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Nondestructive testing

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Nondestructive testing (also called NDT, nondestructive evaluation, NDE, and nondestructive inspection, NDI) is testing that does not destroy the test object. NDE is vital for constructing and maintaining all types of components and structures. To detect different defects such as cracking and corrosion, there are different methods of testing available, such as X-ray (where cracks show up on the film) and ultrasound (where cracks show up as an echo blip on the screen). This article is aimed mainly at industrial NDT, but many of the methods described here can be used to test the human body. In fact methods from the medical field, where there tends to be more development funding available, have often been adapted for industrial use, as was the case with Phased array ultrasonics and Computed radiography.

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.

[edit] The need for NDT

It is actually 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 molding 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) don't 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:

  • aircraft skins need regular checking to detect cracks;
  • underground pipelines are subject to corrosion and stress corrosion cracking;
  • pipes in industrial plants may be subject to erosion and corrosion from the products they carry;
  • concrete structures may be weakened if the inner reinforcing steel is corroded;
  • pressure vessels may develop cracks in welds;
  • the wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires and other damage is important.

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 — a function that is now carried out by instrumentation and referred to as the acoustic impact technique. In the cowboy days, it was quite common for a gun to kill the shooter rather than the person they were aiming at. From the 1992 Clint Eastwood western Unforgiven, here's a quote that reflects historical reality:

Little Bill Daggett: "... Bob's as good as dead because ... Corky ... takes careful aim and BAM!, the cylinder explodes in that Walker Colt he was carrying; a failing common to that model. It would have been better if Corky had two guns..., 'cause Bob walks over and shoots him."

Too bad for "Corky" that in those days X-ray was not available to allow the gun-maker to check for defects.

[edit] Notable events in early industrial NDT

  • 1854 Hartford, Connecticut: a boiler at the Fales and Gay Gray Car works explodes, killing 21 people and seriously injuring 50. Within a decade, the State of Connecticut passes a law requiring annual inspection (in this case visual) of boilers.
  • 1895 Wilhelm Conrad Röntgen discovers what are now known as X-rays. In his first paper he discusses the possibility of flaw detection.
  • 1880 - 1920 The "Oil and Whiting" method of crack detection is used in the railroad industry to find cracks in heavy steel parts. (A part is soaked in thinned oil, then painted with a white coating that dries to a powder. Oil seeping out from cracks turns the white powder brown, allowing the cracks to be detected.) This was the precursor to modern liquid penetrant tests.
  • 1920 Dr. H. H. Lester begins development of industrial radiography for metals. 1924 — Lester uses radiography to examine castings to be installed in a Boston Edison Company steam pressure power plant [1].
  • 1926 The first electromagnetic eddy current instrument is available to measure material thicknesses.
  • 1927 - 1928 Magnetic induction system to detect flaws in railroad track developed by Dr. Elmer Sperry and H.C. Drake.
  • 1929 Magnetic particle methods and equipment pioneered (A.V. DeForest and F.B. Doane.)
  • 1930s Robert F. Mehl demonstrates radiographic imaging using gamma radiation from Radium, which can examine thicker components than the low-energy X-ray machines available at the time.
  • 1935 - 1940 Liquid penetrant tests developed (Betz, Doane, and DeForest)
  • 1935 - 1940s Eddy current instruments developed (H.C. Knerr, C. Farrow, Theo Zuschlag, and Fr. F. Foerster).
  • 1940 - 1944 Ultrasonic test method developed in USA by Dr. Floyd Firestone.
  • 1950 J. Kaiser introduces acoustic emission as an NDT method.

(Source: Hellier, 2001) Note the number of advancements made during the WWII era, a time when industrial quality control was growing in importance.

[edit] Applications

NDT is used in a variety of settings that covers a wide range of industrial activity.

[edit] 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.

[edit] Elements

Regardless of application or method, all nondestructive testing shares the same basic elements:

Source 
Source provides a medium for testing.
Modification 
The probing material must get modified due to variation in the source.
Detection 
A detector which will determine the changes on the probing medium.
Indication 
The response (blip on a screen) or evidence from an examination
Interpretation 
A method of interpreting indications.

