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Technical Discussions
Rolf Diederichs
Director, Editor
NDT.net, Germany, Joined Nov 1998, 603

Rolf Diederichs

Director, Editor
Joined Nov 1998
00:41 Sep-02-1996
Near field operation by use of Hardface probes

Hello Mr. Splitt,

Today is your day!
I hope you will get a lot of questions.
Sound propagation is part of our program, so I have a question about this.

Hardface probes are designed to operate in direct contact method.
The probes doesn't have inside a delay, actually they are immersion
probes providing a wear resistance protection layer.

The physics of sound beam showing that within the near field
many interferences, by means of maxima and minima of the sound preasure.

Because of this interferences I am concerned if this technique could be used.
My further questions are.

1. Can I do a proper flaw detection

2. Need the indications a special interpretation

3. Can I use it for wall thickness measurements with no problem

4. Could you explain something general about the nearfield operation, also with immersion



R & D, -
Private, Germany, Joined Nov 1998, 38


R & D, -
Joined Nov 1998
00:42 Sep-05-1996
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.


Rolf D.
Director, Editor
NDT.net, Germany, Joined Nov 1998, 603

Rolf D.

Director, Editor
Joined Nov 1998
01:04 Sep-05-1996
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.

Rolf Diederichs

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.


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