In this paper, effect of strain rate on the tensile strength of high-modulus type poly-p-phenylene benzobisoxazole (PBO) fiber was investigated in monofilament tests. Tensile tests were carried out at a gauge length of 12.5 mm. The strain rate ranged from 6.7×10-4 to 3.2×10-1 s-1. It was found that the tensile strength removed size effect in diameter direction of PBO fiber was represented by a two parameter Weibull distribution under all strain rate conditions. The mean tensile strength at high strain rate range from 4.0×10-2 to 3.2×10-1 s-1 was about 8 % higher than that at low strain rate range from 6.7×10-4 to 1.3×10-2 s-1. In addition, it was found by SEM observation that there were distinct differences between fracture surface image of specimen at high strain rate range and at low strain rate range, i.e., the crack length in fiber direction at high strain rate range was much shorter than that at low strain rate range. However the crack grew along boundaries of microfibrils at the low strain rate range, the crack grew not only along the boundaries but also in the microfibrils at the high strain rate range. Therefore, the increase in tensile strength at the high strain rate range was caused by the difference in the path of crack propagation.
