2006 Issue

www.utahengineerscouncil.org 22 UEC JOURNAL February 2006 load transfer points to the supporting ve- hicle structures. The latch plate is the com- ponent that inserts into the buckle, routing the strap across the shoulder and lap of the occupant. It is usually constructed of stampedmetal “tongue” plate (the load car- rying frame) that is over-molded with plas- tic (for a smoother interface with the strap). In normal everyday use, any substances that Figure 2 – Visually similar marks on latch plate (a) rough-looking build-up of grime, and (b) melted and displaced plastic from loading. Figure 1 – Visually similar marks on strap (a) scratches in thin film of grime, and (b) clothing smears from occupant loading. Jon E. Bready is a senior mechanical engi- neer with Collision Safety Engineering, LC in Orem, Utah; the company specializes in automobile accident reconstruction and crashworthiness analysis. He has a gradu- ate degree in Mechanical Engineering from BYU. He has performed research in sev- eral areas related to accident reconstruc- tion, and published several peer-reviewed articles with the various organizations in- cluding ASME. He has expertise in the fields of automobile accident reconstruc- tion, seat belt restraint system performance and usage, occupant kinematics analysis, and photogrammetry. Figure 3 – Visually similar marks on D- ring (a) chronic wear groove, and (b) abraded plastic from loading.] collect on the strap from handling and/or from the vehicle environment can transfer to and accumulate on the latch plate slot surfaces (see Figure 2a). Under load in a collision, the strap can adjust between the shoulder and lap lengths resulting in heat and pressure sufficient to melt and displace the plastic on the surface (see Figure 2b). The “D-ring” is the loop that situates the strap above the shoulder, and is attached to the “pillar” of the vehicle. This loop often sustains chronic wear to the degree of wearing grooves in the plastic. Careful ex- amination is sometimes required to discern between this wear and visually similar abra- sions from loading (see Figure 3). Correct evaluation of seat belt usage in a crash will be consistent with the physical evidence and the results from the analysis performed with respect to the other ele- ments of the case analyzed by the expert. ■ Sealt Belt Safety — continued ▲ Roll Over Crashes — continued ▲ maneuver; the corresponding tangential speed is assumed equivalent to the actual maximum vehicle cornering speed. An alternative method of calculating the speed loss during the pre-rollover phase is “conservation of en- ergy” which is a function of the vehicle’s changing “side slip” angle and characteristics of the tire-ground interaction, andcanaccount for the affects of braking or acceleration. The “trip” phase is the process leading up to and the condition in which the vehicle’s angular and vertical momentum becomes sufficient to situate the vehicle’s center of mass over and beyond the tire- ground contact location or “balance point”. Where tire marks or soil furrowing tracks end abruptly is where the vehicle “trips” and becomes airborne as it overturns. Rollover initiation has traditionally been referred to as the “trip” but is somewhat of a misnomer because mechanisms sufficient to cause a vehicle to overturn require high forces sustained for sufficient duration be- fore a vehicle will overturn, as opposed to quick loss of balance such as when a person stumbles by catching his foot. In the rollover phase, all kinetic energy remaining after the trip is gradually dissi- pated through a number of vehicle-ground impacts which damages the vehicle and scene, and leaves behind a “trail” of various vehicle parts and contact patterns along the path of the rolling vehicle. Consequently, there is potentially a vast amount of physi- cal evidence available from the rollover F E A T U R E Seat Belt Safety Where tire marks or soil furrowing tracks end abruptly is where the vehicle “trips” and becomes airborne as it overturns.