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07:06 Apr-15-2005

David Forsyth

R & D
TRI/Austin,
USA,
Joined Nov 2001
41
defect vs flaw vs discontinuity

This issue of semantics came up in another thread. It is one which has been much discussed in some groups I participate in. It has also been addressed by ASNT for example, who require use of the word "discontinuity" in their publications.

In my current opinion, which has evolved over time (covering up for use of words defect and flaw in previous papers of mine), the appropriate terminology is "discontinuity". The words "defect" and "flaw" imply that there is a some judgement made as to the serviceability of the object under inspection, if an object contains defects or flaws, is it not defective or flawed, and therefore unfit for service?

It is in fact NOT the role of the inspector to determine whether an object under inspection is fit for service. The inspector performs an inspection, which may yield indications of discontinuities in the object under test. It is the role of the structural engineer to provide the guidelines by which this indication is interpreted to determine fitness for service.

Discontinuities exist in all materials, as noted by another poster in the previous thread. Some are beneficial, alloying elements for example. Some are benign under the expected usage of the object under inspection, therefore are not "defects".

All inspectors should be aware that for your own protection, the use of the word "flaw" or "defect" sets you up for trouble. If you find a "flaw" or "defect" during an inspection of an object, and this object is not removed from use or repaired, you may end up on the wrong side of a lawsuit.

My two cents.

Cheers, Dave.


 
08:11 Apr-15-2005

John O'Brien

Consultant, -
Chevron ETC ,
USA,
Joined Jan 2000
278
Re: defect vs flaw vs discontinuity Oh Oh David

you fall into the same trap. Different industries have specific definitions and explanations for these words. You refer to an Inspector not making a decision. In the Oil & gas Industry we expect an Inspector to make a decision but not an examiner. Examiners perform NDE and sentance where an established reject criteria exists. In this case a flaw becomes a defect once it exceeds a stated threshold value.

This stresses that all personnel be they examiners or inspectors should 'read' the documents relevent to the task in hand and use the terminology as specified in each application. If no terminology is specified use what you deem appropriate but add the definition to your report.


 
08:12 Apr-15-2005

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1198
Re: defect vs flaw vs discontinuity Dave:
You are correct, the issue has had heated debates.

Normally used NDT terms are defined in several Standards. ASTM is one I am familiar with. They have a document, E-1316, that has these items covered.
In alphabetical order:

defect, n—one or more flaws whose aggregate size, shape, orientation, location, or properties do not meet specified acceptance criteria and are rejectable.

discontinuity, n—a lack of continuity or cohesion; an intentional or unintentional interruption in the physical structure or configuration of a material or component.

flaw, n—an imperfection or discontinuity that may be detectable by nondestructive testing and is not necessarily rejectable.

imperfection, n—a departure of a quality characteristic from its intended condition.


The discontinuity term is, from my perspective, ANY break in the homogenous characteristics of a volume. But since it may be a geometric change that is actually intended, so I find it far to general.

In fabrication we rarely achieve "perfection". Even diamonds may be considered "flawed". But a small flaw in a 2 carat diamond does not detract from its worth.

In our volumetric inspections, flaws or imperfections are the usual things we look for in NDT. When we find one we as technicians must use the rules imposed to decide if the condition is acceptable or not. But the rules are not as practical as you indicated (i.e. fitness for purpose). Instead, many acceptance criteria are arbitrary so we use the threshold as one method of decision making, or a form of length assessment based on a threshold.

For awhile I saw what seemed to be a push in some circles to "purge" the word flaw and replace it with discontinuity. However, when the definitions are a clear as indicated above in a document used to make these "legal" judgements, I think the lawyers are the ones that need a lesson in NDT, not the NDT techs needing legal advice.

My two cents worth too.

Ed

----------- Start Original Message -----------
: This issue of semantics came up in another thread. It is one which has been much discussed in some groups I participate in. It has also been addressed by ASNT for example, who require use of the word "discontinuity" in their publications.
: In my current opinion, which has evolved over time (covering up for use of words defect and flaw in previous papers of mine), the appropriate terminology is "discontinuity". The words "defect" and "flaw" imply that there is a some judgement made as to the serviceability of the object under inspection, if an object contains defects or flaws, is it not defective or flawed, and therefore unfit for service?
: It is in fact NOT the role of the inspector to determine whether an object under inspection is fit for service. The inspector performs an inspection, which may yield indications of discontinuities in the object under test. It is the role of the structural engineer to provide the guidelines by which this indication is interpreted to determine fitness for service.
: Discontinuities exist in all materials, as noted by another poster in the previous thread. Some are beneficial, alloying elements for example. Some are benign under the expected usage of the object under inspection, therefore are not "defects".
: All inspectors should be aware that for your own protection, the use of the word "flaw" or "defect" sets you up for trouble. If you find a "flaw" or "defect" during an inspection of an object, and this object is not removed from use or repaired, you may end up on the wrong side of a lawsuit.
: My two cents.
: Cheers, Dave.
------------ End Original Message ------------




 
08:27 Apr-15-2005

Juan J. Amado

Engineering, Inspection
Arco Industrial, S.A.,
Panama,
Joined Nov 2001
44
Re: defect vs flaw vs discontinuity ----------- Start Original Message -----------
: This issue of semantics came up in another thread. It is one which has been much discussed in some groups I participate in. It has also been addressed by ASNT for example, who require use of the word "discontinuity" in their publications.
: In my current opinion, which has evolved over time (covering up for use of words defect and flaw in previous papers of mine), the appropriate terminology is "discontinuity". The words "defect" and "flaw" imply that there is a some judgement made as to the serviceability of the object under inspection, if an object contains defects or flaws, is it not defective or flawed, and therefore unfit for service?
: It is in fact NOT the role of the inspector to determine whether an object under inspection is fit for service. The inspector performs an inspection, which may yield indications of discontinuities in the object under test. It is the role of the structural engineer to provide the guidelines by which this indication is interpreted to determine fitness for service.
: Discontinuities exist in all materials, as noted by another poster in the previous thread. Some are beneficial, alloying elements for example. Some are benign under the expected usage of the object under inspection, therefore are not "defects".
: All inspectors should be aware that for your own protection, the use of the word "flaw" or "defect" sets you up for trouble. If you find a "flaw" or "defect" during an inspection of an object, and this object is not removed from use or repaired, you may end up on the wrong side of a lawsuit.
: My two cents.
: Cheers, Dave.
------------ End Original Message ------------

Very interesting thread developing here, below is my contribution:

Indication: A signal or response from the method or apparatus. Could be the result of something in the specimen under investigation, some external agents, faulty equipment, or intrinsic to the equipment itself (noise)

Discontinuity: A detectable change in the material under study that generates a signal or response. This could be due to some undesirable quality, or to a normal state or behavior of the specimen ( i.e. graininess in a casting)

Flaw: A discontinuity that can be characterized as having undesirable qualities but that is within the boundaries of what the specifications state for rejection

Defect: A discontinuity that is unacceptable to the quality level established (written) by the specification (or tacit) by some known service condition. Important to notice is that quality is not always related to serviceability. A customer can order a part with a specified level of quality that will far exceed the rigors of service, in this case, quality below the required level cannot be accepted under the argument that the part will still perform well under service conditions.

Hope this helps,

Juan Amado



 


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