Protecting occupants during passenger vehicle rollover crashes

Rollover crashes have been acknowledged as one of the most dangerous forms of passenger vehicle crashes (NHTSA, 2006b). The detailed review of the research literature presented in Chapter 2 and cited throughout this thesis, has identified rollover to be a significant social issue in both the US and Australia due to the high proportion of road fatalities and serious injuries this crash mode accounts for. These findings are evidence of an inherent lack of adequate rollover crashworthiness design in modern day vehicles, which in turn has led to a low degree of occupant protection and the high serious injury and fatality rates observed in these crashes. The literature review also identified that a significant number of injuries result from occupants being ejected. While researchers have demonstrated a good understanding of how to prevent such ejections, and therefore the resulting injuries, there remains conflict and confusion when research focus turns to the serious and fatal injuries of contained occupants, particularly when considering spinal neck injuries. This conflict is primarily a result of a series of differing findings of researchers regarding the cause of these injuries. These conflicting findings result from the development of two seemingly contradictory theories which attempt to explain how spinal neck injury is inflicted on contained occupants during rollovers. This has in turn confused opinions on the best method to improve the rollover crashworthiness design of vehicles for close to forty years. As a result, the number and proportion of rollover fatalities has remained high in Australia, and in the US has increased from the mid-1980s to today. Driven by the need for a consistent philosophy underpinned by strong scientific evidence, the research presented in this thesis was aimed at better understanding the injury mechanisms, particularly spinal neck injury mechanisms, which lead to fatalities and serious injuries. This was seen as vital to any improvement in the rollover crashworthiness design of vehicles. To this end, the research presented within this thesis focused on answering three key research questions: 1. What are the injury mechanisms for contained and seat-belted occupants in rollover crashes? 2. Does roof deformation caused by the structural failure of a weak roof lead to severe and/or fatal spinal neck injury to contained and seat-belted occupants? 3. What key components of a vehicle’s roof must be considered when designing a vehicle that protects it’s occupants against a severe spinal neck injury during a rollover crash? From these research questions, the following research hypotheses are developed as a further guide for the work presented. - There is a causal link between the severe and fatal spinal neck injuries of contained and seat-belted occupants of a rollover crash and the strength of a vehicle’s roof; - The mechanism of roof deformation that occurs during a passenger vehicle rollover crash influences the severity of spinal neck injury a contained and seat-belted occupant is subjected to. Chapter 3 presents a review of Australian road crash data and identifies that rollovers in Australia accounted for 24% of all passenger vehicle road fatalities for 2004 and 2005, a higher proportion than had been identified in previous research. Importantly, the rollover crash mode accounted for a higher proportion of crashes than either front or side impact vehicle-to-vehicle crashes. The Chapter 3 review of road crashes concludes that the injury profiles and injury mechanisms leading to fatalities on Australian roads, for the years investigated, were similar to those reported in previous US investigations. This in turn leads to the finding that similar limitations in vehicle design are evident in the vehicle fleet in both countries. This is true despite the fact that US road users are thought to be adequately protected by a mandatory rollover standard, namely FMVSS 216. This then also leads to the conclusion that this standard is ineffective in protecting passenger vehicle occupants during rollovers. To address the underlying conflict regarding the causation of those spinal neck injuries observed to occur to contained and seat-belted occupants in rollover crashes, the research presented in Chapter 4 focuses on identifying and comparing the different roof deformation mechanisms where these injuries occur. Some of these roof mechanisms are identified through the review of previous research while others are original, in that this thesis is the first to identify and examine in detail these mechanisms. In all, five mechanisms are identified and discussed: - Diving (which significantly is directly related to occupant/vehicle kinematics rather than the degree of roof deformation), - “Progressive wave” roof buckle, - Roof side sway “match-boxing”, - Roof “inward fold”, and - Roof “outward