Thoracic Injury Risk in Frontal Car Crashes with Occupant Restrained with Belt Load Limiter

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In France and in other countries, research shows that the highest proportion of severe injuries and fatalities to restrained occupants occurs in frontal impacts. The oldest occupants involved in severe frontal impacts often suffer the worst thoracic injuries due to the seat belt. In France in the 1970's, some cars were equipped with load limiters making it possible to observe the relationship between force applied and occupant age with regard to thoracic risk. It has become essential, due to the ability of newer vehicles to better withstand intrusion in frontal impacts, to limit the restraint forces of seat belts that cause severe thoracic injuries, especially in elderly occupants. To address this risk, a restraint system combining belt load limitation and pyrotechnic belt pretension [the Programmed Restraint System (PRS)] has been installed in Renault cars since 1995. From static and dynamic tests performed with the load limiter, it is possible to determine the shoulder belt force applied to the occupant. 89 accident cases with equivalent energy speed of 40-80 km/hour, involving frontal collisions with cars equipped with the PRS, are reported in this paper. This study's purpose was to establish, for belted occupants, thoracic injury risk as a function of occupant age and the load applied at shoulder level. For 50% of thorax risk of AIS3+, the force for all ages is 6.9 kN. Results were obtained for 256 occupants representing age distribution similar to that of front seat occupants of the French accident file. Shoulder belt load appears to be in accordance with the occurrence of chest injuries. Other relationships between real-world accidents and post-mortem human subjects and between Hybrid III thoracic injury measurements and shoulder belt load are also investigated. This study confirms that a 6 kN force level is not enough to protect a larger proportion of the population. It is posited that a belt load limitation of 4 kN, combined with a specifically designed airbag, would protect 95% of those involved in frontal impacts from thorax injuries of AIS3+.