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Technical Discussions
00:28 Jul-28-2006
Clad Pipe - Metallurgically Bonded

How accurate can you measure the wall thickness using a zero degree probe on Clad pipe? Would you adopt a 2Mhz or 5Mhz zero degree probe? Is there an alterantive?

Tom Nelligan
retired, USA, Joined Nov 1998, 390

Tom Nelligan

Joined Nov 1998
02:08 Jul-28-2006
Re: Clad Pipe - Metallurgically Bonded
First, I assume that your total pipe thickness is under an inch or so with the cladding representing 25% or less of the total thickness.

With a properly set up precision thickness gage like a Panametrics-NDT Model 35 and an appropriate contact transducer, we can normally measure metal pipe wall thickness to an accuracy of +/- 0.001" or better. However in this sort of case the measurement accuracy is potentially affected by the small difference in sound velocity between the cladding and the base metal. The effective sound velocity of a two-metal sandwich is the weighted average of the velocities of the two metals. As the thickness of one or the other varies, changing the ratio between them, the composite velocity varies too. Because the accuracy of any ultrasonic thickness measurement is only as good as the velocity calibration, this introduces some uncertainty.

If you know the sound velocity of the cladding and the substrate, and the range of thickness for each, then you can easily calculate the worst-case uncertainty. Calculate the weighted average velocity of the sandwich, and then calculate the error that would result from measuring either of the two metals alone over the appropriate thickness range. That's your absolute worst case, which you're unlikely to ever see. As a practical matter, velocities of typical claddings like inconel and substrates like carbon steel alloys are within 5% of each other, and the substrate metal will always be the majority of your sound path, so in practice the potential uncertainly seldom exceeds a couple percent.

I'd have to know the specific thickness range involved to recommend an optimum transducer, but assuming a maximum thickness around an inch, a 5 MHz, 0.5" diameter contact transducer would be my first choice.

Corrosion gages and flaw detectors will be slightly less accurate than precision gages, but only on the order of +/- 0.002" with proper setup, so the potential velocity variation is still the limiting factor for accuracy.

Neil Burleigh
Krautkramer Australia Pty Ltd, Australia, Joined Dec 2002, 153

Neil Burleigh

Krautkramer Australia Pty Ltd,
Joined Dec 2002
04:06 Jul-30-2006
Re: Clad Pipe - Metallurgically Bonded
The 1st series questions we would ask would relate to the material type of the cladding. Is it ferrous or non ferrous? Is the clad material coarse grained? Is there much difference acoustically between the clad material and the base material?
Next we would be asking do you want to measure the total thickness, just the clad thickness or the thickness of the base pipe?
We would need to look at all these factors before even considering your questions.

Neil Burleigh

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: How accurate can you measure the wall thickness using a zero degree probe on Clad pipe? Would you adopt a 2Mhz or 5Mhz zero degree probe? Is there an alterantive?
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