Modelling creep tests in HMPE fibres used in ultra-deep-sea mooring ropes

Due to its low density and high strength, HMPE (high modulus polyethylene) fibres are being increasingly used in synthetic ropes for offshore mooring. Nevertheless, the occurrence of creep at sea temperature can be a shortcoming for its practical use. Creep tests performed at different load levels in a sub-system of the HMPE rope (yarn) are frequently used as a first step to obtain some information about the susceptibility to creep deformation at a given temperature. The present paper is concerned with the phenomenological modelling of creep tests in HMPE yarns. In this macroscopic approach, besides the classical variables (stress, total strain), an additional scalar variable related with the damage induced by creep process is introduced. An evolution law is proposed for this damage variable. The predicted lifetimes and elongations of HMPE specimens in creep tests at different load levels and room temperature are compared with experimental results showing a good agreement.