Nondestructive testing

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Nondestructive testing (NDT) is a wide group of analysis techniques used in science and technology industry to evaluate the properties of a material, component or system without causing damage. [1] The terms nondestructive examination (NDE), nondestructive inspection (NDI), and nondestructive evaluation (NDE) are also commonly used to describe this technology. [2] Because NDT does not permanently alter the article being inspected, it is a highly valuable technique that can save both money and time in product evaluation, troubleshooting, and research. The six most frequently used NDT methods are eddy-current, magnetic-particle, liquid penetrant, radiographic, ultrasonic, and visual testing. [3] NDT is commonly used in forensic engineering, mechanical engineering, petroleum engineering, electrical engineering, civil engineering, systems engineering, aeronautical engineering, medicine, and art. [1] Innovations in the field of nondestructive testing have had a profound impact on medical imaging, including on echocardiography, medical ultrasonography, and digital radiography.

Contents

Various national and international trade associations exist to promote the industry, knowledge about non-destructive testing, and to develop standard methods and training. These include the American Society for Nondestructive Testing, the Non-Destructive Testing Management Association, the International Committee for Non-Destructive Testing, the European Federation for Non-Destructive Testing and the British Institute of Non-Destructive Testing.

NDT methods rely upon use of electromagnetic radiation, sound and other signal conversions to examine a wide variety of articles (metallic and non-metallic, food-product, artifacts and antiquities, infrastructure) for integrity, composition, or condition with no alteration of the article undergoing examination. Visual inspection (VT), the most commonly applied NDT method, is quite often enhanced by the use of magnification, borescopes, cameras, or other optical arrangements for direct or remote viewing. The internal structure of a sample can be examined for a volumetric inspection with penetrating radiation (RT), such as X-rays, neutrons or gamma radiation. Sound waves are utilized in the case of ultrasonic testing (UT), another volumetric NDT method – the mechanical signal (sound) being reflected by conditions in the test article and evaluated for amplitude and distance from the search unit (transducer). Another commonly used NDT method used on ferrous materials involves the application of fine iron particles (either suspended in liquid or dry powder – fluorescent or colored) that are applied to a part while it is magnetized, either continually or residually. The particles will be attracted to leakage fields of magnetism on or in the test object, and form indications (particle collection) on the object's surface, which are evaluated visually. Contrast and probability of detection for a visual examination by the unaided eye is often enhanced by using liquids to penetrate the test article surface, allowing for visualization of flaws or other surface conditions. This method (liquid penetrant testing) (PT) involves using dyes, fluorescent or colored (typically red), suspended in fluids and is used for non-magnetic materials, usually metals.

Analyzing and documenting a nondestructive failure mode can also be accomplished using a high-speed camera recording continuously (movie-loop) until the failure is detected. Detecting the failure can be accomplished using a sound detector or stress gauge which produces a signal to trigger the high-speed camera. These high-speed cameras have advanced recording modes to capture some non-destructive failures. [4] After the failure the high-speed camera will stop recording. The capture images can be played back in slow motion showing precisely what happen before, during and after the nondestructive event, image by image.

Applications

NDT is used in a variety of settings that covers a wide range of industrial activity, with new NDT methods and applications, being continuously developed. Nondestructive testing methods are routinely applied in industries where a failure of a component would cause significant hazard or economic loss, such as in transportation, pressure vessels, building structures, piping, and hoisting equipment.

Weld verification

Section of material with a surface-breaking crack that is not visible to the naked eye.
Penetrant is applied to the surface.
Excess penetrant is removed.
Developer is applied, rendering the crack visible. Ressuage principe 2.svg
  1. Section of material with a surface-breaking crack that is not visible to the naked eye.
  2. Penetrant is applied to the surface.
  3. Excess penetrant is removed.
  4. Developer is applied, rendering the crack visible.

In manufacturing, welds are commonly used to join two or more metal parts. 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. For example, the base metal must reach a certain temperature during the welding process, must cool at a specific rate, and must be welded with compatible materials or the joint may not be strong enough to hold the parts together, or cracks may form in the weld causing it to fail. The typical welding defects (lack of fusion of the weld to the base metal, cracks or porosity inside the weld, and variations in weld density) could cause a structure to break or a pipeline to rupture.

Welds may be tested using NDT techniques such as industrial radiography or industrial CT scanning using X-rays or gamma rays, ultrasonic testing, liquid penetrant testing, magnetic particle inspection or via eddy current. In a proper weld, these tests would indicate a lack of cracks in the radiograph, show clear passage of sound through the weld and back, or indicate a clear surface without penetrant captured in cracks.

