A Methodology for Accelerated Thermo-Mechanical Fatigue Life Evaluation of Advanced Composites

Thermo-mechanical fatigue and natural aging due to environmental conditions are challenging to simulate in an actual test with advanced fiber-reinforced composites, where their fatigue and aging behavior are little understood. Predictive modeling of these processes is challenging. Thermal cyclic tests take a prohibitively long time, although the strain rate effect can be scaled well for accelerating the mechanical stress cycles. Glass fabric composites have important applications in pipes, aircraft, and spacecraft structures, including microwave transparent structures, impact-resistant parts of the wing, fuselage deck and many other load-bearing structures. Often additional additively manufactured features and coatings on glass fabric composites are employed for thermal and anti-corrosion insulations. In this paper, we employ a thermo-mechanical fatigue model based on an accelerated fatigue test and life prediction under hot-to-cold cycles. Thermo-mechanical strain-controlled stress evolution is modeled and tested for fitting fatigue model parameters over thermal cycles under different creep stresses. The model accounts for damage mechanics-based treatment of stiffness degradation up to a limiting inelastic strain up to endurance limit stress, and strength degradation in the process of damage to crack initiation. The strain evolution and stiffness degradation are monitored, and fatigue strength degradation behavior is predicted using the constitutive model. A scheme for remaining user life (RUL) prediction is developed and the scheme is validated using different thermo-mechanical cycles as compared to the data used for fitting the constitutive model parameters. This study limits the fatigue damage to crack initiation in simple flexure and temperature cycles for specific micro-damage coalescence to interlaminar fracture. To generalize the life prediction methodology, a scheme based on finite element stress analysis-based progressive damage methodology is employed, which can be employed for complex composite structures involving different complex damage mechanisms and final failure modes.