|Rolf Diederichs |
NDT.net, Germany, Joined Nov 1998, 604
Can TOFD replace conventional UT on welding?
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?
APPLICATION OF MECHANIZED ULTRASONIC INSPECTION TO MANUALLY WELDED PIPELINE GIRTH WELDS
Ultrasonic testing and image processing for in-progress weld inspection
|Ed Ginzel |
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1252
Re: Can TOFD replace conventional UT on welding? : 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
: APPLICATION OF MECHANIZED ULTRASONIC INSPECTION TO MANUALLY WELDED PIPELINE GIRTH WELDS
: Ultrasonic testing and image processing for in-progress weld inspection
TOFD has been proposed as a viable option to Pulse-echo methods by some practitioners. However, it suffers from several shortcomings that can limit its effectiveness in some situations.
The lateral wave is usually used as a reference indicating the entry surface. It has the typical ringdown associated with pulse length at any interface. This ring time is a function of several factors including probe damping, element nominal frequency and angle of incidence. Near surface defects are either not detected or not possible to size due to the dead zone formed by the lateral wave. Advocates of the method point out that a disruption of the lateral wave can be seen where near surface nonfusion and undercut are present, however, the advantage of precise depth assessment associated with TOFD advantages is not seen here.
In a situation where a high-low (poor fitup or mismatch) exists between two welded pieces, the transmitted pulse forms two signals off the opposite wall, one from the higher side arriving prior to the one from the lower. The amount of high-low can be accurately assessed but if a defect occurred in the lower of the two sides the backwall signal from the higher side could obscure the defect.
In both cases a pulse-echo technique can be adapted to inspect these areas to provide improved coverage where TOFD is not ideal.
In the specific application described for pipeline girthweld inspections the problems are compounded by the relatively thin wall used. Typically 9mm to 16mm wall thicknesses are used in North American. The lateral wave duration for the 4 MHz probe used by one of the service providers is about 5mm. On a 9mm wall over 50% of the metal is not ideally inspected using TOFD. On a 16mm wall the situation improves but it is not totally corrected. I have managed to reduce this dead zone to about 2mm using a 15 MHz highly damped probe but grain structure starts to be detected. It is possible to apply digital signal processing to effectively cancel the lateral wave by adding a phase-reversed equivalent signal to the lateral wave signal. This is an attempt to look under the entry "noise" and see near surface signals. This is done off line and would therefore require more than doubling the evaluation time.
TOFD is more than just a useful addition. Because TOFD is not as sensitive to defect orientation it is a necessity to assess irregular volumetric flaws more common to manual welding. Specifications normally require assessment of indications based on amplitude which, although reasonable for planar flaws of a fixed orientations, suffers for non ideal reflection surfaces. Until now, pipeline inspections were satisfied with the pulse-echo techniques as the welding process was a mechanized Gas Metal Arc Welding process where primarily planar nonfusion was the concern.
Standards and proposed standards for TOFD exist but these are not referenced in the pipeline industry. For example, British Standard BS 7706 is a Guide for using TOFD and prEN583-6. There is a draft for Acceptance Criteria for TOFD based on workmanship and is mentioned in Insight (April 1997 vol. 39 #4) by F.Dijkstra et al.
|Rolf Diederichs |
NDT.net, Germany, Joined Nov 1998, 604
Re: Can TOFD replace conventional UT on welding?
Image of situation where TOFD is not ideal
Ed Ginzel provided this image for further explanation
of his message.
It was explained a situation where TOFD is not ideal:
"In a situation where a high-low (poor fitup or mismatch) exists between two welded pieces, the transmitted pulse forms two signals off the opposite wall, one from the
higher side arriving prior to the one from the lower. The amount of high-low can be accurately assessed but if a defect occurred in the lower of the two sides the backwall
signal from the higher side could obscure the defect.
In both cases a pulse-echo technique can be adapted to
inspect these areas to provide improved coverage where TOFD is not ideal".
|Udo Schlengermann |
Standards Consulting, Germany, Joined Nov 1998, 176
Re: Can TOFD replace conventional UT on welding? echnique 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 partswith 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 criticaldefects 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.
|H. Wuestenberg |
R & D
BAM Berlin, Germany, Joined Nov 1998, 26
Re: Can TOFD replace conventional UT on welding? The debate about advantages and drawbacks of the application of the TOFD approach for ultrasonic weld inspection should not forgot the original reasons for the introduction of the ultrasonic weld inspection during the 60th of this century.
The mayor advantage at that time had been the better crack detection potential of the ultrasonic method against x-ray techniques in view of an increased use of steels and welding technologies with a remarquarble risc of diverse cracking phenonema (e.g. cold cracking, transverse cracks etc. ). An ultrasonic technique based on the matching between the orientation of the beam and possible crack surfaces and/or based on the corner effect together with a suitable choice of the amplitude related sensitivity had been the back bone for a successful application of the ultrasonic weld inspection. The listed interactions are using a reflection rather than a diffraction at the defect that means the zero-th order diffraction instead of higher order diffractions. This is an important fact because the reflection (=zero-th order diffraction) can be reduced to fairly simple basic physical laws enabling us to predict the response from cracklike or other defects with reliable assumptions. Especially worst case conditions that means the influences reducing the echo answers from cracks can be taken into account (e.g. misorientation, mode conversions etc.). This is a distinct advantage against all other techniques using weaker interactions based on higher order diffractions, because in most cases they can only be calculated for idealized conditions like e.g. the response from a crack tip. Residual stress conditions, corrosion products between the crack boundaries or crack branching having a tremendous influence on the crack tip diffraction can only be estimated from experimental data but not be predicted and therefore not taken into account during a sensitivity setting procedure or the choice of suitable angles of incidence. This is until today the mayor reason why TOFD approaches will havedifficulties to guarantee under worst case assumptions the reliable detection of cracks in the before mentioned steel and welding technology combination with a certain risc of cracking. In addition also for the TOFD the dependency of an indication from the possible misorientation of linear diffraction sources have to be considered.
Fortunately modern welding procedures have in the past strongly reduced the probability of cracking bringing into the foreground again conventional quality deficiencies in welds like slags and porosity. For this situation the TOFD methods may be valuable tools because they can easely detect corresponding defects. This explains partly the obvious success of NDE vendors applying TOFD and should be considered during the application of TOFD. But one should not oversee the limitations for a reliable crack detection, which could result in severe riscs, if this technique will be used for welds with a somehow increased potential of cracks. Such a situation can nowadays not be excluded given the fact that a global economy orders welded products every where, even on places, where the experience with the cracking potential is not always present.
The possibility of TOFD methods to size crack dimensions once they are detected is not discussed here. This is of course an important advantage of all crack tip based sizing approaches and is not specific to the TOFD technique.