Papers by Author: A.Toshimitsu Yokobori Jr.

Abstract: The hydrogen diffuses and accumulates at the stress concentration area like a crack tip and it causes hydrogen embrittlement. To clarify the mechanism of hydrogen embrittlement, it is important to obtain the hydrogen concentration behavior. However, experimental detection is not feasible due to the high diffusivity of hydrogen and numerical analyses have been preceded. In this paper, by using a finite element and finite difference coupled method at which the diffusion analysis is performed by FDM coupled with the stress analysis by FEM, the analyses of hydrogen diffusion were conducted under cyclic loading conditions.

Abstract: Most heat resisting materials in structural components are used under multi-axial stress conditions and under such conditions ductile materials often exhibit brittle manner and low creep ductility at elevated temperature. Creep crack initiation and growth properties are also affected by multi-axial stress and it is important to evaluate these effects when laboratory data are applied to structural components. Creep crack growth tests using circumferential notched round bar specimens are a simple method to investigate multi-axial stress effects without using complicated test facilities. Creep crack growth tests have been performed using a 12CrWCoB turbine rotor steel. In order to investigate the effects of multi-axial stress on creep crack growth properties, the tests were conducted for various notch depths at 650°C. The circumferential notched round bar specimen showed brittle crack growth behaviour under multi-axial stress conditions. Creep crack growth rate was characterized in terms of the C* parameter. A 12CrWCoB turbine rotor steel has been tested using circumferential notched round bar specimens with different multi-axiality. Circumferential notched round bar specimens show increased brittle creep crack growth behaviour due to the multi-axial stress condition. Creep crack growth properties could be predicted by allowing for the decrease of creep ductility under multi-axial conditions.

Abstract: Previously, we proposed stress corrosion cracking model on the basis of interaction of dislocation and hydrogen around a crack tip to predict discontinuous cleavage-like fracture during stress corrosion cracking (SCC) for ductile fcc alloys. Furthermore, we conducted numerical analyses using this proposed model. In the analysis, hydrogen was treated as a static cluster. However, actually, both of hydrogen and dislocations move with interaction each other. Therefore, in this paper, a physical model of dislocation and hydrogen dynamics with interaction was proposed. And the behavior of interaction between dynamic dislocations and hydrogen was investigated. On the basis of this analysis, the discontinuous cleavage-like fracture during SCC for ductile fcc alloys was clarified.

Abstract: Hydrogen embrittlement causes a crack initiation and brittle fracture. Therefore, it is important to conduct theoretical analysis to predict hydrogen distribution on the basis of reasonable physical model. In this paper, numerical analysis on the hydrogen diffusion and concentration at the localized stress field was conducted to clarify the behavior of hydrogen concentration and the sensitivity of hydrogen embrittlement. Furthermore, on the basis of this analysis, the effect of post weld heat treatment on the release of concentrated hydrogen was analyzed and optimum condition of post and pre heat treatment on the welding was investigated.