[edit] Reliability and statistics

Its testing reliability is defined by its relationship to a perfect gold standard test, which measures the value of the test object without fail, denoted by a binary variable, termed OK/not_OK, whose values are elements in the set {OK, not_OK}. By definition, the nondestructive test errs when its measurement of OK/not_OK differs from the measurement of the same quantity by the perfect gold standard test. The Cartesian product of the sets of values respectively measured by the perfect gold standard test and the nondestructive test defines the four, possible events that belong to a measurement: a "false negative" occurs when the test object is not_OK but it is identified as OK by NDT; a "false positive" occurs when the test object is OK but it is identified as not_OK by NDT; similarly, a "true positive" occurs when the test object is not_OK and it is identified as not_OK by NDT and a "true negative" occurs when the test object is OK and NDT identifies it as OK.

[edit] Defect detection tests

NDT's defect detection tests attempt to detect defects (e.g., cracks, intergranular corrosion, pits and inclusions) before they can cause structural failure, leaks or other, unfavorable outcomes. The existence of a "probability of detection," corresponding to the probability of a true positive given a true positive OR false negative (inclusive disjunction implied by OR) is more often asserted for these tests than delivered by their design. Similarly, the existence of a "probability of false call," corresponding to the probability of a false positive given a false positive OR true negative is more often asserted than delivered. This topic is amplified below.

[edit] Violations of probability theory

Oldberg and Christensen (1995) and Oldberg (2005) report that today's defect detection tests violate probability theory empirically. Often, a probability of false call is not defined. Sometimes, the "probability" of detection is not a probability. In these ways, the testing reliability is ill-defined.

The false impression that the testing reliability is well defined can be established by the use of terms that imply the preservation of probability theory under conditions in which probability theory is empirically violated. An article that is posted by the U.S. Nuclear Regulatory Commission, 2000 at its Web site provides a case in point. In the article, the agency applies the terms "probability of detection," "signal" and "noise" to a Defect Detection Test that is used in the safety inspections of nuclear power reactors. The terms "signal" and "noise" imply that probability theory is unconditionally preserved, but Oldberg and Christensen (1995) demonstrate that it is only conditionally preserved in this test. The term "probability of detection" implies that probability theory is preserved, given the event of a true positive OR false negative but Oldberg and Christensen (1995) demonstrate that, for this test, this is not true.

Some of the consequences of this kind of terminological abuse are: a) it appears that the testing reliability can be determined when it can't b) it appears that the expected utility of testing can be determined when it cannot and c) it appears that decisions can be made about testing on a basis that is considered rational by decision analysts when one's ability to make such decisions is actually crippled. In a defect-sensitive technology such as nuclear power, a further consequence might be significant, unnecessary loss of life and property damage.

Potential and actual users of Defect Detection Tests should also be aware of the fact that the expected utility of such a test depends upon a) the nature of the statistical populations to be tested and b) the utilities of the user. As the populations are undefined and the associated utilities vary by user, claims that testing with a particular technology has a greater, expected utility than non-testing or that testing with a particular technology as a greater, expected utility than testing with another technology should be met with skepticism.

[edit] See also

[edit] References

[edit] NDT Books

  • Bray, D.E. and R.K. Stanley, 1997, Nondestructive Evaluation: A Tool for Design, Manufacturing and Service; CRC Press, 1996.
  • Chuck Hellier, Handbook of Nondestructive Evaluation, McGraw-Hill Professional; 2001
  • Peter J. Shull, Nondestructive Evaluation: Theory, Techniques, and Applications, Marcel Dekker Inc., 2002.
  • ASTM International, Annual Book of ASTM Standards, Volume 03.03 Nondestructive Testing
  • ASNT, Nondestructive Testing Handbook

[edit] Statistics in NDT

[edit] External links

[edit] General NDT, Online Information

[edit] NDT Published Journals

[edit] NDT Societies

[edit] NDT Trade Sites

[edit] NDT Research

[edit] Collections of NDT Links

[edit] NDT Events

[edit] See also

Inspection | Maintenance testing | Reliability engineering | Quality control | Product certification | Risk based inspectionde:Werkstoffprüfung es:Ensayo no destructivo fr:Contrôle non destructif he:בדיקות לא הורסות it:Controlli non distruttivi nl:Niet-destructief onderzoek ru:Неразрушающий контроль uk:Неруйнівний контроль vi:Kiểm tra không phá hủy

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