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Information about Nondestructive Testing

Nondestructive testing (NDT), also called 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 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.

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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

  • 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 http://www.robots4welding.com/articles.php?tag=615.
  • 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.
Enlarge picture
An example of Ultrasonic Testing (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 [1]) and Oldberg (2000 [2] and 2005 [3]) 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

  • 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

External links

NDT journals

NDT research institutes

See also

Phased Array (PA) ultrasonics is an advanced method of ultrasonic testing that has applications in medical imaging and industrial nondestructive testing, originally pioneered by Albert Macovski of Stanford University.
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Computed Radiography (CR) uses very similar equipment to conventional radiography except that in place of a film to create the image, an imaging plate is used. Hence, instead of taking a film into a darkroom for developing in chemical trays, the imaging plate is run through a
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In destructive testing, tests are carried out to the specimen’s failure. These tests are generally much easier to carry out, yield more information, and are easier to interpret than nondestructive testing.
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Forensic science (often shortened to forensics) is the application of a broad spectrum of sciences to answer questions of interest to the legal system. This may be in relation to a crime or to a civil action.
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A testing reliability is a set of two probabilities, the definition of which varies by field. In medicine, the sensitivity and specificity are conventionally used. In the field of defect detection testing, the probabilities of detection and false call are conventionally used.
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Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld puddle
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Molding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a mold.

A mold or mould is a hollowed-out block that is filled with a liquid like plastic, glass, metal, or ceramic raw materials.
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Casting is a manufacturing process by which a liquid material such as a suspension of minerals as used in ceramics or molten metal or plastic is introduced into a mould, allowed to solidify within the mould, and then ejected or broken out to make a fabricated part.
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Pipeline transport is a transportation of goods through a pipe. Most commonly, liquid and gases are sent, but pneumatic tubes that transport solid capsules using compressed air have also been used.
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Corrosion is breaking down of essential properties in a material due to reactions with its surroundings. In the most common use of the word, this means a loss of an electron of metals reacting with water and oxygen.
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Stress corrosion cracking (SCC) is the unexpected sudden failure of normally ductile metals or tough thermoplastics subjected to a constant tensile stress in a corrosive environment, especially at elevated temperature (in the case of metals).
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factory (previously manufactory) or manufacturing plant is an industrial building where workers manufacture goods or supervise machines processing one product into another.
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Erosion is displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement in response to gravity or by living organisms (in the case of bioerosion).
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Reinforced concrete, also called ferroconcrete in some countries, is concrete in which reinforcement bars ("rebars") or fibers have been incorporated to strengthen a material that would otherwise be brittle.
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Rebar, a portmanteau for reinforcing bar or reinforcement bar, is common steel bar, an important component of reinforced concrete and reinforced masonry structures.
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A pressure vessel is a closed, rigid container designed to hold gases or liquids at a pressure different from the ambient pressure. The end caps fitted to the cylindrical body are called heads.
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This article is concerned with a particular type of suspension bridge, the suspended-deck type. For an index to the several types see suspension bridge types.


A suspension bridge
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Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.

Vibration is occasionally desirable.
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Structural loads are forces applied to a component of a structure or to the structure as a unit.

In structural design, assumed loads are specified in national and local design codes for types of structures, geographic locations, and usage.
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Metal fatigue redirects here. For the computer game, see Metal Fatigue.


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Mechanical failure modes
Buckling
Corrosion
Creep
Fatigue
Fracture
Melting
Thermal shock
Wear
Yielding
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Acoustic Emission (AE) is a naturally occurring phenomenon whereby external stimuli such as mechanical loading generate sources of elastic waves. AE occurs when a small surface displacement of a material is produced.
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automobile (from Greek auto, self and Latin mobile moving, a vehicle that moves itself rather than being moved by another vehicle or animal) or motor car (usually shortened to just car) is a wheeled passenger vehicle that carries its own motor.
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An engine is something that produces an output effect from a given input. The origin of engineering however, came from the design, building and working of (military "engines") because before such devices came to be employed in battles there were very few mechanical devices used.
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chassis (plural: "chassis") (IPA: /ˈʃæːsiː, ˈtʃæːsiː/) consists of a framework that supports an inanimate object, analogous to an animal's skeleton, for example in a motor vehicle or a
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Aviation refers to all activities involving the operation of heavier-than-air aircraft, machines designed for atmospheric flight. The term also describes the organizations and regulatory bodies as well as the personnel related with the operation of aircraft and the industries
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Please help [ improve this article] by expanding this section.
See talk page for details. Please remove this message once the section has been expanded. (tagged since March 2007)

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For the novel by Michael Crichton, see Airframe (novel).


Airframe means the mechanical structure of an aircraft[1] and as generally used does not include the engines.
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space frame is a truss-like, lightweight rigid structure constructed from interlocking struts in a geometric pattern. Space frames usually utilize a multidirectional span, and are often used to accomplish long spans with few supports.
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A reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more pistons to convert pressure into a rotating motion. This article describes the common features of all types.
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jet engine is an engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton's . This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets and pump-jets, but in common usage, the term generally refers to a
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