Tensile fracture surface analysis for boron fiber

In order to discuss the relationship between the fracture strength and defect size, tensile tests were performed on two kinds of boron fibers fabricated by CVD method, and their fracture surfaces were examined by a scanning electron microscope. The Weibull plots of tensile strength for boron fibers were divided into three regions; the lowest strength one, medium strength one and highest strength one. Fractography revealed that in the lowest strength region the defect size was large, while in the highest strength region no defect could be observed. The defect size was linearly related to the fracture strength, showing the applicability of fracture mechanics. The critical stress intensity factor for the surface defects was about twice larger than that for the inner defects observed near core tungsten fiber. This means that the difference of residual stress would be about 1.5 GPa from near fiber surface to near core tungsten fiber.