An investigation of inertial unlatching of side-release seat belt buckles using computational modelling

This paper reports on the modelling of an RCF-67 (invented by Robert C. Fisher [RCF]) buckle and will demonstrate the modal analysis of the spring. A computational model of the side-release RCF-67 buckle is developed using the MADYMO code. This accurate model is used to demonstrate the inadequacy of the computational or analytical model to demonstrate unlatching due to a short-duration pulse, unless the modal properties of the spring are considered. This study clearly indicates that the first resonant mode shape at 2000 Hz shall facilitate the unlatching mechanism. The auto spectrum of the input transient excitation with pulse duration less than 1 ms is capable of exciting a broad range of frequency, including 2000 Hz. The operating deflection shape (ODS) is large when the input excitation puts energy near the resonant frequency, and the ODS is dominated by the mode shape at that resonant frequency. The modal analysis provides an insight regarding the design changes, as required, making this buckle inertial unlatching proof. This then will allow the development of a spring that will not allow the buckle to release under short-duration, high-magnitude dynamic loads.