment incur a skin injury, 47 % of these injuries are attributed directly to the airbag itself (Jernigan, 2001). Of these injuries, 41% occur to the face (Cocke, 1992, Duma et al. 1996, Murphy et al., 2000, Rozner, 1996, Smally et al., 1992, and Smock and Nichols, 1995), 3% occur to the neck (Hansen et al., 1999, Morrison et al., 1998, and Steinmann et al., 1992). 11% occur to the chest and abdomen (Beckerman and Sama, 1995), 42% occur to the upper extremities (Freedman et al., 1995, Huelke et al., 1995, Burton, 1994, and Molia and Stroh, 1996), and 2% occur to the lower extremities A Injury Biomechanics Research 158 (Weinman, 1995). While these skin injuries may not be life threatening, nearly 300,000 occupants incur an airbag induced skin injury every year in the U.S. Because this number is so large, there is considerable motivation for investigating the possibility of airbag cushion modification in order to reduce the risk of skin injury. Early research done by Kikuch showed that injury severity is directly proportional to the total pressure exerted onto the skin by an airbag (Kikuch et al., 1975). This was found by deploying airbags onto the shaved regions of rabbit heads; however, no specific injury criterion was recommended. Reed performed studies on human volunteers to elucidate the potential for skin abrasion caused by airbag deployment (Reed et al., 1992). The volunteers exposed the front of their lower leg to a deploying airbag. The magnitude of the pressure exerted onto the volunteers by the airbag was determined using an instrumented leg-form placed in the same position as the volunteer’s leg to insure they would be exposed to similar pressures. The response time of the load cells used on the instrumented leg form were too slow to capture the event; therefore, Reed utilized Fuji pressure film to record the resulting pressure. Reed concluded that normal loading alone, as measured by the pressure film, induced enough pressure to cause skin abrasions. Two forms of skin abrasion injury criteria were presented: peak pressure from the Fuji film of 1.75 MPa (2490 psi), or a peak leading edge airbag velocity of 85 m/s (190 mph). Reed also established a method for utilizing these injury criteria by placing Fuji Pressure Film on the surface of a PVC pipe that was exposed to an airbag deployment in a test configuration that assumed a normal loading injury mechanism (Reed et al., 1993). Sugimoto utilized a similar test configuration, but recommended a slightly lower peak velocity criterion on 70 m/s (157 mph) (Sugimoto et al., 1994). The purpose of the current study is to develop a new methodology for comparing skin abrasions caused by different airbag fabric and seam types due to shear loading. While the previous work by Reed presents an injury criterion for normal pressure alone, it does not characterize all types of airbag loading. In particular, the type of loading that would occur in the region of the airbag seam should be tested and evaluated. It is suggested that there are two general injury mechanisms for airbag induced skin injuries: normal pressure from perpendicular contact of the airbag with the face and thoracic areas, and shear loading as the airbag expands and interacts with the upper extremities (Figure 1). The purpose of this paper is to develop a new methodology for comparing abrasions caused by different airbag fabric and seam types with respect to shear loading. Figure 1: Two proposed injury mechanisms for airbag induced skin abrasions: normal loading of the face and chest versus shear loading of the upper extremities. A New Methodology for Investigating Airbag Induced Skin Abrasions 159 METHODS Previous research done by Reed found that airbag velocities above 85 m/s (190 mph) resulted in skin abrasions for normal loading (Reed et al., 1992). Based on this criterion, the target fabric velocity for the current study was established at 89 m/s (200 mph). The experimental setup was designed to accelerate the airbag fabric to this velocity as it based below a tissue sample (Figure 2). The skin mount assembly held the skin in place and contained the necessary instrumentation to allow for contact pressure measurements. A pneumatic impactor fired an aluminum projectile that struck the airbag fabric mount in order to achieve the required velocity for the tests. Figure 2: Orientation of skin and airbag material for abrasion testing. A pneumatic impactor was used to accelerate the fabric mount up to the target velocity of 89 m/s (Figure 3). This was accomplished by accelerating an aluminum slug down the impactor barrel using compressed air stored in the air tank. The skin tester assembly was mounted on linear bearings, which allowed the skin tester to translate in order to absorb the recoil induced by the slug (Figure 2). Figure 3: Pneumatic impactor used to achieve a relative velocity between the skin and airbag fabric of 200 mph. Eight different airbag fabric and seam types were obtained and prepared by cutting into a rectangular shape and mounting to a polycarbonate plate, or fabric mount (Figure 3). The Injury Biomechanics Research 160 mounting was such that the fabric was held in tension along all four directions. The airbag fabric was attached to the fabric mount and positioned for testing. A control test was also performed using the fabric mount without fabric attached to it. Figure 4: Polycarbonate block with fabric ready for testing. Porcine skin tissue was used because of its scheduled post-mortem availability. A comparative study was performed between human cadaveric tissues and the porcine specimens to compare the thickness of the different layers of the skin. Before the skin tissue was removed from the porcine subjects, square outlines were made on the skin with the inside dimensions of the clamping bracket. This made it possible to apply pretension to the skin back into its original biaxial state of stress once inside the aluminum bracket (Figure 5a). Once the tissue was secured in the aluminum bracket, the assembly was mounted to the tissue stage. The tissue stage was prepared with Fuji film sheets (Medium Fujifilm Prescale Mono Sheet Type, Fuji Photo Film, Tokyo, Japan) and Flexi-Force pressure sensors (FlexiForce A101 100lb, Tekscan, South Boston, MA)(Figure 5b). A total of four sensors were used, each with a sensing diameter of 9.5 mm.