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Adam
NDT Inspector,
NDT Inspection, USA, Joined Aug 2014, 9

Adam

NDT Inspector,
NDT Inspection,
USA,
Joined Aug 2014
9
16:52 Aug-26-2014
Aluminium 5052 UTSW exit point issue - please help

I am looking for anyone who may have an idea of what is going wrong, Myself and 5 other NDT inspectors are now stumped on what is going on with a Shearwave test peice. We have all tried using different Scans from epoch 600's to GE 35's with alum wedges 70 deg, using 2.25mhz, 1/2 diam.
No matter what we have all done including adjusting the velocity, The EXIT point is just not at the right point.
PLEASE anyone that has had this problem with Aluminium and has figured it out I am looking for an answer to why the exit point is OFF.
Thanks

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

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
17:17 Aug-26-2014
Re: Aluminium 5052 UTSW exit point issue - please help
In Reply to Adam at 16:52 Aug-26-2014 (Opening).

The effect that you are seeing is due to the anisotropic grain structure of aluminum. It has been documented for many years, but many inspectors are unfamiliar with the phenomenon. The basic problem is that the center of energy of the beam (which is what you use to peak a signal on an IIW block) in an anisotropic medium does not trave along the same path as the axis normal to the front of the wave. In other words, the front of the wave travels along one straight-line path, but the center of energy on that wave is deflected along a different line. You can find a very detailed explanation of the physics involved at this link:

http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=2265&context=qnde

The effect is especially noticeable with 70 degree wedges, which is what you have been using.

1
 
 Reply 
 
adam
NDT Inspector,
NDT Inspection, USA, Joined Aug 2014, 9

adam

NDT Inspector,
NDT Inspection,
USA,
Joined Aug 2014
9
17:50 Aug-26-2014
Re: Aluminium 5052 UTSW exit point issue - please help
In Reply to Tom Nelligan at 17:17 Aug-26-2014 .

what would i do to prove to the government where my exit point may be or what it is exactly?

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

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
17:58 Aug-26-2014
Re: Aluminium 5052 UTSW exit point issue - please help
In Reply to adam at 17:50 Aug-26-2014 .

My personal opinion is that angle beam testing of aluminum, especially at high angles, has to allow for some inherent uncertainty in depth or distance measurements due to the physics effects that are described in the paper that I linked. But it's always up to the user to decide what is appropriate for his/her particular situation, so I'm afraid I can't answer your second question.

 
 Reply 
 
Adam
NDT Inspector,
NDT Inspection, USA, Joined Aug 2014, 9

Adam

NDT Inspector,
NDT Inspection,
USA,
Joined Aug 2014
9
18:41 Aug-26-2014
Re: Aluminium 5052 UTSW exit point issue - please help
In Reply to Tom Nelligan at 17:58 Aug-26-2014 .

Tom thank you for your help.

The question still stands for anyone that can tell how to prove the exit point on an inspection. We have a +/- 3 deg allowance, but i still need to prove the exit point?

Again if anyone has dealt with this and has been able to prove the exit point using a 70 deg wedge on aluminium 5052 series, I am looking for an answer.

Thanks.

 
 Reply 
 
Joe Buckley
Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT, United Kingdom, Joined Oct 1999, 526

Joe Buckley

Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT,
United Kingdom,
Joined Oct 1999
526
20:46 Aug-26-2014
Re: Aluminium 5052 UTSW exit point issue - please help
In Reply to Adam at 18:41 Aug-26-2014 .

The exit point is a function of the probe, not what its testing, you should be able to show that on a good quality carbon steel test piece, (the angle will be out) then transfer to the aluminium one to prove the angle.

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1286

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1286
20:53 Aug-26-2014
Re: Aluminium 5052 UTSW exit point issue - please help
In Reply to Adam at 18:41 Aug-26-2014 .




Adam, Tom has identified the problem well. Anisotropy essentially means the metal has different velocities in different directions. Long ago, during my certification exam, we were given an old 8x9mm 70° probe to inspect an aluminium weld. We were also given an IIW-Type block made of aluminium to identify the exit point and actual angle. What a shock to determine that the exit point was outside the probe housing (never could get a true angle as a result)!
Probes are usually marked for steel at 3240m/s. When we assess the angle in aluminium which is nearer to 3100m/s (on average because of the anisotropy Tom noted) it could drop to nearer 64°. To add to the problem, anisotropy is dependent on the rolling process and direction. As the beam moves through the grain structure at some angle, it actually encounters different velocities so the beam bends. If the velocity started slow and gradually increased the beam would be banana shaped. The attached image is intended to give you an idea of the process comparing the expected path in steel to what MIGHT be happening in a varying velocity aluminium block. Clearly this represents a problem for accurate plotting of an indication. The only solution I might recommend is to re-locate the flaw with a zero-degree probe. Remember, the bending of the beam might ALSO be in the lateral direction so it might be skewed off left or right. If you have an indication you will have a rough idea how far from the probe it is then scan the area over that to locate the surface coordinates and you will have a better idea of depth using the compression mode.
1
 
 Reply 
 
Adam
NDT Inspector,
NDT Inspection, USA, Joined Aug 2014, 9

Adam

NDT Inspector,
NDT Inspection,
USA,
Joined Aug 2014
9
23:43 Aug-26-2014
Re: Aluminium 5052 UTSW exit point issue - please help
In Reply to Ed Ginzel at 20:53 Aug-26-2014 .


Thank you Tom Nelligan and Ed Ginzel, your helpful information is very much appreciated, we once again took out both IIW blocks for steel and Aluminium and found the using a Aluminium Wedge and 70deg ducer on the steel had a plus 2 degree gain (72deg) and once again on the Aluminium IIW there was a 5 degree drop (65deg) we are still going to have to find a way to prove to the Navy that it is caused by the aluminium anisotopic grain. but with the information you have helped with it might help to prove our results.
thanks again

 
 Reply 
 
Amy
NDT Inspector, - -
USA, Joined Jan 2009, 87

Amy

NDT Inspector, - -
USA,
Joined Jan 2009
87
21:51 Aug-27-2014
Re: Aluminium 5052 UTSW exit point issue - please help
In Reply to Adam at 23:43 Aug-26-2014 .

Adam,
I had the same issue the first time I tried to find the exit point. It was very frustrating. It's rare that I have to do shear wave on aluminum, but every time is a challenge. My best advise is radiography :) ....only kidding. Use multiple angles to verify your indications are really where and what you think they are and use caution that your indications are true. I find that the material, especially in an aluminum weld, will occasionally create a surface wave. Finger dampen the surface and wipe the couplant from the front. Also porosity and oxide inclusions are more prevalent in aluminum welds, and although does not show a strong signal, sometimes will attenuate the sound. Good luck.

 
 Reply 
 

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