tenting”. Using video footage of the Malibu II rollover crash tests, carried out by General Motors (GM), it was identified that the diving mechanism (Moffatt 1975) does indeed occur in both strong and weak roof vehicles. Further, it is shown that the severity of impact to an occupant during the diving mechanism is related to the kinematics of the vehicle and the movement of the occupant within the vehicle throughout the rollover event. Using the Malibu II crash tests, it is shown that the maximum head impact velocity reaches around 2.0 to 3.0 m/sec, when considering the diving mechanism. Through further analysis of the Malibu II rollover crash test footage the remaining mechanisms, each of which involves different forms of roof deformation due to structural failure, are each shown to be associated with a potential for head impact velocities greater than 3.0 m/sec. Using the analysis of the roof deformation and the recorded results of the Hybrid III Anthropomorphic Test Device (ATD) used during the Malibu II rollover crash test series, it is concluded that structural failure(s) and the resulting roof deformation, lead to an increased risk of severe and fatal spinal neck injury for contained and seat-belted occupants. Chapter 5 extends the work of Chapter 4 by identifying and discussing real world rollover crash cases where the roof mechanisms introduced in Chapter 4 can be identified and the injuries to contained occupants exposed to these mechanisms. Two studies are presented, the first being a review of Australian single vehicle fatal rollover crashes involving contained occupants and the second being a review of US single vehicle rollover crashes involving contained occupants that recorded all levels of injury severity. These studies lead to a series of important findings. These include that: all but one of the mechanisms identified in Chapter 4 could be observed from post crash photos of vehicles that had rolled over; in the case of strong roof vehicles (defined as vehicle with a FMVSS Strength-to-vehicle Weight Ratio >4.0) where occupants are most likely to be subjected to the diving mechanism, there are no occupants that sustained fatal injuries; severe (AIS 3+) injuries to the head and neck were more prevalent in crashes where structural failure of the vehicle’s pillars and header (i.e. roof structure) was observed; lower frequency of injury to the head and spinal neck is observed for high roof strength vehicles in the US; occupants seated on the front far-side of the vehicle demonstrate those characteristics (considering injury profile and frequency) of those occupants who are contained and killed in fatal rollover crashes; and finally a lower frequency of structural failure of pillars is concluded to lead to an increased level of protection for contained and seat-belted occupant’s, as observed in rollover crashes involving high roof strength vehicles. Chapter 6 analyses the Hybrid III ATD results from the Malibu II series of FMVSS 208 dolly rollover crash tests, as they relate to occupant neck injury risk. As a precursor to this, an investigation and review of the neck loading process including theoretical analyses and computer modeling of inverted drop test experiments involving Hybrid III ATDs are presented. The work also identifies that spinal neck injury risk as measured by such an ATD is directly related to the head impact velocity to which the ATD was subjected. Through an investigation examining cadaver test data, Chapter 6 also identified a series of spinal neck risk relationships. These risk relationships can be related either to the velocity, momentum or kinetic energy of the head impact. The strength of the relationship between each injury risk variable and cadaver neck fracture is identified and from this it is shown that the variable which demonstrated the relationship of highest strength is head impact momentum. These results show that an increase in intrusion velocity, such as that observed when structural failures occur in vehicle roofs, increases the risk of severe spinal neck injury for contained occupants. Chapter 6 then presents a comparison of spinal neck injury risks for all Potentially Injurious Impacts (defined by Orlowski et al. 1985) recorded by the Hybrid III ATDs during the Malibu II rollover crash tests. ATD-measured spinal neck injury risks are identified using the risk relationships established for head impact velocity, momentum and kinetic energy as well as the risk relationship identified by the National Highway Traffic and Safety Administration’s (NHTSA), i.e. the Neck Injury Criterion (NIC). It is shown that ATDs in the un-reinforced (production) roof vehicles record higher average and peak spinal neck injury risks than ATDs in the reinforced roof vehicles. This trend is observed using all injury risk measures, indicating an increased serious injury risk for contained and seat-belted occupants of low roof strength vehicles.