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Technical Discussions
Doug Mendes
NDT Inspector, R & D
Imetrix Inc., USA, Joined Oct 1999, 7

Doug Mendes

NDT Inspector, R & D
Imetrix Inc.,
USA,
Joined Oct 1999
7
09:18 Dec-08-1999
Double hulled structures

I'm interested in using ultrasonics to inspect a structure that has two steel plates which have a water path between the two plates. I need to inspect one of the plates for corrosion and pitting through the water column and the other plate. I only have access to one side of this structure which complicated matters. The plates are less than 1/2'' (12.7mm) thick but the water path is unknown. My question is could this inspection be performed and what if any special equipment would be needed? The water path would be known before the inspection. Any help would be grreatly appreciated.


 
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M. William Moyer
M. William Moyer
02:17 Dec-08-1999
Re: Double hulled structures
Another approach is to use a pitch catch setup, particularly if you are inspecting in immersion. Set up the transducers for 45 deg
shear in the steel. The spacing should be such that you can geometrically focus the transducer array at about 3-4 inches (for 0.5 inch steel).
Move the transducer pair toward the part until they are geometrically focused on the back surface of the first hull. This signal will peak up. Then
continue to move in until you are focused on the surface of interest. Because of the angles, the multiple reflections will be very small and only the
signal of interest will be seen. This technique works well on many multilayer inspections.
The technique is described in "Ultrasonic Inspection Using the Pitch-Catch Technique," M. W. Moyer,Materials Evaluation, Vol 37, Number 5, April, 1979,
pg. 51.

M. William Moyer
ult@y12.doe.gov.

:
: : I'm interested in using ultrasonics to inspect a structure that has two steel plates which have a water path between the two plates. I need to inspect one of the plates for corrosion and pitting through the water column and the other plate. I only have access to one side of this structure which complicated matters. The plates are less than 1/2'' (12.7mm) thick but the water path is unknown. My question is could this inspection be performed and what if any special equipment would be needed? The water path would be known before the inspection. Any help would be grreatly appreciated.

: This is an application that we've run into on several previous occasions, with mixed results. The biggest complication is that multiple echoes from the outer plate and the water gap will be superimposed over the echoes from the near and far wall of the inner plate. Whether this will be a major problem or just an annoyance will depend on the exact thickness of the outer plate and the exact length of the water path and thus the spacing of the echoes. If you're lucky, the echoes from the inner plate will occur between, rather than on top of, other signals. (But Murphy's Law usually insures that everything will be superimposed!)

: In that case, you might investigate the possibility of using digital signal processing to subtract the outer plate and water path echoes from the wave train. Unfortunately, that too gets complicated if the outer plate thickness and water gap length are varying from point to point, since that will change the wave pattern that you need to subtract.

: --Tom Nelligan




 
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