Early detection of thermal fatigue damage using ultrasonic shear waves

High-cycle thermal fatigue (HCTF) can occur in nuclear power plant (NPP) piping where turbulent fluid mixing produces rapid, localised temperature fluctuations. If undetected, the accumulated damage may initiate cracks, as highlighted by the Civaux 1 reactor incident in 1998. Existing monitoring methods rely on external temperature sensors, but fail to capture high-frequency (1 Hz) fluctuations because the pipe wall acts as a low-pass thermal filter. A noninvasive technique for HCTF monitoring that overcomes the thermal filtering effect is desirable given the potentially deadly repercussions of cracks in NPPs.Ultrasonic velocity and attenuation are known to be sensitive to thermal fatigue. However, experimental studies to date were limited to post-fatiguing measurements. This study investigated whether thermal-fatigue-induced changes in velocity and attenuation could be detected during thermal cycling using a permanently installed transducer. Cyclic thermal transients of up to 600 °C were used to thermally fatigue three samples of austenitic stainless steel, with two taken to failure (cracking). Fatigue damage was confirmed using shear and Rayleigh wave measurements.The ultrasonic shear velocity decreased as the number of thermal transients experienced increased for each sample, as expected. However, the relationship between the magnitude of the decrease and the number of transients was not straightforward. The attenuation increased following crack initiation; ≈ 30% and ≈ 150% for shear polarisation parallel and perpendicular to the (expected) crack direction, respectively. Whilst further validation under representative conditions and geometries is required, these results demonstrate the potential for HCTF monitoring using changes in velocity and attenuation.