Nonlinear Viscoelastic Formulation of Spinal Ligament

Soft tissues exhibit viscoelastic behavior, which includes time-dependent creep and stress relaxation, and hysteresis in a loading cycle. Changes in the viscoelastic properties of soft tissues such as spinal ligaments under dynamic loading can cause the damages. In this study viscoelastic behavior of spinal ligaments is investigated by considering two different quasi-linear viscoelastic models under dynamic loading for creep and stress relaxation. After developing equations, the results of formulation were compared with the results of experimental data in the literature and finally, the viscoelastic model that had more accurate behavior to the results of experiments, was choose as the appropriate model of spinal ligament. For this purpose, obtained data by Hingoryani in an experimental study (related to creep and relaxation tests on rabbit medial ligament) were plotted in a log-log graph. According to the graphs, it was found that the strain rate decreased with higher levels of stress and relaxation rate decreased with higher levels of strain. According to the results, present formulation and the obtained constants of the equations had acceptable accordance with the experimental results, and therefore these equations can be used for spinal ligaments with acceptable accuracy.