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In this paper, tensile and fatigue properties of poly-p-Phenylene benzobisoxazole (PBO) fiber have been investigated. The tensile test was carried out with a gauge length of 12.5 mm and displacement rate of 0.5 mm/min. The tensile strength data were analyzed in terms of the Weibull probability distribution function. Fatigue test was performed using an electromagnetic force fatigue tester. The fatigue testes were carried out at a frequency of 10 Hz with three stress ratios of 0.1, 0.5, and 0.7. It was found from the tensile strength test that the distribution of PBO fiber strengths expressed as Weibull distribution function with 2 parameters. The test results clearly showed that the fatigue behavior appeared in the S-N characteristic curves. The relation between the stress amplitude and the fatigue life depended on the stress ratio. However, the relation between the maximum stress and the fatigue life was independent of the stress ratio, and the maximum stress was most suitable parameter to describe the fatigue strength at the different stress ratios. Consequently, it became clear that the fatigue strength of the PBO fibers was not cycle dependent, but is time dependent. 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.

In order to investigate very high cycle fatigue properties of Zr55Al10Ni5Cu30 bulk amorphous alloy, rotating bending fatigue tests were conducted at room temperature by using 5 different series of Zr55Al10Ni5Cu30 alloy. The differences of material processing conditions are manufacturer, product year, material size and minimum diameter of specimen. Although all specimens fractured from surface, duplex S-N characteristics were observed for each series of the material. Time strength distributions at N=104 for short life region and at N=107 for long life region were well approximated by normal distribution. The entire S-N property accepting the normalized stress amplitude by the time strength at N=104 or N=107 has shown more clearly duplex S-N characteristics. In addition, P-S-N properties were also estimated from the standard deviation of time strength distributions at N=104 or N=107. Based on the observation of fracture surface by scanning electron microscope (SEM), it is confirmed that every fracture surface was consisting of typical three regions such as multi-facet region, stable crack growth region and instantaneous fracture region.