Modeling creep response for HMPE ropes by a viscoelastic damage model based on fractional derivative theory

High modulus polyethylene (HMPE) ropes are being increasingly applied to the mooring systems of deep and ultra -deep water floating structures. Nevertheless, due to the viscoelasticity of fiber materials, HMPE ropes exhibit creep behaviors and, in some cases, even creep failure, which poses a great concern regarding the reliability of mooring systems. To describe the whole creep process of HMPE ropes, a viscoelastic damage model is proposed based on the fractional derivative theory. By characterizing the HMPE material using a damaged spring and a fractional dashpot in series, the creep equation for HMPE ropes is established and the method for model parameter identification is proposed as well. Experimental creep data of HMPE strands available in the literature are used to validate the present viscoelastic damage creep model. The simulation results are in good agreement with experimental data, confirming that the model can effectively describe the damage -creep coupled behaviors of HMPE ropes at various loading levels. Finally, a sensitivity analysis is conducted regarding the fractional derivative parameters. The developed model is anticipated to act as a reference for studying the long-term reliability of HMPE mooring lines.