The need for NDT
It is very difficult to weld or mold a solid object that has the 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 non-destructive 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;
Reinforced 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.
Finished machined parts, such as bearings, that have newly been assembled can be tested for missing pieces, such as a ball or roller bearing, or grease within the housing non-destructively with a checkweigher. A roller motor for a conveyor can be tested for the proper level of oil, without disassembling the finished product. Thousand of manufactured products can benefit from this form of testing.
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.
Use of X-rays for NDT is a common way of examining the interior of products for voids and defects, although some skill is needed in using radiography to examine samples and interpret the results. Soft X-rays are needed for examining low density material like polymers, composites and ceramics.
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 "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 .
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.
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)
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.
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