by Stacy M. Imler, Ph.D., P.E., Exponent, Inc.
The first rollover-activated side curtain airbags (RSCAs) were offered by Ford Motor Company in their 2002.5 model year Ford Explorer/Mercury Mountaineer 4 door sport utility vehicles, manufactured after March 4, 2002. The purpose of this technology was to provide incremental benefit to belted occupants in rollover crashes. Implementation of this technology into production vehicles involved rigorous developmental and testing work to design, develop, and test the overall system which included the curtain airbag, seatbelt pretensioners, restraint control module, and platform-specific algorithms for sensing and deployment, all integrated within a specific vehicle platform. Since the introduction of RSCAs, vehicle manufacturers have continued to incorporate this technology into their vehicle fleet at a steady rate with a sharp increase in market insertion starting in the 2010 model year (Figure 1, N
HTSA, DOT HS 811 882, 2014).
RSCAs are passive supplemental restraint systems to seatbelt use. Analysis of field accident data demonstrates that seatbelt use is highly effective in prevention of occupant ejection and reduction of serious and fatal injury in rollovers. For example, Malliaris and Digges (SAE 1999) found that 98.8% of belted pickup rollover occupants did not sustain serious or greater injury. Further, they reported serious and greater injury rates of 1.4% to 3.1% for belted passenger vehicle occupants in rollover crashes.
Initial development of RSCAs was directed at providing incremental head protection through cushioning, as well as a level of supplemental containment through reduced portal size. These objectives were balanced with the goal of minimizing injury potential associated with deployment and occupant interaction with the device itself (e.g., the system must “do no harm”). The resulting systems have evolved and continue to evolve in response to regulatory efforts, but have finite coverage, finite energy capacity, and finite head cushioning capacity. These concepts are demonstrated in the National Highway Traffic Safety Administration’s (NHTSA’s) guided impactor testing of production systems, wherein it has been demonstrated that the greatest retentive capacity occurs in the regions where the curtain is supported by vehicle structures (e.g., at the upper rear aspect for the front window positions). The lowest retentive capacity occurs at the unsupported perimeter of the curtain airbags or in areas of limited coverage. In the region of lower retentive capacity, tests have demonstrated motion of the guided impactor well beyond the glass plane and boundaries of the RSCAs, resulting in ejection of the impactor. More generally, this testing demonstrates that forceful occupant loading into an inflated RSCA results in movement of the RSCA into and through the window plane and can result in occupant ejection.
Published research examining field accident data has shown that an occupant’s injury risk cannot be reduced to zero through implementation of a particular safety countermeasure, particularly for occupants involved in severe crashes. Analysis of initial field accident data which includes vehicles equipped with RSCAs demonstrates estimates of fatality reduction by approximately 20% to 40% (Padmanaban and Fitzgerald, IRCOBI 2012; NHTSA, 2014). Examination of the distribution of rollovers shows that the average rollover crash in the field involves less than 2 quarter revolutions (Gloeckner et al., AAAM 2007), and it is well established that occupant injury potential increases with corresponding increases in the number of quarter revolutions (Moore et al., AAAM 2005). Accordingly, to estimate RSCA efficacy in multiple roll events, the NHTSA specifically incorporated this relationship with recognition of reduced RSCA efficacy with increased exposure for belted, partially ejected occupants (NHTSA FRIA, 2011). Field accidents contain examples of fatal injury as a result of partial ejection even in the presence of a deployed RSCA. These examples as well as those encountered in litigation, demonstrate that for belted occupants, the presence of a deployed RSCA cannot preclude partial ejection or fatal injury.
In addition to ejection related injuries, research examining the effects on occupant kinematics and occupant loading in the presence of RSCAs has been performed in the context of catastrophic neck injuries, also referred to as “diving” injuries, sustained during rollover crashes as a result of torso augmentation at vehicle-to-ground impact. It has been shown in spin testing as well as in full-scale rollover testing that the presence of an RSCA does not prevent the up-and-out motion of a belted dummy. Further, the presence of an RSCA does not prevent the head from coming into contact or close proximity to the interior of the roof nor does it prevent the head-neck-torso alignment needed for “diving” injuries (Heller et al., SAE 2015; Newberry, Imler, et al., SAE 2014). Rollover component system testing demonstrated that the use of pretensioners and RSCAs did not preclude head contact with the roof and had a limited effect on the dummy neck loading at roof to-ground impacts (McCoy, SAE 2010). These tests demonstrate the potential for occupants to sustain catastrophic neck injury even in the presence of a deployed RSCA.
