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Technical Discussions
Kris smith
Kris smith
06:59 Jan-10-2020
Sonda air scan

Currently I have been experimenting with a sonda air scan system using 225 kHz transducers. My question to this forum is I am noticing my signals amplitude is been weakened when moving from a 32 ply composite bonded panel to a a 12 ply composite bonded panel. With my UT pulse echo experience my signals amplitude is greater when moving to thinner material. I was hoping someone might have some insight. Thank you.

Regards- Kristopher Smith

Joe Buckley
Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT, United Kingdom, Joined Oct 1999, 528

Joe Buckley

Consultant, ASNT L-III, Honorary Secretary of BINDT
United Kingdom,
Joined Oct 1999
10:47 Jan-10-2020
Re: Sonda air scan
In Reply to Kris smith at 06:59 Jan-10-2020 (Opening).

Hi Kris.

N.B. Feel free to contact me directly (joe@levelxndt.com) - I've been working with the Sonda for over 20 years.

For relatively thin composite panels, especially at low frequency, material attenuation with distance is only a tiny part of the picture, and is usually overwhelmed by other factors such as differences in the material structure and the surface condition, it is also possible to get slight resonance effects when testing at zero degrees, although I suspect both panels are a bit thin for that at 225 kHz.

Are the panels believed to be otherwise identical?

18:16 Jan-14-2020
Re: Sonda air scan
In Reply to Joe Buckley at 10:47 Jan-10-2020 .

Wow – have you searched in an archive of historic NDT equipment? I had the chance to work with the Sonda Airscan in the 1990ties. I just went to the QMI Homepage and wondered about the publications they have linked: Youngest from WCNDT Rome 2000.
Using the Airscan we soon got stuck because it has a very narrow band signal with long ring down. With this there rise quickly significant problems with resonance and constructive/destructive interference as the pulse is usually much longer than it needs to travel through the material. Pulse attack (rising pulse) interferes with pulse decay from the first and the second and even following reflections on typical CFRP samples, so the reflections are not separate pulses but merge with part of the initial pulse. (Just thinking if it would be helpful to evaluate the signals with pulse thermography methods… I remember a guy from Russia later working at GE central labs, who did a similar step with pulse eddy current with very interesting results – Yuri Plotnikov. Pulse Thermography is understood as thermal wave imaging, so here is also the pulse length larger than the travel time through the material. FFT or wavelet transformation might help maybe….)
It is not uncommon, that if you apply ACUT (air coupled Ultrasound) in the Airscan way e.g. on a step wedge with delamination, you get in one step an signal decrease at the location of the delamination, at another steps – differing in thickness - you recognize a significant signal increase (>6db) where the transducer stays at the delamination compared to the area without delamination. Checking out the conditions you quickly realize the reason: constructive interference. As the sound of low frequencies usually is not fully blocked by an airgap (which a delamination is) it is easy to get funny effects.
Resonance effects are the reason, that you usually emit short pulses in general in ultrasonic testing, even in through transmission (TT) mode. With long pulse also you quickly get standing waves.
The long pulses with very narrow bandwidth are tribute to the large energy losses at each Interface between transmitter piezo and receiver piezo as each interface shows drastic acoustic impedance changes. So a long time the ACUT transducer avoid any damping to save energy but the tribute is a long ring down of the pulses (you quickly recognize this if you look at the front-page of the Airscan brochures showing this extreme long pulse - http://www.arorandt.com/wp-content/uploads/2018/07/Air-Coupled-Test-Instrument.pdf)
I think if you need to step into ACUT inspection, the Airscan is not any longer state of the art. In the meantime other companies in that field have much improved the technique. Either through replacing the old-fashioned solid piezo transducers through well adapted composite transducers with improved matching layers, optimized beam forming, better efficiency to turn electric signals in sound and back. Also increasing bandwidth and shortening the pulses.
And in the recent past we got new types of transducers/receivers e.g. the thermo-acoustic ultrasonic transducers, the Ferro-Electret (piezo-electret) ultrasonic transducers, and the XARION laser-acoustic microphones. Even the replacement of the receiver piezo by a kind of laser vibrometer / interferometer is used successfully.
You may find links to the above mentioned improvements in following papers: https://www.ndt.net/index.php?docID=19481&source=docs&target_url=article/wcndt2016/papers/fr1d2.pdf ) mentioned developments.
Also see:

The industrial state of the art is now much beyond the Airscan as you can get even ACUT array transducers. One of the spectacular applications of ACUT is the tail beam inspection of EUROCOPTER with ACUT (https://www.ndt.net/index.php?docID=13806&source=docs&target_url=article/aero2012/papers/tu2b3.pdf) or the large fairing for the Ariane Rocket satellite starting system manufactured by RUAG (https://www.robo-technology.de/robo/en/references/space-technology)

18:18 Jan-14-2020
Re: Sonda air scan
In Reply to J.B. at 18:16 Jan-14-2020 .

Just realized a mistake:
I wrote:
"Pulse attack (rising pulse) interferes with pulse decay from the first and the second and even following reflections on typical CFRP samples, so the reflections are not separate pulses but merge with part of the initial pulse."

It is vice versa!


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