Acceleration Amplification in Safety Belt Buckle Systems

Multi-planar rollover accidents report belt usage rates that are noted to be significantly lower than that of planar crash modes.¹ One explanation for this dramatic difference in reported belt usage in rollovers as compared to planar crashes is that highway patrol officers often conclude that an ejection would not have occurred had the occupant been belted. Restrained occupants can be ejected from the vehicle if the seat belts fail to restrain occupants as a result of belt spool out or buckle unlatching.^2,3 Previous studies have documented the susceptibility of certain safety belt buckles to inertially unlatch.^4,5,6,7 Under sufficient vertical accelerations, top button safety belt buckles have inertially unlatched in testing which would obviously negate the safety belts ability to restrain the occupant. This study expands on the mechanisms that could lead to inertial release under vertical loading in safety belt buckle systems with stiff attachments to the vehicle floor. An audio transducer was attached to a vehicle floorpan to induce accelerations at variable frequency and amplitude. Accelerations were recorded at the floorpan and safety belt buckle body simultaneously. Accelerations measured at the buckle body were up to 13 times greater than the accelerations measured at the floor pan for frequencies up to 8.5 kHz. The present study provides a first step to better understand the injury biomechanics by quantifying the accelerations at the floor pan and safety belt buckle.