09:49 Sep-11-1996 Robert A. Dau Engineering Milky Way Jewels, USA, Joined Nov 1998 40
Re: Is the GE specification good and still relevant? Rolf -
Near surface resolution remains a difficult area to specify because the actual parameters you need depend on what size defect you need to detect and on the signal-to- noise you need to reliably detect it. Certainly the GE specification will not be adequate in some cases, it will be in others. So my answer is it depends.
One very good way to make a specification for a particular test is to pin everything down. Specify the near surface resolution in terms of a reference FBH a certain distance from the surface and the dB of signal- to-noise you need. The GE specification is of this type but not completely rigorous because it doesn't specify how you measure SN. If you want to detect 0.4 mm FBH 1.2 mm into steel. The GE spec. is good although putting a more "scientific" definition on the two wave amplitudes is desirable.
If you want to apply the GE spec. to another material that is more attenuative for example, then the GE spec. will be inappropriate. Specifying the near surface independently from the application is as far as I can see beyond our current knowledge. The "do everything in steel and translate to other materials method" is of limited use to most ultrasonic users because there are no "translation" codes available to do that. Indeed there is not much data on attenuation as a function of frequency out "there."
I might also add that near surface resolution is more complex than most ultrasonic measurements because it is a function of almost every parameter you can think off. Center frequency, band width (damping), material attenuation, element diameter, focal length, instrument setting, and water path all change near surface resolution. So I think you are stuck with using the GE Spec. and "translating" it experimentally to your needs or of creating a spec. specific to your own needs.
: The GE spec describes the probe's and equipment's capability :for its near to surface resolution. : See also GE Spec in UT A-Z.
: For my opinion this specification let to much parameters open, : for instant there is not any dB value mentioned about the : signal noise ratio at the flaw's echo. Is this true?