Tensile and Fatigue Strength of High-Modulus Type Poly-p-Phenylene Benzobisoxazole (PBO) Fiber

Abstract

In this paper, tensile and fatigue strength of high-modulus (HM) type (258GPa modulus) poly-p-phenylene benzobisoxazole (PBO) fiber improved tensile modulus by heat-treatment have been investigated. The tensile tests of a monofilament were carried out at a gauge length of 12.5mm and deformation rate of 0.5mm/min. 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. Standard modulus (As spun : AS) type (187GPa modulus) PBO fiber that authors reported its properties before was employed as standard specimen. It was found that the tensile strength of HM type PBO fiber was well represented by a two-parameter Weibull distribution, and indicated a size effect in diameter direction. The tensile strength of HM type PBO fiber on the basis of the concept of effective volume was higher than that of AS type fiber. The fatigue strength of HM type PBO fiber was higher than that of AS type fiber over all fatigue lives. 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. Therefore, it was found that the maximum stress was useful to describe the fatigue lives at the different stress ratios. Additionally, the factor governing fatigue fracture tended to vary from the mean stress to stress amplitude at the low stress ratio because crystalline regions increased by heat-treatment and fiber became brittleness.