Plant life management activities of LWR plants in Japan

Subsurface stresses in welded structures increase the likelihood of fatigue cracks and environmental induced material degradation. The ability to evaluate stresses at the surface as well as in the interior of welded structural members would substantially increase the accuracy of structure life estimation. The critically refracted longitudinal (LCR) wave is a bulk longitudinal mode that travels within an effective depth underneath the surface. It may be used to detect in-plane subsurface stresses in the structures. In the past, experiments of using the LCR wave in the cold-rolled steel plates have proved its ability of evaluating near surface stresses. The designated LCR wave data was able to predict the existence of surface compressive stress in a 19-mm (0.75-in.) thick cold-rolled steel plate. This residual compressive stress was confirmed by the blind-hole-drilling method, and was effectively removed by a subsequent heat treatment. This paper presents a study of internal stress measurement in a long 75 x 25 mm^2 (3 x 1 in.^2) steel bar under four-point bending. A side by side stress evaluation was conducted, using conventional strain gauge and 2.25 or 5 MHz LCR probe set. At various stress levels the 5.0 MHz data almost followed the surface stain gage data and showed a far greater travel time change than that indicated by 2.25 MHz data. This is consistent with the calculated four-point bending stress distribution, in which high stress will be distributed near the outer surface where the high frequency signal is propagating. For each frequency, the active LCR wave will effectively interact with the internal stress within a defined depth range and generate a measurable change on the travel time. This technique has high potential to predict internal stress level in welded structures and mechanical components.