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Technical Discussions
Jacques Parent
Jacques Parent
05:03 Mar-15-2000
Pitch and catch, Tip diffraction

I have an application where I need to confirm and size a longitudinal crack, ID initiated, on a 12.75" OD pipe, wall thickness 2.343".
The crack is running between tube holes which are separated by by ~3" of steel.

We are thinking of a pitch and catch arrangement.
We will "pitch" a shear wave, of appropriate angle, in the circumferential direction, to excite the bottom opening of the crack on the ID. (1/2", 2.25 MHz)
We are hoping to "catch" the Tip Diffracted signal, with a normal beam probe, positioned directly above the crack location. By using Time of Flight in the absolute mode, we are hoping to measure the ligament above the crack tip and therefore the crack size (depth).

1) Any comments?
2) Any experience with something similar?
3) Any suggestions on the shear and/or normal probes specifications?
4) Any other ideas?

Thanks
jp


    
 
 Reply 
 
Mark Davis
Teacher, And Consultant
University of Ultrasonics, Birmingham, Alabama, USA, Joined Mar 2000, 85

Mark Davis

Teacher, And Consultant
University of Ultrasonics, Birmingham, Alabama,
USA,
Joined Mar 2000
85
05:21 Mar-15-2000
Re: Pitch and catch, Tip diffraction
Dear Jacques,

This is Mark Davis. Have you worn out your egg from all the NDT Stress???

I discussed this inspection with Dick Grey. It is really a very easy inspection, but the most important part is a calibration block. ID creeping waves and tip diffraction are your best bets. Detect and qualitiatively size with ID Creeping wave. Then you can back it up with Tip Difrraction either foward scatter or back scattering technique. Again, the calibration block is most important in my opinion.

Please call me or have Dick call me.

Good Luck,

J. Mark Davis

PS. I would also look at a 3.5 MHZ or a 4 or 5 Mhz to get a tip with good signal to niose ratio.

: I have an application where I need to confirm and size a longitudinal crack, ID initiated, on a 12.75" OD pipe, wall thickness 2.343".
: The crack is running between tube holes which are separated by by ~3" of steel.

: We are thinking of a pitch and catch arrangement.
: We will "pitch" a shear wave, of appropriate angle, in the circumferential direction, to excite the bottom opening of the crack on the ID. (1/2", 2.25 MHz)
: We are hoping to "catch" the Tip Diffracted signal, with a normal beam probe, positioned directly above the crack location. By using Time of Flight in the absolute mode, we are hoping to measure the ligament above the crack tip and therefore the crack size (depth).

: 1) Any comments?
: 2) Any experience with something similar?
: 3) Any suggestions on the shear and/or normal probes specifications?
: 4) Any other ideas?

: Thanks
: jp




    
 
 Reply 
 
Gary
Gary
07:27 Mar-15-2000
Re: Pitch and catch, Tip diffraction
: I have an application where I need to confirm and size a longitudinal crack, ID initiated, on a 12.75" OD pipe, wall thickness 2.343".
: The crack is running between tube holes which are separated by by ~3" of steel.

: We are thinking of a pitch and catch arrangement.
: We will "pitch" a shear wave, of appropriate angle, in the circumferential direction, to excite the bottom opening of the crack on the ID. (1/2", 2.25 MHz)
: We are hoping to "catch" the Tip Diffracted signal, with a normal beam probe, positioned directly above the crack location. By using Time of Flight in the absolute mode, we are hoping to measure the ligament above the crack tip and therefore the crack size (depth).

: 1) Any comments?
: 2) Any experience with something similar?
: 3) Any suggestions on the shear and/or normal probes specifications?
: 4) Any other ideas?

: Thanks
: jp




    
 
 Reply 
 
Gary Woodside
NDT Inspector,
Visual Testing Services Ltd., United Kingdom, Joined Feb 2000, 8

Gary Woodside

NDT Inspector,
Visual Testing Services Ltd.,
United Kingdom,
Joined Feb 2000
8
07:34 Mar-15-2000
Re: Pitch and catch, Tip diffraction
Sounds similar to ligament cracking experienced in UK power stations.

A videoscope with measurement capability can measure the crack wher it is visible in both holes. The mid point of the crack may be deeper but is in proportion to both sides. As the cracks in this instance are a result of thermal cycles - it is the rate of propagation which is most important in deciding when to reject the unit.

Regards

Gary

by : : I have an application where I need to confirm and size a longitudinal crack, ID initiated, on a 12.75" OD pipe, wall thickness 2.343".
: : The crack is running between tube holes which are separated by by ~3" of steel.