As with considerations for implementation of all safety countermeasures, the incremental benefits in safety provided by RSCAs need to be balanced with the goal of minimizing occupant injury potential related to deployment (i.e., the safety countermeasure should “do no harm”). As supported by field accident data, there are limitations to the efficacy of rollover curtain technologies, particularly in high severity rollover crashes. Further, the design and performance goals need to be balanced with the inherent risks, to result in a system which will increase occupant safety. However, the resulting system cannot mitigate all serious injuries. For a particular rollover crash, the occupant injury outcome related to the performance of the rollover curtain technology is dependent on the vehicle-, occupant-, and crash specific parameters.
FMVSS No. 226: Ejection Mitigation
On January 19, 2011, nearly a decade after vehicle manufacturers first introduced RSCAs into production vehicles, Federal Motor Vehicle Safety Standard (FMVSS) No. 226, “Ejection Mitigation”, was established “to reduce the partial and complete ejection of vehicle occupants through side windows in crashes, particularly rollover crashes.” As detailed in the Final Rule, ejection mitigation countermeasures are required to limit the outboard displacement of a projected headform to 100 mm (3.9 in) beyond the inside surface of the window glazing of the portal. The NHTSA anticipated that manufacturers will modify their existing side curtain airbags through increased window coverage, increased inflation duration, and tethering geometry changes to meet the standard. The agency asserted, “full window opening coverage was key to the effectiveness of the curtain in preventing ejection.” The phase-in schedule for FMVSS 226 requires that a percentage of vehicles meet the new requirement beginning September 1, 2013, and will require that all new vehicles meet the standard by September 1, 2017.
To test for the 100 mm displacement criterion, an 18 kg (40 lb) headform is projected at impact speeds of 20 kph (12.4 mph) and 16 kph (10 mph), at 1.5 and 6 seconds, respectively, following curtain airbag deployment. As per the Final Rule, these tests “replicate the forces that an occupant can impart to the curtain during the rollover event as well as during side impacts.” Impact target locations are determined based on the vehicle specific geometry of the side daylight openings. Pertaining to the specific ejection mitigation countermeasure, the standard “does not allow the use of movable glazing as the sole means of meeting the displacement limit of the standard (i.e., movable glazing is not permitted to be used without a side curtain air bag).” Further, the second impact, executed at 6 seconds following curtain deployment, must be performed with the glazing retracted or removed from the daylight opening.
Rollover-activated side curtain airbags were first available in late 2002 model year vehicles, but only after extensive development and testing performed by component and vehicle manufacturers. Since then, introduction of this technology into the vehicle fleet has followed a phase-in approach for multiple reasons, including technological challenges, still-limited field performance data, and the potential for unintended consequences. As this technology continues to evolve, design and performance goals need to be balanced with the inherent risks, to result in a system which will increase occupant safety. However as with all safety countermeasures, the resulting system cannot mitigate all serious injuries. Evaluation of occupant injury outcome related to the performance of RSCA technology in a specific rollover crash is dependent on the vehicle-, occupant-, and crash specific parameters.
Citation abbreviations: SAE – Society of Automotive Engineers; IRCOBI – International Research Council on Biomechanics of Injury; AAAM – Association for the Advancement of Automotive Medicine.
About the Author: Dr. Stacy Imler is a Managing Engineer in Exponent’s Atlanta office. Dr. Imler is a licensed engineer who specializes in injury biomechanics and her work includes evaluation of the effects of existing or hypothetical safety countermeasures such as airbags and seatbelts on injury outcome. She earned her undergraduate degree in Mechanical Engineering at Lehigh University and her Masters and PhD in Mechanical Engineering at Georgia Tech.