Toshihito Ohmi

Suppression of fatigue crack propagation with hydrogen embrittlement in stainless steel by cavitation peening

In the use of hydrogen energy, the hydrogen embrittlement should be investigated, and it is necessary to improve reliability and safety of machine components which are used in hydrogen environment. The hydrogen sensitivity of material depends on material structure, defect and stress distribution. There is a possibility that the hydrogen invasion into material and the fatigue crack propagation with hydrogen embrittlement can be suppressed by introducing compressive residual stress by us- ing surface modification such as cavitation peening. The cavitation peening is a one of the peening technique, and enhancement of the fatigue strength of the mechanical components and structural materials by cavitation peening have been revealed. In this study, the austenite stainless steel JIS SUS316L with precrack were charged by a cathodic hydrogen charging method, and the fatigue test with and without hydrogen charge were conducted by a plate bending fatigue test. The results demonstrated that the fatigue crack propagation with hydrogen embrittlement can be greatly suppressed by cavitation peening.

Micro mechanics based on vacancy diffusion coupled with damage mechanics related to creep deformation and prediction of creep fracture life

AbstractIn situ observations of crack growth and damage progression were conducted under creep conditions for P92 and titanium aluminides inter-metallic compound. A proposed analysis of stress induced particle diffusion was applied to stress induced vacancy diffusion. Results obtained from this analysis were successfully correlated with the experimental behaviour of macroscopic damage progression and a theoretical characteristic of creep deformation was derived. It was found to be in good agreement with experimental characteristics of creep deformation. Furthermore, the experimental characteristics of creep damage progression which concern voids and micro crack formations at grain boundary were found to be well correlated with those of deformation. From these results, correlation between vacancy diffusion in nano-scale, creep damage in mezzo-scale and creep deformation in macro-scale were successfully realized.

The application of the analysis of potential driven particle diffusion to the strength of materials

The problems of diffusion in solid are widely treated as those of numerical analyses such as mechanisms of hydrogen diffusion which causes embrittlement, boundary micro crack initiation and growth under high temperature creep condition and stress and electro-migration due to vacancy diffusion in LSI circuit. In this paper, the significance of α multiplication method which we proposed to analyze the particle diffusion problems under potential gradient and the results obtained from this method were discussed for the problems of hydrogen embrittlement, vacancy diffusion which results in creep crack damage and deformation, and line failure in LSI circuit. Furthermore, temperature characteristic of the sensitivity of particle diffusion and concentration such as hydrogen and vacancy were discussed based on this proposed analysis.

Flow Controllability of Hydrogen Diffusion Driven by Local Stress Field for Steel under Cyclic Loading Condition

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.

Analysis of Stress Induced Voiding Using by Finite Element Analysis Coupled with Finite Difference Analysis

In this paper, the vacancy migration in Cu interconnect of large scale integration caused by stress induced voiding was calculated using the α multiplication method. Then, the effect of weight coefficient, α, on stress induced vacancy diffusion analysis was investigated and the validity of the α multiplication method was verified. Furthermore, the method of the vacancy diffusion analysis coupled with thermal stress analysis which can consider the history of thermal stress due to temperature changes was proposed. The results of the vacancy diffusion analysis coupled with the thermal stress analysis were compared with the analytical results of the vacancy migration without the effect of history of thermal stress. As a result, the maximum site of vacancy accumulation was found to be qualitatively in good agreement between them. However, the quantitative value of maximum vacancy concentration obtained by the vacancy diffusion analysis coupled with thermal stress analysis was found to be much higher and the vacancy distribution is found to be much more localized.

