Development of a new method for determining residual stress by microscopic measurement of the distance between markers

The Stress Corrosion Cracking (SCC) has been observed abroad in several PWRs' control rod drive mechanism nozzles. Although it is considered that the residual stress is the essential cause of SCC, the mechanism is not known yet. For the purpose of measuring residual stress at the portion where the SCC was observed, no suitable conventional technique are available. In this paper, new technique developed for measuring residual stress around the location of SCC is proposed. The technique consists of the following steps: stamping markers with diamond tips and measuring the distance between two marks using measuring microscope before and after releasing the stress. The residual stress can be calculated from the value of the strain, i.e. change in the distance. It has been confirmed that this technique gives the same accuracy as the conventional method using strain gauses. (author)