## 3 Results  
Figure 3 shows the obtained stress-strain curve for a single molecule. The curve can be divided into three zones describing three different mechanisms. Zone I (form \(\epsilon_{i}=0\) to \(\epsilon_{1}=1.3\%\)): in this phase, the molecule was stretched along its principal axis. The small resulting stress was due to the change of the angle energy. This phase is hardly visible on the stress-strain curve since it needed an extremely slow strain rate to allow for atoms to relax. Zone II (form \(\epsilon_{1}=1.3\%\) to \(\epsilon_{2}=31.8\%\)): in this phase, the molecule was uniformly stretched. The elastic constant was \(\sim\)7.9 GPa and resulted mainly from the bond stretching below the hyperelastic critical distance r\({}_{1}\). Zone III (form \(\epsilon_{2}=31.8\%\) to \(\epsilon_{3}=52\%\)): in this phase, a sharp change in strength occurred as the bond distance reached the hyperelastic critical distance r\({}_{1}\) (elastic constant was \(\sim\)47.1 GPa). The molecule continued to stretch uniformly until reaching the breaking point. The molecule broke when interatomic distances reached the breaking distance \(r_{b}\). Figure 3: Current computed stress–strain variation in the single TC molecule along with [PERSON] et al. [42] model prediction. Figure 4 shows the stress-strain curve for the collagen fibril for different crosslink densities. The overall trend of the curves is consistent with previous work: increasing crosslink densities increased both the ultimate tensile strength and the ultimate tensile strain of the fibril. Similar to the single molecule, the angles on the fibril were first straightened since the angle energy was much smaller than the bond and pairwise energies. Then, with increasing strain, the internal stress started building up in the structure up to a threshold where the pairwise forcefield could no longer resist the shear forces induced between molecules. As a result, the molecules started sliding and stress started to decrease. When crosslinks were present in the fibril, they provided additional resistance to the shearing between molecules and, therefore, retarded the sliding threshold, increasing the ultimate tensile strain and stress. Nonlinear curve fitting of our data collected from molecular dynamics simulation was used to evaluate the material characteristics of the fibril for native and degraded tropocollagen crosslinks. The goodness-of-fit (GOF) value (coefficient of determination, given as mean, standard error (SE)) for the fitted data was 0.91 \(\pm\) 0.04 for the crosslink failures. For all cases, the fitted curves provided satisfactory fits to the MDS data. Table 4 shows the average optimal input fibril parameters for degradation processes. The mechanical properties of the fibrils influenced the aggregate behavior of the ACL at full extension. Tropocollagen crosslinks degradation slightly influenced the elastic stress of the ACL as observed during the axial tension that ranged between 0 and 6%. The effect of this degradation became remarkable when the axial strain varied between 6 and 8% (Figure 5). However, the ACL yield stress was substantially influenced by tropocollagen crosslink failure. The yield stress was approximately reduced by 90% when \(\beta\) varied gradually from 100% to 0 (Figures 5 and 6). However, a minimal variation was observed on the elastoplastic behavior of the ACL when the amount of crosslink failure was more than 80% (\(\beta\) = 20% or less). Under pre-yielding loading conditions (7.5% axial strain), ligaments stresses distributions were nonuniform at all degradation stages (Figure 7). The maximum axial stresses ranged from \(\sim\)10 MPa to \(\sim\)20 MPa. The largest stress values were located at the posterolateral side near the tibial junction, and the smallest values were at the anteromedial area near the femoral junction (Figure 7). Figure 4: Strain–stress curve of the collagen fibril as a function of the amount of crosslink (\(\beta\)) predicted using the coarse-grained model under axial tensile testing.