Re: TOFD notches Hello Ed
I hope you dont mind some clarification comments on the document status first. (I had to look this up, cos I was not sure)
CEN upgraded the CEN/TC 121 committee document 377 to Technical Specification CEN/TS 14571:2004 "Welding - Use of time-of-flight-diffraction technique (TOFD) for examination of welds" on 11th July 2004 for "provisional application" with an initial limited validity of 3 years.
BSI along with 27 other European countries are required to announce the existence of the document in the same way as for an European Standard and to make it available promptly at the national level in "an appropriate form". BSI has done this as a Draft for Development - DD CEN/TS 14751:2004.
BSI's National foreword emphasises in bold print "This publication is not to be regarded as a British Standard". It expatiates that "..the time-of-flight technique has not been widely used for these applications (weld testing). It should be applied on this provisional basis, so that information and experienceof its practical application may be obtained".
After 2 years CEN members are requested to submit their comments, particulalrly on the question whether it can be converted into an European Standard. If so agreed then all conflicting National Standards must be withdrawn upon its full issue as an EN.
My input to your question now, Ed. Table 1, Section 5 of the DD (if I may call it that) specifies Examination Levels of which there are 4, viz. A thru D of increasing reliability. Column 4 of the table is "reference block for sensisitivity settings (see 10.1.4)" and stipulates there is a requirement for such for examination levels B thru D. All examination levels are applicable for pre-service inspections (yer pays yer money, yer takes yer choice!). Level D is for in-service inspection.
Section 10 "Range and sensitivity settings" and 10.1.4. "Sensitivity settings" specifies the hierarchy of lateral wave, backwall, grain noise as you stated in your post. It then continues that for reference levels B thru D "the sensitivity shall be VERIFIED by use of.." blocks containing either real flaws or machined discontinuities. Section 10.3.4. "Reference reflectors" gives the number required, i.e t = 6 - 25 mm minimum of 3, t > 25mm minimum of 5 - and Fig A4 shows a notched reference block and Fig A5 with side-drilled holes, all at various depths. Which is used seems to be a matter of agreement or perhaps referencing code.
10.2. "Checking of the settings" requires a check at least every 4 hours and sfter com[pletion of the test. and refers to Table 3 which requires that, for a sensitivity deviation from the reflector > 6dB, then "Settings shall be corrected and all examinations carried out since the last valid check shall be repeated".
So my understanding is that once the test sensitivity has been set by use of lateral, backwall or grain noise then the maximum screen height of each notch's analogue signal should be recorded along with the original system gain setting. If at the original gain setting any of the analogue responses diminish by > 6 dB then settings shall be corrected and all examinations carried out since the last valid check shall be repeated. There is a similar clause for the range for a deviation > 0,5mm or 2% of depth-range (whichever is greater). So the location of the notches will need to be initially measured for depth accuracy and recorded and checked at least every 4 hours.
I haven't tried it yet - our TOFD equipment is spanking new and not been used in anger yet - but I'll certainly post expereiences once we build up experience.
As for the 60 degree tip shape (could be difficult to machine and to verify), I dont know!
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: I am trying to determine the function of the reference notches as described in the various TOFD documents.
: BS-7706 indicates the notch as one of three options (grain noise and side drilled holes being the other options). BS-7706 suggests a near sided sharp pointed notch machined (EDM) to half the wall thickness. The sensitivity based on the diffracted signal being set to a set value (e.g. 80%FSH).
: EN583-6 indicates only a sensitivity setting based on 5% grain noise. It then lists the diffracting notches in the Annex but does not state how they are used, simply that:
: "The sensitivity setting can now be checked making use of representative flaws or diffracting artificial defects in a reference block as described in Annex A. The results can be used to justify
: reducing the gain setting or give warning that the signal-to-noise ratio is insufficient."
: The new EN Specification 121377 uses the Lateral wave set between 40-80% for single zones, the backwall set between 18-30dB over FSH where the lateral wave is not practical or present, and grain noise set at 5-10% for all other conditions. Reference blocks that include the 60° pointed notches are described in the Informative Annex A. Yet there seems to be no described method of using these targets. There are references in the document to using these for "verifying" sensitivity, for determining "adquacey of the testing (e.g. coverage and sensitivity)". But nowhere does it explain how these are to be done.
: I think there are a few good reasons to have TOFD "notches", but the sensitivity verification items seem to be a nonissue when the lateral wave, backwall and grain structure noise are all valid limits indicating we should use maximum sensitivity possible.
: The notches provide a method of verifying adequate beam coverage on the far side to carry out a single centred pass for a D-scan or if we are to use scans offset from the centreline because the beam divergence is not adequate for HAZ coverage.
: The notches also provide a reasonable method of determining the near and far-surface resolutions (deadzones) so the operator can decide if a higher frequency may be required.
: Can anyone suggest why the notches are required for sensitivity and why the specific tip shape is imposed (i.e. 60° point)?
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