What is NDT?

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NDT is the abbreviation for Non-destructive testing. NDT refers to a testing method applied to materials or workpieces that does not damage them or affect their performance or usability in the future. Through the use of NDT, defects present both internally and on the surface of materials or workpieces can be detected; the geometric features and dimensions of workpieces can be measured; and the internal composition, structure, physical properties, and condition of materials or workpieces can be determined. NDT can be applied in various areas such as product design, material selection, manufacturing, finished product inspection, and in-service inspection (maintenance), playing a key role in optimizing quality control and reducing costs. NDT also helps to ensure the safe operation and/or effective use of products. NDT encompasses many methods that can be effectively applied, and the most commonly used NDT methods include: radiographic testing, ultrasonic testing, eddy current testing, magnetic particle testing, penetrant testing, visual inspection, leak detection, acoustic emission testing, and fluoroscopic testing. Since various NDT methods each have their own scope of application and limitations, new NDT methods are continually being developed and applied. Generally, as long as it meets the basic definition of NDT, any physical, chemical, or other possible technical means can be developed into an NDT method. In our country, the term \"non-destructive testing\" was initially referred to as flaw detection or non-destructive flaw detection; the various methods used for this purpose were also called flaw detection methods, such as radiographic testing, ultrasonic testing, magnetic particle testing, penetrant testing, and so on. This term or notation has been widely spread and used to this day, with a prevalence rate no less than that of the term \"non-destructive testing\". Abroad, the English term corresponding to the phrase \"non-destructive testing\" is mostly identical in its first part, that is, the word \"non-destructive\", but the spelling of its second part varies from place to place. In Japan*, it is customary to write it as “inspection”. In Europe, many countries** used to write it as “flaw detection”, but now they uniformly use “testing”. In the United States, apart from also using “testing”, it seems that “examination” and “evaluation” are preferred. When combined with the first part, the abbreviations formed are NDI, NDT, and NDE; in Chinese, these correspond to various terms and expressions such as non-destructive testing, non-destructive inspection (non-destructive examination), non-destructive inspection, non-destructive testing, and non-destructive evaluation. In fact, these various English terms and their corresponding Chinese equivalents all have the same meaning; they are synonyms. To this end, the International Organization for Standardization’s Technical Committee on Non-Destructive Testing (ISO/TC 135) developed and issued a new international standard (ISO/TS 18173:2005) aimed at unifying these terms in their various forms and expressions, clarifying that they refer to the same concept and are synonyms, all equivalent to non-destructive testing. Different ways of writing are simply due to differences in linguistic habits. Therefore, as a standardized term, it is recommended to use the term “non-destructive testing”; the corresponding English term recommended is “Non-destructive testing”. For the names of various non-destructive testing methods, it is also recommended to use the term “testing,” such as radiographic testing, ultrasonic testing, magnetic particle testing, penetrant testing, eddy current testing, and so on. When translating, English terms such as inspection, examination, and evaluation that are used in conjunction with Non-destructive should all be translated as “non-destructive testing”; it is advisable to avoid using expressions like “non-destructive flaw detection,” “non-destructive inspection,” “non-destructive examination,” or “non-destructive evaluation.” This translation method is equally applicable to the translation of names of various non-destructive testing methods. Note: Terms such as “inspection”, “examination”, and “evaluation” should be translated as “detection” only when rendering the names of non-destructive testing and its methods. In all other cases, the translation should be determined based on the original text and customary Chinese usage. I. Conventional non-destructive testing methods include: 1. Ultrasonic Testing (abbreviated as UT); 2. Radiographic Testing (abbreviated as RT); 3. Magnetic Particle Testing (abbreviated as MT); 4. Penetrant Testing (abbreviated as PT); 5. Eddy Current Testing (abbreviated as ET); 6. Time of Flight Diffraction (abbreviated as TOFD); 7. Phased Array Ultrasonic Testing (abbreviated as PAUT); 8. Visual Testing (abbreviated as VT).
