Enhancement of Fatigue Damage due to Environmental Variation and its Recovery Treatment

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Fatigue properties of steel are strongly affected by environmental conditions, in which hydrogen plays an essential role. The clarification of the mechanism and measures for reducing the susceptibility have been subjects of many studies. We have proposed a new model for hydrogen embrittlement of steel that claims enhanced formation of vacancies during plastic deformation with the aid of hydrogen and the coagulation of vacancies that leads to the decrease in the ductile crack growth resistance. Based on the model, the aim of the present studies is to investigate particularly die effect of variations of environmental conditions on the susceptibility to hydrogen-related feilure. Firstly, defects created during fatigue have been detected using hydrogen as a probe, revealing the formation of vacancies as a fector of fetigue damage. Interactions between fatigue damage and hydrogen result in degradation of fatigue properties and enhanced susceptibility to hydrogen-related failure. On the same … More basis, in a delayed fracture test of a high strength steel cyclic variations of the applied stress and hydrogen-charging current density and their synergetic effect have been confirmed to enhance the susceptibility. Annealing of pre-fatigued specimens at temperatures as low as 200℃ recovered the enhancement of the susceptibility. Evolution of hydrogen-related failure in a biomedical material, a shape-memory NiTi alloy that is uses in oral cavity, has been revealed. The proposed model has been directly proved with transmission electron microscopy using specimens fabricated by means of a focused ion beam method from just beneath fracture surface. Crack growth associated with local amorphization has been revealed, supporting the increased density of vacancies. Hydrogen microprint technique has been also applied for revealing local distribution of defects that trap hydrogen. Dynamic observation of electron diffraction has been successfully applied for revealing the surface structure of specimens in nanometer-scale. Less