Effects of stress waveform and water absorption on the fatigue strength of angle-ply aramid fiber/epoxy composites

The influences of stress waveform and water absorption on the tension–tension fatigue fracture behavior were investigated in ±45° angle-ply laminates of aramid fiber reinforced epoxy matrix composite. For dry specimens, the fatigue strength under negative pulse waveform was higher than that under the positive pulse waveform. Rotation of fibers to the longitudinal direction, which resulted from creep deformation caused by the cyclic loading superimposed on the maximum stress hold time, decreased the compliance, thereby increasing the fatigue life under the negative pulse waveform. Water absorption degraded the fiber/matrix interfacial strength and caused the swelling of the matrix, which resulted in decreases in the static tensile strength and fatigue strength. Although the strength under the negative pulse waveform was slightly higher than that under the positive one, the influence of stress waveform on fatigue strength was smaller in wet specimens.