Development of a Mechanical Model for Rear Impacts: Evaluation of Volunteer Responses and Validation of the Model

Doctoral thesis, 2000

The objective of this research has been to develop a biofidelic crash-test dummy to facilitate design of seat systems with enhanced occupant protection in rear-end collisions. These frequent collisions may cause soft-tissue neck injuries to occupants in struck vehicles. These injuries, although mainly classified as minor, often lead to long-lasting complaints and large societal costs. There is currently no biofidelic mechanical dummy available for dynamic seat tests. Validation data for such a dummy are at present inadequate. Further, the knowledge of the injury mechanisms causing these short- and long-term complaints is incomplete and consequently injury criteria are sparse. Two aspects of the rear-impact injury enigma are studied in this thesis: volunteer kinematics, and development and validation of a dummy. In the volunteer studies, two series of rear-impact tests were performed to enable biofidelity assessment of models. Human kinematic responses in flexible laboratory seats with head restraints and in a rigid seat without head restraint were determined. High-speed video recorded the motions; accelerometers measured head and T1-vertebra accelerations. In the second series neck loads as well as the kinematics were estimated. It was found that, the torsos ramped up along the inclined seat backs during the impacts. The T1 vertical displacements were approximately twice that of the pelvis due to straightening of the thoracic kyphosises in interaction with the seatback. In the first series the neck underwent flexion motion in several of the volunteers during the first 100 ms, followed by an extension motion. In the second series, all volunteers' upper-neck regions underwent flexion motion, then most of the lower-neck regions underwent extension motion simultaneously as their upper-neck region remained flexed. Most of the volunteers lower- and upper-neck regions underwent extension motion. As a basis for the development of a new dummy, bio-mechanical guidelines for rear-impact dummies were established by reviewing the literature. The first production version of the new rear impact dummy, the BioRID I, is based on the Hybrid III but it had a new segmented spine that was fitted with neck-muscle substitutes and a new soft torso. The kinematics of the BioRID I appeared to be more humanlike in rear impacts compared to the Hybrid III. A new prototype, the BioRID P3, was then developed with improved neck-muscle substitutes and a softer thoracic spine and torso. The kinematic response of this prototype is close to those of the volunteers. The dummy also provides repeatable test results and the design appears to be reproducible. This prototype has been further developed and made into a production version, the BioRID II. Seat-system tests with biofidelic dummies, such as the BioRID II, in combination with appropriate injury criteria, will possibly facilitate the design of new seat-system designs with enhanced occupant protection in rear-end collisions.