During the quality assurance process, an inspector or QA Technician might use words like “flaw,” “discoloration,” or “blemish.” At first glance, one might assume that if a part had a “flaw” that it was defective and therefore unusable. The aforementioned descriptors are commonly used synonyms for the word "defect.” However, in the world of nondestructive testing, “defect” and its synonyms are not necessarily interchangeable.
For example, a part may have a blemish or a flaw in it and still not be ‘defective" in the sense that its usefulness for its intended purpose will be impaired. A spot or stain would have no effect on the service performance of a connecting rod of an automobile engine and would certainly not make it defective but on stainless steel exterior panel, such a spot might make the panel unusable and in turn render it defective.
So, how is a “defect” accurately defined in the language of nondestructive testing? Initially, everything found during MPI testing is classified as an “indication”. Then, based on what is causing the indication (part geometry, discontinuity, etc.) the indication is classified as relevant or non-relevant. Once an indication is determined to be relevant, then the acceptance criteria must be referenced in order to determine if the indication causes the part to be rejected.
Before we dive further into how a defect is appropriately categorized, we must consider also the state that occurs prior to a defect being defined. Here we find another common term: "discontinuity".
“Discontinuity” addresses the condition, or an abnormal occurrence on a part, before it is determined whether it is a defect or not. The cause of magnetic particle indications is in all cases is a discontinuity- whether physical or magnetic. And if we exclude those discontinuities that are present by design and consider only those present in the metal by accident or the result of some manufacturing process, these may still not in all cases make the part defective in the sense that its service performance will be affected unfavorably. Therefore, we can conclude that a discontinuity is not necessarily a defect. It is a defect only when it will interfere with the performance of the part or material in its intended service. And a discontinuity which may make one part defective may be entirely harmless in another part designed for a different service. Further - as in the case of the panel - what would be only a blemish in most cases may become a defect in a product in which appearance is a major factor in acceptable service use.
There are a number of possible ways of classifying discontinuities that occur in ferromagnetic materials and parts. For many years it has been customary to classify discontinuities for the purpose of magnetic particle testing (as well as for other nondestructive testing methods) according to their source or origin in the various stages of production of the metal, its fabrication and its use.
Below are the commonly used categories for organizing magnetic particle discontinuities and what each of those categories entails:
Inherent
This group of discontinuities is present in metal as the result of its initial solidification from the molten state, before any of the operations to forge or roll it into useful sizes and shapes has begun.
Learn more about inherent discontinuities here
Primary Processing
When steel ingots are worked down into usable sizes and shapes such as billets and forging blanks, some inherent defects may appear. But the rolling and forging processes may themselves introduce discontinuities which in many cases constitute defects. Primary processes are those which work the metal down, by either hot or cold deformation, into useful forms such as bars, rod and wire, and forged shapes. Casting is another process usually included in this group since, though it starts with molten metal, it results in a semi-finished product. Welding is similarly included for comparable reasons.
Learn more about primary processing discontinuities here
Secondary or Finishing Processing
In this group are those discontinuities associated with the various finishing operations, after the part has been rough-formed by rolling, forging, casting or welding. Discontinuities may be introduced by machining, heat treating, grinding and similar processes.
Learn more about secondary processing discontinuities here
Service
The fourth major classification of discontinuities comprises those which are formed or produced after all fabrication has been completed and the part has gone into service. The objective of magnetic particle testing to locate and eliminate discontinuities during fabrication, is to put the part into service free from defects. However, even when this is fully accomplished, failures in service still occur as a result of cracking caused by in use conditions.
Learn more about service discontinuities here
Now that we have established what discontinuities are, we must consider our original question: what is a defect and how do we define it within the nondestructive testing world?
The word "defect" is correctly applied only to a condition which will interfere with the safe or satisfactory service of the particular part in question. An abnormality on a part at any stage is not automatically a defect, but rather, as explained above, it is a discontinuity which can be categorized based on the stage of production in which abnormality occurred. Once that discontinuity has been identified, then it can be determined if it will have an impact on a part’s performance in which case it is then classified as a defect.
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