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Technical Discussions
John
John
03:22 Nov-28-2006
why TOFD cursor is parabola?

I want to know why TOFD cursor is parabola?and what decide the parameter of the curve?is there any equation?Thanks a lot

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

Ed Ginzel

R & D, -
Materials Research Institute,
Joined Nov 1998
1307
03:28 Nov-28-2006
Re: why TOFD cursor is parabola?
YunXi:
Typically TOFD uses a relatively small diameter element. The pressure boundary of such a probe can be approximated by a cone. Now imagine passing such a cone past a small point diffractor in its path. The point will first be detected far away as the edge of the cone detects the flaw. As the probe is moved along its straight scan path the flaw gets closer until the flaw is directly across from the probe. As the probe continues its scan movement past the flaw the process reverses and the flaw gets further away. The effect is like taking a slice through the cone. If you were to cut a cone at some point parallel to the central axis the remaining cross section is the definition of a parabola!

The actual shape of the beam boudary is defeined by a Bessel function but the effect is effectively the same as a cone.

The same effect can be seen on the shape of small targets in pulse-echo but the process is called backscatter instead of TOFD (forward scatter).

Ed

----------- Start OriginalMessage -----------
: I want to know why TOFD cursor is parabola?and what decide the parameter of the curve?is there any equation?Thanks a lot
------------ End Original Message ------------

John
John
03:04 Nov-29-2006
Re: why TOFD cursor is parabola?
Some guys use two phase array probe as TOFD,would you like tell me that is there any differene between typical TOFD?

----------- Start Original Message -----------
: YunXi:
: Typically TOFD uses a relatively small diameter element. The pressure boundary of such a probe can be approximated by a cone. Now imagine passing such a cone past a small point diffractor in its path. The point will first be detected far away as the edge of the cone detects the flaw. As the probe is moved along its straight scan path the flaw gets closer until the flaw is directly across from the probe. As the probe continues its scan movement past the flaw the process reverses and the flaw gets further away. The effect is like taking a slice through the cone. If you were to cut a cone at some point parallel to the central axis the remaining cross section is the definition of a parabola!
: The actual shape of the beam boudary is defeined by a Bessel function but the effect is effectively the same as a cone.
: The same effect can be seen on the shape of small targets in pulse-echo but the process is called backscatter instead of TOFD (forward scatter).
: Ed
:
: : I want to know why TOFD cursor is parabola?and what decide the parameter of the curve?is there any equation?Thanks a lot
------------ End Original Message ------------

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

Nigel

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
09:00 Nov-30-2006
Re: why TOFD cursor is parabola?
I have very much appreciated the Platonic dialogue between Yun Xi and Ed G on aspects of phased array. I would not intervene but Ed's last post has a broken link as of my attempt to access it.

So can I (without deep knowledge of the subject) supposition that the wedges will have been machined so that the incident angle of the L-wave from the transducer was approximately 30 degrees (to generate a 45 degree shear-wave in steel). But a focal law of the phased array probe could be constructed to generate a suitable range of angles in the wedge to produce L-waves in the test material. Thus if the PA probe produced a 16 degree beam in the shearwave wedge this would give a 45 degree L-wave in steel. Assuming similar acoustic velocities between the phased-array wedge and the attached "shear-wave" wedge would mean more sound transmission than refraction or reflection at the intrface. Thus the experienced PA focal-law designer would be able to stipulate which incident angles to generate to produce the suitable L-waves in the test material.

Hopefully Ed will return or other experienced people will write to either confirm or refute my supposition and perhaps add some other information.

Thanks guys.

John
John
06:32 Dec-07-2006
Re: why TOFD cursor is parabola?
Thank you Nigel and Ed, I understand that I can generate LW with SW wedge.
Another question: when I use two PA probes as TOFD, what is the difference between
TOFD and Pitch&Catch?
Thanks

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

Nigel

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
09:06 Dec-09-2006
Re: why TOFD cursor is parabola?
Hi John

Can we assume we are using standard and not PA wedges. It may not have any significance but i will feel more comfortable about that.

