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While Destructive Testing usually provides a more reliable assessment of the state of the test object, destruction of the test object usually makes this type of test more costly to the test object's owner than nondestructive testing. Destructive testing is also inappropriate in many circumstances, such as Forensic investigation. That there is a tradeoff between the cost of the test and its reliability favors a strategy in which most test objects are inspected nondestructively; destructive testing is performed on a sampling of test objects that is drawn randomly for the purpose of characterizing the Testing Reliability of the nondestructive test. THE NEED FOR NDT It is very difficult to Weld or Mold a solid object that has no risk of breaking in service, so testing at manufacture and during use is often essential. During the process of Casting a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture. During their service lives, many industrial components need regular nondestructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example:
Over the past centuries, swordsmiths, blacksmiths, and bell-makers would listen to the ring of the objects they were creating to get an indication of the soundness of the material. The wheel-tapper would test the wheels of locomotives for the presence of cracks, often caused by Fatigue — a function that is now carried out by instrumentation and referred to as the Acoustic Impact Technique . NOTABLE EVENTS IN EARLY INDUSTRIAL NDT
''(Source: Hellier, 2001) Note the number of advancements made during the WWII era, a time when industrial quality control was growing in importance.'' APPLICATIONS NDT is used in a variety of settings that covers a wide range of industrial activity.
METHODS AND TECHNIQUES NDT is divided into various ''methods'' of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various ''techniques''. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT. (UT) on blade roots of a V2500 IAE Aircraft Engine . ''Step 1'': The UT probe is placed on the root of the blades to be inspected with the help of a special Borescope tool (video probe). ''Step 2'': Instrument settings are input. ''Step 3'': The probe is scanned over the blade root. In this case, an indication (peak in the data) through the red line (or gate) indicates a good blade; an indication to the left of that range indicates a crack.]]
TERMINOLOGY ;Indication : The response or evidence from an examination, such as a blip on the screen of an instrument. ;Interpretation : Determining if an indication is of a type to be investigated. For example, in electromagnetic testing, indications from metal loss are considered flaws because they should usually be investigated, but indications due to variations in the material properties may be harmless and nonrelevant. ;Flaw : A type of discontinuity that must be investigated to see if it is rejectable. For example, porosity in a weld or metal loss. ;Evaluation : Determining if a flaw is rejectable. For example, is porosity in a weld larger than acceptable by Code ? ;Defect : A flaw that is rejectable — i.e. does not meet acceptance criteria. Defects are generally removed or repaired. (''Source: ASTM E1316 in 'Vol. 03.03 NDT'') RELIABILITY AND STATISTICS Oldberg and Christensen (1995 and Oldberg (2000 [http://www.nrc.gov/reading-rm/doc-collections/gen-comm/reg-issues/2000/ri00022.html#_1_6 and 2005 [http://www.ndt.net/article/v10n05/oldberg/oldberg.htm]) argue that tests to evaluate defect detection capability are frequently based on misapplications of probability theory. Oldberg and Christensen object to the common method of determining probability of detection, which is to divide the total number of test flaws by the number of flaws detected. This method ignores other possible results that should be included in the set, such as indications from non-existent flaws. FURTHER READING Books
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