Automotive Textiles
Force is energy divided by stretch. Every millimetre of elongation the webbing does not give is force the occupant takes instead.
— N
Energy divided by the stretch distance available
A constant-force idealisation: real webbing has a rising load-extension curve and a pretensioner and load limiter reshape it deliberately, which is exactly how the force is held below both the webbing and the occupant limits. Load utilisation above 100% means this configuration cannot restrain the impact. Occupant protection is regulated and must be validated by sled and crash testing — this is not a substantiation tool.
Seatbelt Webbing Energy Dissipation & Elongation Modeler — free, with the formula and a worked example, at Textile School.