Jankauskas, Audrius; Kaunas University of Technology; Lithuania
Jankauskas, A.; Kaunas University of Technology; Lithuania
Mazeika, L.; Kaunas University of Technology; Lithuania
Nageswaran, C.; TWI Ltd; United Kingdom
Session: Civil Infrastructure - ET, UT
Time: 16:40 - 17:00
The welds of the steel pipes operating under high temperatures and pressure can be affected by creep damage which usually occurs in heat affected zones. The creep damage starts to develop from creep cavities possessing dimensions of a few micrometres, which are growing in time. In the later stages of development, the creep originated cracks are relatively easy detectable. In this case, due to the expected fast grow of cracks the frequent inspection is needed in order to avoid formations of leakage or breaks. So, the detection of the damage in earlier stages is needed. However, many of the conventional NDT methods are not able to detect early stages of creep damage due to the small dimensions of the voids, which are measured in tens of micrometres. The objective of the work presented was to develop and to investigate NDT technique which enable detection of creep damage in early stages of its development.
Investigations carried out have demonstrated that the most promising approach is to use high frequency ultrasonic waves. It was shown by modelling using diffraction model of the focused transducer that detection of small non-uniformities in steel is possible only using focused transducers with frequencies higher than 20MHz. However, even in this case strong amplification (up to 80-90dB) of signals was required using low noise amplifiers. The proposed NDT technique for creep damage detection was based on the scanning of the weld area using high frequency focused transducer. In the second stage the acquired signals were processed using proposed signal analysis algorithm, which is based on relating the properties of the backscattered noise to the characteristics of the metal structure. As the results of processing, the boundaries of the parent metal, heat affected zone and weld metal in ultrasonic image can be indicated. Application of proposed technique for investigation of the samples exposed to different levels of creep damaged revealed that it was possible not only to detect early stage of creep damage but even level of the damage could be estimated. The experimental results were verified using metallographic analysis, which completely validated the proposed ultrasonic approach.