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Characterization of the effects of Water and Disbond on Ultrasonic Signals in Honeycomb Composite Structures
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19:02 Jan-06-2011

wolfgang Bisle

R & D, Project leader & Group Leader InService Inspect.,
Airbus Deutschland GmbH,
Germany,
Joined Jul 2000
35
1308 views
Characterization of the effects of Water and Disbond on Ultrasonic Signals in Honeycomb Composite Structures
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This article shows a typical student sight of that theme with limited practical background. I think the student is not responsible about the quality of the approach but the tutor should have better insight to guide a young fellow.
The solution proposed is not state of the art. I recently did a presentation on the ATA NDT Forum covering a very similar subject - NDT Toolbox for Honeycomb Sandwich Structures/
http://www.airlines.org/SafetyOps/EM/Documents/2010%20NDT%2018%20Toolbox%20for%20Honeycobm%20Sandwich%20Structures.pdf
The use of neutron radiography is for me a totally overpowered method, the same can be rached by thermography (long pulse, flash, modulation) - see my presentation, we showed several very successfull options - as well with radiography. A well trained inspector quickly can interprete singal from both method, especially Thermography is a cheap and highly portable solution, where you must not disassemble control surfaces from the aircraft. This is especially valid in case of vertical structures like rudders.
Another approach is single side applied Ultrasonic - using low frequencies (250-800KHz), for detecting disbonds this method is limited to nearly parallel-skin honeycomb structures as it uses a guided wave approach resonating with the cell walls. For detecting fluid ingress it works also on tapered structures - see the mentioned presentation. We lack currently powerfull UT array transducers in the range of 0.5..1 MHz, now only one Imasonic transducer nearly fits our needs (but others are catching up); In case the mentioned Guided Wave approach is finally completely transferred to Array Transducers, there will be an extremly fast inspection method available, even coupling problems on aged and contaminated surfaces can be overcome wtih a new revolutionary way of coupling - we are working on this.
But neutron radiography and squirter UT is not necessary and inspecting wet is overcome by aircoupled UT or as others say "NCU" see
http://www.dr-hillger.de/Inhalte/Englisch/Luftultraschall_e.html
 
 
19:12 Jan-07-2011
Thomas Krause
Re: Characterization of the effects of Water and Disbond on Ultrasonic Signals in Honeycomb Composite Structures
In Reply to wolfgang Bisle on 19:02 Jan-06-2011 (Opening).

Wolfgang, I agree that water ingress is detectable by various forms of ultrasonics and thermography. However, to my knowledge only through-transmission ultrasonics has the potential to determine whether or not disbond is present when water is also present in the adjoining honeycomb cells. This was the point of the presentation.

 
 
15:48 Jan-10-2011

Wolfgang Bisle

R & D, Project leader & Group Leader InService Inspect.,
Airbus Deutschland GmbH,
Germany,
Joined Jul 2000
35
Re: Characterization of the effects of Water and Disbond on Ultrasonic Signals in Honeycomb Composite Structures
In Reply to Thomas Krause on 19:12 Jan-07-2011 .

Download ATA NDT Forum 2007 Hicken - Rudders structural inspectionsPresented

Dear Thomas,
Sorry but I guess you are not following the technology applied to aircraft, especially not to Airbus aircraft.
Since a lot of years there are procedures included in the NonDestructive Tesing Manuals of all Airbus families, using the approach I described in my posting. It has also been presented in several ATA NDT Fori (or shall I say Forums) by my colleagues and so it can be said it is industry standard. Even if you go back to the homepage of Olympus you will find the method described there as UT bond testing for rear side debonding of sandwiches.
If you go for
http://www.olympus-ims.com/en/ndt-application/183-id.209715294.html
and head down to the paragraph "Conventional Ultrasonic Testing" the principle of the method is described there.
Dr. Hillger offers a special system for those inspections, if you look to the last page of this flyer
http://www.dr-hillger.de/PDF/LF_Ultrasonics.pdf
you will see a picture using this technique an Al honeycomb structures.
Refer also to this article:
http://www.ndt.net/article/ecndt98/aero/011/011.htm
and the DAMDOS Project.
Some of the older presentation of e.g. ATA NDT Forum 2007 are not available on the net anymore.

So as you see, sometime industry is maybe far ahead of research - call e.g. the colleagues of Air Canada, they should have used this method already....
Or ask the colleagues of Olympus in Quebec, we have discussed this method already years ago with them, you see the result is the application note in their Homepage.

Summing up: Detecting water with single sided UT in Honecomb is simple methods, which is industry standard. (If you focus on the capability to differentiate between indication showing only a disbond or showing only the presence of water:
With the method I presented here, you can do this differentiation only of the cells are horizontal so that water has contact with the surface where you place the transducer. If you place the transducer on a horizontal placed sandwich where the cells are situated vertical, disbond and water ingress may produce the same indication (in theory the water completely damps the signal, a disbond reduces between 6 and 12 dB., But in reality such structures have lot of variations that this differentiation cannot be made. So the threshold in our procedures is always signal reduction of more than 6 dB, after adjustment on the structure (not on a separate reference block) as production of sandwich has a big variance and reference blocks never (!!!) show the same behavior like real in service structure. We have tested this method now between high arctic and tropical rain forest - from dessert to seaside, on new but also on aged structures - a POD is existing - look for the ATA NDT Forum presentation I have referenced

http://www.airlines.org/SafetyOps/EM/Documents/2010%20NDT%2018%20Toolbox%20for%20Honeycobm%20Sandwich%20Structures.pdf

This presentation may give you especially on Foil 13..16 the necessary information
I uploaded also the 2007 presentation.

 
 
18:04 Jan-26-2011

Norm Woodward
Engineering, ,
U.S. Air Force,
USA,
Joined Nov 1998
76
Re: Characterization of the effects of Water and Disbond on Ultrasonic Signals in Honeycomb Composite Structures
In Reply to wolfgang Bisle on 19:02 Jan-06-2011 (Opening).

Wolfgang-

I agree that neutron radiography (N-ray) should be considered "overpowering," but this is generally the opinion of nearly everyone who doesn't own one, and canÂ’t afford one. As the student said, he was hoping to offer a solution to the current problem, that the scheduled detection of disbonds by TTU was being baffled in the presence of water. The use of N-wave was only to graphically demonstrate that he was able to separate disbonds from water, and he was not suggesting N-ray to routinely detect either.

BTW, one of the few advantages of N-ray is that it is not hindered by such nuisances as unaligned (metallic) honeycomb cells and the presence of fasteners, thereby eliminating the problems you seemed to have with x-ray. [It does, however, share the problem of separating water from globs of organic sealants/adhesives.]

I am glad you are enthralled with thermography, though your advocacy of such enhancements such as baking and freezing introduces of such unintentional consequences as core blowouts due to sudden water evaporation, or core ruptures due to water freezing and expanding.

With all due respect to your commercial aviation background, from the military prospective of doing the best with what you have on hand, I thought the student handled his mission admirably.

Respectfully yours,
Norm

 
 
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