Development of an Injury Criteria for Iliac Wing Injuries Under Distributed Frontal Loading

Introduction: Lap belt induced injuries to the region of the pelvis between the anterior superior iliac spine (ASIS) and anterior inferior iliac spine (AIIS) in frontal impacts have been denoted in literature for almost 50 years. While it is an uncommon injury in the field, this injury is one that can be found within the current fleet of vehicles. However, the risk of this injury may increase with the development of Highly Automated Vehicles (HAVs). New, unconventional seating configurations may occur due to the presence of open, spacious interiors. In some vehicle concepts, the instrument panel and knee bolster components would be moved farther away from the occupant to allow for better comfort. Such a position may lower the efficacy of airbags, and may even bring about their removal. However, if an occupant were to be in a crash in this style of vehicle, the seatbelt system will provide all of the restraint of the occupant. Increased lap belt forces would likely cause an increase of the occurrence of the iliac wing injuries; yet, no experimental studies have been conducted to specifically understand at what force these injuries are caused.
Goals of Study: The main goal of this study is to develop an injury criteria for the iliac wings under frontal lap belt loading conditions. To achieve the goal of developing an injury criteria, I first need to recreate similar fractures using a similar loading/injury mechanism. I will design an experiment to characterize belt-to-pelvis loading with the goal of recreating the same fracture at the same isolated lap-belt-to-pelvis orientation that caused fracture in Richardson et al 2020. After replicating these fractures on a full scale test, I will then design a component level test to measure the tolerance of fracture on isolated pelvic wings, and do so across a variety of pelvises. I will then use the tolerance data to perform a statistical analysis to predict risk across the population to achieve the desired injury criteria.
Methods of Study: I developed a test fixture that was capable of replicating mid-sled test postures where pelvic fractures occurred. Three whole-body PMHS tests were run; two sustained fracture between the ASIS and AIIS on one of the two iliac wings. From there, two, subinjurious tests were run on a fourth PMHS to capture iliac wing strain data at varying lap-belt-to-pelvis angles. I then created a simpler, component-level test environment to isolate loading on denuded iliac wings between the ASIS and AIIS at the same lap-belt-to-pelvis angles from the first set of tests. Testing was completed on the fourth PMHS pelvis to relate the boundary conditions and load response on the component-level test to the full-scale test environment. After that, 20 pelvis wings were loaded to failure, and development of an injury risk function was performed.
Results of Study: The lap belt loading rate and injury type sustained by the PMHS in the first testing environment matched those of the sled test series where the targets came from. Two of the three PMHS sustained pelvis injuries at the targeted location, while the third subject submarined. Twenty two pelvic wings were tested to failure at two lap-belt-to-pelvis angles; nineteen of them sustained injuries similar to those found in literature and the full-scale test environment. A survival analysis was completed using the tolerance data, and an injury risk function was developed using a Weibull distribution cumulative distribution function. From this analysis, a 50% probability of injury correlated to an iliac wing force of ~4500 N. While loading angle did not have a significant effect on the fracture tolerance of the pelvis, a weighted bone density metric was shown to be a significant predictor of injury risk.
Impact of Thesis: The testing in this thesis was the first to specifically investigate the tolerance of the iliac wings to lap belt loading. While the injury is not currently common in the field data, it has the potential to become much more common with the occupant environments predicted to be available with HAVs. Restraint manufacturers and automobile OEMs can begin to use the results of this study, and specifically IRF developed in this thesis, to guide the development of occupant restraints and other injury countermeasures. Further work will be needed to relate load at the iliac wing to lap belt tension load; however this thesis provides the first injury tolerance data of the iliac wings under frontal loading.