Fatigue Performance and Novel Life Predictive Modeling of Thermoplastic <scp>GFRPMMA</scp> Bars

ABSTRACT The excellent corrosion resistance of FRP bars makes them a promising alternative to steel reinforcement in coastal and bridge concrete structures. While thermosetting GFRP bars are widely used, research on thermoplastic GFRP bars, particularly their fatigue performance, remains limited. This study conducted static mechanical tests and 27 tension–tension fatigue experiments on glass fiber reinforced polymethyl methacrylate (GFRPMMA) bars. The static properties were quantified as ultimate tensile strength (UTS) = 1155.8 MPa, interlayer shear strength (ISS) = 18.5 MPa, and tensile modulus E = 65.8 GPa. Fatigue tests systematically explored the effects of upper limit stress, stress range, mean stress, and stress ratio on fatigue behavior. Results showed that the upper limit stress was the dominant factor, with higher stress levels markedly reducing fatigue life. Fatigue failure involved fiber fracture, matrix cracking, and fiber–matrix interfacial debonding, progressing from the bar surface inward. Normalized stiffness degradation and energy dissipation were strongly correlated, with a critical stiffness of 92.1% corresponding to a critical lifespan of 92.6%, suggesting that real‐time energy monitoring can provide quantitative indicators of fatigue damage. A Δ–S–N predictive model, incorporating upper limit stress and stress range, was developed. The model achieved high accuracy, with a relative error &lt; 10% and coefficient of variation of 4% in logarithmic space, providing a quantitative framework for predicting fatigue life and guiding the design of thermoplastic GFRPMMA bars in engineering applications.