Abstract
In this paper, tensile and fatigue strength of high-modulus type (258GPa modulus) poly-p-phenylene benzobisoxazole (PBO) fiber have been investigated. The tensile tests of a monofilament were carried out with a gauge length of 12.5mm. The fatigue tests of a monofilament were carried out to determine the S-N property at a frequency of 10Hz with three stress ratios of 0.1, 0.5 and 0.7. It was found that the fatigue lives of PBO fiber showed a large scatter. The relation between the stress amplitude and fatigue life depended on the stress ratio. However, the relation between the maximum stress ratio and fatigue life was independent of the stress ratio, the effect of mean stress on the fatigue life was relatively large. Therefore, it was found that the maximum stress was useful to describe the fatigue lives at the different stress ratios. In addition, it is found by SEM observation that there are distinct differences between the surface image of tensile test specimen and that of fatigue test specimen, i.e., the crack length in the fatigue test specimens are much longer than that in the tensile test specimens.
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