Welding techniques may also be actively monitored with acoustic emission techniques before production to design the best set of parameters to use to properly join two materials. [5] In the case of high stress or safety critical welds, weld monitoring will be employed to confirm the specified welding parameters (arc current, arc voltage, travel speed, heat input etc.) are being adhered to those stated in the welding procedure. This verifies the weld as correct to procedure prior to nondestructive evaluation and metallurgy tests.

Structural mechanics

Structure can be complex systems that undergo different loads during their lifetime, e.g. Lithium-ion batteries. [6] Some complex structures, such as the turbo machinery 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. The resulting sets of differential equations are then used to derive a transfer function that models the behavior of the system.

In NDT, 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.

Reference standards, which are structures that intentionally flawed in order to be compared with components intended for use in the field, are often used in NDT. Reference standards can be with many NDT techniques, such as UT, [7] RT [8] and VT.

Relation to medical procedures

Chest radiography indicating a peripheral bronchial carcinoma. Thorax pa peripheres Bronchialcarcinom li OF markiert.jpg
Chest radiography indicating a peripheral bronchial carcinoma.

Several NDT methods are related to clinical procedures, such as radiography, ultrasonic testing, and visual testing. Technological improvements or upgrades in these NDT methods have migrated over from medical equipment advances, including digital radiography (DR), phased array ultrasonic testing (PAUT), and endoscopy (borescope or assisted visual inspection).

Notable events in early academic and industrial NDT

(Basic Source for above: 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

An example of a 3D replicating technique. The flexible high-resolution replicas allow surfaces to be examined and measured under laboratory conditions. A replica can be taken from all solid materials. Replica system for nondestructive testing.JPG
An example of a 3D replicating technique. The flexible high-resolution replicas allow surfaces to be examined and measured under laboratory conditions. A replica can be taken from all solid materials.

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.

Personnel training, qualification and certification

Successful and consistent application of nondestructive testing techniques depends heavily on personnel training, experience and integrity. Personnel involved in application of industrial NDT methods and interpretation of results should be certified, and in some industrial sectors certification is enforced by law or by the applied codes and standards. [19]

NDT professionals and managers who seek to further their growth, knowledge and experience to remain competitive in the rapidly advancing technology field of nondestructive testing should consider joining NDTMA, a member organization of NDT Managers and Executives who work to provide a forum for the open exchange of managerial, technical and regulatory information critical to the successful management of NDT personnel and activities. Their annual conference at the Golden Nugget in Las Vegas is a popular for its informative and relevant programming and exhibition space

Certification schemes

There are two approaches in personnel certification: [20]

  1. Employer Based Certification: Under this concept the employer compiles their own Written Practice. The written practice defines the responsibilities of each level of certification, as implemented by the company, and describes the training, experience and examination requirements for each level of certification. In industrial sectors the written practices are usually based on recommended practice SNT-TC-1A of the American Society for Nondestructive Testing. [21] ANSI standard CP-189 outlines requirements for any written practice that conforms to the standard. [22] For aviation, space, and defense (ASD) applications NAS 410 sets further requirements for NDT personnel, and is published by AIA – Aerospace Industries Association, which is made up of US aerospace airframe and powerplant manufacturers. This is the basis document for EN 4179 [23] and other (USA) NIST-recognized aerospace standards for the Qualification and Certification (employer-based) of Nondestructive Testing personnel. NAS 410 also sets the requirements also for "National NDT Boards", which allow and proscribe personal certification schemes. NAS 410 allows ASNT Certification as a portion of the qualifications needed for ASD certification. [24]
  2. Personal Central Certification: The concept of central certification is that an NDT operator can obtain certification from a central certification authority, that is recognized by most employers, third parties and/or government authorities. Industrial standards for central certification schemes include ISO 9712, [25] and ANSI/ASNT CP-106 [26] (used for the ASNT ACCP [27] scheme). Certification under these standards involves training, work experience under supervision and passing a written and practical examination set up by the independent certification authority. EN 473 [28] was another central certification scheme, very similar to ISO 9712, which was withdrawn when CEN replaced it with EN ISO 9712 in 2012.

In the United States employer based schemes are the norm, however central certification schemes exist as well. The most notable is ASNT Level III (established in 1976-1977), which is organized by the American Society for Nondestructive Testing for Level 3 NDT personnel. [29] NAVSEA 250-1500 is another US central certification scheme, specifically developed for use in the naval nuclear program. [30]

Central certification is more widely used in the European Union, where certifications are issued by accredited bodies (independent organizations conforming to ISO 17024 and accredited by a national accreditation authority like UKAS). The Pressure Equipment Directive (97/23/EC) actually enforces central personnel certification for the initial testing of steam boilers and some categories of pressure vessels and piping. [31] European Standards harmonized with this directive specify personnel certification to EN 473. Certifications issued by a national NDT society which is a member of the European Federation of NDT (EFNDT) are mutually acceptable by the other member societies [32] under a multilateral recognition agreement.

