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02:38 Mar-05-1998

Rolf Diederichs

Director, Editor, Publisher, Internet, PHP MySQL
NDT.net,
Germany,
Joined Nov 1998
598
Is the time-reserval mirror technique industrial applied or applicable?

The current NDTnet issue (1998 No. 3, News and Abstract) reports on this sophisticated technique developed at the Laboratoire Ondes et Acoustique (Paris).

Tim-Reversal-Mirrors (TRM):
This technique is a completely different approach of the focusing problem through liquid-solid interfaces. It improves the focusing of the ultrasound, particularly for samples with complex shapes, and makes it possible to detect small defects with a greater degree of certainly than conventional UT. The technique is self adaptive and requires only the presence of a flaw in the sample of interest. It is based on the concept of time reversal of ultrasonic field. Acoustic Tim-Reversal-Mirrors (TRM) are made of large transducer arrays, allowing the acoustic field reflected by a defect to be sampled, time reversed and re-emitted. Such a time reversal mirror allows one to convert a divergent reflected wave issued from a defect into a convergent wave focusing on this defect. They are, among many other applications (e.g., medicalapplications), powerful instruments for nondestructive testing of solid media. It was shown the ability to detect very small defects in scattering samples (.4mm flat bottom hole at 140mm depth in forged titanium samples).
(Ref: UT Encyclopedia, shows also an image of its principle http://www.ndt.net/article/ut_az/ut_t/ut_t.htm#trm)

The Engine Titanium Consortium (ETC) carried out comprehensive work of ultrasonic detectability of embedded hard-alpha defects. The study delivered greatly superior performance in detection of defects by ultrasonic multizone technique, which is a conventional technique using multiple ultrasonic transducers, each designed to focus at a particular depth, operated with parallel channel instrumentation. The results indicate also that a lower cost alternative for zoned inspection ultrasonic hardware may be available with phased array ultrasonic transducers instead of multiple transducers. (more at http://www.ndt.net/abstract/asntf97/031.htm)

I would like to discuss the following issues:

1. Who can report on really industrial applied TMRs and not only experimental work?
2. No doubt of TMR's high sensitivity, but what NDT applications really needs it?

Side bar:
The aerospace industry prefers a risk based inspection instead of doing efforts to find all smallest defects. Cost-effective methods are obviously welcome. On the other hand a DC 10 crash in Sioux City, Iowa in 1998 was caused by a ' hard-alpha' defect in the titanium alloy.

Rolf Diederichs




 
01:03 Mar-06-1998

Hermann Wuestenberg

R & D
BAM Berlin,
Germany,
Joined Nov 1998
26
Time Reversal Mirror Technique and experiences Concerning the „Time Reversal Mirror“-technique

Dear Mr. Diederichs
Remark´s concerning our experiences with the TRM-technique.
This technique has been presented and discussed since a long time. The „Autofocussing“ procedures have some similarities with this technique.
Since we have applied phased probes for practical applications we have a somewhat different position concerning this approach. Our understanding of the technique reduces it´s importance to the problem of defining a delay time repartition (for transmission and reception) of a phased array probe system in order to produce intentionally a focussing at a given location. We are using several different procedures to achieve this. (But our practical experiences are limited to phased array probe systems with one dimensional arrays). Those methods are:
1. Use of a focussing probe design algorithm, which calculates the transducer curvatures in two planes (plane of incidence and perpendicular to the plane of incidence) taking into account the angle of incidence, the surface curvatures of the object to be inspected, it´s sound velocity values and if needed also anisotropy, water or perspex delay path etc. The transducer curvature and it´s inclination is translated into the delay time repartition needed for the specific angle of incidence and the desired focussing.
2. Use of approximative formulas taking into account surface curvature and delay pathes.
3. Use of a reference target in a testblock to optimize the focussing parameters derived according to 1. or 2. This approach is essentially valuable for complicated geometries, but it requires a testblock well adapted to the object geometry and material.
Especially for cases with perspex wedges to be adapted to surface curvatures this is a helpful procedure and supports also the performance demonstration towards third party observers. The optimization can be enhanced by the use of evolution theory based algorithms.
Only the third method has some similarity with the socalled TRM-technique. Most of our focussed efforts are based on point 1.
I am not convinced that the application of the TRM technique outside of this purpose (that means the determination of optimal focussing parameters) is a real need. Some ideas published in connection with this method remember the case where the target is painted after the arrival of the arrow.

Today, I will use the opportunity of a visit of M. Fleury at the BAM to discuss this problem with him.



