Experimental investigation for the theory of environment assisted cracking on local environmental conditions

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A theoretical formulation of crack growth rate based on the film rupture/slip dissolution mechanism was proposed by the head of investigators of this project. However, it is difficult to evaluate crack growth rate quantitatively because of diversities of mechanical, metallurgical, environmental and their interaction parameters. Measurements of local conditions around crack tip were required for the parameters included in the theoretical formulation, aiming which quantitative information of the local environmental condition was the most important. Accordingly, this project was started from 1997 and main achievements can be summarized as follows.(1) Although the sampled volume could be reduced from 50μL to 2μL by the improvement of the micro-sampling system, it was still needed to reduce the sampled volume. A novel sampling method was developed in this study and optimization was performed. Maximum resolution could be improved to about 5ppb of ClィイD1-ィエD1 and SOィイD32-(/)4ィエD3. The experim … More ents mentioned above was performed on the specimens made of both low alloy and stainless steels in high temperature, high pressure water environments.(2) Local physical properties of surface film formed due to the local active environment were measured by atomic force microscope and magnetic force microscope, and it turned out that electrical and magnetic properties of films could be changed in different environment. In-situ contact electric resistance (CER) measurements were performed in high temperature, high pressure water as an in-situ local measurement of local physical properties of surface film. Results showed that CER of the film depends on corrosion potential, temperature and materials.(3) Results showed that the parameter "m" for dissolution characteristics of new bare surface was very important, which depends strongly on crack tip solution chemistry. The effect of crack tip strain rate on parameter "m" was taken into account in the theoretical formulation. Unknown parameters were obtained by inverse analysis. The results of theoretical analysis were in good agreement with experimental results. Less