Fig. 5-3 Stress-strain curve of graphite material The slope of the stress-strain curve in the region where residual strain is produced, as illustrated by point Q, is smaller than that for no residual strain, indicated by point P. The deformation resistance of the graphite becomes smaller as residual strain is produced.

Fig. 5-4 Indentation load-depth curve of graphite materials Typical indentation load-depth curve when a diamond indentor is used on graphite materials to which no initial strain, and 0.2% simulated strains is applied. Since the deformation resistance of the material becomes smaller when simulated residual strain is applied, the indented depth becomes larger and the hardness values are smaller. Investigating hardness values make it possible to deduce the amount of residual strain produced.

Since the reactor core outlet temperature of the High-Temperature Test Reactor (HTTR) is about 950 degrees cent., graphite materials which have excellent heat resistance are used for the reactor core components. Since the dimension of graphite components change during neutron irradiation, residual strain accumulates in the graphite structures during reactor operation. It is therefore necessary to know the amount of residual strain that accumulates in the graphite components during their service life. Some methods to measure residual strain are in use, such as the "cutting method," which estimates strain based upon the deformation which occurs when the residual strain is relieved by cutting the component, and the x-ray diffraction method. However, these methods are not adequate for use on graphite components with complex shapes in which residual strain has been accumulated due to neutron irradiation. The radioactivity of such components also requires that a technique consistent with remote handling be applied. Consequently, a method has been developed which makes use of a change in the deformation resistance of graphite materials under loading when residual strain has been produced. The change in deformation resistance is measured by indenting the surface with a diamond indentor where residual strain is detected. The integrity of components used under severe conditions is maintained by applying this method to in-service inspection of the HTTR core graphite structure.

Reference M. Ishihara et al., Development of a Method for Measuring Residual Strain Based on Small Deformation Characteristics of Graphite Materials, Nippon Kikaigakkai Ronbunshu, 62(A) (602), 2305 (1996).

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