Re: Near field operation by use of Hardface probes

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Re: Near field operation by use of Hardface probes
Posted by: Splitt , E-mail: Sp@kk-a-col.mhs.compuserve.com, on September 05, 1996 at 10:42:45:
In Reply to: Near field operation by use of Hardface probes posted by : Rolf Diederichs on September 02, 1996 at 10:41:00:
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.
Regards

Follow Ups:

Re: Near field operation - a further discussion Rolf D. 21:04:54 9/05/96 (size: 1364) (1)

Re: Near field operation - a further discussion Splitt 14:15:54 9/07/96 (size: 1360) (0)

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: 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. : Regards

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