Fatigue behavior and failure of three-dimensional six-directional braided composites under tension

Carbon/epoxy-three-dimensional (3D) six-directional braided composites are fabricated and tensile fatigue behavior are reported. The results show that the composites have good fatigue properties and limit with small braiding angle. Fatigue life decreases with increasing the stress level. Dynamic modulus decreases and rate of stiffness degradation is also restricted by cycle number. Moreover, there are two regions on the hysteretic curve where the failure strain suddenly increases. Fracture morphology indicates that fatigue cracks spread from the outside to inside of the material and enter fiber/matrix interface, then to the fibers. The damage degree becomes more serious with increasing the stress level. At low stress levels, composite shows small damage and flush fracture. At high stress levels, it exhibits shear fracture along the braiding angle.