Detection and quantification of pipe damage from change in time of flight and phase

The use of ultrasonic guided waves for damage detection in pipes is continuously increasing. Generally longitudinal (axial symmetric) modes are excited and detected by PZT (Lead Zirconate Titanate) transducers in transmission mode for this purpose. In most studies the change in the received signal strength with the extent of damage has been investigated while in this study the change in the phase and the time-of-flight (TOF) of the propagating wave modes with the damage size is investigated. The cross-correlation technique is used to record the small changes in the TOF as the damage size varies in steel pipes. Dispersion curves are calculated to carefully identify the propagating wave modes. Differential TOF is recorded and compared for different propagating wave modes. Feature extraction techniques are used for extracting phase and time-frequency information. The main advantage of this approach is that unlike the recorded signal strength the TOF and the phase are not affected by the bonding condition between the transducer and the pipe. Therefore, if the pipe is not damaged but the transducer-pipe bonding is deteriorated then although the received signal strength is altered the TOF and phase remain same avoiding the false positive alarms of damage.