For me pitch and catch is a bulk wave technique relying on reflection from a surface and conforming to Snell's Law. My main exposure to pitch-and-catch is using tandem probes maintained at a set distance apart in a frame. It may be top-down or bottom-up, 3-leg or 5-leg. Top down means the transmitted probe interrogates a zone of the sidewall on its first leg, bottom-up on its second-leg (reflected from the ID). I use a triangle shape as a means of memorising the set-up - to look at a top fill zone with a top down the probes should be set close together in the frame, to look at a bottom fill the probes should be set further apart.) It is the inverse for bottom-up set-ups.

A pitch-catch set-up in a similar configuration to a TOFD set-up (transmitter and receiver on opposite sides of the weld) should by definition be a "shadow" technique, i.e. the signal amplitude is set up on a clean section of pipe. During the weld scan any defect will mask some of the incident energy causing a drop in the received sound intensity on the opposite side of the weld.

TOFD relies on diffraction of the bulk wave into a spherical wave radiating out from the tip defect in all directions. As with pitch-catch some sound energy will be reflected from a reflector but it will not be incident upon the receiver. Any reflected back wall signals will take longer to travel and thus show up later than the diffracted signals. Thus the characteristic TOFD display shows at the top the lateral creep wave travelling in the most direct route just sub-surface - this gives the top surface dead zone. At the bottom of the display is the reflected longitudinal wave reflected from the backwall or the root of the weld - arguments about whether this implies a backwall dead zone rage. I think that we acknowledge TOFD is capable of revealing root or backwall defects but there may be occasions where such defect indications are masked by the reflected backwall indication. Beyond this are reflected shear waves. In between the top and bottom surfaces any received signals should be due to diffraction.

I hope others (Ed) correc tmy mistakes or fill-in my omissions.

Regards

Nigel

----------- Start Original Message -----------
: Thank you Nigel and Ed, I understand that I can generate LW with SW wedge.
: Another question: when I use two PA probes as TOFD, what is the difference between
: TOFD and Pitch&Catch?
: Thanks
------------ End Original Message ------------

John
John
03:30 Dec-11-2006
Re: why TOFD cursor is parabola?
Ok, I confirm that my understanding is true or not: pitch&catch and TOFD both can use longitudinal wave, but pitch&catch technology is based on reflection signal, TOFD use diffraction signal. But how to distinguish the signal is generate by reflection or diffraction?
Thanks

----------- Start Original Message -----------
: Hi John
: Can we assume we are using standard and not PA wedges. It may not have any significance but i will feel more comfortable about that.
: For me pitch and catch is a bulk wave technique relying on reflection from a surface and conforming to Snell's Law. My main exposure to pitch-and-catch is using tandem probes maintained at a set distance apart in a frame. It may be top-down or bottom-up, 3-leg or 5-leg. Top down means the transmitted probe interrogates a zone of the sidewall on its first leg, bottom-up on its second-leg (reflected from the ID). I use a triangle shape as a means of memorising the set-up - to look at a top fill zone with a top down the probes should beset close together in the frame, to look at a bottom fill the probes should be set further apart.) It is the inverse for bottom-up set-ups.
: A pitch-catch set-up in a similar configuration to a TOFD set-up (transmitter and receiver on opposite sides of the weld) should by definition be a "shadow" technique, i.e. the signal amplitude is set up on a clean section of pipe. During the weld scan any defect will mask some of the incident energy causing a drop in the received sound intensity on the opposite side of the weld.
: TOFD relies on diffraction of the bulk wave into a spherical wave radiating out from the tip defect in all directions. As with pitch-catch some sound energy will be reflected from a reflector but it will not be incident upon the receiver. Any reflected back wall signals will take longer to travel and thus show up later than the diffracted signals. Thus the characteristic TOFD display shows at the top the lateral creep wave travelling in the most direct route just sub-surface - this gives the top surface dead zone. At the bottom of the display is the reflected longitudinal wave reflected from the backwall or the root of the weld - arguments about whether this implies a backwall dead zone rage. I think that we acknowledge TOFD is capable of revealing root or backwall defects but there may be occasions where such defect indications are masked by the reflected backwall indication. Beyond this are reflected shear waves. In between the top and bottom surfaces any received signals should be due to diffraction.
: I hope others (Ed) correc tmy mistakes or fill-in my omissions.
: Regards
: Nigel
:
: : Thank you Nigel and Ed, I understand that I can generate LW with SW wedge.
: : Another question: when I use two PA probes as TOFD, what is the difference between
: : TOFD and Pitch&Catch?
: : Thanks
------------ End Original Message ------------

