A Multiscale Analytical Model for Tensile Fatigue of Reinforced Thermoplastic Pipes Based on Continuum Damage Mechanics

To accurately predict the tensile fatigue life of reinforced thermoplastic pipes (RTPs) and clarify the mechanism of fatigue damage evolution, this research proposes a multiscale analytical model based on continuum damage mechanics (CDM). Built upon the three‐dimensional constitutive relations of RTPs and the load–deformation compatibility conditions, the model solves the stress–strain responses of individual plies at the macroscopic level and achieves cross‐scale stress decomposition from plies to fiber/matrix phases via a bridging model. Within a cycle increment framework, the CDM evolution equations are used to iteratively update stresses and damage variables, enabling prediction of residual stiffness evolution and fatigue life. Damage parameters are calibrated using composite fatigue experiments, and the predicted fatigue life and key responses are validated against experiments and finite element simulations. The results demonstrate that the proposed model can predict the tensile fatigue life of RTPs under different load ratios with good accuracy and further reveal stress redistribution and damage evolution laws across different scales, providing an efficient multiscale analysis framework for interpreting RTP fatigue mechanisms and conducting structural assessment.