Effects of resin matrix and specimen diameter on the long-term properties of carbon fiber reinforced polymer bars used for mooring lines under marine environment

Carbon fiber reinforced polymer bars based mooring lines are under consideration for fixing marine floating structures in far-reaching areas but their long-term durability in seawater is strongly controlled by the resin matrix and bar geometry. This study evaluated pultruded ultra-performance carbon fiber reinforced polymer bars manufactured with two bisphenol-A epoxy matrices and different diameters under 20-80 °C artificial seawater. Water absorption/desorption, mechanical properties, thermomechanical properties, and microstructural evolution were characterized by short-beam shear and flexural tests, nanoindentation, dynamic mechanical analysis, thermogravimetric and differential scanning calorimetry, scanning electron microscopy, and Fourier-transform infrared spectroscopy. All bars exhibited Fickian moisture transport, with coefficients of determination above 0.986. Temperature was the primary accelerating factor. For Heerzi resin-based 7 mm bars, the diffusion coefficient increased by approximately 26.6 times and the equilibrium moisture content increased by approximately 3.3 times when temperature rose from 20 to 80 °C. Dow resin-based bars showed lower equilibrium water uptake and weaker moisture retention than Heerzi resin-based bars, while larger-diameter bars exhibited higher apparent diffusion coefficients but lower equilibrium moisture contents. Seawater aging mainly reduced short-beam shear strength, flexural properties, and glass transition temperature through matrix plasticization, resin hydrolysis, and fiber-matrix interfacial debonding, whereas carbon fibers remained comparatively stable. The eXtreme Gradient Boosting algorithm gave the best prediction of absorption and desorption, with coefficients of determination above 0.99 in both training and testing datasets. These findings provide a mechanistic and predictive basis for matrix selection, size optimization, and service-life evaluation of carbon fiber reinforced polymer mooring-line components in marine environments.