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Nondestructive testing
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Non-destructive testing (NDT) is an analysis technique used in scientific fields to determine the state or function of a system by comparing a known input with a measured output, without the use of invasive approaches like disassembly or
failure testing. Because NDT does not require the disabling or sacrifice of the system of interest, it is a highly-valuable technique that saves both money and time in product evaluation, troubleshooting, and research.
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Encyclopedia
Non-destructive testing (NDT) is an analysis technique used in scientific fields to determine the state or function of a system by comparing a known input with a measured output, without the use of invasive approaches like disassembly or
failure testing. Because NDT does not require the disabling or sacrifice of the system of interest, it is a highly-valuable technique that saves both money and time in product evaluation, troubleshooting, and research. Common NDT methods include acoustic testing, liquid penetrant testing, and radiographic testing. NDT can be used with any isolated input / output system, and is a commonly-used tool in forensic engineering, mechanical engineering, electrical engineering, civil engineering, systems engineering, and medicine.
Background and Principles
NDT involves comparing a known input to a measured output and comparing to a known model - does not require the sacrifice of the physical system, as would be the case with disassembly, dissection, or failure testing. Such methods are known as Non-Destructive Testing techniques. Because the physical system does not need to be sacrificed or damaged for NDT tests, such techniques are valued for saving time and money. On the other hand, NDT methods do not always reveal hidden defects, and skill is usually needed in interpreting the results. Such NDT methods are important in litigation because the material evidence is preserved intact.
Methods
NDT methods usually rely on use of electromagnetic radiation to examine samples. Initially, this includes most kinds of microscopy to examine external surfaces in detail. The examination is often reasonably obvious especially when different light sources are used. Thus glancing light on a fracture surface will reveal details not immediately obvious to sight. The inner parts of a product can be examined using other kinds of radiation which can penetrate the material, such as X-rays or ultrasound. Contrast between a defect and the bulk is always an important consideration, and may be enhanced by using liquids for example to penetrate fatigue cracks, provided that the liquid has absolutely no effect on the sample being examined.
Applied NDT Examples
Weld Verification
In manufacturing, welds are commonly used to join two or more metal surfaces. Because these connections may encounter loads and fatigue during product lifetime, there is a chance that they may fail if not created to proper specification. During the process of casting a metal object, for example, the metal may shrink as it cools, which may introduce voids or cracks inside the structure. Some typical weld defects that need to be found and repaired in order to ensure the safe operation of a product are: lack of fusion of the weld to the metal, porous bubbles inside the weld, and variations in weld density, all of which could cause a structure to break or a pipeline to rupture.
Welds may be tested using NDT techniques such as industrial radiography using X-rays or neutrons, liquid penetrant testing and other methods such as acoustic emission. In a perfect weld, these tests (the system input) would produce known results (such as a known radiographic response, or a clean penetrant surface). Tests that produce differing results may indicate flaws that would otherwise cost money, time, and even lives in the case of structures such as buildings or vehicles. It is important to record results from several different angles to be able to detect flaws in the weld.
Structural Mechanics
Structures can be complex systems that undergo different loads during their lifetime. Some complex structures, such as the turbomachinery in a liquid-fuel rocket, can also cost millions of dollars. Engineers will commonly model these structures as coupled second-order systems, approximating dynamic structure components with springs, masses, and dampers. These sets of differential equations can be used to derive a transfer function that models the behavior of the system.
In NDT testing, the structure undergoes a dynamic input, such as the tap of a hammer or a controlled impulse. Key properties, such as displacement or acceleration at different points of the structure, are measured as the corresponding output. This output is recorded and compared to the corresponding output given by the transfer function and the known input. Differences may indicate an inappropriate model (which may alert engineers to unpredicted instabilities or performance outside of tolerances), failed components, or an inadequate control system.
Radiography in Medicine
As a system, the human body is difficult to model as a complete transfer function. Elements of the body, however, such as bones or molecules, have a known response to certain radiographic inputs, such as x-rays or magnetic resonance. Coupled with the controlled introduction of a known element, such as digested barium, radiography can be used to image parts or functions of the body by measuring and interpreting the response to the radiographic input. In this manner, many bone fractures and diseases may be detected and localized in preparation for treatment. X-rays may also be used to examine the interior of mechanical systems in manufacturing using NDT techniques, as well.
Notable events in early industrial NDT
- 1854 Hartford, Connecticut: a boiler at the Fales and 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 "" 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 .
- 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.
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.
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)
Penetrant testing : Non-destructive test typically comprising a penetrant, a method of excess removal and a developer to produce a visible indication of surface-breaking discontinuities.
(Source: ISO 12706:2000, Note: To be replaced by ISO/DIS 12706 (2008-03).)
Reliability and statistics
Defect detection tests are among the more commonly employed of non-destructive tests. The evaluation of NDT reliability commonly contains two statistical errors. First, most tests fail to define the objects that are called "sampling units" in statistics; it follows that the reliability of the tests cannot be established. Second, the literature usually misuses statistical terms in such a way as to make it sound as though sampling units are defined. These two errors may lead to incorrect estimates of probability of detection. .
Further reading
- NDT.net,
- 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
- ASNT,
See also
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