II. Non-conventional non-destructive testing techniques include: 1. Acoustic Emission Testing (abbreviated as AE or AT); 2. Leak Testing (abbreviated as LT); 3. Optical Holography (abbreviated as OH); 4. Infrared Thermography (abbreviated as IT); 5. Microwave Testing; 6. Computed Tomographic Testing (abbreviated as CT); 7. Visual Testing (abbreviated as VT); 8. Computed Radiography Testing (abbreviated as CR); 9. Magnetic Flux Leakage (abbreviated as MFL); 10. Digital Radiography (abbreviated as DR).
III. Advantages and disadvantages of common testing methods: 1. Radiographic Testing – This is a technique that utilizes the attenuation of radiation (such as X-rays, gamma rays, neutrons, etc.) as it passes through materials or workpieces, in order to detect internal structural discontinuities. Radiation passing through the material or workpiece induces varying degrees of exposure on the X-ray film depending on its intensity, thereby producing an image of internal discontinuities. 1.1 Radiography (RT) is a non-destructive testing method that uses X-rays or gamma rays to penetrate the test specimen, with film serving as the medium for recording information. It is the most fundamental and widely used type of non-destructive inspection method. 1.2 Principle of radiographic inspection: Radiation can penetrate materials that are invisible to the naked eye and expose the film. When X-rays or gamma rays illuminate the film, they, just like ordinary light, cause silver halides in the film’s emulsion layer to develop latent images. Since materials with different densities have varying absorption coefficients for radiation, the energy of the radiation reaching different parts of the film varies as well; thus, defects can be identified by examining the differences in darkness across various areas of the film after darkroom processing. 1.3 The advantages and limitations of radiography are summarized as follows: 1.3.1 It enables the acquisition of intuitive images of defects, with accurate qualitative assessment, as well as relatively precise quantitative measurement of length and width ； 1.3.2 Detection results are directly recorded and can be preserved for a long time ； 1.3.3 It has a high detection rate for volume defects (porosity, slag inclusions, tungsten inclusions, burn-through, undercutting, weld beads, pits, etc.), while for area defects (incomplete penetration, lack of fusion, cracks, etc.), improper photography angles can lead to missed detections ； 1.3.4 It is suitable for inspecting workpieces with thin thicknesses but not those with thick thicknesses, as inspecting thick workpieces requires high-energy radiation equipment, and its inspection sensitivity decreases as the thickness increases ； 1.3.5 Suitable for inspecting butt welds, but not suitable for inspecting fillet welds as well as plates, bars, forgings, etc ； 1.3.6 It is relatively difficult to determine the position and size (height) of defects in the thickness direction of the workpiece ； 1.3.7 High detection costs and slow speed ； 1.3.8 Possessing radiobiological effects, non-destructive testing ultrasonic flaw detectors can kill biological cells, damage biological tissues, and jeopardize the normal functioning of biological organs. 1.4 The characteristics of RT are that it provides more accurate qualitative results and produces intuitive images suitable for long-term storage. However, its overall cost is relatively high; radiation is harmful to the human body, and the inspection speed is relatively slow. 2. Ultrasonic testing (UT): When ultrasonic waves propagate through the material being inspected, the acoustic properties of the material and any changes in its internal structure have an impact on the propagation of these waves. The technique that involves detecting the extent to which the ultrasonic waves are affected in order to understand the material’s properties and structural changes is known as ultrasonic testing. 2.1 Principle of ultrasonic operation: It is mainly based on the propagation characteristics of ultrasonic waves in the test specimen. 2.1.1 The sound source generates ultrasonic waves, which are then introduced into the test specimen in a certain manner ； 2.1.2 Ultrasonic waves propagate through the test specimen and interact with the specimen material as well as any defects within it, thereby causing changes in their direction of propagation or characteristics ； 2.1.3 The modified ultrasonic waves are received by the detection equipment, where they can be processed and analyzed ； 2.1.4 Evaluate whether there are defects in the specimen itself and within it, as well as the characteristics of those defects, based on the characteristics of the received ultrasonic waves. 