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Technical Discussions
mark
mark
21:13 May-06-2013
locus curve in tofd

Dear colleagues

can anybody explain me the concept of locus curve in tofd and its importance?

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1307

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1307
23:39 May-06-2013
Re: locus curve in tofd
In Reply to mark at 21:13 May-06-2013 (Opening).

Mark, the locus of equal time is merely a curve along which a flaw's depth could occur between the TOFD probes and be assessed at the same depth. The importance relates to the perceived depth. Our calculations for flaw depth assume that the flaw is always at the midpoint of the TOFD probes. But the locus indicates the other depths it might have if the flaw is not actually at the midpoint between the probes.

 
 Reply 
 
David Hermanutz
David Hermanutz
14:04 May-09-2013
Re: locus curve in tofd
In Reply to Ed Ginzel at 23:39 May-06-2013 .

Hi Mark,

Get a piece of string.

Fix the end of the string at two positions at any given PCS.

Pull the string taught with a pencil in the shape of a triangle, then draw the "locus curve" with your pencil while keeping the string taut.

The location of a defect is based on the transit time of the diffracted signal. Which can be equal, longer on the Tx side or longer on the Rx side depending on the axial position.




1
 
 Reply 
 
massimo carminati
Consultant, AUT specialist
IMG Ultrasuoni Srl, Italy, Joined Apr 2007, 691

massimo carminati

Consultant, AUT specialist
IMG Ultrasuoni Srl,
Italy,
Joined Apr 2007
691
17:06 May-09-2013
Re: locus curve in tofd
In Reply to David Hermanutz at 14:04 May-09-2013 .

Good example, David I've never thought to use this in my trainings...

 
 Reply 
 
Sufian
Sufian
01:14 Sep-21-2018
Re: locus curve in tofd
In Reply to David Hermanutz at 14:04 May-09-2013 .

Hi Sir.

Sory, may you show in video or picture. I am still not clear about this.
If possible.

 
 Reply 
 
Anmol Birring
Consultant,
Birring NDE Center, Inc., USA, Joined Aug 2011, 832

Anmol Birring

Consultant,
Birring NDE Center, Inc.,
USA,
Joined Aug 2011
832
20:51 Sep-21-2018
Re: locus curve in tofd
In Reply to massimo carminati at 17:06 May-09-2013 .

Follows the equation of a ellipse

1
 
 Reply 
 
Henry Herrera
R & D,
UT Quality, Canada, Joined Jun 2000, 19

Henry Herrera

R & D,
UT Quality,
Canada,
Joined Jun 2000
19
00:10 Sep-25-2018
Re: locus curve in tofd
In Reply to mark at 21:13 May-06-2013 (Opening).

Download

Hi Mark,
That's an interesting question.
Please have a look at the attached pdf, I have done some calculations for you to make it simpler.
I'm referring you to Fig 2.10 from Charlesworth and Temple (1989), but there is a self-contained explanation on the pdf.
In essence is this:
From figure 2.10 page 36. You have Tx and Rx which are the foci of an ellipse. The dotted line is the locus of points. This curve means that any pair of rays coming from the Tx to the curve and back to the Rx traveled the same distance.

Physically it means that if your reflector is at the center between Tx and Rx the depth will be correct. But if the diffraction source is away from the center, in any point along the locus, it will still produce the same reflection. That's the ambiguity in depth for any point in that curve.
I also shared the Matlab code to reproduce what I did. You can run it in Octave as well.

Maximo: David's idea is a great analogy, but be aware of stretching the string beyond the Tx and Rx. Those are the focal points, and the curve goes a bit further.

Hope this helps,
Regards,
Henry

 
 Reply 
 
Henry Herrera
R & D,
UT Quality, Canada, Joined Jun 2000, 19

Henry Herrera

R & D,
UT Quality,
Canada,
Joined Jun 2000
19
04:15 Sep-25-2018
Re: locus curve in tofd
In Reply to Anmol Birring at 20:51 Sep-21-2018 .

Hi Sufian,
I didn't notice this was an old post and you just reactivated it.
It is what Anmol says, follow the equations of an ellipse.
That's what did in my previous reply. I hope this helps you understand the rationale behind the TOFD locus curve.

Regards,
Henry

 
 Reply 
 
Daniel D.
Daniel D.
07:44 Sep-26-2018
Re: locus curve in tofd
In Reply to mark at 21:13 May-06-2013 (Opening).

zoom image



Dear Mark,
maybe the picture can help you. The TOF (time of flight) by line ABC and by line ADC it's equal. But there is different in depth and in position between probes. E.g. with two TOFD probes (with parallel scan) you can't determine position of indication between probes. You can find more information int the book Introduction to Phased Array Ultrasonic Technology Applications (https://www.olympus-ims.com/en/books/) page 74, chapter 2. Best regards,

Daniel Dopjera
 
 Reply 
 

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