Experimental and numerical analysis of seat belt bunching phenomenon

In current cars, loops are commonly used to redirect the webbing which reels out from the retractor to the passengerʹs shoulder. Some types of pillar loops, also called D-rings, lead to a non-systematic instability. The webbing, which should scroll without hindrance through the D-ring, laterally shifts, bunches and produces the overturning of the ring. In this paper, this so-called seat belt bunching phenomenon is parsed during a first step with sled test campaigns data. The results of designs of experiments are analysed and discussed. To expertize this instability issue, an innovative fixture is exploited during a second step to reproduce the phenomenon in a fully controlled manner for dynamic and quasi-static loadings. To assess these sub-system tests, a Digital Images Correlation system is employed to evaluate the strain distribution of seat belt webbing during the bunching phase. Based on these local measurements, a correlation of a Finite Element model of seat belt bunching is achieved using a new shell element for webbing fabric, before proposing an explanation of the phenomenon.