Canada also implements an ISO 9712 central certification scheme, which is administered by Natural Resources Canada, a government department. [33] [34] [35]

The aerospace sector worldwide sticks to employer based schemes. [36] In America it is based mostly on AIA-NAS-410 [37] and in the European Union on the equivalent and very similar standard EN 4179. [23] However EN 4179:2009 includes an option for central qualification and certification by a National aerospace NDT board or NANDTB (paragraph 4.5.2).

Levels of certification

Most NDT personnel certification schemes listed above specify three "levels" of qualification and/or certification, usually designated as Level 1, Level 2 and Level 3 (although some codes specify Roman numerals, like Level II). The roles and responsibilities of personnel in each level are generally as follows (there are slight differences or variations between different codes and standards): [25] [23]

Terminology

The standard US terminology for Nondestructive testing is defined in standard ASTM E-1316. [38] Some definitions may be different in European standard EN 1330.

Indication 
The response or evidence from an examination, such as a blip on the screen of an instrument. Indications are classified as true or false. False indications are those caused by factors not related to the principles of the testing method or by improper implementation of the method, like film damage in radiography, electrical interference in ultrasonic testing etc. True indications are further classified as relevant and non relevant. Relevant indications are those caused by flaws. Non relevant indications are those caused by known features of the tested object, like gaps, threads, case hardening etc.
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. [38]

Reliability and statistics

Probability of detection (POD) tests are a standard way to evaluate a nondestructive testing technique in a given set of circumstances, for example "What is the POD of lack of fusion flaws in pipe welds using manual ultrasonic testing?" The POD will usually increase with flaw size. A common error in POD tests is to assume that the percentage of flaws detected is the POD, whereas the percentage of flaws detected is merely the first step in the analysis. Since the number of flaws tested is necessarily a limited number (non-infinite), statistical methods must be used to determine the POD for all possible defects, beyond the limited number tested. Another common error in POD tests is to define the statistical sampling units (test items) as flaws, whereas a true sampling unit is an item that may or may not contain a flaw. [39] [40] Three key parameters in reliability evaluation of nondestructive testing methods are a50, a90 and a90/95. a50 and a90 refer to length of defects that your desired nondestructive testing technique is capable to detect with 50% and 90% probabilities, respectively. If a 95% degree of confidence is also considered in determination of defect length with 90% probability, it is a90/95. [41] Guidelines for correct application of statistical methods to POD tests can be found in ASTM E2862 Standard Practice for Probability of Detection Analysis for Hit/Miss Data and MIL-HDBK-1823A Nondestructive Evaluation System Reliability Assessment, from the U.S. Department of Defense Handbook.

See also

Related Research Articles

Inspection organized examination or formal evaluation exercise

An inspection is, most generally, an organized examination or formal evaluation exercise. In engineering activities inspection involves the measurements, tests, and gauges applied to certain characteristics in regard to an object or activity. The results are usually compared to specified requirements and standards for determining whether the item or activity is in line with these targets, often with a Standard Inspection Procedure in place to ensure consistent checking. Inspections are usually non-destructive.

Dye penetrant inspection (DP), also called liquid penetrate inspection (LPI) or penetrant testing (PT), is a widely applied and low-cost inspection method used to check surface-breaking defects in all non-porous materials. The penetrant may be applied to all non-ferrous materials and ferrous materials, although for ferrous components magnetic-particle inspection is often used instead for its subsurface detection capability. LPI is used to detect casting, forging and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.

Magnetic particle inspection non-destructive method used to detect defects in ferrous materials

Magnetic particle Inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and shallow subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current, which may be either alternating current (AC) or some form of direct current (DC).

Plastic welding welding of semi-finished plastic materials

Plastic welding is welding for semi-finished plastic materials, and is described in ISO 472 as a process of uniting softened surfaces of materials, generally with the aid of heat. Welding of thermoplastics is accomplished in three sequential stages, namely surface preparation, application of heat and pressure, and cooling. Numerous welding methods have been developed for the joining of semi-finished plastic materials. Based on the mechanism of heat generation at the welding interface, welding methods for thermoplastics can be classified as external and internal heating methods, as shown in Fig 1.

