Seatbelt constitutive model, invoked by using *MAT_SEATBELT, in LS-DYNA provides features to model the loading and unloading characteristics from a uni-axial test. Parameter LLCID provides ability to model the loading curve which allows the definition of force as a function of engineering strain. Parameter ULCID, provides ability to model the unloading curve again allowing the definition of force as a function engineering strain. Inherently, LS-DYNA models the seatbelt as an elastic-plastic where the first non-zero point on the loading curve, LLCID, is taken as the yield force/stress. Unloading always following the loading curve when the force level is below the first non-zero value. Unloading int the post-yield region is done by first shifting the unloading curve, ULCID, along the x-axis (striain) until it intersects the current yield point on the loading curve before unloading along the unloading curve. This is shown in the following figure.
In certain cases when the unloading is extremely nonlinear, then the shifting of the unloading curve could yield conditions where the unloading curve could intersect the loading curve at more that one location. In such cases the unloading occurs as shown below.