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Re: Can TOFD replace conventional UT on welding?
Posted by: Udo Schlengermann , E-mail: uschlengermann@krautkramer.de, on May 23, 1997 at 14:00:00:
In Reply to: Can TOFD replace conventional UT on welding? posted by : Rolf Diederichs on May 20, 1997 at 16:55:43:
: Many advantages of the TOFD (Time of Flight Diffraction) method
: are reported in literature.
: Since the method works fast it can be an economically alternative to pulse echo UT or X-ray.
: What are the draw backs especially for welding testing?
: Is the method just a useful addition?
: Is there already a standard established?
: Rolf Diederichs
: References:
: APPLICATION OF MECHANIZED ULTRASONIC INSPECTION TO MANUALLY WELDED PIPELINE GIRTH WELDS
: http://www.ultrasonic.de/article/wsho0597/ginzel3/ginzel3.htm
: Ultrasonic testing and image processing for in-progress weld inspection
: http://www.ultrasonic.de/article/shaun/shaun.htm
This is an answer by Udo Schlengermann to the questions of Rolf Diederichs
„Can TOFD replace conventional UT on welding?"
and give also some additional details to the reply of Ed Ginzel:1. The British Standard 7706 (1993) ‘Guide to calibration and setting-up of the ultrasonic time-of-flight (TOFD) technique for the detection, locating and sizing of flaws’ shows some general examples of weld testing using TOFD but gives no acceptance criteria and no special calibration parameters or reference blocks for these applications.
Because of this lack of experience the European Standard ENV 583-6 (1997) „Time-of-flight diffraction technique as a method for defect detection and sizing" is only a preliminary standard (symbol V) for a three years period. More experience is needed
· on calibration procedures,
· on detection capability,
· on locating capability.
The capability of TOFD to size defects is out of question.
After that period there will be a ballot wether EVN 583-6 should be cancelled or revised into a ‘real’ European Standard.
2. The usual TOFD procedure uses wide ultrasonic beams to image not only diffracted signals but also the lateral wave on the surface and the reflection at the backsurface simultaneously. Scanning of areas with wide beams of course needs less time than scanning with narrow beams, but spatial resolution is better with narrow beams. So data collection on site by TOFD is faster than most conventional methods. But TOFD images have to be evaluated afterwards offline and this needs a lot of time and a big catalogue of typical TOFD images and experienced experts which slow down the process and increase cost compared to common techniques.
3. As Ed. Ginzel pointed out already, TOFD is a technique for precise depth assessment. This is because a digital flaw detector has a precise clock to measure time-of-flight. But to determine locations by TOFD, i.e. by time differences , some important assumptions have to be made.
· There must be clear signals
But diffracted signals are very weak, at least 20 dB less than reflected signals.
Not all natural defects, especially typical weld defects, generate diffracted signals, so TOFD will not detect them.
· The ultrasound velocity must be independent of the direction of propagation.
This is fulfilled by forged parts with low structural noise. But not by anisotropic materials and coarse grained materials. Also some rolled steels (thermomechnical processed) are anisotropic. So TOFD will mislocate signals in these materials and evalution of the image will be wrong.
4. Diffracted signals are only generated at edges (circular waves around the point of origin). But because wide sound beams are used with TOFD the interaction of this diffractor with the beam generates a long arch of possible locations of the point of origine (showing the extension of the beam, not the shape of the flaw). An untreated TOFD B-scan image therefore does not show a reconstruction of the defect, but only possible locations of special points of a defect. This problem can be solved by a synthetic aperture focussing algorithm (SAFT) But in practice today TOFD testing is done without it.
5. As Ed Ginzel already mentioned, the lateral wave and the backwall echo generate permanent dead zones for possible flaw signals close to these surfaces. This problem of course can be treated by signal processing procedures, e.g. by convolution, but it is not done till now.
6. When asking wether a testing technique can be replaced by another, the main task is wether a method can detect and evaluate the critical flaws of the object.
Here TOFD has some limitations, because cracks are most critical close to surfaces, and TOFD has dead zones there.
On the basis of fracture mechanics plane cracks are very critical to structures, but TOFD detects only the end of cracks by diffraction. Wether diffracting points are connnected (dangerous crack) or isolated must be evaluated using other criteria (phases of signals, shadowing effects on the reflected signal or on the lateral wave).
7. When ultrasonic testing has to be done for safety reasons, it is a high risk to replace a proofed technique by another one which seems to be faster or cheaper.
Safety and reliability of the testing are much higher if methods are used which offer strong interaction with critical defects for a safe detection and correct location.
For cracks these physical phenomena are reflection at planes or corners, reflection of surface waves or wave mode conversion.
Diffraction is a weak interaction and not all natural defects show diffraction.
Therefore in safety related testing TOFD cannot replace existing techniques which allow detection and locating of defects with higher reliabilty.
Udo Schlengermann
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