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641 views
00:16 Nov-02-2000

Mario Belo

NDT Inspection
Trenes Buenos Aires,
Argentina,
Joined Jan 2000
7
Corrections calculation

Dear experts:

Is it possible, to determine the transference corrections (surface and attenuation) from a reference block with endless wall (V1/5) to an unknown flaw (smaller than the beam) in a certain piece with different acoustic characteristics and a different surface, which does not present backwall, through the correspondent AVG (DGS) diagram?
An example would be welcome.

Best regards,

Mario Belo



 
09:35 Nov-14-2000

Udo Schlengermann

Consultant, -
Standards Consulting,
Germany,
Joined Nov 1998
172
Re: Corrections calculation reply by Udo Schlengermann

If you calibrate on a block which has different material and surface than the test object, this is not possible, for the following reasons:
Four different losses determine the signal amplitude:
1. divergence - depending on the frequency, the roughness and the size of the contact area.
2. attenuation - in reference block and test object.
3. transmission losses - if the materials are different for both objects (acoustical impedances).
4. reflectivity of reflectors - calibration reflector and detected discontinuity.

The DGS diagram is designed for a full size contact (effective transducer diameter) and a material without attenuation.
Most standards therefore allow DGS method only if the curvature compared to transducer size is below a limit.
Transfer losses on curved, rough objects therefore are only valid for a given contour and roughness, and a defined distance of reflectors, i.e. they change with distance, if attenuation is not comparably equal on both objects.
Before using transfer losses, they have to be determined on a test block having the same curvature and the same roughness as the test object, containing the same reflector (e.g. backwall) at the same distance than your calibration block. Then losses no 4. and 3. are constant in both cases and the remaining difference to the DGS-curve is the transfer loss (attenutation and transmission losses).
It is therefore necessary to have at least one reflector of known size and distance in the test object. You cannot determine transfer losses using an unknown reflector.

Kind regards

Udo Schlengermann
++++++
: Dear experts:

: Is it possible, to determine the transference corrections (surface and attenuation) from a reference block with endless wall (V1/5) to an unknown flaw (smaller than the beam) in a certain piece with different acoustic characteristics and a different surface, which does not present backwall, through the correspondent AVG (DGS) diagram?
: An example would be welcome.

: Best regards,

: Mario Belo




 


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