Crack propagation analysis of SiCf/SiC composites by gas permeability measurement

Permeability of helium gas through the NITE-SiCf/SiC composites after applying tensile stress was measured experimentally in a vacuum apparatus. Tensile stress equal to 1.1–1.2 times the proportional limit stress (PLS) was applied parallel to the direction of the reinforced fiber. Results of the permeability experiments revealed that the permeability rapidly increased when threshold stress was applied on the specimens. The permeability of helium gas was governed by the narrowest diameter of the permeation pathway. In the case of NITE composites, the diameter of the pathway was calculated to be below 0.65 μm. The NITE composites exhibited superior performance even when the applied stress was greater than 1.2 times the PLS. Fiber bundles considerably magnified the permeability of helium gas because of the relatively large pore size of the intra-fiber bundles. Transverse cracks propagated with increasing stress and they connected fiber bundles when the applied stress was 1.10–1.15 times the PLS.