Modeling nonlinear time-dependent behaviors of synthetic fiber ropes under cyclic loading

Attributed to the time-dependent property, synthetic fiber ropes employed as mooring lines in the taut leg mooring system may present special mechanical behaviors such as the dynamic stiffness, creep-recovery and stress relaxation, which directly affect the dynamic response and fatigue performance of the mooring system. In the present work, a new stress-strain constitutive model, which can fully take into account the loading history and the time-dependent property of synthetic fiber ropes under cyclic loading, is proposed based on the Schapery׳s theory and Owen׳s rheological theory. The present model is capable of quantifying the change-in-length property of fiber ropes under cyclic loading reported by Flory et al. (2007), and can be incorporated into the commercial software for mooring analysis. Detailed methods for identifying the model parameters are also proposed. In order to examine the accuracy of the present model, the dynamic stiffness and hysteresis loop of aramid and polyester ropes under cyclic loading are simulated by the present model, and are used to compare with the measured data and empirical expression. The good agreement proves that the present model can well simulate nonlinear time-dependent behaviors of synthetic fiber ropes under cyclic loading.