Re: Pitch and catch, Tip diffraction

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Re: Pitch and catch, Tip diffraction
Posted by: Gary Woodside (PID_528), E-mail: md@vtservices.fsnet.co.uk, on March 15, 2000 at 17:34 :
In Reply to: Re: Pitch and catch, Tip diffraction posted by : Gary on March 15, 2000 at 17:27:
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

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Comments:
: 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

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