 
08:14 Mar-07-1998
Mathias Fink
Re: Is the time-reserval mirror technique industrial applied or applicable? Dear Rolf Diederichs

First of all, I want to congratulate you for the fantastic work you do with UT Encyclopedia and the UT forum.

Concerning your two questions.
1- We have only investigate very carrefully the TRM techniques in our laboratory on Titanium billets coming from SNECMA. We have a lot of experimental data obtained in our lab on different defects of different sizes and depths. The work have been done with 2D arrays (128 elements) working at 5 Mhz and we are able to detect Flat bottom holes of .4mm diameter in the core of titanium billet (140mm depth).
We cannot report on really industrial applied TRMs because , to our knowledge, no other TRM electronics has been built in other groups. We are looking to find a company to industrialize the electronic prototype, which differs from usual multielements electronic by the fact that it needs programmable transmitter on each channel and some particular processing in the receive mode as described in a recent article that have been published in theissue of Physics World ( Mars 1998) " Ultrasound puts material to the test".

2. Because TRM focusing works as a matched filter to the propagating media and to defect responses it allows for medium with distorted beam a vey efficient approach to detect small defects. It allows also errors in the location of the array compared to the sample. For example in technology involving multi zone inspections with a set of Fermat surface transducers, any error on the positioning can have severe consequence in the detectability of defect.
Did NDT applications needs very low defect detection? I cannot respond to this question because my knowledge in metalurgy is very limited.

3. However, to respond to Hermann Wuestenberg, I dont agree with his last comment (the target is painted after the arrival of the arrow!!!). When you use an array of transducers and when you have to detect some defect in a medium; where you d'ont know the exact geometry, you have to choose some beam forming processes to obtain from the setof A lines that you obtained (on each A lines, the echo from the defect may be hidden by the echoes from the microstructure). If your beam-forming use a wrong set of delays, you will never discriminate the defect echo from the noise. This is what we observed with small defects located in the core of titanium billet (where the beams are distorted and were the scattering noise is important).
There is another point where TRM is more efficient than just focusing with delay time repartition. It is the case of dispersive guide- waves like lamb waves. In this case the use of delay time corrections is not enough to focus ( you need a different delay for each frequency), and with R.K. Ing we have shown, that even if a pulse gradually lengthens as it propagates, the TRM compensates for all the pulse dispersion and allows to detect defect with a very good signal to noise. A paper describing this technique has been accepted for publications by IEEE transactions UFFC (R. Ing, M. Fink: Time-reversed lamb waves).

Best regards
Mathias Fink




 
04:20 Mar-11-1998
Gerard Fleury
Re: Is the time-reserval mirror technique industrial applied or applicable? Dear Mr Diederichs,

Thank you for giving the opportunity to discuss this fascinating topic of Time Reversal Miror. Although the Time Reversal Miror concept lies beyond the domain of transducers it illustrates the large potentiality of new systems using array transducers.

This innovative technique is so different from all classical approaches and methods that a special attention is certainly needed to avoid any misunderstanding.To better understand the TRM, many papers have been published and I would suggest that Mathias Fink communicates to the Forum a list of articles. A recent and very interesting article has been published in the English Journal Physics World; if it is possible and notably if the Journal agrees it would be good to have a copy of this article available in the Forum.

The TRM requires an amount of special means and notably special electronics to drive indivual elements of an array with time reversed waveforms. At the moment, this is obviously a source of practical or economical limitations for accessing this technique. However it looks extremely promising and exciting results have been obtained in various configurations. The technique can also be used in a combination with classical approaches and notably with other autofocusing techniques. This combination could help the user to be fully confident in case he would like to make sure, following the warning of Mr Wuestenberg, that the "target is (not) painted after the arrival of the arrow" .

At Imasonic we had the pleasure to build special transducers to produce the TRM effect and we have been pleased to see the evolution of this approach and the growing enthousiasm and commitment of Mathias FINK and his team in spite of the numerous difficulties and the frequent septicism they had to face.
At last, if somebody would like to have a feeling of the coming applications of TRM, I would strongly recommend to visit Mathias Fink and his colleagues at the Laboratoire Ondes et Acoustique. This gives a truly impressive and prospective view of several future NDT and medical systems.

Gerard Fleury

: The current NDTnet issue (1998 No. 3, News and Abstract) reports on this sophisticated technique developed at the Laboratoire Ondes et Acoustique (Paris).

