00:42 Sep-05-1996 Splitt R & D, - Private, Germany, Joined Nov 1998 38
Re: Near field operation by use of Hardface probes Near field operation by use of hardface probes
Hello Mr. Diederichs
Thank you for your first questions in this workshop.
Measuring in the near field zone of probes seems to be a critical task. Fortunately due to the definite bandwidth of the probes the modulation of the field is not so severe as for monofrequent probes. With a broad band probe the maxima and minima are smoothed out.
1. Flaw evaluation
1.1 DGS method
As documented by the DGS curves in the Krautkramer probe data sheets, flaw evaluation in the near field is possible. The shortest distance between probe and flaw is only limited by the ringing of the probe after the transmitter pulse (near resolution).
1.2 Reference reflector method
With this method you can evaluate any flaw in comparison to a reference reflector without knowing details about the sound field structure.
2. Special interpretation
In both cases you have to keep in mind that in the near field range the maximum flaw indication generally is off axis, i.e. you will detect a small reflector two or more times when you scan the probe transverse to the flaw position.
3. Wall thickness measurement
For wall thicknes measurement in the near field range we need high resolution / broad band probes. As the reflector (back wall echo) normally exceeds the sound beam cross section the sound field modulation will be averaged. Therefore, the wall thickness measurement is not influenced by the near field structure.
4. The above explanations are not restricted to hardface probes but are genarally valid for all types of probes.
01:04 Sep-05-1996 Rolf D. Director, Editor, Publisher, Internet, PHP MySQL NDT.net, Germany, Joined Nov 1998 602
Re: Near field operation - a further discussion Hello Mr. Splitt, Hello Workshop participants,
thanks for your answer, however I have still some more questions regarding the near field operation.
You said "With a broad band probe the maxima and minima are smoothed out", what will be the dB figure approximately?
Further you said "...you will detect a small reflector two or more times when you scan the probe transverse to the flaw position."
That actually means that the sound pressure curve isn't so much smoothed out as you said it before. Or is there just a minima at the beam axis?
I remember while adjusting a probe on a pipe I received at two positions a maxima echo amplitude, if entry echo or backwall echo I don't remember. My question is: In what position should the probe be adjusted? If I use a maxima the probe will not stand in the 'correct' angle to the pipe axis. The application is a wall thickness measurement as well as a flaw detection.
By the way, here a hint for the Germans. Mein Englisch ist auch nicht vom Besten, ohne ein Spell check program für meine englischen Texte wäre ich aufgeschmissen Sehr gut eignet sich der Spell Anywhere von GIR software. Ich hatte ihn früher mal über CompuServ geladen. Dieses oder ähnliches ist aber sicherlich auch im Internet zu beziehen.