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Technical Discussions
mehdi
Engineering, Advanced NDT
Iran, Joined Feb 2012, 25

mehdi

Engineering, Advanced NDT
Iran,
Joined Feb 2012
25
08:41 Nov-06-2013
tofd-spatial resolution

Dear colleagues
I want to know what is spatial resolution in TOFD and is it effective to change it in es-beamtool while designing scan plan for importing to td-scan?

even locus curve position is changeable in es-bemtool. is it effective too?

 
 Reply 
 
Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
13:12 Nov-06-2013
Re: tofd-spatial resolution
In Reply to mehdi at 08:41 Nov-06-2013 (Opening).

Mehdi

I have checked the index of the TOFD bible (Charlesworth and Temple) and there is no mention of it, nor even of resolution. It refers to spatial averaging, coherence and frequency though.

Best regards

 
 Reply 
 
frank ding
frank ding
13:24 Nov-06-2013
Re: tofd-spatial resolution
In Reply to mehdi at 08:41 Nov-06-2013 (Opening).

The resolution for TOFD have 2 direction: one is in thickness direction, and the other is in scan direction. It depended on the frequence, size, damping of probe; PCS etc.

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1282

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1282
14:01 Nov-06-2013
Re: tofd-spatial resolution
In Reply to mehdi at 08:41 Nov-06-2013 (Opening).

Mehdi, Spatial resolution is defined in the old EN 583-6

10.1.5 Spatial Resolution
The spatial resolution (R) is a function of depth and can be calculated by:
R = [C2(Td + Tp)2/4 - S2]½ - d

This is the approximation of the ring-time at some depth less than the backwall in TOFD and essentially indicates the smallest flaw height you can expect to resolve. If you assume the depth to be zero you effectively calculate the lateral wave dead-zone depth.
The Locus Curve indicates the range of actual depths that a flaw might have if it is not positioned at the exact mid-point between the probes. In some cases your weld cap may prevent the probes being used for a parallel scan to see the maximum depth (which occurs on the curve when the flaw is located at the mid-point of the probes). An effective way of overcoming this limitation is to use 2 probes in tandem as receivers. Placing the locus curves at the depths of the flaw as indicated in the 2 TOFD B-scan displays, the point of crossing of the two curves indicates the true depth and true offset of the flaw.

 
 Reply 
 
Stan
NDT Inspector,
Canada, Joined Jan 2009, 31

Stan

NDT Inspector,
Canada,
Joined Jan 2009
31
17:08 Nov-06-2013
Re: tofd-spatial resolution
In Reply to Ed Ginzel at 14:01 Nov-06-2013 .

Hi Ed:
In thinking about what you said in regard to using two receiver probes, the same results should be achievable if two offset scans, as defined in ASME V Article 4 Mandatory Appendix III, were used, provided the same indication could be seen in each data set. If this was tried using two different set ups, e.g for a two zone scan, with both set ups centered on the weld, then there would be two points of intersection, equally distant from the center of the weld, but I believe that the depth of the intersecting points should be the correct flaw depth. Is this correct Ed.

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1282

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1282
18:59 Nov-06-2013
Re: tofd-spatial resolution
In Reply to Stan at 17:08 Nov-06-2013 .

zoom image
Stan, indeed a 2 zone TOFD could provide a crossing point using the locii. However, if they are symmetrically placed about the centreline there are 2 crossing points of the loci so you have improved the estimate of depth, but you have not identified the side of the weld. With an offset scan (or the extra receiver) the loci cross at one point. In the attached image I used a 2 zone TOFD on a 35mm wall with a 60° pair crossing at 23mm and the 70° at 10mm. The "X" I placed at 20mm depth and we see that the 2 loci would indicate scan displays would indicate depths of 20.5mm and 21mm and there would be a crossing point of 20mm... but on both sides of the weld. The off-set probe would indicate the depth at about 20.2mm and the crossing point of the loci at 20mm depth would occur on only the right side of the centreline.
 
 Reply 
 
Mehdi
Engineering, Advanced NDT
Iran, Joined Feb 2012, 25

Mehdi

Engineering, Advanced NDT
Iran,
Joined Feb 2012
25
08:58 Nov-09-2013
Re: tofd-spatial resolution
In Reply to Ed Ginzel at 18:59 Nov-06-2013 .

Hi Ed
Thanks for replying.

There is an option in ESBeamtool to change the position of locus curve in the same PCS of probes.is it really possible to set the locus curve where to be?
or for a fix PCS of probes the location of locus curve is fixed too?

Regards.

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1282

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1282
14:42 Nov-09-2013
Re: tofd-spatial resolution
In Reply to Mehdi at 08:58 Nov-09-2013 .

Mehdi, in a 3-probe setup such as I illustrated, you would know the PCS of the probes. The locus of equal time is what ESBT is drawing for each probe-pair. The curve of equal time has a maximum depth at the midpoint. To use the curves to locate the flaw position you will need to know the depth to the same flaw as determined from your 2 TOFD displays. In my example I used 20.5mm and 21mm. Remember, in each TOFD display we use the first approximation that the flaw is at the midpoint of the probes. Therefore, to use the loci we place the curves at the depths we read from the individual TOFD displays. The crossing point of the curves indicates the actual location of the flaw.

 
 Reply 
 

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