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This paper reports the bending fatigue behavior of three-dimensional five-directional braided T-shaped composite from finite element analyses and experimental characterizations. The braided composite microstructure was divided into five types of unit cell models, that is, interior cell, surface cell, corner cell, interior cell in joint region, and corner cell in joint region. A user-defined material subroutine was developed to characterize the unit cells properties, damage accumulation, and failure criterion of the T-beam under different stress levels. The stiffness degradation curves and bending displacement curves were obtained from the finite element analysis to show the three stages of fatigue developments, that is, matrix cracks, interface debonding, and fiber breaking. The stress and strain concentration areas were found in the middle of the flange and the web of the T-beam composites. The high strength reinforced fibers are recommended to add in the middle of the flange and the web for improving the bending fatigue resistance. And also, we hope the mixed unit-cell model could be extended to the other braided composite structures under quasi-static or cyclic loadings.

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Fiber-reinforced polymer matrix composites during engineering applications can be influenced by critical environmental conditions (e.g., water and temperature), probably causing a negative impact on the mechanical performance. In this work, the hygrothermal aging behavior and flexural property of carbon fiber (CF)-reinforced polyethylene terephthalate glycol (PETG) composites were investigated at different aging temperatures and days. The CF/PETG composites were fabricated using the hot-press molding method. The mathematical fitting model and finite element analysis method were employed to reveal the hygrothermal aging behavior and the moisture absorption evolution. After 90 days of aging, the water uptakes of the specimens at 25°C, 45°C, and 60°C are 0.55%, 0.62%, and 1.67% for PETG and 0.47%, 0.54%, and 1.52% for CF/PETG, respectively. The flexural strengths of the CF/PETG composites at 25°C, 45°C, and 60°C after 90 days of aging decrease by 28.6%, 30.2%, and 32.2% as compared with the non-aging composites, respectively. The water uptakes of the CF/PETG composites tend to increase with the increase in cyclic aging time interval. The flexural strengths of the composites with the cyclic aging intervals of 2, 5, 10, and 15 days first decrease and then increase slightly. The knowledge gained in this study will benefit future design optimization of the CF/PETG composite material and lay the foundation for its engineering application.