Abstract
Round bar specimens of short glass fiber reinforced polybuthyleneterephthalate (PBT) were fatigued under axial-load controlled conditions with six different stress ratios including negative ratio. The tensile mean stress decreased the fatigue strength at a constant fatigue life and the modified Goodman relation gave a dangerous estimate for stress ratios larger than 0.7. The relation between the fatigue strength and the mean stress was expressed by Gerber parabolic relation except that the strength at zero mean stress was equal to the creep strength, but not to the tensile strength. A new method for predicting the mean-stress effect on the fatigue strength was proposed by using creep data and the fatigue strength at one stress ratio. Under cyclic loading with high mean stresses, the creep phenomenon became predominant and the ratcheting deformation increased with the number of cycles. The ratcheting deformation became larger for larger mean stresses at the same stress amplitude and for larger stress amplitudes at the same mean stress. The compliance of the specimen increased with the number of cycles. Fatigue cracks started from the center of the specimen and propagated outward. Fractographic observation revealed the feature of fatigue fracture surfaces made under high stress ratios resembled those of creep fracture surfaces, showing ductile extension of plastic matrix. On the other hand, fatigue fracture surfaces under low stress ratios were rather flat accompanied by pull-out of fibers.