: Tim-Reversal-Mirrors (TRM):
: This technique is a completely different approach of the focusing problem through liquid-solid interfaces. It improves the focusing of the ultrasound, particularly for samples with complex shapes, and makes it possible to detect small defects with a greater degree of certainly than conventional UT. The technique is self adaptive and requires only the presence of a flaw in the sample of interest. It is based on the concept of time reversal of ultrasonic field. Acoustic Tim-Reversal-Mirrors (TRM) are made of large transducer arrays, allowing the acoustic field reflected by a defect to be sampled, time reversed and re-emitted. Such a time reversal mirror allows one to convert a divergent reflected wave issued from a defect into a convergentwave focusing on this defect. They are, among many other applications (e.g., medical applications), powerful instruments for nondestructive testing of solid media. It was shown the ability to detect very small defects in scattering samples (.4mm flat bottom hole at 140mm depth in forged titanium samples).
: (Ref: UT Encyclopedia, shows also an image of its principle http://www.ndt.net/article/ut_az/ut_t/ut_t.htm#trm)

: The Engine Titanium Consortium (ETC) carried out comprehensive work of ultrasonic detectability of embedded hard-alpha defects. The study delivered greatly superior performance in detection of defects by ultrasonic multizone technique, which is a conventional technique using multiple ultrasonic transducers, each designed to focus at a particular depth, operated with parallel channel instrumentation. The results indicate also that a lower cost alternative for zoned inspection ultrasonic hardware may be available with phased array ultrasonic transducers instead of multiple transducers. (more at http://www.ndt.net/abstract/asntf97/031.htm)

: I would like to discuss the following issues:

: 1. Who can report on really industrial applied TMRs and not only experimental work?
: 2. No doubt of TMR's high sensitivity, but what NDT applications really needs it?

: Side bar:
: The aerospace industry prefers a risk based inspection instead of doing efforts to find all smallest defects. Cost-effective methods are obviously welcome. On the other hand a DC 10 crash in Sioux City, Iowa in 1998 was caused by a ' hard-alpha' defect in the titanium alloy.

: Rolf Diederichs




 
01:56 Mar-16-1998

Rolf Diederichs

Director, Editor, Publisher, Internet, PHP MySQL
NDT.net,
Germany,
Joined Nov 1998
598
Article on Time-Reserval Mirror technique We are pleased to receive the copyright of the Institute of Physics UK
for an online version of the article:
Matthias Fink 'Ultrasound puts materials to the test'
ISBN: 0953-8585, Physics world FEB 01 1998 v 11 n 2
The url will be http://www.ndt.net/article/0498/fink/fink.htm

The April issue's articles will focus on NDT/UT of welding.
Nevertheless this article will appear as well,
so we'll have new material to continue our discussion on TRM.

Acoustic Tim-Reversal-Mirrors (TRM) principle is based on the concept of time
reversal of ultrasonic field. TRMs are made of large transducer arrays,
allowing the acoustic field reflected by a defect to be sampled,
time reversed and re-emitted.

Rolf Diederichs



 
07:07 Apr-08-2000
.
Re: Article on Time-Reserval Mirror technique : We are pleased to receive the copyright of the Institute of Physics UK
: for an online version of the article:
: Matthias Fink 'Ultrasound puts materials to the test'
: ISBN: 0953-8585, Physics world FEB 01 1998 v 11 n 2
: The url will be http://www.ndt.net/article/0498/fink/fink.htm

: The April issue's articles will focus on NDT/UT of welding.
: Nevertheless this article will appear as well,
: so we'll have new material to continue our discussion on TRM.

: Acoustic Tim-Reversal-Mirrors (TRM) principle is based on the concept of time
: reversal of ultrasonic field. TRMs are made of large transducer arrays,
: allowing the acoustic field reflected by a defect to be sampled,
: time reversed and re-emitted.
:
: Rolf Diederichs




 
07:08 Apr-08-2000
.
Re: Article on Time-Reserval Mirror technique : We are pleased to receive the copyright of the Institute of Physics UK
: for an online version of the article:
: Matthias Fink 'Ultrasound puts materials to the test'
: ISBN: 0953-8585, Physics world FEB 01 1998 v 11 n 2
: The url will be http://www.ndt.net/article/0498/fink/fink.htm

: The April issue's articles will focus on NDT/UT of welding.
: Nevertheless this article will appear as well,
: so we'll have new material to continue our discussion on TRM.

: Acoustic Tim-Reversal-Mirrors (TRM) principle is based on the concept of time
: reversal of ultrasonic field. TRMs are made of large transducer arrays,
: allowing the acoustic field reflected by a defect to be sampled,
: time reversed and re-emitted.
:
: Rolf Diederichs



 


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