Comparison of Dynamic and Static Web Loop Deployment Forces

Over the past 40 years since seat belts were first required in the front seats of passenger cars in the United States, numerous features have been incorporated in these systems. One such feature is the web loop. Vehicles manufacturers have used web loops in numerous vehicles over the years and have indicated that they modify kinematic and/or absorb energy. A web loop consists of a length of seat belt webbing held in place with a pattern of stitches designed to tear out, or rip, at a predetermined load. This concept is similar to the fall protection lanyards or load limiting parachute risers, both of which are designed to reduce shock loading on the user. While generally these systems are tested using a slow application of force, in a crash, the loads are applied more rapidly. This has presented a question as to how the deployment force varies when the stitching is loaded more rapidly. This paper will present the results of a study in which web loop equipped seat belt systems from various manufacturers were tested both quasi-statically and dynamically to compare the levels at which web loops deploy. Additionally, the results of these tests are compared to the belt loads measured during full vehicle crash testing. The results indicate good correlation between the quasi-static, dynamic, and crash test deployment loads for web loops.