Influences of stress waveform and wet environment on fatigue fracture behavior of aramid single fiber

An investigation has been carried out concerning the influences of stress waveform and wet environment on the fatigue fracture behavior of aramid single fiber, Kevlar 49. Tension–tension fatigue tests were conducted under sinusoidal, negative pulse and positive pulse waveforms for the fiber preconditioned in laboratory air and in wet air. The fatigue strength of Kevlar 49 tested in wet air was lower than that in laboratory air, and the fatigue strength in laboratory air under negative pulse waveform was higher than those under other two waveforms. The fibers broke with fiber splitting for the tensile tests and the fatigue tests under sinusoidal and positive pulse waveforms in air. Under negative pulse waveform, however, the fibers whose fatigue lives were longer than 105 stress cycles broke with transverse crack. The final fracture morphology under negative pulse waveform corresponded to the transverse undulations observed by an atomic force microscope (AFM).