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Technical Discussions
BONNOT Fabien
BONNOT Fabien
08:39 Dec-01-2013
TOFD phase inversion

Hello everybody

I'm a french ultrasonic testing teacher (classic UT and spécificals technics like TOFD and phased array) and I have one comprehension problem with the TOFD phase inversion (it's not in the formation program).

I would like to know what 's the physical principle of this phase inversion between the two diffraction echoes (top and bottom) of a crack.

Some people talk about the PROGLER théory.

I know that physically there is always a phase inversion when a plan wave move in a circular wave (the diffraction) but why there is an inversion between the top and bottom diffraction (I think it's not an interference problem between them : very short impulsion and possibly high dimension of the crack so impossibility interference of the tow diffraction).

Thank a lot for help on this subject.

BONNOT Fabien

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1300

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1300
17:58 Dec-01-2013
Re: TOFD phase inversion
In Reply to BONNOT Fabien at 08:39 Dec-01-2013 (Opening).

Fabien, I think you might imagine this as the effect of the reflection coefficient.
If the reflecting material is acoustically soft then Z2 is small compared to Z1 and (Z2-Z1)/(Z2+Z1) gives a negative number, hence phase inversion. This is the condition for the pulse reflecting off the top of the flaw (and off the backwall). But the lateral wave and the signal from the bottom of the flaw are not reflections, but are instead the original beam being bent (diffracted), hence it has the phase opposite that of the "reflected" component.

1
 
 Reply 
 
BONNOT Fabien
BONNOT Fabien
17:41 Dec-02-2013
Re: TOFD phase inversion
In Reply to Ed Ginzel at 17:58 Dec-01-2013 .

Thank a lot for this very quick answer

I’me agree with the big difference acoustic impedance between steel and air so an opposite phase between lateral and backwall wave (reflection). But I don’t understand why are you talking about a reflection wave at the top of the flaw (it’s a diffraction wave like the bottom wave of the flaw). with finally two diffractions on top and bottom of a crack, why is there a phase inversion.

I’ve another question : there is a phase inversion between lateral and backwall wave (big acoustic different) but why isn’t there a difference between the lateral wave and a porosity wave (the same different acoustic impedance wave).

Sorry for all those interrogations but I’ve some doubt between the theory and the practice

Good week

BONNOT.F

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1300

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1300
23:28 Dec-03-2013
Re: TOFD phase inversion
In Reply to BONNOT Fabien at 17:41 Dec-02-2013 .

zoom image



zoom image



Fabien, it really does not matter if you call it reflected or diffracted. In medical it would be considered "forward scattered". The uploaded image is a photoelastic illustration of a 60° phased-array L-mode pulse having just passed a 1x3mm rectangular notch in glass. The incident pulse leads with a darker shade and I refer to it as negative. The spherical L-mode off the upper tip of the notch is seen to be phase-reversed (leading with a bright shade). The second image shows the pulse a bit later in time. The arc of the L-mode pulse is seen to extend up to the surface (leading with negative phase) and there it is called the lateral wave. But that same pulse, as it bends around the lower tip, has the same negative phase as it forms the spherical arc off the lower tip.
1
 
 Reply 
 
BONNOT Fabien
BONNOT Fabien
15:54 Dec-04-2013
Re: TOFD phase inversion
In Reply to Ed Ginzel at 23:28 Dec-03-2013 .

Thank a lot for this explication and those pictures. I understand now.

Have a good week.

Fabien

 
 Reply 
 
Rajeev J
, ADVANCED NDT ENGINEER (PAUT & TOFD)
India, Joined Oct 2014, 16

Rajeev J

, ADVANCED NDT ENGINEER (PAUT & TOFD)
India,
Joined Oct 2014
16
06:48 May-05-2017
Re: TOFD phase inversion
In Reply to Ed Ginzel at 23:28 Dec-03-2013 .

very interesting to read these conversations and got an clear idea about phase reversal but still i wish to know about one thing,

if a second medium is weaker for eg. crack in steel then the phase of top tip echoe will be positive
if i have a tungsten inclusion in the steel which is having higher Z value to steel then the phase reversal suppose to be same as lateral wave right?

is it possible to check with your simulations sir...?

