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Technical Discussions
Carlos Correia
R & D, - -
UCV & EGROUP, Venezuela, Joined Oct 2008, 120

Carlos Correia

R & D, - -
UCV & EGROUP,
Venezuela,
Joined Oct 2008
120
15:41 Mar-25-2010
DAC in Phase Array Sectorial mode

Hello NDT Brothers.
Ultrasonic inspection of Girth Pipe Welds, following ASME code, requires a calibration with a block made of the same pipe material, same geometry and same thermal history. The DAC curve shall be build with ID and OD notches (Figure T-434.3, ASME Sec. V, Art. 4). Using Phased Arrays in sectorial mode, many focal laws could be involved. The question is:
Which is the better focal law to build the DAC?
Or, Do we need to build more than one DAC?
Finally, I would like to put over the table an hypothetical situation: if we detect an indication with a maximum Amplitude at X degree (X focal law), and the DAC was made with the focal law relative to Y-degree, and the amplitude of the indication for the Y-degree is less than the DAC limit (20% DAC):
Does a new DAC need to be constructed for the Y-degree?
I cannot find any requirement in the ASME code relative to this situation.
Thank you.

 
 Reply 
 
Alex
Alex
11:13 Mar-26-2010
Re: DAC in Phase Array Sectorial mode
In Reply to Carlos Correia at 15:41 Mar-25-2010 (Opening).

Hi!
To say what is better probably it is impossible, optimum (if it is short) will be such which will allow to reveal defects of a seam a direct beam.
As to a hypothetical situation such effect is possible because of Echo-transmittance (from Krautkramer).Therefore the estimation should be made on the first DAC.
Also check up calibration and beam focusing.
Regards, Alex.

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1304

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1304
12:44 Mar-26-2010
Re: DAC in Phase Array Sectorial mode
In Reply to Carlos Correia at 15:41 Mar-25-2010 (Opening).

Carlos: you have a good habit of pointing out the problems that ASME has not suitably dealt with!
Calibration targets on curved surfaces have long been problematic. The notch is perhaps only a convenient option for machining but it is not so great for uniform sensitivity. In fact, for the Basic block for plate weld inspections there is also a notch but it is not used in DAC constructions as it is in piping. ASME has you calibrate on the SDHs and the function of the notch is only mentioned in Appendix B-465 (...indications from the notch shall be "considered" when evluating reflectors at the opposite surface).
I think the appropriate reference in ASME here would relate to the reference reflectors. T-434.1.1 states "An alternative reflector may be used provided that the alternative reflector produces a sensitivity equal to or greater than the specificed reflector." If you cannot do an E-scan using a single angle but must instead use an S-scan, I think you can satisfy the intent of the Code to keep a high sensitivity by using the DAC (TCG) that would be more sensitive than the single angle option (e.g. if your would typically use a 45° beam as the single angle technique then the 60° angle would probably require more gain than the 45° if using a 45-65° S-scan.
Again, this question raises another point to resolve in the ASME (and perhaps other) Codes.

 
 Reply 
 
luis
Engineering,
USA, Joined Sep 2008, 25

luis

Engineering,
USA,
Joined Sep 2008
25
15:15 Mar-29-2010
Re: DAC in Phase Array Sectorial mode
In Reply to Carlos Correia at 15:41 Mar-25-2010 (Opening).

Carlos, how are you my firend?

Please see my response below, I hope this can help you,

Ultrasonic inspection of Girth Pipe Welds, following ASME code, requires a calibration with a block made of the same pipe material, same geometry and same thermal history. The DAC curve shall be build with ID and OD notches (Figure T-434.3, ASME Sec. V, Art. 4). Using Phased Arrays in Sectorial mode, many focal laws could be involved. The question is:



Which is the better focal law to build the DAC? A focal law is only a set of angle used in an inspection. It would depend on the item to be inspected.



Or, Do we need to build more than one DAC? On Olympus Omniscan you can build a TCG that adjust the sensitivity of each angle in the sectorial scan to your reflector. This is code compliant.



Finally, I would like to put over the table an hypothetical situation: if we detect an indication with a maximum Amplitude at X degree (X focal law), and the DAC was made with the focal law relative to Y-degree, and the amplitude of the indication for the Y-degree is less than the DAC limit (20% DAC):



Does a new DAC need to be constructed for the Y-degree? I would say on piping (curved surfaces) yes. However, most companies I know inspect the x-axis with encoded phased array but do the y-axis or transverse scan manually (often with conventional UT due to wedge curvature). But it doesn’t clearly state this in the code.



Regards

 
 Reply 
 
Carlos Correia
R & D, - -
UCV & EGROUP, Venezuela, Joined Oct 2008, 120

Carlos Correia

R & D, - -
UCV & EGROUP,
Venezuela,
Joined Oct 2008
120
01:42 Mar-30-2010
Re: DAC in Phase Array Sectorial mode
In Reply to Ed Ginzel at 12:44 Mar-26-2010 .

Thank you very much Ed. Your words encourage me to follow my study of these particular articles in ASME Code.

You well known my personal point of view regard 20% DAC acceptance criteria; I deeply appreciate your help with this topic.

Thank you too Luis for your comments.

 
 Reply 
 

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