We investigate the molecular features of high-performance gel-spun ultrahigh-molecular-weight polyethylene UHMWPE fibers (SK75, invented and manufactured by DSM) and propose a multiscale structural model that describes the organization of molecules from the unit cell to the filament level, based on X-ray diffraction in static and dynamic conditions (during tensile testing). The model emphasizes the discontinuous nature of the crystalline phase, which is embedded in a percolating amorphous phase and connected by tie molecules running through the amorphous phase. The tie molecules play a critical role in the tensile properties (e.g., Young’s modulus and sonic modulus) of the material. We analyze the micromechanics of the material during tensile deformation and show that, in the elastic regime, the stress-transfer mechanisms (e.g., tie molecules) are so efficient to realize a homogeneous stress distribution through the various length scales (from filament level to unit cell level). Plastic deformation of filaments begins with shear break-up of crystals that triggers or is triggered by an unusual, not well explored, deformation mode of the orthorhombic unit cell (contraction of the a-axis with simultaneous expansion of the b-axis). We also show that the morphological model with discontinuous crystalline phase provides a logical base for the interpretation of the sonic modulus of UHMWPE fibers. Realignment of molecules in the noncrystalline regions of the material can explain the remarkable increase of the sonic modulus measured during tensile tests.

For a number of years, the creep performance of standard High Modulus Polyethylene (HMPE) fiber types has limited their use in synthetic offshore mooring systems. In 2003, a low creep HMPE fiber was introduced and qualified for semi-permanent MODU moorings. This paper reports on the introduction of a new High Modulus Polyethylene fiber type with significantly improved creep properties compared to other HMPE fiber types, which, for the first time, allows its use in permanent offshore mooring systems, for example for deep water FPSO moorings. Industry guidelines and standards mentioning HMPE creep are briefly discussed, and results on fiber and rope creep experiments reported. Laboratory testing has shown that ropes made with the new fiber type retain the properties characteristic of HMPE such as high static strength and stiffness and yarn-on-yarn abrasion resistance.

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In order to predict the lifetime of fibre rope mooring lines it is essential to be able to predict their behaviour under tension fatigue. Creep failure is known to be a major contributor to fatigue in synthetic fibres and models to predict creep failure are well-established. We show that expansion of such models to varying loading conditions allows the prediction of the fatigue performance. However, it is difficult to design tests to quantify the fatigue performance for HMPE ropes since often premature failure occurs due to external abrasion and viscous heating due to too high testing frequencies or amplitudes. This paper presents a testing methodology which allows tensile fatigue lifetime to be evaluated by testing at higher temperature to avoid premature abrasion failure. We also show that when the temperature evolution due to viscous heating is properly accounted for the modelling framework presented can be effectively used to describe the premature failure occurring due to this heating effect. Results from tests on yarns and small ropes are presented, and a predictive model for rope fatigue lifetime has been validated.

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