Regards,
Rajeev J

 
 Reply 
 
John Pitcher
John Pitcher
09:11 May-05-2017
Re: TOFD phase inversion
In Reply to BONNOT Fabien at 08:39 Dec-01-2013 (Opening).

Further to the above replies, here is a section from the original TOFD training notes written by AEAT/Sonomatic many years ago.

3.3.3.3 Phase Relationships
An A-scan is reproduced in Figure 3.7 and contains the lateral wave and back wall signals. When a wave in a medium with a higher acoustic impedance is reflected at the interface to a lower acoustic impedance there is a phase change of 180 degrees (e. g. steel to water or steel to air) This means that if the waveform starts with a positive cycle before it hits the wall it will start with a negative cycle after reflection from the wall.



Figure 3.7 A-scan with no Defect Present

When a defect is present the situation shown in figure 3.8 occurs. The signal from the top
of the defect acts as if it had undergone a reflection from a back wall and has a phase change of 180 degrees, i.e. the phase is like a back wall and starts with a negative cycle. The bottom of a defect, however, acts as if the wave runs round the bottom without a phase change and the phase of the signal is like the lateral wave, i.e. it starts with a positive cycle. Theory shows that if two diffraction signals have opposite phase they must have a continuous crack between them. Theory also shows that in a few cases the top and bottom diffraction signals may not have a phase change of 180 degrees, but in general they will. Thus the recognition of phase change is very important for characterising signals and for making the most accurate defect sizing. For example two signals may be present which are from two slag lines rather than a single crack. In this case there will not be a phase change. Slag lines and pores are generally to thin to produce separate top and bottom signals.
Because the number of observed cycles in a signal very much depends on the amplitude of the signals it is often difficult to recognise the phase. This is especially true for the back wall which is generally saturated. In these situations it is important to sit the probes on the sample being examined or a calibration block and turn the gain down so that the back wall or any other difficult signal has the same screen height as the defect signals and then increase the gain making a note of how the signal grows with respect to the order of the phases. It is sometimes easiest to concentrate on the two or three most predominant cycles.
It is because the phase information is important that it is necessary to collect the TOFD un-rectified signals digitally.

 
 Reply 
 
Anmol Birring
Consultant,
Birring NDE Center, Inc., USA, Joined Aug 2011, 805

Anmol Birring

Consultant,
Birring NDE Center, Inc.,
USA,
Joined Aug 2011
805
15:03 May-05-2017
Re: TOFD phase inversion
In Reply to BONNOT Fabien at 08:39 Dec-01-2013 (Opening).

There are many articles on the web. This is covered under acoustics. Google search "reflection of sound phase change" See also http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/reflec.html

 
 Reply 
 
Mario Talarico
NDT Inspector,
Italy, Joined May 2010, 423

Mario Talarico

NDT Inspector,
Italy,
Joined May 2010
423
15:41 May-05-2017
Re: TOFD phase inversion
In Reply to BONNOT Fabien at 08:39 Dec-01-2013 (Opening).

I signal this simulation of the influence of the impedance change on the reflexion and transmission signal in the second medium.

http://pages.iu.edu/~kforinas/WJS/TransmissionJS.html

greetings
mario

 
 Reply 
 
Rajeev J
, ADVANCED NDT ENGINEER (PAUT & TOFD)
India, Joined Oct 2014, 16

Rajeev J

, ADVANCED NDT ENGINEER (PAUT & TOFD)
India,
Joined Oct 2014
16
12:44 May-08-2017
Re: TOFD phase inversion
In Reply to Ed Ginzel at 17:58 Dec-01-2013 .

dear Mr. Zincel,

I would like to clarify one thing from your reply,

If the same wave is hitting on a reflector how for diffraction it will be opposite and for bended or reflected wave it will be different ..?

how this equipment knows whether the signal is reflected or diffracted?

