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UBC Theses and Dissertations

An investigation into low speed rear impacts of automobiles Thomson, Robert William


A substantial number of whiplash injuries are reported for motor vehicle accidents which produce little or no structural damage to the automobile. These injuries are predominantly associated with rear-end type accidents affecting passengers of the struck vehicle. Since passengers of the striking vehicles are not reporting as many injuries for the same accidents, occupant and vehicle dynamics experienced during low speed-rear impacts were proposed to be a major source of the whiplash claims. A review of previous research revealed that little information exists for this type of accident. In general, vehicle safety research and government regulations have been directed towards occupant mortality - not injury - in frontal collisions. Occupant dynamics research has been limited to sled testing, using modified seat structures, or out-of-date vehicle models. Full scale, rear impact, crash testing has concentrated on high impact speeds (above 30 km/h) where significant structural deformation occurs. A research program was designed to investigate the occupant and vehicle dynamics during low speed - rear impacts. Experimental research was undertaken to document the structural performance of vehicles, noting the impact speeds necessary to initiate the crush mechanisms in the rear portion of the vehicle. To facilitate this testing, a pendulum impactor, based on the government test procedures, was designed and built to consistently reproduce impact speeds below 20 km/h. A total of 56 rear impact tests were conducted with 1977-1982 Volkswagen Rabbits. The vehicle wheels were locked to represent a vehicle stopped in traffic - the most commonly reported whiplash producing accident. An anthropometric test dummy was used to represent a front seat passenger during the tests. High speed video recordings of the tests were digitized to provide kinematic information on the occupant and vehicle response. Accelerometers were incorporated into the last 24 tests to monitor the acceleration levels at the bumper mount, seat mount and within the dummy. Information obtained from this testing suggested that permanent structural damage was only visible when an impact speed between 14 and 15 km/h was experienced by the vehicle. Very little frame deformation occurs for impact speeds below this value. Below this threshold, the vehicle frame can be considered rigid; vehicle response being dominated by the compliance of the bumper and suspension systems as well as sliding of the locked wheels. The accompanying occupant response was a differential rebound of the head and shoulders off the seatback and head restraint. This relative motion between the head and torso was evident in each test and increases the potential for injury. Typical occupant response observed consisted of an initial loading and deflection of the seatback due to the occupant's inertia followed by the release of this stored spring energy as the occupant was catapulted forward. It is this elastic behaviour of the seatback which is the likely cause of whiplash injury. Resulting head velocities were found to be in the order of 1.5 - 2 times the resulting vehicle speed. Initial occupant postures which increased the distance between the torso and seatback tended to increase the dynamic loading experienced by the passenger. Analytical modelling of the vehicle was initiated as the groundwork for full occupant-vehicle simulation. A finite element model of the vehicle frame, bumper, and suspension was created. Previously obtained empirical information suggested that a non-linear bumper and suspension system connected to a rigid frame would be an acceptable approximation. A parametric analysis of bumper stiffness and braking conditions was conducted in a 30 simulation matrix. General kinematic trends of the tests were observed in the simulations, however, limitations in the material properties introduced a much stiffer response than that experimentally observed. Results from this study show that little protection is offered to an occupant during a rear end collision. Impact energy management within the vehicle may not be adequate to prevent injury. Improved occupant protection requires the highly elastic behaviour of the vehicle frame and seatback to be attenuated. This will eliminate the amplification of vehicle motion through the seatback to the occupant.

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