Ultrasonic testing testing techniques based in the propagation of ultrasonic waves in the object or material tested

Ultrasonic testing (UT) is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz, and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials. A common example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion.

Welder certification, is a process which examines and documents a welder's capability to create welds of acceptable quality following a well defined welding procedure.

Phased array ultrasonics

Phased array ultrasonics (PA) is an advanced method of ultrasonic testing that has applications in medical imaging and industrial nondestructive testing. Common applications are to noninvasively examine the heart or to find flaws in manufactured materials such as welds. Single-element probes, known technically as monolithic probes, emit a beam in a fixed direction. To test or interrogate a large volume of material, a conventional probe must be physically scanned to sweep the beam through the area of interest. In contrast, the beam from a phased array probe can be focused and swept electronically without moving the probe. The beam is controllable because a phased array probe is made up of multiple small elements, each of which can be pulsed individually at a computer-calculated timing. The term phased refers to the timing, and the term array refers to the multiple elements. Phased array ultrasonic testing is based on principles of wave physics, which also have applications in fields such as optics and electromagnetic antennae.

Time-of-flight diffraction ultrasonics

Time-of-flight diffraction (TOFD) method of ultrasonic testing is a sensitive and accurate method for the nondestructive testing of welds for defects. TOFD originated from tip diffraction techniques which were first published by Silk and Liddington in 1975 which paved the way for TOFD. Later works on this technique are given in a number of sources which include Harumi et al. (1989), Avioli et al. (1991), and Bray and Stanley (1997).

Industrial radiography

Industrial radiography is a method of non-destructive testing where many types of manufactured components can be examined to verify the internal structure and integrity of the specimen. Industrial Radiography can be performed utilizing either X-rays or gamma rays. Both are forms of electromagnetic radiation. The difference between various forms of electromagnetic energy is related to the wavelength. X and gamma rays have the shortest wavelength and this property leads to the ability to penetrate, travel through, and exit various materials such as carbon steel and other metals.

Rail inspection

Rail inspection is the practice of examining rail tracks for flaws that could lead to catastrophic failures. According to the United States Federal Railroad Administration Office of Safety Analysis, track defects are the second leading cause of accidents on railways in the United States. The leading cause of railway accidents is attributed to human error. The contribution of poor management decisions to rail accidents caused by infrequent or inadequate rail inspection is significant but not reported by the FRA, only the NTSB. Every year, North American railroads spend millions of dollars to inspect the rails for internal and external flaws. Nondestructive testing (NDT) methods are used as a preventive measures against track failures and possible derailment.

Electromagnetic acoustic transducer

Electromagnetic acoustic transducer (EMAT) is a transducer for non-contact acoustic wave generation and reception in conducting materials. Its effect is based on electromagnetic mechanisms, which do not need direct coupling with the surface of the material. Due to this couplant-free feature, EMATs are particularly useful in harsh, i.e., hot, cold, clean, or dry environments. EMATs are suitable to generate all kinds of waves in metallic and/or magnetostrictive materials. Depending on the design and orientation of coils and magnets, shear horizontal (SH) bulk wave mode, surface wave, plate waves such as SH and Lamb waves, and all sorts of other bulk and guided-wave modes can be excited. After decades of research and development, EMAT has found its applications in many industries such as primary metal manufacturing and processing, automotive, railroad, pipeline, boiler and pressure vessel industries, in which they are typically used for nondestructive testing (NDT) of metallic structures.

Weld quality assurance is the use of technological methods and actions to test or assure the quality of welds, and secondarily to confirm the presence, location and coverage of welds. In manufacturing, welds are used to join two or more metal surfaces. Because these connections may encounter loads and fatigue during product lifetime, there is a chance they may fail if not created to proper specification.

Thermographic inspection refers to the nondestructive testing of parts, materials or systems through the imaging of the thermal patterns at the object's surface. Strictly speaking, the term thermography alone, refers to all thermographic inspection techniques regardless of the physical phenomena used to monitor the thermal changes. For instance, the application of a temperature sensitive coating to a surface in order to measure its temperature is a thermographic inspection contact technique based on heat conduction where there is no infrared sensor involved. Infrared thermography on the other hand, is a nondestructive, nonintrusive, noncontact mapping of thermal patterns or "thermograms", on the surface of objects through the use of some kind of infrared detector.

Shearography

Shearography or Speckle pattern shearing interferometry is a measuring and testing method similar to holographic interferometry. It uses coherent light or coherent soundwaves to provide information about the quality of different materials in nondestructive testing, strain measurement, and vibration analysis. Shearography is extensively used in production and development in aerospace, wind rotor blades, automotive, and materials research areas. Advantages of shearography are the large area testing capabilities, non-contact properties, its relative insensitivity to environmental disturbances, and its good performance on honeycomb materials, which is a big challenge for traditional nondestructive testing methods.

