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Re: LI

Posted by: Ed , E-mail: amexpat2006@hotmail.com, on June 27, 2006 at 04:47 :

In Reply to: Re: LI posted by : David Profile , E-mail: david_davies_100@hotmail.com, on June 27, 2006 at 02:15 :

----------- Start Original Message -----------
: : Can anyone differenciate between the and the linear indication. Also explain as interpretation point of view
: : regards
: It is my belief that a is normally a form of linear indication. I usually define linear as having a length greater than 3 times the width. Interpret firstly into relevant and non relevant, then into rounded, linear, aligned etc, then look for the type of discontinuity - slag, pore, LOF, etc. The manufacturing process and part history should help here.
: The code or specification should give some guidelines on this. Sometimes it is required to differentiate between types, sometimes just being linear is enough to reject.
: Actual techniques of interpretation depend on the NDT method you are using and the product and manufacturing process.
------------ End Original Message ------------

A code will give you a definition and sometimes even illustrations describing flaws. It should also provide acceptance/rejection criteria for them, but it will not give you guidelines on how to differentiate between them.

That is the ultimate responsibility of the end user and comes down to the "experience" of the NDT technician. The technician should be able to pass a practical examination which should include identifying different types of indications.

It is assumed the technician is trained, qualified and certified before he/she performs the examinations independently. In fact, that is usually a code requirement.

Most codes identify cracks as cracks and linear indications as linear indications, or more specifically, slag, LOF, IF, IP, etc. But a crack is always identified as a crack. There is a reason for that. Cracks are the most detrimental type of defects you can have, whereas, linear indications can be accepted, depending on their vertical height and/or length.

That is with the exception of ASME Section XI, which does clearly state "Linear" or "Rounded". The Nuclear industry doesn't like to use the word "crack". ASME Section XI uses height vs. length for acceptance or rejection of flaws using Aspect Ratios, regardless of whether it is a crack or anything else. Length is grater than 3x width. In fact, the only place I have ever seen the work "crack" in Section XI was under Visual Examinations.

However, I do agree, sometimes differentiating can be difficult, a decision must be made. This is true, even if the NDE Level III has to make the decision. After all, the Level III is responsible for the results.

You haven't stated what method you are using. If you are using UT, you have some options. Try confirming it with RT. Alternatively, you could try using ToFD or try vertical sizing of the indication. If you use a notch and increase the dB until you can see the notch tip, you should be able to see a crack tip. Again, this comes down to experience.

I think ToFD is you best bet, though.

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