INVESTIGATION OF THE FLEXURAL FATIGUE BEHAVIOR OF UNIDIRECTIONAL CARBON FIBER-REINFORCED COMPOSITES

Bending fatigue is a common loading mode in composite structures. Existing research has primarily focused on bending fatigue under positive stress ratios, while the damage-evolution mechanisms under negative stress-ratio bending fatigue remain insufficiently clarified. This study presents a systematic experimental investigation into the bending fatigue mechanical behavior of bogie-grade T700/7901 composites with different fiber orientations. Under fatigue loading with stress ratios R = 0.1 and R = −1, hysteresis loops and dissipated energy density are analyzed to elucidate the governing mechanisms; together with fractographic observations, the primary determinants of fatigue life are quantitatively identified. The results demonstrate that, at a fixed load level, a negative stress ratio shifts the bending fatigue S–N curve of the composite to lower lives, reducing life by 1-2 orders of magnitude. Moreover, a negative stress ratio load promotes extensive damage on both the tensile and compressive surfaces of the laminate, markedly altering the hysteresis loop and increasing the dissipated energy density, thereby accelerating damage evolution. By analyzing the correspondence between dissipated energy density and fatigue life, a clear linear mapping relationship between the two is identified. Under the same stress ratio, the fatigue life of 0° fiber–oriented composites is substantially higher than that of other orientations; the 45° fiber–oriented composites exhibit longer fatigue life than the 30° and 15° orientations. These findings indicate that, in the design of composite structures subjected to bending, particular attention should be paid to bending load cases with negative stress ratios. The present study provides guidance for the bending fatigue design of fiber-reinforced composites.