Fig. 3.
Constitutive model for a single collagen fiber. λ represents the fiber stretch while σ is the fiber stress. The fiber does not carry any stress until the recruitment stretch λr is reached. Once stretched beyond this point, the fiber behaves as a linear elastic rod with an elastic modulus of Ecol. At a stretch of λp, the fiber stress-stretch curve exhibits a peak which is equivalent to Scol, the fiber strength. Fiber stress decreases monotonically in a linear fashion after this peak to a stress of zero at a stretch of λmax = λp + 0.1. Free parameters of the fiber constitutive model are λr, Ecol, and Scol. λp can be computed from these parameters as λp = λr + Scol/Ecol. A representation of the collagen fiber in each stretch region is shown in red. Arrows denote the loading-unloading paths.