: : We are thinking of a pitch and catch arrangement.
: : We will "pitch" a shear wave, of appropriate angle, in the circumferential direction, to excite the bottom opening of the crack on the ID. (1/2", 2.25 MHz)
: : We are hoping to "catch" the Tip Diffracted signal, with a normal beam probe, positioned directly above the crack location. By using Time of Flight in the absolute mode, we are hoping to measure the ligament above the crack tip and therefore the crack size (depth).

: : 1) Any comments?
: : 2) Any experience with something similar?
: : 3) Any suggestions on the shear and/or normal probes specifications?
: : 4) Any other ideas?

: : Thanks
: : jp




    
 
 Reply 
 
Udo Schlengermann
Consultant, -
Standards Consulting, Germany, Joined Nov 1998, 177

Udo Schlengermann

Consultant, -
Standards Consulting,
Germany,
Joined Nov 1998
177
08:58 Mar-16-2000
Re: Pitch and catch, Tip diffraction
he crack is running between tube holes which are separated by by ~3" of steel.

: We are thinking of a pitch and catch arrangement.
: We will "pitch" a shear wave, of appropriate angle, in the circumferential direction, to excite the bottom opening of the crack on the ID. (1/2", 2.25 MHz)
: We are hoping to "catch" the Tip Diffracted signal, with a normal beam probe, positioned directly above the crack location. By using Time of Flight in the absolute mode, we are hoping to measure the ligament above the crack tip and therefore the crack size (depth).

: 1) Any comments?
: 2) Any experience with something similar?
: 3) Any suggestions on the shear and/or normal probes specifications?
: 4) Any other ideas?

: Thanks
: jp




    
 
 Reply 
 
Jacques Parent
Jacques Parent
04:20 Mar-16-2000
Re: Pitch and catch, Tip diffraction
Thank you for the suggestion

What would you expect the minimum detectable crack size (length and depth) to be?

What would you use for calibration block design?

Saw cuts OK or should we go EDM notches?

10%, 20%, 40%, 60%, 80% of wall thickness OK?

How long would you make the calibration "cracks"?

Regards
jp




    
 
 Reply 
 
J. Mark Davis
Teacher, And Consultant
University of Ultrasonics, Birmingham, Alabama, USA, Joined Mar 2000, 85

J. Mark Davis

Teacher, And Consultant
University of Ultrasonics, Birmingham, Alabama,
USA,
Joined Mar 2000
85
06:59 Mar-16-2000
Re: Pitch and catch, Tip diffraction
: Thank you for the suggestion

: What would you expect the minimum detectable crack size (length and depth) to be?

This is really dictated by the Critical Flaw Size. What does the FM people say. 10% of t and 1/2 inch long???

: What would you use for calibration block design?

This should be a mock up of the actual design or a close design to incorporate the geometric limitations.

: Saw cuts OK or should we go EDM notches?

Saw cuts or notches are ok. You just need a calibration reflector to calibrate the time of flight of the UT system. With of the notch reflector is a consideration; too wide a notch and you could get tip diffracted energies from both notch edges. I suggest not widths to be between 1 to 3 mm. Small as practical.

: 10%, 20%, 40%, 60%, 80% of wall thickness OK?

Yes, but the rememebr the critical flaw size. If it is 5% then add a 5%.

: How long would you make the calibration "cracks"?


Again, what is the critical flaw size. Is length the concern or depth?
: Regards
:jp




    
 
 Reply 
 
Udo Schlengermann
Consultant, -
Standards Consulting, Germany, Joined Nov 1998, 177

Udo Schlengermann

Consultant, -
Standards Consulting,
Germany,
Joined Nov 1998
177
09:03 Mar-20-2000
Re: Pitch and catch, Tip diffraction
dth of the block, i.e. at least three beam diameters. But as said above, it is possible to detect much shorter cracks.

If you have to detect a given minimum crack depth, e.g. of 0,05t, then I recommend make also a cut of 5% depth.
Additionaly, to convince all non-experts, it would be nice to demonstrate the technique on a real crack, e.g. an enforced crack on a tensile specimen.

Kind regards
Udo Schlengermann
--------------------------

Thank you for the suggestion

: What would you expect the minimum detectable crack size (length and depth) to be?

: What would you use for calibration block design?

: Saw cuts OK or should we go EDM notches?

: 10%, 20%, 40%, 60%, 80% of wall thickness OK?

: How long would you make the calibration "cracks"?

: Regards
: jp




    
 
 Reply 
 

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