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

Nigel

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
02:42 Dec-11-2006
Re: why TOFD cursor is parabola?
Amplitude. Any diffracted signals will be very weak (-30dB from reflected signals). Why doesn't your company buy a simple TOFD package with data processing software and then, with a reference block, you can see for yourself exactly what it will do.

----------- Start Original Message -----------
: Ok, I confirm that my understanding is true or not: pitch&catch and TOFD both can use longitudinal wave, but pitch&catch technology is based on reflection signal, TOFD use diffraction signal. But how to distinguish the signal is generate by reflection or diffraction?
: Thanks
: : Hi John
: : Can we assume we are using standard and not PA wedges. It may not have any significance but i will feel more comfortable about that.
: : For me pitch and catch is a bulk wave technique relying on reflection from a surface and conforming to Snell's Law. My main exposure to pitch-and-catch is using tandem probes maintained at a set distance apart in a frame. It may be top-down or bottom-up, 3-leg or 5-leg. Top down means the transmitted probe interrogates a zone of the sidewall on its first leg, bottom-up on its second-leg (reflected from the ID). I use a triangle shape as a means of memorising the set-up - to look at a top fill zone with a top down the probes should be set close together in the frame, to look at a bottom fill the probes should be set further apart.) It is the inverse for bottom-up set-ups.
: : A pitch-catch set-up in a similar configuration to a TOFD set-up (transmitter and receiver on opposite sides of the weld) should by definition be a "shadow" technique, i.e. the signal amplitude is set up on a clean section of pipe. During the weld scan any defect will mask some of the incident energy causing a drop in the received sound intensity on the opposite side of the weld.
: : TOFD relies on diffraction of the bulk wave into a spherical wave radiating out from the tip defect in all directions. As with pitch-catch some sound energy will be reflected from a reflector but it will not be incident upon the receiver. Any reflected back wall signals will take longer to travel and thus show up later than the diffracted signals. Thus the characteristic TOFD display shows at the top the lateral creep wave travelling in the most direct route just sub-surface - this gives the top surface dead zone. At the bottom of the display is the reflected longitudinal wave reflected from the backwall or the root of the weld - arguments about whether this implies a backwall dead zone rage. I think that we acknowledge TOFD is capable of revealing root or backwall defects but there may be occasions where such defect indications are masked by the reflected backwall indication. Beyond this are reflected shear waves. In between the top and bottom surfaces any received signals should be due to diffraction.
: : I hope others (Ed) correc tmy mistakes or fill-in my omissions.
: : Regards
: : Nigel
: :
: : : Thank you Nigel and Ed, I understand that I can generate LW with SW wedge.
: : : Another question: when I use two PA probes as TOFD, what is the difference between
: : : TOFD and Pitch&Catch?
: : : Thanks
------------ End Original Message ------------

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

Ed Ginzel

R & D, -
Materials Research Institute,
Joined Nov 1998
1307
03:15 Dec-11-2006
Re: why TOFD cursor is parabola?
True, amplitude is normally lower for diffracted waves but that is also true of reflections in off-axis portions of the beam. But the question is misleading. "Pitch-catch" simply refers to a probe configuration. Reflection or diffraction refers to the way in which a wavefront interacts with a boundary. TOFD, although we may use upper and lower "tip diffracted signals" to attempt vertical extent sizing, also detects the backwall via "reflection". Also, the large area presented by an interpass nonfusion will provide TOFD users a large amplitude signal with no upper and lower tip resolvable. Therefore TOFD uses "reflection" to detect (and characterise) interpass nonfusion.

