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- since 1996 -
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Technical Discussions
J.Spencer
Engineering,
Canada, Joined Jan 2013, 10

J.Spencer

Engineering,
Canada,
Joined Jan 2013
10
00:59 Jan-23-2013
Low Velocity problem

I am doing shear wave inspection on a velocity of 2460ms (Long) and 885ms (Shear). Can I use the longitudinal for angle beam testing or can someone suggest a wedge material that could be applied to this issue?

 
 Reply 
 
Michel Couture
NDT Inspector,
consultant, Canada, Joined Sep 2006, 869

Michel Couture

NDT Inspector,
consultant,
Canada,
Joined Sep 2006
869
02:22 Jan-23-2013
Re: Low Velocity problem
In Reply to J.Spencer at 00:59 Jan-23-2013 (Opening).

J.

It would be better if you were to give more info on what you are trying to do. But right off, something is wrong. Shear velocity is most of the time close to half of the longitudinale. Yours is about one third. Care to elaborate?

 
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Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
11:04 Jan-23-2013
Re: Low Velocity problem
In Reply to J.Spencer at 00:59 Jan-23-2013 (Opening).

Hi Josh

A blast from the past! Hope you are well.

I'm a bit surprised that with all your experience you give so little information and expect your peers (or juniors, old fella!) to be able to answer! Or maybe not.

How have you measured both L and T-wave velocity so accurately? Or is that from literature? What is the material - lead? Is it some kind of solder?

To refract shear waves you'll obviously need wedge material with Vlong < Vshear in the material (885 m/s). Big ask AND material attenuation. What frequency transducer you working with.

X and Y cut piezoelectric materials can be directly coupled to the material, obviating the need for a wedge, but you'll need specialist advice on this. Go to PE transducer experts or academics.
Longitudinal velocity in wedge material, of course

 
 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
13:29 Jan-23-2013
Re: Low Velocity problem
In Reply to J.Spencer at 00:59 Jan-23-2013 (Opening).

As suggested by Michel and Nigel, more information would be nice. As noted by Nigel, the acoustic velocity of the shear mode at 885m/s is rare for any metal (perhaps indium, I have a publication with its compression mode at 2220m/s and shear at 874m/s). With metallic characteristics similar to lead, it may have similar high attenuation so even getting a shear mode through it could be problematic. It seems you are concerned with obtaining an incident angle that will provide a positive refracted angle. Even if you could get a useful shear mode into the material in question using an immersion setup, the closest low velocity liquid would be something like carbon-tetra-chloride at 940m/s.

 
 Reply 
 
J.Spencer
Engineering,
Canada, Joined Jan 2013, 10

J.Spencer

Engineering,
Canada,
Joined Jan 2013
10
14:48 Jan-23-2013
Re: Low Velocity problem
In Reply to Ed Ginzel at 13:29 Jan-23-2013 .

Sorry about giving so little information... I'm trying to inspect HDPE and the velocities were from literature (885ms seemed a bit low to me as well). My goal is for PA coverage on my Cap & Root area and ToFD for the volume. Any help would be great...

I have a few approaches in mind but anything that can get me away from using water column wedges would be nice.

Nigel, I hope all is well ;)

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

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
15:07 Jan-23-2013
Re: Low Velocity problem
In Reply to J.Spencer at 14:48 Jan-23-2013 .

If you're testing polyethylene with wedges designed for steel, the shear mode is irrelevant since you will get only a low angle longitudinal wave. The L-wave velocity in HDPE is less than the L-wave velocity in typical wedge materials, so there is no mode conversion and the coupled L-wave is refracted downward rather than upward. You can calculate the exact refracted angle in a given case from Snell's Law using L-wave velocities.

 
 Reply 
 
Andrew Hurrell
Consultant, Ultrasonic Transducer Production
Precision Acoustics Ltd, United Kingdom, Joined Apr 2000, 25

Andrew Hurrell

Consultant, Ultrasonic Transducer Production
Precision Acoustics Ltd,
United Kingdom,
Joined Apr 2000
25
16:43 Jan-23-2013
Re: Low Velocity problem
In Reply to J.Spencer at 14:48 Jan-23-2013 .

Just checked my data files for HD PE and if anything both velocities are a little on the high side. I have values closer to 2390 and 815 m/s respectively for long and shear. However, as with all polymers there can be quite some variation. For elastic solids where there is relatively little variation of bulk and shear modulus as a function of temperature, a ratio 2:1 for long:shear velocity is not unreasonable. However modulii of polymers can show a much greater range of long:shear velocities and these can also be very temperature dependant

To add to Tom's comment. Actually whenever a longitudnal wave is incident upon a solid material at anything other than 90 degrees there will be mode conversion. However I agree with the point that you need Snell's Law will quickly show you which wave componts are either refracted or will undergo total internal reflection at the wedge interface. Certainly we have had a number of customers who have had requested custom wedges for optimum propagation into polymeric materials.

Final point to bear in mind is that many polymers can be very lossy particularly at higher frequencies. Thus you may need to consider keeping the operating frequency as low as is practical to try and preserve what signal you have

Good luck with your meausrements

 
 Reply 
 
Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
17:01 Jan-23-2013
Re: Low Velocity problem
In Reply to Andrew Hurrell at 16:43 Jan-23-2013 .

Josh

Might be a suitable article for your application.

http://www.ndt.net/article/wcndt2012/papers/533_wcndtfinal00533.pdf

Nigel

 
 Reply 
 
Jon Wallis
NDT Inspector, -
Netherlands, Joined Feb 2010, 626

Jon Wallis

NDT Inspector, -
Netherlands,
Joined Feb 2010
626
01:01 Jan-24-2013
Re: Low Velocity problem
In Reply to J.Spencer at 14:48 Jan-23-2013 .

For your information there is a euronorm for the examination of welds in thermoplastics, EN13100.
Part 2 deals with radiography and part 3 deals with ultrasonic examination.
Hope this helps.

 
 Reply 
 

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