2.2 Advantages of ultrasonic testing: 2.2.1 Suitable for non-destructive testing of various materials such as metals, non-metals, and composite materials ； 2.2.2 It has strong penetration ability, enabling the detection of internal defects in specimens within a relatively wide thickness range. For metal materials, it can detect thin-walled tubes and plates with a thickness of 1–2 mm, as well as steel forgings several meters in length ； 2.2.3 Relatively accurate defect localization ； 2.2.4 High detection rate for area-type defects ； 2.2.5 High sensitivity, capable of detecting defects with very small sizes within the test specimen ； 2.2.6 It features low detection costs and fast speed, lightweight equipment, no harm to humans or the environment, and is convenient for use on-site. 2.3 Limitations of ultrasonic testing: 2.3.1 Further research is still needed to achieve accurate qualitative and quantitative assessment of defects in the test specimen ； 2.3.2 It is difficult to perform ultrasonic testing on specimens with complex shapes or irregular profiles ； 2.3.3 The location, orientation, and shape of defects have a certain impact on the detection results ； 2.3.4 Materials, grain size, etc. have a significant impact on detection ； 2.3.5 When using the common manual Type A pulse reflection method for detection, the results are not intuitive, and there are no direct written records of the detection outcomes. 2.4 Applicability of ultrasonic testing: 2.4.1 In terms of the materials of the objects to be inspected, it can be used for metals, non-metals, and composite materials ； 2.4.2 In terms of the manufacturing process of the components to be inspected, it can be applied to forgings, castings, welded parts, bonded parts, etc ； 2.4.3 In terms of the shape of the object to be inspected, it can be used for sheets, bars, tubes, etc ； 2.4.4 In terms of the size of the object to be inspected, the thickness can range from as low as 1 mm to several meters ； 2.4.5 In terms of the location of the defect, it can be either a surface defect or an internal defect. 3. Magnetic Particle Inspection (MT) 　3.1 Principle of Magnetic Particle Inspection: When ferromagnetic materials and workpieces are magnetized, the presence of discontinuities causes local distortions in the magnetic field lines on the surface and near the surface of the workpiece, resulting in a leakage magnetic field. This leakage field attracts magnetic particles applied to the surface of the workpiece, forming magnetic traces that are visible under appropriate lighting; thus, the location, shape, and size of the discontinuities can be identified through magnetic particle inspection. 3.2 Applicability and limitations of magnetic particle testing: 3.2.1 Magnetic particle inspection is suitable for detecting discontinuities on the surface and near the surface of ferromagnetic materials that are very small in size and have extremely narrow gaps – such as cracks that are 0.1 mm long and only a few microns wide – discontinuities that are difficult to detect visually. 3.2.2 Magnetic particle testing can be used to inspect raw materials, semi-finished products, finished components, as well as components in service. It can also be applied to sheets, profiles, tubes, bars, welded parts, cast steel parts, and forged steel parts. 3.2.3 Defects such as cracks, inclusions, grain boundaries, white spots, folds, cold shuts, and porosity can be detected. 3.2.4 Magnetic particle testing cannot be used to inspect austenitic stainless steel materials or welds made with austenitic stainless steel electrodes, nor can it be used to inspect non-magnetic materials such as copper, aluminum, magnesium, and titanium. Shallow surface scratches, deeply buried holes, as well as delaminations and folds with an angle of less than 20° to the workpiece surface are difficult to detect. 4. Penetrant Testing (PT) 4.1 Basic principle of liquid penetrant testing: After a penetrant containing a fluorescent dye or a colored dye is applied to the surface of a part, under the effect of capillary action, it penetrates into any surface defects after some time has passed ； After removing the excess penetrant from the surface of the part, a developer is applied to that surface. Again, due to capillary action, the developer attracts the penetrant remaining in the defects; this penetrant then returns into the developer. Under certain light sources (ultraviolet light or white light), the traces of penetrant in the defects become visible, appearing as yellow-green fluorescence or bright red, thereby allowing the shape and distribution of the defects to be detected. 4.2 Advantages of penetrant testing: 4.2.1 Can detect various materials, metallic and non-metallic materials ； Magnetic, non-magnetic materials ； Processing methods such as welding, forging, and rolling ； 4.2.2 It has high sensitivity (able to detect defects 0.1μm wide). 4.2.3 It provides intuitive display, is easy to operate, and has low detection costs. 4.2.4 Disadvantages and limitations of penetrant testing: 4.2.5 It can only detect defects with surface openings ； 4.2.6 Not suitable for inspecting workpieces made of porous, loose materials and workpieces with rough surfaces ； 4.2.7 Penetrant testing can only detect the surface distribution of defects, making it difficult to determine their actual depth; as a result, it is hard to provide a quantitative assessment of these defects. The detection results are also greatly influenced by the operator. 