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sibu
NDT Inspector,
contractor staff at punj lloyd ltd, India, Joined Mar 2016, 27

sibu

NDT Inspector,
contractor staff at punj lloyd ltd,
India,
Joined Mar 2016
27
15:59 Dec-05-2016
why we are not considering after first backwall echo if any echo are coming in TR probe

Dear friends,
plz explain ...all staff are told we need to check only before 1 BWE only after this what will Happen??in case of many echo are coming .....

 
 Reply 
 
James Scalf
NDT Inspector,
Global Integrity, Canada, Joined Oct 2012, 274

James Scalf

NDT Inspector,
Global Integrity,
Canada,
Joined Oct 2012
274
13:43 Dec-06-2016
Re: why we are not considering after first backwall echo if any echo are coming in TR probe
In Reply to sibu at 15:59 Dec-05-2016 (Opening).

Sibu,

The reason T/R they normally only look before the first back wall echo is that signals that might occur after this could be the result of reflections caused by one sonic wave impacting another. This reduces the overall velocity of the returning sound wave and could result in signals that are erroneous and non-relevant.
To say this is always the case would be wrong as I know of a couple of procedures where we specifically look for signals returning shortly after the back wall however this is not the norm.
If you find that you are getting a lot of signals after the back wall or even small signals before the back wall in some cases try adjusting the Pulse Repetition Rate (PRF) of your instrument or ring time and this should clean up or discriminate better between relevant and non-relevant indications.

Hope this helps. Cheers...

James

 
 Reply 
 
Godfrey hands
Consultant,
PRI Nadcap, United Kingdom, Joined Nov 1998, 304

Godfrey hands

Consultant,
PRI Nadcap,
United Kingdom,
Joined Nov 1998
304
09:00 Dec-12-2016
Re: why we are not considering after first backwall echo if any echo are coming in TR probe
In Reply to James Scalf at 13:43 Dec-06-2016 .

Sibu,
Another factor which may be responsible for erroneous echoes after the first BWE is mode conversion, where the sound beam goes to the back wall as Longitudinal, and most of it is reflected as Longitudinal, but a small amount is converted to Transverse wave.
This transverse wave will then return to the probe at a slower velocity than the Longitudinal wave, arriving after the BWE.
You can calculate where this signal will appear from the path length and relative velocities.

Godfrey

 
 Reply 
 
Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
07:32 Dec-13-2016
Re: why we are not considering after first backwall echo if any echo are coming in TR probe
In Reply to sibu at 15:59 Dec-05-2016 (Opening).

Cibu

Have you asked the supervisor who issued you this instruction?

Why would you want to consider any echoes displayed after that from the backwall? Or to put it another way - do you have any doubts about your probes capability to complete the inspection task at hand using the information displayed between the sound entry point into the test material and the BWE, i.e. following the instruction given you by your supervisor??

You can either ask or read about the design of twin crystal probes and you will note that they are artificially 'focused' by being set at a slight angle to each other - 'toeing-in'. I am not a probe designer but İ suspect at n angle of around 2 to 3 degrees. Thus beyond this 'focal length' sensitivity to reflectors directly under the probe decreases sharply.

Your supervisor has given you a good instruction - you could have asked him/her the question why.


 
 Reply 
 

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