Automotive crashes are a leading cause of death in the US, with side-impacts being the most fatal. Thoracic injuries are among the leading causes of such fatalities in small, elderly females. Thus, a series of six realistic side-impact post-mortem human surrogate (PMHS) sled tests were conducted. Results showed serious thoracic injuries in all PMHS, despite current side-impact anthropomorphic test devices (ATDs) predicting a <10% probability of such injuries. While this is partially due to the inaccuracy of injury risk scaling, it is hypothesized that combined thoracic loading, anteriorly-posteriorly (A/P) from the seatbelt pretensioner and laterally from the side airbag, increased rib fractures despite less-than-expected lateral chest deflection.
The objectives of this study were to investigate the following in small, elderly female PMHS: 1) discrepancies between existing injury risk assessment tools and actual injury outcomes; 2) the effects of seatbelt pretensioners on thoracic injury; and 3) the effect of combined loading from the seatbelt pretensioner and side airbag on thoracic injury in side-impact.
Prior to PMHS testing, SID-IIs ATD tests were run to match conditions to the previous study. A foam-padded pneumatic lateral impactor was used as a repeatable airbag surrogate. Three PMHS were tested, each under three different loading conditions: 1) A/P-only (seatbelt pretensioner), 2) lateral-only (airbag surrogate), and 3) combined loading (both). Anterior and posterior aspects of right and left ribs were instrumented with strain gages to detect potential fractures and fracture timing. Chest deflection was measured by axillary- and xiphoid process- level chestbands. Seatbelt load and spine motion were also measured. An anatomical dissection was completed after each test series to document all injuries.
Each PMHS first underwent an A/P-only test, resulting in A/P chest compressions of 7-12% and no rib fractures. While the goal was to conduct one lateral-only and combined loading test per PMHS, number of tests and impact energy varied due to detected rib fractures or lack thereof. PMHS 1 only experienced one 4.9 m/s lateral-only impact due to 5 rib fractures occurring at 11% half-lateral compression. PMHS 2 underwent three lateral-only and two combined tests. Half-lateral compressions ranged from 4-8% for lateral-only and 7-8% for combined tests; 2 rib fractures occurred after the third lateral-only impact at 3.2 m/s. PMHS 3 underwent two lateral-only and one combined test. Half-lateral compressions measured 6-9% for the lateral-only and 7% for combined. PMHS 3 had 6 rib fractures in the second lateral-only impact at 3.0 m/s.
Poor prediction of input impact energy from SID-IIs tests suggested low ATD biofidelity. A/P-only tests revealed that pretensioners alone do not likely cause rib fractures in fragile occupants. The high frequency of rib fractures in lateral-only tests restated the need for demographic-specific injury risk criteria. Strain gage data in combined tests showed initial tensile strain in the impact-side ribs that became compressive, indicating dynamic loading. Combined loading also reduced rib strain magnitude despite constant chest deflections. While these findings may imply an increased thoracic injury risk under combined loading, future work is needed to fully understand the underlying injury mechanism.