Dynamic stiffness evolution of synthetic yarns for offshore mooring under progressive cyclic loading

The dynamic mechanical response of synthetic fibers used in offshore mooring is strongly influenced by cyclic loading and load history. This study investigates stiffness and energy-dissipation evolution in multifilaments of high-modulus polyethylene (HMPE), polyamide (PA), and polyester (PET) through dynamic tests with progressive loading until rupture. The experimental protocol was based on ISO-18692 dynamic stiffness stage, consisting of successive blocks of 100 sinusoidal cycles at 0 . 1 Hz , with constant load variation of Δ F = 10 % of Yarn Break Load (YBL). The response was evaluated using cycle-by-cycle normalized stiffness, normalized secant stiffness referred to the origin, dissipated energy, and block-based stiffness formulations. For normalized stiffness, PA and PET exhibited progressive stiffening with cycling, whereas HMPE showed two-stage behavior with initial stiffening followed by reduction at high-load levels. Secant stiffness revealed decreasing behavior for HMPE, increasing behavior for PA, and an intermediate response for PET, highlighting the influence of accumulated deformation on global mechanical response. Dissipated energy further differentiated the materials, with increasing dissipation for HMPE, decreasing dissipation for PA, and a low-value and stable response for PET. The results demonstrate that stiffness interpretation depends on adopted formulation and analysis scale, providing insights into the dynamic behavior of synthetic yarns for offshore mooring applications.