Conditioning monitoring by microstructural evaluation of cumulative fatigue damage
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I.
INTRODUCTION
AS older nuclear power plants approach 40 years of service, evaluating prefatigue damage is becoming more and more important. Now that the reactors built in the 1950s have reached the age of retirement, according to design rules such as the ASME Boiler and Pressure Vessel Code,[1] the high cost of replacing them has promoted queries as to why utilization of these power plants has to be suspended if they are still largely fit for service. The ability to extend the life safely while avoiding forced outages and expensive repairs would be facilitated by inspection techniques that could aid in predicting when fatigue microcrack initiation was about to occur in power plant structures. If a correlation between prefatigue damage and the microstructural changes could be obtained, the accuracy of usable life assessment would be enhanced, corrective measures could be made more efficient, and the reliability of the refurbished plants would be ensured. Earlier studies of microstructural behavior of pressure vessel steels, such as SA508, suggest that the accumulation of fatigue strains prior to the development of visually observable cracking could be related to misorientation change of dislocation cell structures, as measured by selected area diffraction (SAD).[2,3,4] Many alloys are normally free from these cell structures before exposure to fatigue, and although these cells subsequently develop during cycling, they do not exhibit the stability in size observed in SA508.[5] C. FUKUOKA, Research Engineer, Materials Technology Department, and Y.G. NAKAGAWA, Manager, Technical Planning Group, are with the Research Laboratories, Ishikawajima-Harima Heavy Industries Company, Ltd., Tokyo 135, Japan. J.J. LANCE, Target Leader, is with O & M Cost Control Technology, Electric Power Research Institute, Charlotte, NC 28262, R.N. PANGBORN, Professor and Associate Dean, is with the College of Engineering, Pennsylvania State University, University Park, PA 16802. Manuscript submitted October 24, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
BACKGROUND
The SAD method used in the studies mentioned in the introduction is schematically illustrated in Figure 1. An SAD is performed for individual grains having a ^111& diffraction orientation. In each grain, there are a large number of dislocation cells. After placing a selector aperture to allow inspection of a few cell structures, small differences of crystallographic orientation among the cells appear as small displacements of the diffraction spots; this pattern is then recorded on a negative film. The selector aperture is then placed in the neighboring area, and the diffraction pattern is recorded on the same film. This is repeated five times, exposing one negative to the diffraction spots of five different areas. The maximum angular deviation, u, on the negative where the five diffraction patterns are superimposed, is measured and considered to be a datum. This fivediffraction (5-SAD) superimposition procedure is repeated about 20 times for ^111& oriented grains fou
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