Spherical-impact damage to two silicon nitrides is investigated. Gas-pressure-sintered silicon nitride exhibits an elastic response to impact by spherical partially stabilized zirconia particles, resulting in Hertzian cone-crack initiation in the sintered body. Pressureless-sintered silicon nitride, on the other hand, demonstrated an elastic/plastic response, with median/radical-crack initiation. These differences in behavior are due to their microstructural differences as well as to the different hardness values of the silicon nitrides in relation to those of the PSZ spheres. The postimpact bend strength of silicon nitrides is also degraded when crack length exceeds the inherent flaw size. [
Nissan has been developing and marketing ceramic turbocharger rotors for five years. This paper outlines the major theories and techniques used in ceramic fabrication, joining of ceramic and metal components, and machining of ceramics. It also presents a dynamic stress analysis using DYNA3D and describes techniques used in performing impact damage experiments, reliability evaluation, and lifetime preprediction.
Nissan has been developing and marketing ceramic turbocharger rotors for over five years. This paper outlines the major theories and techniques used in ceramic fabrication, joining of ceramic and metal components and machining of ceramics. It also presents a dynamic stress analysis using DYNA3D and describes techniques used in performing impact damage experiments, reliability evaluation and lifetime prediction NOMENCLATURE A pre-multiplier in crack velocity a crack length B constant D constant E Young's modulus F failure probability H hardness Kc fracture toughness KiCcritical stress intensity factor L load m Weibull modulus N crack velocity exponent P survival probability r radius S material strength t time V velocity ve effective volume Y shape factor p density 6 stress
The silicon carbide (Sic) whisker reinforcement of silicon nitride @LaN,) improves fracture strength and toughness, hardness, and Young's modulus, resulting in higher resistance of the composites to sphere penetration and crack initiation at spherical impact. Sintered Si3N4 shows an elastic/plastic response and initiates mediadradial cracks at 100 m/s impact velocity. SiC-whisker/Si3N4 composites, on the other hand, demonstrate an elastic response, with Hertzian cone crack initiation, only when impact velocity exceeds 280 m/s. The SiC-whisker/Sifi4 composites thus exhibit improved strength degradation versus critical impact velocity characteristics because of improved mechanical properties provided by the SIC whiskers. [
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