초음파 비선형 특성을 이용한 원전 구조재의 열화 손상 평가

Abstract

The oblique incidence technique for ultrasonic nonlinear characterization was studied in stainless steel 316L alloy subjected to high cycle fatigue. A dog-bone plate specimen was prepared to make different fatigue-driven deformation at each position where the stress concentration could occur in the middle of specimen. In addition to the normal transmission technique, the oblique incidence technique which is newly suggested in this study, was used to measure ultrasonic nonlinear parameter. The fatigued specimen shows higher ultrasonic nonlinear parameter than the virgin specimen for both techniques. Ultrasonic nonlinear parameter highly increases in the middle of test specimen where the stress concentration existes. Relative nonlinear parameter has strong correlation with fatigue damage. Consequently, the oblique incidence technique with longitudinal wave can be potential to characterize high cycle fatigue damage. And the influences of grains and precipitates of microstructural evolution on the ultrasonic nonlinearity have been experimentally investigated. The prior-austenite grain and precipitate size are controlled by the variation in austenitizing and tempering conditions in reactor pressure vessel materials of nuclear power plant, SA508 Gr.3 low alloys. The ultrasonic nonlinearity was found to have strong correlations with grains and precipitates since the ultrasonic nonlinear parameter β shows decrease trend with coarsening of grains and precipitates. Although the prior-austenite grain size increased, the β changed little due to the effects of subgrains, packets and laths. For the preciptate effects, the β decreased sharply due to decrease in Mo2C causing the coherency stain in addition to the precipitate size. The results in this study may provide a potential for characterizing the microstructural evolution, grains and precipitates, by measuring the ultrasonic nonlinearity.