Papers by Author: Hitoshi Soyama

Abstract: The objective of this study is to evaluate the effect of hydrogen on the micro-and macro-strain of austenitic stainless steel using X-ray diffraction. When hydrogen is trapped in lattice sites, it can affect both the micro-and macro-strain. The micro-strain was evaluated through fitting profiles to measured X-ray diffraction profile using a fundamental parameter method. The macro-strain, i.e., the residual stress, was evaluated by a 2D method using a two-dimensional PSPC. The experimental samples were charged with hydrogen by a cathodic charging method. The results revealed that the induced residual stress was equi-biaxial and compressive, and that the micro-strain increased. Both of these varied rapidly with increasing hydrogen charging time. Saturation occurred at a compressive stress of around 130 MPa. On reaching saturation, the hydrogen charging was terminated and desorption of hydrogen began at room temperature. Then, the strains decreased and the compressive stress reverted, ultimately, to a tensile stress of 180 MPa. Martensitic transformation occurred due to hydrogen charging and this had a significant effect on the X-ray diffraction profile.

Abstract: Cavitation normally causes severe damage in hydraulic machinery such as pumps and turbines by the impact produced by cavitation bubbles collapsing. Although cavitation is known as a factor of erosion, Soyama et al. succeeded in utilizing impacts of cavitation bubble collapsing for surface modification by controlling cavitating jet in the same way as shot peening. The local plastic deformation caused by cavitation impact enhances the fatigue strength of metallic materials, and the surface modification technique utilizing cavitation impact is called “cavitation peening (CP)”. It is well known that the peening improves fatigue strength by introducing compressive residual stress on the surface, but little attention has been paid to the behavior of fatigue crack growth of the material which was modified by CP. In the present study, the fatigue behavior of austenite stainless steel with and without CP was evaluated by a plate bending fatigue test, and the results revealed that the compressive residual stress introduced by CP suppresses fatigue crack growth rate by 70 % compared to that without CP.

Abstract: Quantitative evaluation of small cracks in stainless steel under water, which simulated the environment of unclear reactor, by using microwave, was demonstrated. The crack depth was evaluated by means of the microwave dual frequency technique, and the crack closure stress was estimated based on the compliance technique.

Abstract: Peening method using cavitation impacts have been developed. In this peening method, cavitation bubbles were generated by a cavitating jet. In order to increase peening intensity, a low speed water jet was injected around a cavitating jet, as the water jet swept away residual bubbles, which weaken the cavitation impact. In the present paper, improvement of fatigue strength of stainless steel by using a cavitating jet with an associated water jet was investigated compared with the results of conventional cavitating jet. It was concluded that the cavitating jet with an associated water jet can peen the stainless steel more effectively compared with the conventional cavitating jet.