Time of flight diffraction technique TOFD - an alternative to radiography?

Review of a regional DGZfP working group meeting.

TOFD Lecture - Podium Discussion

By Rolf Diederichs

Mr. Wilfried Hueck [1], the chairman of this DGZf working group [2], opened the 188th meeting in Dortmund on 13 May, 1997. It was a very interesting topic that attracted about 100 participants to the meeting.

TOFD lecture:

Mr. Udo Schlengermann * of Krautkraemer introduced the TOFD technique. He was one of the creators of the TOFD pre-standard 583 part 6 [3] and was also involved in the development of the British Standard BS 7709 1993 [4].

Here is an outline of Mr. Schlengermann's remarks:

• Evaluation of diffracted signals was not invented by TOFD. It is possible to apply only one probe to measure crack depth by use of the diffracted signals at a crack tip. However that is very imprecise since velocity is unknown and the angle relates to the material. It is also possible to measure crack depth using of the corner effect together with the diffracted signal [5]. That is not TOFD either.
  
• Diffracted waves have a different velocity than reflected longitudinal waves.
  
• The TOFD method only evaluates diffracted echoes which are 20dB smaller than the reflected echoes.
  
• The location of a flaw can be determined at the time of flight minimum, but that is more or less imprecise.
  
• Diffraction is stronger for longitudinal waves than for transversal waves.
  
• The standard is the use of longitudinal probes, 50° to 70°, small crystal, with widely spread sound beam to cover the whole defect.
  
• The two diffracted signals of the crack tip are generated with a 180° phase shift. The distance between the two signals on the time scale is nonlinear.
  
• TOFD always uses RF signals to display images, (minus = white, plus = black), although colors could be used.
  
• Images showing arcs which shapes can be used to illustrate the geometry of the flaw. Image post processing was applied to improve the image. It has always been assumed that images of defects can be better interpreted with TOFD than with A-scan images. However, this side-by-side-comparison disproves that.
  
• A disadvantage is that the gain must be very high, which produces a very high back wall echo and it is is not suitable for coarse grained materials.
  
• An image catalog has to be established. Example for a weld: Lack of root penetration, lack of side wall fusion, slag, porosity.
  
• The probe frequency should be 10 MHz or higher, frequencies under 5 MHz are not applicable.
  
• The Brit. Standard 7706 [4] uses a calibration block with a V notch. The instrument gain is not defined. The EN pre-standard 586-6 uses side drilled holes with slits to the surface [3].
  
• TOFD can perform more exact sizing than the reflection method.
  
• Crack edges must be sharp, and they aren't always!

O. A. Barbian - Heckhäuser - Prof. V. Deutsch - Dr. Bouma - K.-H Gischler

Podium Discussion: In the podium discussion, the majority of participants warned of disadvantages of the TOFD method. TOFD can solve specific problems but can not replace other established methods. The panel's major statements are summarized herein: O. A. Barbian , Pipetronix Cracks with cleft edges are almost impossible to see. TOFD is very undefined. It could be good applied as an in-service inspection but an exact protocol for all parameters must be established. TOFD is good for sizing. Generally speaking, the physics should be investigated first, and the standard developed after that. Heckhäuser , Gottfeld It's a good method for homogeneous materials. It was successfully applied for turbine blade testing. A precondition is that the crack edges are very clear. Since TOFD is fast it can be good for repeated testing. Prof. V. Deutsch , Karl-Deutsch TOFD works similar to the Delta Technique [6]. Some specific cracks can not be found. First comes the method and then we look for an application - that is not the right way; in the past it was the other way around. Dr. Bouma , RTD (NL) He has specialized in TOFD since 1993, and works with a team of 10 at RTD. He introduced the good results of the Netherlands welding institute (NIL) study. TOFD showed 90% POD in contrast to pulse echo, which showed only 50%. (The audience was critical of these results; the majority felt the study was not objective.) It is possible to test welds of 6 mm part wall thickness. The technique must be investigated more. TOFD is not a technique for all tasks. K.-H Gischler , Gischler He mentioned some bad experiences with tests performed on parts under 10 mm wall thickness. The method is a good add-on but can not replace other methods. Dr. Ewert , BAM The test standard should be fulfilled; that should be the priority, not "the cheaper method wins."

When the day started, the official theme was "Is TOFD an alternative to radiography?"
By closing time it was clear that TOFD is far from and a more appropriate title for our Theme of the Day might have been "Is TOFD a reliable testing method at all?" NDTnet - September 1997, Vol.2 No.09

1. Mr. Wilfried Hueck,
   VEBA Oel AG,
   D-45876 Gelsenkirchen,
   +49-209-366-2406, Fax +49-209-366-3491.
   
2. About the 19 DGZfP Regional Groups
   See also : Meeting of the chairmen of regional DGZfP groups
3. The European TOFD standard draft ENV 583-6
   
4. The British TOFD standard BS 7706
   
5. Ultrasonic crack depth measurement of surface bracking cracks in piping
   By T. Just, G. Csapo, TÜV Nord, Hamburg (D )
6. TOFD related terms in the UT Encyclopedia: |delta technique| |diffraction| |TOFD| |sizing technique|

Speaker

Udo Schlengermann
is principal investigator in the Application Laboratory of Krautkraemer
with over 30 years experience in ultrasonic testing applications.
In 1975 he received the Berthold award for outstanding research in the field of
transducer's beam propagation. He is also a Fellow of the DGZfP and the DGM.
Krautkraemer GmbH Co.
Robert-Bosch-Str. 3, P. 0. Box 13 63
D-50354 Huerth
Phon: +49-2233-6010, Fax: +49-2233-601402
Email: uschlengermann@krautkramer.de
Homepage: NDTnet Exhibition

For more information see: TOFD in UTonline 09/97