Car-to-Car Side Impacts: Occupant Injuries, Test Methods, and the Development and Evaluation of Protective Systems
Doctoral thesis, 1994
The present work addresses the problem of injuries following car-to-car side collisions. The injuries in this type of impact have been evaluated to form the basis for the development of test methods and for the principles of protective systems.
Side impacts give rise to relatively more severe injuries than frontal impacts. The study has shown that the neck and the legs of the struck side occupant need to be better protected due to a high risk of permanent disabilities. The head, chest and abdomen/pelvis need to be better protected from a risk of life-threatening injuries. It has also been found that chest injuries increase significantly with age.
Improved protection in side impacts can be achieved with car body/door reinforcements and compliant bolstering (padding and/or airbag) on the inside of the door. In order to develop and evaluate the principles of such protective systems a subsystem sled test method that simulates full-scale test conditions has been developed. The BioSid dummy was used. The use of a chest airbag (8 litre and unventilated) resulted in significantly lower loadings to the chest compared to (50 mm thick) compliant chest padding of polyethylene foam type, and significantly lower loadings to the head, neck and abdomen than a stiff reference door. Pelvis padding (75 mm thick) of the same material effectively reduced the pelvic loads. Both the acceleration based chest injury criterion TTI and the deformation based criterion VC were considerably reduced compared to a stiff door. The head ejection through the side window frame was reduced by an average of 30 mm.
To further enhance the protection level, the airbag was increased to 12 litres in volume to cover both the chest and the abdomen down to the door armrest level. The effect of bag pressure and bag ventilation was investigated at two side impact severities. The ventilation of the bag reduced the maximum chest deflection by 30% and the maximum chest viscous criterion, VC, by 50% (comparison was made with the same bag without ventilation). A suitable initial bag (over) pressure was found to be about 40 kPa, when the loading of the abdomen was also taken into consideration. The deflection based criteria were found to be very sensitive to the door velocity. The stiffness of the bag corresponds to the tolerance level of the thorax of elderly people. The 12 litre ventilated side airbag resulted in 30-40% lower chest deflection and about 60% lower VC than 50 mm of another compliant chest padding material (Ethafoam 220 of polyethylene foam). TTI was 30-45% lower. The findings from the evaluation of the airbag (size, initial pressure, ventilation) are also applicable to seat mounted side airbags of the type Autoliv has developed together with Volvo. To further improve the protection of the head/neck in side impacts a supplementary protective device "the curtain" at the side window has been proposed.
In an investigation carried out together with the Birmingham University it was found that the optimum location for a side airbag sensor is the rear lower quadrant of the front door. A sensor located in this sector would cover almost 90% of the impacts with risk of AIS 3+ injuries. A pendulum subsystem test method for the evaluation of sensors for side airbags has been developed and validated against low speed full-scale tests. The method has been used for evaluation of a pyrotechnical sensor. The sensor fires at impact speeds above 15-20 km/h, when there is a risk of life threatening (AIS 3+) injuries to struck side occupants and not at low impact speeds (7 km/h), when the risk of injury is minimal.
The principle finding in the study was that protective systems in side impacts must be softer than the corresponding human body segments. This was best accomplished with a ventilated side airbag for the chest and the abdomen (down to the armrest level) and padding for the pelvis and the leg. The proposed "curtain" will further enhance the head/neck protection.