Interface-controlled hygrothermal-fatigue degradation of hybrid FRP prestressing strands: Experiments, molecular dynamics, and Bayesian factor inference

Fiber reinforced polymer (FRP) cables are promising alternatives to steel bridge cables. Hybrid FRP offers a balance of performance and cost, while its performance evolution under coupled fatigue and environmental conditions remains insufficiently understood. In this study, six types of strands were fabricated, i.e., T700 Carbon FRP (CFRP), T300 CFRP, three uniformly hybridized carbon-glass composites (24 K 1:1; 12 K 1:1; 12 K 2:1), and Glass FRP (GFRP). The experimental program consisted of static tensile and transverse shear tests, followed by tensile fatigue tests and post-fatigue residual mechanical evaluation. The fatigue tests were conducted at 70% of the ultimate tensile strength under three environmental conditions: room temperature and humidity, high temperature and room humidity (60 °C/30% RH), and high temperature and high humidity (60 °C/98% RH). Static results showed that the 12 K 2:1 hybrid achieved 77.38% of the tensile strength of T300 CFRP (at only 68.7% of its cost), indicating superior cost-effectiveness. Under room-temperature and hot-dry fatigue conditions, the FRP strands generally maintained high residual tensile and shear strength. Under the hot-humid condition (60 °C/98% RH), degradation became more pronounced and was strongly dependent on strand configuration. The 12 K 1:1 UH and 12 K 2:1 UH strands retained 84.67% and 89.82% of their tensile strength, respectively, whereas the 24 K UH strand failed before completing the target fatigue cycles. The residual shear strength showed greater environmental sensitivity than the residual tensile strength, suggesting that interfacial degradation played a dominant role in the hygrothermal-fatigue damage process. MD simulations further revealed that hygrothermal exposure promotes water migration, which enlarges free volume and accelerates interfacial sliding, underscoring the importance of an interface-oriented hybrid design for FRP cables.