The British Institute of Non-Destructive Testing or BINDT is a professional body for engineers and other technical professionals involved in non-destructive testing and condition monitoring in the United Kingdom. The Institute was founded in 1976, by amalgamation of the Society of Non-Destructive Examination (SONDE) and the NDT Society of Great Britain (NDTS), which were both founded in 1954.

The American Society for Nondestructive Testing, Inc. or ASNT is a technical society for nondestructive testing (NDT) professionals. ASNT evolved from The American Industrial Radium and X-Ray Society which was founded in 1941. Its headquarters is located in Columbus, OH, and there are 70 local sections in the United States and 14 local sections in other countries.

Shantou Institute of Ultrasonic Instruments Co., Ltd (SIUI) is China's biggest manufacturing base of ultrasound with longest history. SIUI goes through a technology development of A-B-C-D-E. Each step has driven the industry progress in China. A: A mode ultrasound equipment. B:B mode ultrasound imaging equipment. C:Color Doppler. In 1997, SIUI manufactured its first color Doppler system Apogee 800.D:Real-time 3D/4D ultrasound. In 2008, SIUI developed the real-time 3D/4D technology, and released a series of real-time 3D/ 4D color Doppler systems. E: Elastography.

DIN EN ISO 9712:2012 is a certification issued by the German institute for standardization. It certifies personnel working in Non-destructive testing. This standard evaluates and documents the competence of personnel whose tasks require knowledge of non-destructive tests. The certification process is performed by authorized independent certification bodies, such as Sector Cert, DQS, TÜV, DEKRA etc. They can be applied at the German accreditation body.

Welding of advanced thermoplastic composites is a beneficial method of joining these materials compared to mechanical fastening and adhesive bonding. Mechanical fastening requires intense labor, and creates stress concentrations, while adhesive bonding requires extensive surface preparation, and long curing cycles. Welding these materials is a cost effective method of joining concerning preparation and execution, and these materials retain their properties upon cooling, so no post processing is necessary. These materials are widely used in the aerospace industry to reduce weight of a part while keeping strength.

A variety of non-destructive examination (NDE) techniques are available for inspecting plastic welds. Many of these techniques are similar to the ones used for inspecting metal welds. Traditional techniques include visual testing, radiography, and various ultrasonic techniques. Advanced ultrasonic techniques such as time of flight diffraction (TOFD) and phased-array ultrasonics (PAUT) are being increasingly studied and used for inspecting plastic pipeline welds. Research in the use of optical coherence tomography (OCT) and microwave reflectrometry has also been conducted.

References

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  36. R. Marini and P. Ranos: "Current Issues in Qualification and Certification of Non-Destructive Testing Personnel in the Aerospace Industry", ECNDT 2006 - Th.3.6.5
  37. AIA-NAS-410: "Aerospace Industries Association, National Aerospace Standard, NAS Certification and Qualification of Nondestructive Test Personnel"
  38. 1 2 ASTM E-1316: "Standard Terminology for Nondestructive Examinations", The American Society for Testing and Materials , in Volume 03.03 NDT, 1997
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  40. T. Oldberg (2005). "An Ethical Problem in the Statistics of Defect Detection Test Reliability". 10 (5). NDT.net.Cite journal requires |journal= (help)
  41. Abolfazl Zolfaghari, Amin Zolfaghari, Farhad Kolahan (2018) "Reliability and sensitivity of magnetic particle nondestructive testing in detecting surface cracks of welded components”, Nondestructive Testing and Evaluation, 33 (3), 290-300. https://doi.org/10.1080/10589759.2018.1428322

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    • EN 1330-4: Non-destructive testing. Terminology. Terms used in ultrasonic testing (2010)
    • EN 1330-7: Non-destructive testing. Terminology. Terms used in magnetic particle testing (2005)
    • EN 1330-8: Non-destructive testing. Terminology. Terms used in leak tightness testing (1998)
    • EN 1330-9: Non-destructive testing. Terminology. Terms used in acoustic emission testing (2009)
    • EN 1330-10: Non-destructive testing. Terminology. Terms used in visual testing (2003)
    • EN 1330-11: Non-destructive testing. Terminology. Terms used in X-ray diffraction from polycrystalline and amorphous materials (2007)
  • ISO 12706: Non-destructive testing. Penetrant testing. Vocabulary (2009)
  • ISO 12718: Non-destructive testing. Eddy current testing. Vocabulary (2008)