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- since 1996 -
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Technical Discussions
Joey
Joey
17:29 Apr-09-2018
SHEAR WAVE - Aluminum

Hello all I am having issues with dialing in my scope for aluminum shear wave during the initial set up. I am using a USN 60 scope and when I set up for calibration on the IIW block and finding the exit point on my aluminum shoe my maximum reflection is way outside the limits of my "0" and all the tic's. The shoe is brand new and cannot figure whats up. I have tried multiple shoes all new, even went as far as sending a set back because the were jacked but still same issues persist. Does anyone have any ideas or experience with this?

 
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P V SASTRY
R & D, NDT tecniques metallurgy
TAKEN VRS FROM THE POSITION OF SR. DEPUTY GENERAL MANAGER BHEL CORPORATE R&D, India, Joined Jan 2003, 195

P V SASTRY

R & D, NDT tecniques metallurgy
TAKEN VRS FROM THE POSITION OF SR. DEPUTY GENERAL MANAGER BHEL CORPORATE R&D,
India,
Joined Jan 2003
195
20:57 Apr-09-2018
Re: SHEAR WAVE - Aluminum
In Reply to Joey at 17:29 Apr-09-2018 (Opening).


Are you using an IIW block made of steel or Aluminum.

Best wishes

P V SASTRY

 
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Joey
Joey
21:23 Apr-09-2018
Re: SHEAR WAVE - Aluminum
In Reply to P V SASTRY at 20:57 Apr-09-2018 .

P V SASTRY - I am using an aluminum wedge, and an aluminum IIW/ NAVSEA block. My transducer is 1/2" 5MHz.
-- UPDATE -- I have also tried to calibrate off just the NAVSEA block with the side drilled holes using back reflections but after I find the second reflection and scan the rest of the holes everything is 1/4" off. While attempting this I had to disregard the exit point and the angle, I am assuming at this point they are right on the money with a 70 degrees wedge with the exit point right on, I am trying different avenues to approach this.

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

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1286
21:31 Apr-09-2018
Re: SHEAR WAVE - Aluminum
Joey, this is a relatively old topic on NDT.net. You can find a similar thread at http://www.ndt.net/forum/thread.php?admin=&forenID=0&msgID=54828&rootID=54827#54828
and I direct your attention to the explanation provided by Tom Nelligan and I posted a simple sketch trying to illustrate the effect.
 
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Joey
Joey
23:09 Apr-09-2018
Re: SHEAR WAVE - Aluminum
In Reply to Ed Ginzel at 21:31 Apr-09-2018 .

Ed Ginzel - I did go back and search previous threads and found http://www.ndt.net/forum/thread.php?admin=&forenID=0&msgID=54828&rootID=54827#54828 that thread before posting. I read thru all the responses and it does not really give an answer, more of reasoning that it is flawed due to grain structure and some information behind the argument. At this point I doubt what I am looking for can be answered with a simple solution.

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

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1286
17:39 Apr-10-2018
Re: SHEAR WAVE - Aluminum
In Reply to Joey at 23:09 Apr-09-2018 .

zoom image



Joey, When you state that you are using an aluminium wedge I suspect you do NOT mean that the wedge is made of aluminium but is a regular plastic refracting wedge intended to refract in aluminium at the angle indicated on the "shoe". The published acoustic velocity in the shear mode in aluminium varies for different alloys typically anywhere from 2900m/s to 3100m/s. But this is simply a nominal value. In the same part the value changes with direction. In an isotropic material like the steel IIW block (Calibration Block 1) the heat treatment assures a very uniform velocity with respect to direction of beam propagation.

Since your probe is intended for use on aluminium, an assessment of exit point and refracted angle on a steel IIW block is challenging since steel will have a shear velocity about 250m/s faster than aluminium. That means placing this 70° probe on the steel IIW block will refract the beam near the second critical angle. If the wedge is the replaceable type perhaps you can use a similar 70° refracting wedge intended for steel and set up the instrument for a 125mm or 250mm range. Then, change back to the aluminium refracting wedge. If they are from the same manufacturer and have a similar design, the delay difference should be minor. Place the probe, with the refracting wedge intended for aluminium, back on the steel IIW block with the zero exit point aligned with the centre mark of the 100mm radius. You should be able to see a small shear wave response near 100mm range on the screen and another signal (the Rayleigh wave) at about 110mm, allowing for any small difference in delay due to path length difference in the wedges.
The peak amplitude of the shear signal should occur with the probe indicated exit point aligned with the centre mark for the 100mm radius.

If you have a 60° wedge for your aluminium applications you should experience a similar situation with the peak signal on the aluminium IIW-type block occurring when the probes is well away from the indicated centre of the 100mm radius. However, if you carry out the comparison using the steel IIW block you will be able to see a more distinct response because the incident angle for 60° in aluminium will result in about a 70° beam in steel.
To illustrate the effect of anisotropy you can do a simple experiment. On your aluminium IIW-type block, place a 0° L-wave probe on the 100mm thickness and calibrate the instrument to display a 300mm range (i.e. 3 backwall signals). Then place the same probe at the end of the aluminium IIW-type block and direct the beam to get a response from the step 200mm from the end (as illustrated in the Beamtool image). The backwall signal from the surface 200mm away will not align with the 200mm distance. This effect will be even more pronounced if you use a 0° shear wave probe.

The issue of anisotropic aluminium IIW-type blocks has long been a concern in the industry. Fred Hotchkiss, when with Panametrics, explained to me how they attempted to minimize (not eliminate) the problem by making small ingots and using only the centre portion of the ingot to fabricate their blocks.
 
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