TOFD data on a 24 inch OD, 1.25 inch thick pipe with ID Indications

Time of Flight Diffraction inspection produces three signals because of the transducer arrangement and operation in a pitch-catch mode.  The first is the lateral wave - caused by propagation of the sound between the transmitter and receiver in a direct path just under the examination surface. The second signal LL, is the primary back wall signal- caused by the direct reflection of the L wave from the back wall. The third signal is the back wall signal created by mode conversions on the ID surface (L wave to S wave and vice versa). The signals occur sequentially at increasing time in the order presented. During a normal scan, the three signals appear consistently. However, when a flaw is present, a signal will appear in the “time window” between the lateral wave and the LL signal. The depth of the flaw can be calculated from the arrival time of the flaw signal to the arrival time of the LL signal and the probe separation distance.

While the Time of Flight Diffraction inspection technique is quite simple and straightforward, there are limitations of the technique. Even as the technique detects defects in weld, its surface and near-surface sensitivity is reduced by the width of the lateral wave signal. The lateral wave signal blanks the near surface defects. The surface and near-surface sensitivity can be improved by reducing the pulse width of the lateral wave signal. The same applies for inspection of welds in thin plates, e.g. less than 10 mm, where the time difference between the lateral wave and LL wave is significantly reduced. The simplest way to increase the time difference between the lateral wave signal and the LL wave signal is to use transducers with the small pulse widths. To improve the examination, the following parameters have to be optimized and incorporated into the procedure.

• Transducer frequency: Higher transducer frequency reduces the pulse width thereby increasing the aperture between the lateral wave signal and LL signal. However, higher frequencies reduce ultrasound penetration.
• Transducer diameter:  Smaller transducer diameter reduces pulse width. However, smaller transducer means lower sensitivity or lower signal to noise ratio.
• Increased damping: Increased transducer damping reduces pulse width. Typical pulse width for TOFD transducers should be approximately 1-½ cycle. However, increased damping reduces sensitivity.
• Probe separation distance: Smaller probe separation will increase the time difference between the lateral wave signal and the LL wave signal. Probe separation is normally set to obtain beam intersection at 2/3 t to t.

TOFD Calibration: The calibration of TOFD should be made in such a way so that it determines the limit of TOFD in both (a) detection and (b) sizing.  The detection limit is determined by diffraction from an edge and the sizing limit requires resolving two signals from the top and bottom of a flaw. The recommended approach for TOFD calibration is to use side drilled holes that produce a signal from the top and bottom of the hole. This response will validate both detection and sizing .  Notches produce only one signal and therefore cannot be used to determine the TOFD sizing limit. TOFD probes with long pulses will pass the detection test on notches but could fail the sizing test on holes. Holes at different thickness level will assure both detection and sizing. There must definitely be a near surface hole to determine the near surface detection limit of the TOFD set up.

Thru-wall Crack in a 1.25 inch pipe.

References: Anmol S. Birring et al., "Ultrasonic Inspections of Welds by TOFD: Codes, Guidelines and Standards," Materials Evaluation, September 2005

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