Another misconception that seems to be occurring in John's "pitch-catch" terminology is relatvie position of the transmitter and receiver. "Pitch-catch" is also termed transmit-receive and separates the functions of the transmitting and receiving elements (as opposed to pulse-echo where the two functions occur in the same element). If the T-R elements are side-by-side the effect is essentially the same as pulse-echo and we are interested in backscattered signals. But the T-R elements may be at any orientation to one another.
Go to the NDT.net online encyclopedia http://www.ndt.net/article/az/ut_idx.htm
and look up pitch-catch, delta technique, and tandem technique as examples.
Ed

----------- Start Original Message -----------
: Amplitude. Any diffracted signals will be very weak (-30dB from reflected signals). Why doesn't your company buy a simple TOFD package with data processing software and then, with a reference block, you can see for yourself exactly what it will do.
: : Ok, I confirm that my understanding is true or not: pitch&catch and TOFD both can use longitudinal wave, but pitch&catch technology is based on reflection signal, TOFD use diffraction signal. But how to distinguish the signal is generate by reflection or diffraction?
: : Thanks
: : : Hi John
: : : Can we assume we are usingstandard and not PA wedges. It may not have any significance but i will feel more comfortable about that.
: : : For me pitch and catch is a bulk wave technique relying on reflection from a surface and conforming to Snell's Law. My main exposure to pitch-and-catch is using tandem probes maintained at a set distance apart in a frame. It may be top-down or bottom-up, 3-leg or 5-leg. Top down means the transmitted probe interrogates a zone of the sidewall on its first leg, bottom-up on its second-leg (reflected from the ID). I use a triangle shape as a means of memorising the set-up - to look at a top fill zone with a top down the probes should be set close together in the frame, to look at a bottom fill the probes should be set further apart.) It is the inverse for bottom-up set-ups.
: : : A pitch-catch set-up in a similar configuration to a TOFD set-up (transmitter and receiver on opposite sides of the weld) should by definition be a "shadow" technique, i.e. the signal amplitude is set up on a clean section of pipe. During the weld scan any defect will mask some of the incident energy causing a drop in the received sound intensity on the opposite side of the weld.
: : : TOFD relies on diffraction of the bulk wave into a spherical wave radiating out from the tip defect in all directions. As with pitch-catch some sound energy will be reflected from a reflector but it will not be incident upon the receiver. Any reflected back wall signals will take longer to travel and thus show up later than the diffracted signals. Thus the characteristic TOFD display shows at the top the lateral creep wave travelling in the most direct route just sub-surface - this gives the top surface dead zone. At the bottom of the display is the reflected longitudinal wave reflected from the backwall or the root of the weld - arguments about whether this implies a backwall dead zone rage. I think that we acknowledge TOFD is capable of revealing root or backwall defects but there may be occasions where such defect indications are masked by the reflected backwall indication. Beyond this are reflected shear waves. In between the top and bottom surfaces any received signals should be due to diffraction.
: : : I hope others (Ed) correc tmy mistakes or fill-in my omissions.
: : : Regards
: : : Nigel
: : :
: : : : Thank you Nigel and Ed, I understand that I can generate LW with SW wedge.
: : : : Another question: when I use two PA probes as TOFD, what is the difference between
: : : : TOFD and Pitch&Catch?
: : : : Thanks
------------ End Original Message ------------

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