During a frontal impact, a normal three-point seat belt allows forward movements, with possible violent contacts with steering wheel and dashboard; this situation can be improved by pretensioning of the belt. Pretensioners are projected to actively withdraw a portion of the belt (about 10-15 cm) at the very beginning of impact, in order to keep the car occupant adherent to his seat. This experimental study was performed on a mechanical pretensioner, on a pyrotechnical pretensioner, on a web-clamping system, and on normal three-point seat belts with and without slack at the moment of impact. Each device has been tested by simulation in a frontal impact at 30 mph with an Hybrid dummy. The following data were looked for: Head Injury Criterion (HIC and HIC36); Head velocity during the restraint action; Head, chest, and pelvis forward displacement; Chest acceleration (greater than 3 msec); and belt loading at the shoulder. The laboratory results demonstrate a good performance for pretensioners; the reduction in HIC and HIC36 is significant; especially when poor wearing conditions are present (slack), when these values are compared with those obtained using three point belts. The dynamic differences of the pyrotechnical and the mechanical pretensioners (mainly in the direction of the retraction force) affect the results, with different tendencies in chest displacement and head criteria. The reduction in pelvis displacement is particularly important in order to prevent submarining and its related abdominal lesions. These characteristics may provide, if correctly linked to the specific deceleration pattern of each car model, better protection to passengers.
Samenvatting