It has only the arrival time from the top and bottom tips of the reflector.

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1300

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1300
14:44 May-08-2017
Re: TOFD phase inversion
In Reply to Rajeev J at 12:44 May-08-2017 .

Rajeev, if you are indeed an Engineer and doing TOFD, you would of course be aware of the fact that the unrectified RF display is used and phase is apparent from the RF signal.

 
 Reply 
 
RAJEEV J
, ADVANCED NDT ENGINEER (PAUT & TOFD)
India, Joined Oct 2014, 16

RAJEEV J

, ADVANCED NDT ENGINEER (PAUT & TOFD)
India,
Joined Oct 2014
16
13:59 May-09-2017
Re: TOFD phase inversion
In Reply to Ed Ginzel at 14:44 May-08-2017 .

Dear Mr. Ed,

I knew phase inversions are apparent in unrectified Tofd display...

In your reply to Mr. Fabien it is clearly mentioned that

If the reflecting material is acoustically soft then Z2 is small compared to Z1 and (Z2-Z1)/(Z2+Z1) gives a negative number, hence phase inversion. This is the condition for the pulse reflecting off the top of the flaw (and off the backwall). But the lateral wave and the signal from the bottom of the flaw are not reflections, but are instead the original beam being bent (diffracted), hence it has the phase opposite that of the "reflected" component.

If the phase inversions are depends upon the signal whether it is diffracted or reflected then how these equipment will understand this..?

What is the real clue for the equipment to display these phase inversions...?
When only wave travel time is available...?

Please advice...

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1300

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1300
14:23 May-09-2017
Re: TOFD phase inversion
In Reply to RAJEEV J at 13:59 May-09-2017 .

Rajeev, now you have me concerned about your understanding of TOFD and the unrectified display. You said you were aware that TOFD uses the unrectified display. Then you state:
What is the real clue for the equipment to display these phase inversions...?
When only wave travel time is available...?

Yet if you are aware that TOFD uses the "unrectified" display, you would not state that "only wave travel time is available". In the unrectified display, phase is also available.
How have you been sizing vertical extent of planar flaws, if it is not by noting the signal phase between upper and lower tip signals?

 
 Reply 
 
Rajeev J
, ADVANCED NDT ENGINEER (PAUT & TOFD)
India, Joined Oct 2014, 16

Rajeev J

, ADVANCED NDT ENGINEER (PAUT & TOFD)
India,
Joined Oct 2014
16
07:16 May-10-2017
Re: TOFD phase inversion
In Reply to Ed Ginzel at 14:23 May-09-2017 .

Dear sir,

In an unrectified display phase is also available as an information to assess the height or vertical extent of the flaw.

The unrectified display is also can be used for the pulse echoe to check the bond quality and to asses some reflectors.

I have been sizing the flaws by keeping the phase inversions in mind to verify whether the signals are originated from the same reflector or not.

let me put my question to you like this,

I am an tofd equipment i am using two probes one is transmitting and other is recieving now an l wave travelled inside a medium and intracted with some boundaries and then some energy is recieved in different intervals with respect to the position of the boundaries from the top surface.

Now my task is to display these inforations in unrectified display where the phase also can be displayed
i can display the depth or relative positions of the reflections or diffraction based on the travel times. But to display the phase what is my source, if the interaction of the wave with different boundaries either diffraction or reflection is the reason for this then, how i (as a machine with unrectified display) can understand this...?

My intention is not to put unnecessary questions to you, all i am trying is to learn something from your suggestions.

Hope you understand...

Thanks for your responses sir...

 
 Reply 
 
John
John
09:15 May-10-2017
Re: TOFD phase inversion
In Reply to Rajeev J at 07:16 May-10-2017 .

