Re: Pitch and catch, Tip diffraction

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Re: Pitch and catch, Tip diffraction
Posted by: Udo Schlengermann (PID_172), E-mail: uschlengermann@krautkramer.de, on March 20, 2000 at 09:03 :
In Reply to: Re: Pitch and catch, Tip diffraction posted by : Jacques Parent on March 16, 2000 at 14:20:
reply by Udo Schlengermann:
To answer your questions:
1. As only the time-of-flight is used to evaluate the crack depth, the temporal resolution determines the minimum crack depth. Using a normal beam probe generating a short pulse (only one wavelength), with a frequency of 5 MHz in steel, the minimum crack depth would be 1,5 mm. With 10 MHz frequency it would be half of this. A crack which is as long as the beam diameter will be safely detected.
2. The calibration block can be a planparallel plate with the same thickness t as the pipe wall. To calibrate the screen of the instrument using backwall echos, this block should have at least 3 steps, e.g. t/4, t/2, 3t/4. The width of the steps should be three times the beam diameter.
3. To demonstrate that crack tips can be detected, I recommend saw cuts of 0,5 mm or 1 mm width with the same depths of t/4, t/2 and 3t/4. But 20%, 40%, 60%, 80% of total thickness t, as you suggested, would be convenient, too.
4. The cuts should be as long as the full width 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

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: reply by Udo Schlengermann: : To answer your questions: : 1. As only the time-of-flight is used to evaluate the crack depth, the temporal resolution determines the minimum crack depth. Using a normal beam probe generating a short pulse (only one wavelength), with a frequency of 5 MHz in steel, the minimum crack depth would be 1,5 mm. With 10 MHz frequency it would be half of this. A crack which is as long as the beam diameter will be safely detected. : 2. The calibration block can be a planparallel plate with the same thickness t as the pipe wall. To calibrate the screen of the instrument using backwall echos, this block should have at least 3 steps, e.g. t/4, t/2, 3t/4. The width of the steps should be three times the beam diameter. : 3. To demonstrate that crack tips can be detected, I recommend saw cuts of 0,5 mm or 1 mm width with the same depths of t/4, t/2 and 3t/4. But 20%, 40%, 60%, 80% of total thickness t, as you suggested, would be convenient, too. : 4. The cuts should be as long as the full width 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

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