Hydrogen-Assisted Cracking Threshold of High-Strength Low-Alloy Steel

Threshold stress intensity factor Kih would be an appropriate index parameter for the cut-off limit in fatigue crack growth analysis in order to prevent hydrogen storage tanks from suffering brittle fractures at hydrogen stations. The Kih factors were evaluated using rising load and constant displacement tests conducted in high-pressure gaseous hydrogen. The following results were obtained:1. Kih measured under rising load was less than Kih measured under constant displacement.2. The difference of Kih increased as tensile strength decreased.3. The difference of Kih may be attributed to the plastic wake of propagating cracks.Hydrogen diffusion behavior at crack tips was also estimated by a finite difference method analysis.Copyright

Hydrogen Diffusion Analysis in the Fatigue Crack Growth Test Under High Pressure Hydrogen

Prevention of hydrogen embrittlement is one of the most important issues for the materials used in hydrogen environment. Various mechanisms for hydrogen embrittlement were proposed, such as the decohesion mechanism and hydrogen enhanced localized plasticity. However, the process of hydrogen diffusion and accumulation into the fracture point is necessary for understanding of hydrogen embrittlement. In this paper, hydrogen embrittlement behavior during the fatigue crack extension test under high pressure hydrogen was first conducted. Then, hydrogen diffusion and accumulation behavior in the crack tip region was analyzed by the numerical simulation based on the Fick’s diffusion theory. Alpha multiplication method which multiplies stress gradient induced terms was found to be valid to realize correct particle diffusion behaviors driven by the stress gradient term. The significance of the concept of alpha multiplication method for the numerical analysis was discussed by comparing to the experimental hydrogen embrittlement behavior.Copyright

The characterization of dominating region of fracture (process region) around a crack tip based on the concept of mechanical similarity and atomic mechanics

Brittle fracture at a crack tip is considered to be caused within a specified local region, that is, process region. However, the determination of this scale has not yet been theoretically clarified. The difficulty of this determination will be due to the wide range scale analysis from the range of nano scale (atomic scale) to macro scale (crack size scale). In this paper, on the basis of the atomic mechanics using super atom and hybrid method of the fractal concept which concerns self-similarity with the proposed analysis of scale projection from macro to nano scales, the disturbed region of atom arrangements around a crack tip were clarified under the local stress field by crack and dislocation. This disturbed region was related to fracture dominating region (process region) which is in good agreement with the scale obtained from experimental consideration by R. Ritchie.

The Quantitative Estimation of the Sensitivity of Corrosion and Hydrogen Environment on Fatigue Crack Growth Rate for Steels

In this paper, the tensile fatigue tests using small compact specimen were conducted by the corrosion and hydrogen embrittlement fatigue testing method which authors have proposed. Based on these results, the quantitative separated estimations of the sensitivity of Corrosion, Hydrogen Embrittlement (HE) and Facet-like Fracture Hydrogen Induced Dislocation Pile-up (F-HIDP) were made by using the acceleration factor, D*, that is the ratio of fatigue crack growth rate to that under atmospheric condition.© 2014 ASME

The Analysis of Hydrogen Diffusion Behaviors Coupled With the Analysis of Heat Transfer Around the Heat Affected Zone of Welding

Hydrogen penetrates into the metal and causes Hydrogen embrittlement due to the increase in hydrogen concentration. This is caused by the local stress fields such as residual stress field at the site of welding or local stress field around a crack tip. It accompanied with incubation time of several hours since the components were exposed to hydrogen atmospheric condition. This incubation time is time lag of hydrogen diffusion and concentration at the site where the hydrogen embrittlement occurs. Therefore, clarification of the hydrogen diffusion behavior is important to prevent from fracture of hydrogen embrittlement.In this paper, the numerical analyses of hydrogen diffusion around weld part including HAZ (Heat Affected Zone) under residual stress coupled with that of heat transfer during the cooling process before and after weld were conducted and the behaviors of hydrogen concentration were analyzed. On the basis of these analyses, the method of heat treatment to prevent from hydrogen concentration at the weld part was investigated. Results obtained by these analyses showed that pre weld heat treatment is effective in the prevention of hydrogen concentration and combined pre weld heat treatment with post weld heat treatment was found to be the most effective treatment.Copyright