Bending Fatigue Behavior Of Four-step 3-D Braided Rectangular Composites Under Various Stress Levels

In 3-D braided composite, a continuous fiber cross three directions (length, width, height) of material, forming 3-D whole net structure which is then impregnated with resin. The composites have strong stiffness and strength along thickness direction, high in-plane shear and interlaminar shear strength, hard hierarchy, and high impact toughness and damage tolerance level.This paper presents the bending fatigue behavior of four-step 3-D rectangular braided composite at different stress levels. The three-point bending fatigue was tested with MTS 810.23 materials tester at the different stress levels. All the tests were run at a stress ratio of 0.1 (R=0.1). The S-N curve and whole stress vs. deflection history were recorded to find the bending fatigue behaviors of the braided composite. Specifically, the area of the stress hysteresis loop in each cycle and the accumulated area were calculated from the whole stress vs. deflection history curves and compared at different stress levels. The energy absorptions during the bending fatigue were compared to analyze the energy dissipation mechanisms during fatigue loading process. The degradation of the stiffness was also calculated to show the fatigue damage. In addition, the damage morphologies of the 3-D braided composite specimens were photographed to compare the failure modes under different stress levels.