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Technical Discussions
Frits Dijkstra
Frits Dijkstra
02:58 Aug-25-1998
Restrictions of TOFD

The Time Of Flight Diffraction (TOFD) method, invented in the UK in the seventies, is now being used in several countries for routine NDT of welds. A method description exists, in the form of prEN 583 part 6. In The Netherlands, acceptance criteria for weld defects detected with TOFD were recently developed in a Joint Industry project, in which a.o. plant owners, manufacturers, NDT service companies and authorities were involved.

Although TOFD has been demonstrated to offer a very good probability of detection, experts agree that the technique is connected with restrictions in flaw detectability near the surface. The extent of the "dead zone" is, as previously explained by Schlengermann and Wuestenberg on this web-forum, influenced by a.o. signal length. In addition, it appears that misalignment (hi-lo) of welds in practice may increase the influence of the dead zone. On the other hand, high-resolution transducers and software algorithms such as straightening and lateral wave removal may help, but not in all cases.

It should be kept in mind that radiography and manual (pulse echo) UT have substantial restrictions as well, which we have comfortably lived with for many years. Therefore, it seems not unreasonable that TOFD is now accepted to replace RT or manual (pulse echo) UT for selected applications. Sometimes, combinations of TOFD with other techniques such as pulse-echo UT and / or magnetic particle inspection (just like other NDT methods sometimes need) will be required to meet defect detection requirements. Equipment allowing such combined inspection (TOFD plus pulse-echo) exists, e.g. RTD Rotoscan or Combiscan or others. Such combinations, if correctly applied, can lead to very reliable NDT, because "the best of two worlds" can be combined, using one technique as a "safety net" for the other.

TOFD is an NDT technique with high potential, and it is used on an increasingly large scale. But, it is not the ultimate solution to every NDT problem.




    
 
 Reply 
 
Jan Verkooijen
Jan Verkooijen
07:24 Sep-23-1998
Re: Restrictions of TOFD
I would like to endorse what Mr. Dijkstra has written
here. As allready stated during my comments at the ECNDT
at Kopenhagen, and also said here by Mr. Dijkstra,
it is clear that Time Of Flight Diffraction
is not the sole answer to all NDT problems in the world.
There are applications, such as the pre-service inspection
of welds, where it has been proven that TOFD is perfectly
capable of replacing an other technique in standard applications.
There are also cases where it is advisable to supplement TOFD
with other techniques should this be necessary to assure
the safety and reliability of the component, just as we
are used to do with the exisiting NDT techniques.
To be able to determine whether/when this is necessary, and which
techniques should be chosen, we still need more objective data
on POD, false call rate, sizing capabilities etc, not only
for TOFD, but also for other NDE techniques. It is
therefore that currently an international program of work is
being prepared to produce such data, to validate acceptance criteria and to
advise on training and certification standards for TOFD.
Only if we all work towards standards which are accepted
and adhered to, we can expect TOFD to become
available to NDT professionals as another valuable
tool to assure the integrity of components in industry.

: The Time Of Flight Diffraction (TOFD) method, invented in the UK in the seventies, is now being used in several countries for routine NDT of welds. A method description exists, in the form of prEN 583 part 6. In The Netherlands, acceptance criteria for weld defects detected with TOFD were recently developed in a Joint Industry project, in which a.o. plant owners, manufacturers, NDT service companies and authorities were involved.

: Although TOFD has been demonstrated to offer a very good probability of detection, experts agree that the technique is connected with restrictions in flaw detectability near the surface. The extent of the "dead zone" is, as previously explained by Schlengermann and Wuestenberg on this web-forum, influenced by a.o. signal length. In addition, it appears that misalignment (hi-lo) of welds in practice may increase the influence of the dead zone. On the other hand, high-resolution transducers and software algorithms such as straightening and lateral wave removal may help, but not in all cases.

: It should be kept in mind that radiography and manual (pulse echo) UT have substantial restrictions as well, which we have comfortably lived with for many years. Therefore, it seems not unreasonable that TOFD is now accepted to replace RT or manual (pulse echo) UT for selected applications. Sometimes, combinations of TOFD with other techniques such as pulse-echo UT and / or magnetic particle inspection (just like other NDT methods sometimes need) will be required to meet defect detection requirements. Equipment allowing such combined inspection (TOFD plus pulse-echo) exists, e.g. RTD Rotoscan or Combiscan or others. Such combinations, if correctly applied, can lead to very reliable NDT, because "the best of two worlds" can be combined, using one technique as a "safety net" for the other.

: TOFD is an NDT technique with high potential, and it is used on an increasingly large scale. But, it is not the ultimate solution to every NDT problem.




    
 
 Reply 
 
Jorden
Jorden
00:38 Feb-03-2007
Osbaldo
http://forum.finddesk.org/animal-health-pet.html


    
 
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