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- since 1996 -
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Technical Discussions
Wei
Wei
03:04 Feb-07-2020
Ultrasound of Composite Prepreg: preventing moisture absorption

Hi,

Background is I am doing an ultrasound pulse-echo inspection of a thick composite prepreg panel using water as couplant. So for example after 3 prepreg layers are laid, an intermediate cure is done followed by Ultrasound inspection to check for defect within these 3 layers. If no defect was found, the next 3 layers is laid onto the cured plies and the process is repeated until the required thickness is reached. The material is Carbon prepreg (epoxy resin).

In cases such as this, may I find out apart from wiping away the water, is there any other additional steps I should take to ensure that the water is completely removed (for example should i heat the part in an oven for a certain duration)? The concern is that any water residue may affect the bonding of the subsequent prepreg layers.

Anybody able to share their experience? Thanks!

 
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J.B.
J.B.
18:41 Feb-07-2020
Re: Ultrasound of Composite Prepreg: preventing moisture absorption
In Reply to Wei at 03:04 Feb-07-2020 (Opening).

Must you use water for coupling????

Not really.
A solution is using an array approach with a Rotoarray. e.g. with Omniscan. O.K. it is advisable to use little water spray to optimize the wheel probe coupling but it must not be much: do it from the opposite side you intend to add the layers, so you do not contaminate the site for laminating. But even check if you can skip the spraying ...

Or use air coupled UT, but here usually you are forced to use through transmission.

Finally the most advanced method would be Laser UT - but it is extremely costly and ... I am not convinced anymore that it is a useful and economic method.

Often proposed is Thermography - some claim even to detect porosity quantitatively, but I am sceptical, that this is already industrial qualified.
As long as you work with Carbon Fibers Thermography is an option to see delaminations and you can also detect layer boundaries and internal structures as well as foreign objects. Be aware of thickness limitations.
A nice variant is vibration loading, as represents a dark field technology - only the disbonds will glow.

By the way:
Usually drying the part for some hours (or for shorter time in an oven) is good for further work.


 
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Wei
Wei
02:22 Feb-10-2020
Re: Ultrasound of Composite Prepreg: preventing moisture absorption
In Reply to J.B. at 18:41 Feb-07-2020 .

Hi J.B.

Thanks for the reply.

In this case, the array approach is not feasible as it is not a flat surface. There is a combination of flat and curved surface. For the curved surface the radius of curvature is small (<1"). In addition, through-transmission is not viable due to the metal mould in which the prepreg is being laid on. So physically I do not have access to the other side of the prepreg itself.

Hence I have adopted contact pulse-echo inspection of such radius. And for now, water is pretty much the only choice I have as gel is messy and very hard to remove completely.

Hope that clarifies.

 
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Simon Amallraja
Consultant, Advanced NDT Methods
Anu Consultants, India, Joined Aug 2013, 40

Simon Amallraja

Consultant, Advanced NDT Methods
Anu Consultants,
India,
Joined Aug 2013
40
14:44 Feb-12-2020
Re: Ultrasound of Composite Prepreg: preventing moisture absorption
In Reply to Wei at 03:04 Feb-07-2020 (Opening).

Hi,
You can check with Laser Shearography to find the locations of water in the component.

 
 Reply 
 
J.B.
J.B.
18:43 Feb-14-2020
Re: Ultrasound of Composite Prepreg: preventing moisture absorption
In Reply to Simon Amallraja at 14:44 Feb-12-2020 .

Wei,

Usually UT scanning would be - of course - the easiest solution. If you would have lots of money I would propose LaserUT especially for the radius area - systems from Tecnatom had been sold to Airbus as well Lockheed Martin uses those machines for inspection of their aircrafts in Fort Worth. But this is a very costly solution and it seems to be commercially also not very successful. So... not really advisable. A way out of the cost problem might be the XARION approach, see https://xarion.com/products/ndt-scanning-system - (Xarion last year has become member of the Airbus BizLab Aerospace Accelerator) If you look for an innovative solution, ceck out with them if it fits ... but it isfar away from you located...

Have you access to shearography? For thinner parts you cab use the usual loading methods of Vacuum or Heat. For thicker (but not too thick) parts vibration loading is a better option. I would recommend systems from ISISYS (also distributed by EDEVIS), this offers a nice vacuum coupled vibration device and a well adapted pulsed laserdiode illumination to to monitor wave fronts.

Hillger & DLR offer a more traditional method to monitor guided waves for delamination checks which is very interesting and works rather good: you excite at a fixed location with ultrasound pulses (in the range of 20KHz to 200KHz depending on your structure just to excite a guided wave) and you scan the part with a receiver, e.g. a electret microphone or if you have more money available, with the XARION laser microphone for much higher bandwidth. You get a full wave field where flaws generate specific interference patterns in the shadow of the flaw.
Check these papers for options: https://www.acoustic-camera.com/fileadmin/acoustic-camera/paper/2013_Boeck_Etal_AK_ImpactDamage_SandwichComposites.pdf, http://www.ecphm2012.com/Portals/98/BB/th2e2.pdf, http://www.idspektr.ru/10_ECNDT/reports/4_01_17.pdf
You may replace the receiver/microphone by a laser vibrometer which makes it even better to deal with curvature. Or even e shearography camera - once Dantech/Ettemmeyer sold a camera capable to do this but it has vanished from their catalogue. VEW (http://www.vew-gmbh.de/en/optical-metrology/golden-eye-shearography-system.html) still offers such a camera, sadly in the english version of the Web page they do not show the vibration loading only in the german version (http://www.vew-gmbh.de/optische-messtechnik/golden-eye-scherografie-system/piezoshaker-fuer-vibrometrieanwendungen.html), the BIAS institute publised 2 papers about the use of the VEW system:
https://pdfs.semanticscholar.org/6f6f/7c57d3972ae2d6469c1b3701c0461dca21af.pdf see paragraph 4.5 Application: Measurement of defects with Lamb waves
https://www.ndt.net/article/laser-ut2008/papers/Focke%20LU2008.pdf

As said, check also thermography, my favorite are the systems from EDEVIS (https://www.edevis.com/content/de/thermography.php) or from AT-Automation Technology - C-CheckIR (https://www.automationtechnology.de/cms/en/mobile-ndt-systems-for-maintenance/), the last is a standard system for some Airbus maintenance inspections (https://www.yumpu.com/en/document/read/23019150/modular-concept).

Finally i coukd imagine, that it is possible with certain resonance methods (MIA, Bondmaster) using couplant free sensors to detect the flaws you need to detect, but usually the minimum size of flaws you can detect is much larger as with conventional UT - you may check out with Olympus Bondmaster, NDT Systems Bondascope, and comparable systems. For automatic scanning you will need a robotic system - so again an costly solution.






 
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