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

由于表面通常是强度和抗破坏性最敏感的部位，为了提高工程材料在各种环境下的疲劳强度和寿命，对机械零件和工程构件的表面进行表面改性。本研究通过回转弯曲疲劳试验，研究了表面陶瓷薄膜、氮化和喷丸处理的金属材料的吉卡疲劳行为，探讨了表面层对疲劳强度的影响规律。（1）高碳铬轴承钢（JIS SUJ 2）、高速工具钢（JIS SKH 51）和低合金钢（JIS SNCM 439）的S-N曲线将断裂模式分为两类，每一类具有不同的裂纹起源。一种是在高应力幅水平和低循环次数下由试样表面上的晶体滑移控制，另一种是由 ...更多信息 在高循环次数下，表面下的非金属夹杂物。这一显著的S-N曲线形状被称为“双S-N曲线”。(2)在高应力幅区，由于表面处理后的硬化和残余压应力的作用，疲劳裂纹的萌生部位由未处理试样的表面向试样的亚表面转移。另一方面，在高周疲劳区，未处理和表面处理的试样之间的亚表面裂纹萌生控制的疲劳寿命没有差异。与表面断裂模式的S-N曲线相比，内部断裂模式的S-N曲线是材料固有的，而表面断裂模式的S-N曲线受表面条件、环境条件等因素的影响。(3)断裂表面的一个独特的特征，被命名为一个粒状亮面（GBF），观察到在附近的非金属夹杂物在地下断裂起源内部的鱼眼区。从断裂力学的观点出发，指出在长时间疲劳过程中，GBF形成后，内部裂纹能够扩展。GBF的形成机理为“球状碳化物的分散脱粘”模型。(4)通过反复拉伸疲劳试验，研究了阳极氧化膜对铝合金疲劳强度的影响。与未处理试样相比，阳极氧化试样的疲劳强度有所下降，这是因为阳极氧化膜在基体金属塑性变形累积形成的临界应变处发生断裂，而裂纹则是在阳极氧化膜的缺陷处诱发的。少

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