A microstructural viscoelastic ligament failure model

This dissertation develops a microstructural ligament failure model that is based on the properties of the collagen fascicle. To account for the sequential fascicle uncrimping and failure, fascicle recruitment and failure distribution are included. A series of mechanical tests were performed on collagen fascicles and a finite strain quasi-linear viscoelastic model was used to fit the fascicle experimental data. With the resulting fascicle properties, the ligament microstructural model was developed using existing posterior longitudinal ligament failure test data. The model was then validated using existing failure data from the three spine ligament types (posterior longitudinal ligament, anterior longitudinal ligament, ligamentum flavum) and two knee ligament types (medical collateral ligament, lateral collateral ligament).; There was a significant strain rate effect in fascicle failure strain (p<0.05), but not in failure force or failure stress. The corresponding average fast-rate and slow-rate failure strains were 0.098 +/- 0.062 and 0.209 +/- 0.081. The average failure force for combined fast and slow rates was 2.25 +/- 1.17 N. Based on observation and R2 value, the microstructural model fit the PLL data well (R2 = 0.76 +/- 0.12). In the model validation study, the PLL microstructural model remained within the average ligament force response corridors with exception of the force at approximately 0.39 though 0.50 strain. The ALL and LF models remained within the force corridors with exception of the forces at approximately 0.30 through 0.49 strain for the ALL and 0.40 through 0.68 strain for the LF. The LCL and MCL models respectively underestimated and overestimated the recruitment phase of the ligament force corridors and remained within the force corridors after the peak force. The results of this dissertation support the hypothesis that the force response of different ligament types can be approximated based on the material and failure behavior of collagen fascicles.