Development of high-functional materials by surface treatment and its evaluation of gigacycle fatigue strength

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Some surface modifications are applied to the surfaces of mechanical elements and engineering components for the purpose of increasing fatigue strength and extending lifetime of engineering materials in various kinds of environments, because the surface is usually the most sensitive part for strength and failure resistance. In this study, gigacycle fatigue behavior of metals surface-treated with ceramics thin film, nitriding and shot-peening is investigated through the cantilever-type rotary-bending fatigue tests and discussed the rules of surface layer on fatigue strength. Main results are summarized as follows :(1) The S-N curve obtained for a high carbon-chromium bearing steel, JIS SUJ2, high speed tool steel, JIS SKH51, and low-alloy steel, JIS SNCM439, clearly classified the fracture modes into two groups each with a different crack origin. One was governed by crystal slip on the specimen surface at high-stress amplitude level and low number of cycles and the other was governed by … More a non-metallic inclusion at subsurface at high number of cycles. The remarkable shape of the S-N curve was named a 'duplex S-N curve'.(2) Fatigue crack initiation site changed from the surface of untreated specimen to the subsurface of the specimen because of hardening and compressive residual stress with surface treatment in the region of high-stress amplitude. On the other hand, no difference in fatigue life controlled by the subsurface crack initiation between untreated specimen and surface treated one was observed in high-cycle region. It was suggested that the S-N curve corresponding to the internal fracture mode is inherent in the material, as compared with the S-N curve of surface fracture mode, which is affected by surface conditions, environmental conditions and so on.(3) A distinctive feature of the fracture surface, named as a granular-bright facet (GBF), was observed in the vicinity of a non-metallic inclusion at the subsurface fracture origin inside the fish-eye zone. From the discussion based on fracture mechanics, it is pointed out that an internal crack is able to grow after formation of the GBF during the long fatigue process. Mechanisms of the GBF formation was proposed as 'disperse decohesion of spherical carbide' model.(4) The effect of anodized coating on fatigue strength of aluminum alloys was investigated under repeated tensile fatigue test. Fatigue strength of the anodized specimens decreased as compared with uncoated one, because the fracture of anodized coating film occurred at the critical strain formed by the accumulation of plastic deformation of substrate metal and crack was induced in the substrate at the flaw of the anodized film. Less