Early detection of fatigue at elevated temperature in austenitic steel using electromagnetic ultrasound transducers

Austenitic pipes and other safety-relevant components of primary circuits in pressurized water reactors are subject to thermo-mechanical load cycles. Temperature gradients, turbulences and vibrations inside the circuit cause both low and high cycle fatigue and lead to microstructural changes and microcracks in the material. As of 2012, direct monitoring of this fatigue has not been implemented in German nuclear power plants. It would therefore be desirable to develop a monitoring concept based on established non-destructive testing techniques which would allow us to detect and Interpret microstructural material changes before macro-cracks occur. Electro-magnetic ultrasonic transducers (EMATs) are a promising technology for that purpose because they are high-temperature compatible and because the ultrasonic signal is sensitive to most microstructural changes that can appear during fatigue. For several years, Fraunhofer IZFP and the Materials Science Institute of TU Kaiserslautern have been closely collaborating in several major projects dedicated to this problem. Both low and high cycle fatigue tests were performed, thus representing the real fatigue processes occurring inside the surge line of a primary circuit. During fatigue tests on austenitic specimens at realistic temperature, ultrasonic in situ measurements were carried out, In which the measured Parameters were the ultrasonic amplitude and time of flight. Characteristic changes in cyclic deformation behavior as well as in these Parameters were observed. It is shown that these results can constitute the basis for a target-oriented structural monitoring concept and proactive aging management.