Dear Rajeev,

May I recommend some light reading on the subject TOFD.
Engineering Application of Ultrasonic Time-of-Flight Diffraction, by Charlesworth and Temple, is an excellent read. It will answer all your questions,

JohnP

 
 Reply 
 
Daniel Braun
Daniel Braun
17:53 May-10-2017
Re: TOFD phase inversion
In Reply to Rajeev J at 07:16 May-10-2017 .

Whilst dealing with the TOFD data interpretation it is necessary to perform the pattern recognition for the imperfections found. The simple examples:

1. for the near surface breaking crack you'll get the lateral wave interruption along with the later coming signal, which' first half wave has the same polarity as lateral wave;

2. for the far surface breaking crack you'll have the back echo interruption along with the signal, which' first half wave has the same polarity as back echo, which is opposite to lateral wave;

3. for the internal crack you'll have 2 signals appearing and disappearing practically simultaneously whilst scanning, the first half wave polarity of the upper tip will have the same polarity as the back echo and the first half wave polarity for the lower tip will have the same polarity as the lateral wave; etc - that's the first reason for using the RF signal presentation and recording. The second one relates to the precision of TOF measurement - upon the signal is identified the TOF definition will be most precise if the measurement cursor is matching with the zero-crossing point

All above will not be possible for the rectified signals

There is very illustrative material available in this article: http://www.sonotronndt.com/NDTWORLD/2012_3/13_21.pdf - it was prepared for the very beginning techs: that what they learn during the first 3 hours in our training class with hands on from the first minute. Despite the source is not in English I presume you'll be able to pass through the A-Scans, which were obtained on the very simple sample - for each A-Scan you may see the probes positioning and the reflectors / diffractors in the sample. Moreover you may manufacture the same sample and try with the regular A-Scan flaw detector provided it may represent the signals in RF form. I'm sure after the simple trials you'll feel better understanding

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

Henry Herrera

R & D,
UT Quality,
Canada,
Joined Jun 2000
19
18:40 May-10-2017
Re: TOFD phase inversion
In Reply to Rajeev J at 07:16 May-10-2017 .

zoom image



Rajeev,
Let’s see if this helps you understand how the phase works in RF signals. This example extends what Ed Ginzel already said in his comment on Dec-02-2013. See his simulation where Ed mentions the back-diffracted positive phase and negative phase. Note, that positive phase is associated with the bright wave and negative is the dark one.

I have created a zero phase pulse and rotated it from 0 degree to 180. Note that zero phase means a wavelet that is perfectly symmetric to the center. This center is the maximum and it corresponds to the bright spot in Ed’s example. Then the wavelet is rotated until this maximum is reversed (180 degrees rotation) which is the dark spot on the negative phase in Ed’s simulation.

This is a simple example with y(t) the wavelet and to rotate it, you just add a constant phase to its spectrum. I.e., Y(f) = FT (y(t)) and then you rotated it as Y(f) = abs(Y(f)) + exp (i * angle(Y(f)) + PhaseShift).
Phase shifts in the figure are 0, 45, 90, 135, and 180 degrees.

John already suggested the book by Charlesworth and Temple. I will add the paper:
Ogilvy, J.A. & Temple, J.A.G., 1983. Diffraction of elastic waves by cracks: application to time-of-flight inspection. Ultrasonics, 21(6), pp.259–269. Available at: http://linkinghub.elsevier.com/
Here are some comments from the paper:
- When a sound wave is incident on a crack tip it may undergo a phase change on diffraction.

This phase changes can be obtained from the scattering amplitude Gp(theta, beta) ) and
“Calculations show that for incident and diffracted compression waves there is always a phase difference of PI between the signals from the top and bottom of a crack. ”

On page 267 of the same paper:
Phase of diffracted signal
"A phase difference of PI exists between the signal diffracted from the top and bottom of a defect, when using compression waves. For shear waves, the phase difference depends on the incident and diffracted angles."
 
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