* This article was posted by Ed Ginzel 
 * (Profile => http://www.ndt.net/wshop/profile/profile.php3?ID=67 )
 * (Stand => http://www.ndt.net/exhibit/stand/14_ky.htm)
Reply at:http://www.ndt.net/wshop/forum/messages-1/10216.html

 * In Reply to: "Re: why TOFD cursor is parabola?"
 * from Nigel on November 30, 2006 at 09:00

Nigel and YunXi:
You have both expressed a valid concern for wedge design. 
True, the PA probe has a limited steering capability and so we "help" the beam with
a natural refracting angle.  
If you read the PA Handbook put out by Olympus you see that the "recommended"
steering limit is within the 6dB beam divergence of the individual elements. 
Therefore the limitation on useful steering is determined by the frequency, the
pitch, and element width.  
You may have read an earlier complaint I had about a Technique using a 0-80 degree
sweep.  It can be done but results are not very useful and any attempt to use
regular features such as DAC or TCG construction will probably not work. 

But a typical probe on a refracting wedge can be expected to provide SOME degree of
steering.  When we try to add focusing to the steering the problems may increase and
thereby reduce the effective range of angles possible.  

But lets use an example;
A 5MHz PA probe with a 0.6mm pitch has an element width less than 0.6mm (but I will
use 0.6mm).  Placed on a wedge with acoustic velocity of 2730m/s this element
prdoces a half angle of divergence of about 21 degrees.  When using a "natural
refraction" from the wedge the incident angle of the wedge is about 37 degrees.  If
I can expect my probe to steer 21 degrees down, this will provide a steering
incident angle of 16 degrees. At 5900m/s in steel this provides about 38 degrees
refracted compression angle. 
Therefore the steering CAN be accomplished.  But working at the edge of the 6dB
envelope will result in very weak results. By steering only 12-13 degrees in the
wedge the 70 degree compression can be made.  
For higher frequncy probes, higher angled wedges, wider elements...the degree of
steering possible MAY not allow the formation of useful compression mode for TOFD. 
Ed


----------- Start Original Message -----------
: I have very much appreciated the Platonic dialogue between Yun Xi and Ed G on
aspects of phased array. I would not intervene but Ed's last post has a broken link
as of my attempt to access it.
: So can I (without deep knowledge of the subject) supposition that the wedges will
have been machined so that the incident angle of the L-wave from the transducer was
approximately 30 degrees (to generate a 45 degree shear-wave in steel). But a focal
law of the phased array probe could be constructed to generate a suitable range of
angles in the wedge to produce L-waves in the test material. Thus if the PA probe
produced a 16 degree beam in the shearwave wedge this would give a 45 degree L-wave
in steel. Assuming similar acoustic velocities between the phased-array wedge and
the attached "shear-wave" wedge would mean more sound transmission than refraction
or reflection at the intrface. Thus the experienced PA focal-law designer would be
able to stipulate which incident angles to generate to produce the suitable L-waves
in the test material.
: Hopefully Ed will return or other experienced people will write to either confirm
or refute my supposition and perhaps add some other information.
: Thanks guys.
------------ End Original Message ------------


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