Study on Effect of Environment on Strength and Life of Micromaterials in Fatigue

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Fatigue test systems were developed to study the fatigue mechanisms in metallic micro-materials. This system has an electro-dynamic actuator or an audio speaker as a loading device. Fatigue tests were conducted with commercially pure iron, or pure aluminum either in air or in sodium chloride solution, and fatigue life and fracture morphologies were observed. In either materials, and in either environment, scattering of fatigue lives were very large compared with conventional size specimens, and they were larger for smaller specimens. The effect of environment was not observed for short life region, and they were appreciable in long life region. Two types of fracture morphologies were observed.For monitoring fatigue damage of micro materials, an A.C. potential method was developed. In iron, the value of potential were almost constant during fatigue process except for just before final fracture, while they continuously increased up to final unstable fracture in aluminum.Fatigue test system for pure aluminum thin film was also developed in the present study. Two types of fatigue test specimen were made by electrolytic polishing and mechanical machining from aluminum foil whose thickness was 12 urn. The strength of specimen made by mechanical machining was larger than that of specimen made by electrolytic polishing. The variation in fatigue life of specimen made by electrolytic polishing was large compared with specimen made by mechanical machining. Both before and after the fatigue tests, slip-bands were observed by using optical microscopy and AFM. The spacing of slip bands in the thin film was found to be much larger than that of the bulk specimen.