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Technical Discussions
anjafo
NDT Inspector
Norway, Joined Aug 2009, 204

anjafo

NDT Inspector
Norway,
Joined Aug 2009
204
19:32 Aug-26-2009
UT WT Fibreglass/Steel Interface

Hi,
Can you do UT through the Fibreglass?

Say the fibreglass is 3mm and bonded to Carbon Steel Metal approx 6mm thick.

Can you inspect the changes in wall thickness in the carbon steel accurately through the glass fibre. Looking for pitting up to 3mm etc.

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

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
23:06 Aug-26-2009
Re: UT WT Fibreglass/Steel Interface
In Reply to anjafo at 19:32 Aug-26-2009 (Opening).

As a general rule, a fiberglass coating in that situation will significantly limit the resolution with which you can test the steel, but the exact effects in a given case will depend on how thick the fiberglass is and also how scattering that specific layup is. Smooth, high density, bubble-free fiberglass is better than thick, rough fiberglass with entrapped air.

If the fiberglass is only a few milimeters thick, you will probably be able to shoot through the fiberglass with a low frequency single element contact transducer (try 2 MHz or 2.25 MHz) and see a backwall echo from the steel that will give a general idea of base metal thickness, but small pits will likely be invisible due to the scattering effects in the fiberglass.

    
 
 
S.V.Swamy
Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex , India, Joined Feb 2001, 787

S.V.Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
787
04:26 Aug-27-2009
Re: UT WT Fibreglass/Steel Interface
In Reply to anjafo at 19:32 Aug-26-2009 (Opening).

Tom Nelligan has already given you a good answer but I would like to ask you one question. Why not try to shoot from the carbon steel side? If the carbon steel is on the outside, wall thickness measurement is easy and if it is inside with a radius of curvature, a perspex adopter fixed to the UT probe should do the trick. Have you tried this approach? Feel free to share more about the application.

Best regards.

Swamy
Quality and NDT expert

    
 
 
anjafo
NDT Inspector
Norway, Joined Aug 2009, 204

anjafo

NDT Inspector
Norway,
Joined Aug 2009
204
08:08 Aug-27-2009
Re: UT WT Fibreglass/Steel Interface
In Reply to S.V.Swamy at 04:26 Aug-27-2009 .

Sometimes thew simple ideas are the best, we will try varying the probe.

It is tank floor inspection, prove up after MFL scanning, so only access from one side.

Im no expert on the layer up of the fibre glass but I can look into it.

    
 
 
John O'Brien
Consultant, -
Chevron ETC , USA, Joined Jan 2000, 278

John O'Brien

Consultant, -
Chevron ETC ,
USA,
Joined Jan 2000
278
14:01 Aug-27-2009
Re: UT WT Fibreglass/Steel Interface
In Reply to anjafo at 08:08 Aug-27-2009 .

This is a common application. Tom is correct lowering the frequency helps. however many tank floors have variable lay up approaches so you may find significant variations in the uniformity of the coating. This may drive your frequency even lower into the 1 MHz range and large beam size so resolution continue s poorer.

    
 
 
anjafo79
NDT Inspector
Norway, Joined Aug 2009, 204

anjafo79

NDT Inspector
Norway,
Joined Aug 2009
204
14:07 Aug-27-2009
Re: UT WT Fibreglass/Steel Interface
In Reply to John O'Brien at 14:01 Aug-27-2009 .

also missing pitts which is a big problem, no?

    
 
 
John O'Brien
Consultant, -
Chevron ETC , USA, Joined Jan 2000, 278

John O'Brien

Consultant, -
Chevron ETC ,
USA,
Joined Jan 2000
278
18:34 Aug-29-2009
Re: UT WT Fibreglass/Steel Interface
In Reply to anjafo79 at 14:07 Aug-27-2009 .

You nee dto define pits. The detection threshold will be a function of frequency for penetration and beam spread etc for sizing. Suggest you get a plate, put targets of the required size in the backside then have the topside FRP coated. Get a selection of probes and then test penetrating power versus beam characteristics until you are confident you can find the desired target on a reliable basis.

    
 
 
Johan van Alphen
Johan van Alphen
21:28 Aug-31-2009
Re: UT WT Fibreglass/Steel Interface
In Reply to John O'Brien at 18:34 Aug-29-2009 .

Hi,

I did a lot of tankfloors with MFL and UT follow-up. Also some coated with glassfibre.
Usually it works fine, even with a dual crystal 4 Mhz probe (MSEB 4).
Thing is that you should have confidence in your MFL indications and do the UT according to it.
What I found difficult is that the interface echo from GRP to steel is about the same height as your backwall echo from the 6mm steel floorplate and appears around the 3mm range. As soon as you hit your corrosion pit, your backwall echo will drop down and you have to increase your dB. This means that your interface echo will get wider and therfore the corrosion echo wil dissapear in the interface echo.
We usually told the client that any corrosion with remaining wallthickness around 3.5mm we could not measure correctly and if he wanted to know the excact thickness we would have to remove the fibreglass.

Good luck!

    
 
 

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