STATIC AND CYCLIC FATIGUE OF GLASS AND SILICON-NITRIDE UNDER TENSILE AND TENSION-COMPRESSION FATIGUE TESTS

Static and cyclic fatigue tests were made under static tensile loading and tension-compression cyclic loading at the stress ratios of R = 0 and -1 on two kinds of silicon nitride (silicon nitride A and B) and borosilicate glass. The tests were made with a specially designed apparatus which was equipped with a device to minimize the load eccentricity. It worked satisfactorily in performing cyclic tension-compression tests at arbitrary stress ratios as well as static tensile tests. The effects of cyclic stress on fatigue strength were investigated. Scarcely any effects of cyclic loading were observed on the strength of silicon nitride A and borosilicate glass, while appreciable degradation in strength due to cyclic loading was observed on silicon nitride B. From the fractographic observation and EDX analysis of the fracture initiation areas, it was shown that the fracture in silicon nitride B occurred by crack growth from defects formed during sintering, while in silicon nitride A, fracture initiation occurred by the cracking formed along the grain boundaries by stress corrosion cracking in the surface layer, and then these cracks linked up to form small cracks and grew by slow crack growth to final failure. The difference between the effect of stress cycling on the strength of silicon nitride A and on that of silicon nitride B, mentioned above, seemed to result from this difference in the fracture initiation process in these materials.