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- since 1996 -

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Technical Discussions
Mark
Australia, Joined Jun 2012, 2

Mark

Australia,
Joined Jun 2012
2
03:30 Jun-06-2012
Material Thickness Testing of Corrugated Steel Tunnel

Hi,

I'm interested to understand if it is possible to thickness test the corrugated steel plate of a tunnel using NDT methods i.e. UT? The tunnel is a 30 year old ARMCO multiplate steel tunnel and requires a structural assessment. As part of the assessment the material thickness of the plate is required and this is unknown. The tunnel is burried with no exposed edges. The pitch of the corrugation is ~200mm and depth ~55mm (see attached).

Any advice anyone can offer would be greatly recieve.

Thanks
Mark

 
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Jon Wallis
NDT Inspector, -
Netherlands, Joined Feb 2010, 626

Jon Wallis

NDT Inspector, -
Netherlands,
Joined Feb 2010
626
07:57 Jun-06-2012
Re: Material Thickness Testing of Corrugated Steel Tunnel
In Reply to Mark at 03:30 Jun-06-2012 (Opening).

If you have access from 1 side of the tunnel (inside) and the visible surface is relatively smooth and corrosion/paint free, you can measure the thickness of steel with ultrasonics. Probe shoes can be shaped to the contour if necessary.
You mention "multiplate", if this means that there are plates laid on top of each other, you will only be able to measure the thickness of the visible plate. Ultrasonic thickness measurements will only measure good steel, not corrosion products and you should expect an accuracy within approx. 0.3 mm.

 
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Mark
Australia, Joined Jun 2012, 2

Mark

Australia,
Joined Jun 2012
2
09:15 Jun-06-2012
Re: Material Thickness Testing of Corrugated Steel Tunnel
In Reply to Jon Wallis at 07:57 Jun-06-2012 .

Thanks Jon.

Multiplate construction is yet to be confimed. I understand the protection system is zinc coating applied during manufacture.

If ultrasonic techniques can be used to determine the wall thickness of a single skin. Once the thickness is determined, I assume any corrosion can be confined on the outer side of the corrugated plate through a reduction in the material thickness when checked using the same ultrasonic technique? Various points (inside tunnel) could be thickness tested and mapped to determine the extent of corrosion (+/- 0.3mm) along the tunnel length? Would you agree this is feasible providing each spot to thickness tested is relatively smooth and free from dirt?

Cheers

 
 Reply 
 
Jon Wallis
NDT Inspector, -
Netherlands, Joined Feb 2010, 626

Jon Wallis

NDT Inspector, -
Netherlands,
Joined Feb 2010
626
11:04 Jun-06-2012
Re: Material Thickness Testing of Corrugated Steel Tunnel
In Reply to Mark at 09:15 Jun-06-2012 .

If there is a zinc coating on the opposite side from where you can measure then no problem, if there is a zinc coating on the measuring side then maybe a problem but not necessarily - contact an ndt company and speak with their level 3 expert.
If you have plate nominal 10mm thick and you measure 6mm, then you have 4mm of corrosion products.
You can take as many point measurements (snapshots) along the length of the tunnel as your budget will allow and you can take a detailed view of a small area (measuring in a grid pattern) again, speak with a local level 3.

 
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Tom Nelligan
Engineering,
retired, USA, Joined Nov 1998, 390

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
15:00 Jun-06-2012
Re: Material Thickness Testing of Corrugated Steel Tunnel
In Reply to Mark at 03:30 Jun-06-2012 (Opening).

"Once the thickness is determined, I assume any corrosion can be confined on the outer side of the corrugated plate through a reduction in the material thickness when checked using the same ultrasonic technique? Various points (inside tunnel) could be thickness tested and mapped to determine the extent of corrosion (+/- 0.3mm) along the tunnel length? Would you agree this is feasible providing each spot to thickness tested is relatively smooth and free from dirt?"

That is correct. Corrosion gaging is one of the most common applications for ultrasonic NDT. Accuracy can actually be better than +/- 0.3 mm if you use a quality corrosion gage with proper calibration. They are readily available from a number of manufacturers including us. I would expect that the zinc coating is a plating that is too thin to be of consequence, however its thickness will be added to that of the steel. In all cases, as Mr. Wallis notes, you will measure the remaining thickness of the metal plate to which the transducer is coupled. In the event of a double wall or overlapping sections, you will measure only the the inner wall thickness.

If you're new to the subject of ultrasonic corrosion gaging, we have an application note on the subject at
http://www.olympus-ims.com/en/applications/corrosion-gaging-dual-element-transducers/



 
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