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Aleksi Näreikkö
R & D,
Finland, Joined May 2016, 1

Aleksi Näreikkö

R & D,
Finland,
Joined May 2016
1
09:37 May-04-2016
Near field calculation validation

Hello everyone,

So I have browsed older forum posts, but couldn't find a similar post, so here goes. We are using linear PAUT to inspect composite laminates and need some validation on our near field calculations. As you know, the near field length is calculated by N=(kL^2f)/4c, where:
k = the near field length correction factor for a rectangular probe
L = Length of the probe. We are using a active aperture of 16 elements with a 0.6 mm element pitch so that equals 16*0.6mm = 9.6 mm
f = 7.5 MHz
c = 1.48 km/s, in water
Then lets assume we are doing immersion testing and want to know where our near field ends in water. We get N=(1.38*9.6^2*7.5)/(4*1.48)=161 mm. Looks feasible when compared to table values for single crystal near fields with the same specification.

Then when we add a 23 mm thick rexolite wedge to the probe and continue with the same scenario. We subtract the thickness of the wedge times the ratio of the speed of sound in the wedge and water from the original length like so -> 161 mm - (23 mm * (1.5/2.35)) = 147 mm. Are these calculations correct?

My second question has to do with the inspection method itself. Assuming our component is a 3 mm thick laminate coated with 2 mm rubber layer on both sides, and we are electronically focusing the beam on top of the laminate which lies 3 mm from the wedge in an immersion tank, making our focusing factor ~0.18, how realiable is our passive (unfocused) direction which also happens to be our inspection direction in terms of sizing flaws? Our worry is, that while being so close in the near field regarding the passive direction of the sound beam, we are unable to get accurate data on reflectors. Is this true and should we change our water path to match the calculated near field length?

Thank you for your time.

-Aleksi

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massimo carminati
Consultant, AUT specialist
IMG Ultrasuoni Srl, Italy, Joined Apr 2007, 691

massimo carminati

Consultant, AUT specialist
IMG Ultrasuoni Srl,
Italy,
Joined Apr 2007
691
16:50 May-05-2016
Re: Near field calculation validation
In Reply to Aleksi Näreikkö at 09:37 May-04-2016 (Opening).

161 mm - (23 mm * (1.5/2.35)) = 147 mm

this is not correct: it has to be 161 mm - (23 mm * (2.35/1.48)) = 124.5 mm

infact, the higher the speed of sound, the shorter the N zone.

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