* 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-1/10206.html

 * In Reply to: Re: why TOFD cursor is parabola?
 * from John on November 29, 2006 at 03:04

Yun Xi
Once a phased array generated pulse leaves the wedge it is not significantly
different from a pulse made using a single element. If the phased array probe
aperature is the same as a single element then the beam characteristics are the
same. 
Phased array generated TOFD provides "convenience" to some users. It is often useful
to have TOFD information even when doing a weld inspection with pulse-echo
techniques; e.g. using Electronic raster scans (E-scans0 or sectorial (S-scans).
This allows the operator a fast method of determining if the pulse-echo signal
originates from a geometric condition or from a real flaw. 

But there are limits. If you want to just do a TOFD examination (without the phased
array pulse-echo) you could design a multi zone TOFD technique with different
frequency probes to optimise results. e.g. a 3mm diameter 15MHz TOFD probe for the
upper 10mm of a weld inspection and a 5MHz 6mm diameter probe for the lower regions.
Phased array generated TOFD would not have the ability to provide the improved upper
zone coverage as its frequency is fixed. Also, there may be physical restrictions of
the Phased Array probe wedge limiting the approach of the wedge to the weld cap.
This may prevent optimum PCS being achieved. 
In spite of the limitations, I have seen some TOFD generated by phased array that is
superior to some of the single element TOFD being provided by others on similar
projects. 

It works the other way around too. 
It is often useful to have pulse-echo information when doing a TOFD technique. TOFD
when used as a single pass D-scan does not provide information about which side of a
weld a flaw occurs. You could do offset TOFD or even B-scan TOFD to determine the
side of the weld a nonfusion was occurring by this may not be an option if the weld
cap is left on and presents a scanning restriction. But pulse-echo can provide the
information quickly. It is all the more convenient if this can be done using a
single pass with a pair of phased array probes

Regards
Ed

----------- Start Original Message -----------
: Some guys use two phase array probe as TOFD,would you like tell me that is there
any differene between typical TOFD?
: Thank you for your reply. 
: 
: : YunXi:
: : Typically TOFD uses a relatively small diameter element.  The pressure boundary
of such a probe can be approximated by a cone.  Now imagine passing such a cone past
a small point diffractor in its path.  The point will first be detected far away as
the edge of the cone detects the flaw.  As the probe is moved along its straight
scan path the flaw gets closer until the flaw is directly across from the probe.  As
the probe continues its scan movement past the flaw the  process reverses and the
flaw gets further away. The effect is like taking a slice through the cone.  If you
were to cut a cone at some point parallel to the central axis the remaining cross
section is the definition of a parabola!
: : The actual shape of the beam boudary is defeined by a Bessel function but the
effect is effectively the same as a cone. 
: : The same effect can be seen on the shape of small targets in pulse-echo but the
process is called backscatter instead of TOFD (forward scatter).
: : Ed 
: : 
: : : I want to know why TOFD cursor is parabola?and what decide the parameter of
the curve?is there any equation?Thanks a lot
------------ End Original Message ------------



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