Fatigue Life Evaluation of a Steel–Prestressed Concrete Hybrid Tower Under Prestress Relaxation Using a Bidirectionally Coupled Damage Model and Neural Network Surrogate

This study investigates the long-term fatigue performance of an in-service 4.55 MW steel–prestressed concrete hybrid wind turbine tower under different tendon relaxation scenarios. The fatigue lives of prestressing tendons and concrete are calculated from finite element stress responses obtained in ABAQUS. A bidirectionally coupled damage model then updates the damage and stiffness of the two materials to determine the overall coupled fatigue life. Based on these results, a Spatial-Topology and Physics-Informed Cascade Network (STPI-Net) is developed for life prediction. The results show that fatigue life reductions vary across tendon relaxation cases and generally increase with prestress loss. Windward-side and simultaneous multi-tendon relaxation cause larger reductions, and damage coupling further reduces overall fatigue life in all studied cases. At 80% prestress loss, the overall coupled fatigue life reaches 0.45 years in the severe three-tendon relaxation case, representing a 28.6% reduction relative to uncoupled life. Scenario-based five-fold cross-validation is further used to evaluate the predictive performance of STPI-Net. The R2 values are 0.8879 for logarithmic tendon fatigue life, 0.9030 for logarithmic concrete fatigue life, and 0.8195 for overall coupled fatigue life. The proposed framework supports the assessment of component and overall fatigue performance under different prestress relaxation conditions.