5. Eddy current testing (ET) is a non-destructive testing method that utilizes the eddy currents induced in a workpiece by ferromagnetic coils to analyze the internal quality of the workpiece. 5.1 Basic principle of eddy current testing: A coil carrying alternating current is placed on the metal plate to be inspected or wrapped around the metal tube to be inspected. At this point, an alternating magnetic field is generated within the coil and in its vicinity, which induces an eddy-current of alternating nature in the test piece; this current is known as an eddy current. The distribution and magnitude of eddy currents depend not only on the shape and size of the coil, as well as the amplitude and frequency of the alternating current, but also on the electrical conductivity and magnetic permeability of the test piece, its shape and size, the distance from the coil, and the presence of cracks or defects on its surface. Therefore, while keeping other factors relatively constant, measuring the changes in the magnetic field caused by eddy currents using a detection coil makes it possible to determine the magnitude and phase changes of the eddy currents in the test specimen. Consequently, information regarding changes in electrical conductivity, defects, material condition, and other physical quantities (such as shape and size), or the presence of defects, can be obtained. However, since the eddy current is an alternating current with a skin effect, the information detected can only reflect the conditions on the surface or near the surface of the test specimen. 5.2 Applications: Depending on the shape of the test specimen and the purpose of the inspection, different types of coils can be used; typically, there are three types: through-type, probe-type, and insertion-type coils. 5.2.1 Through-type coils are used to inspect tubes, bars, and wires; their inner diameter is slightly larger than that of the objects to be inspected. The object in question is moved through the coil at a certain speed, which allows defects such as cracks, inclusions, and pits to be detected. 5.2.2 Probe-type coils are suitable for local inspection of test specimens. When in use, the coil is placed on metal plates, tubes, or other components to detect fatigue cracks on the inner tubes of aircraft landing struts and on turbine engine blades. 5.2.3 Insertion coils, also known as internal probes, are placed into the holes of pipes or parts for inspecting their inner walls. They can be used to assess the degree of corrosion on the inner surfaces of various types of pipes. To improve detection sensitivity, probe-type and insertable coils are mostly equipped with a magnetic core. 5.3 The eddy current method is primarily used for the rapid inspection of metal tubes, bars, and wires on production lines, as well as for flaw detection of large quantities of parts such as bearing balls and valve components (in which case, in addition to eddy current instruments, mechanical devices for automatic loading, unloading, and transportation are also required). It can also be used for material sorting and hardness measurement, as well as for measuring the thickness of coatings and paint films. 5.4 Advantages and disadvantages: During eddy current testing, the coil does not need to be in direct contact with the object being tested, which allows for high-speed inspection and facilitates automation. However, this method is not suitable for parts with complex shapes; it can only detect surface and near-surface defects in conductive materials. Moreover, the test results are prone to being affected by the material itself and other factors. 6. Visual Inspection (VT) Visual inspection is a type of non-destructive testing, also known as visual examination. It is a simple and widely used inspection method aimed primarily at detecting defects on the surface of materials. Visual inspection includes two methods: direct and indirect. Direct inspection is carried out by using the naked eye without any auxiliary tools, while indirect inspection makes use of tools such as magnifying glasses, lenses, endoscopes (mini cameras), and closed-circuit television to assist in the inspection. Visual inspection is a very important non-destructive testing method, yet it is often overlooked. Visual inspection is the foundation of the non-destructive testing industry. For example, visual inspection in current engineering applications can often reveal some relatively obvious problems, such as leaks, high-frequency deviations, corrosion, and misalignment. At the same time, visual inspection can also effectively determine the necessity of conducting more advanced non-destructive testing on specific areas. Visual inspection is a primary method in the first stage of non-destructive testing; it is rarely practiced in China, but is highly regarded internationally. In accordance with international practices, visual inspection is carried out first to ensure it does not affect subsequent tests, after which the four standard tests are performed. For example, for the PCN certification offered by BINDT, there are specific VT Level 1, 2, and 3 assessments, as well as specific requirements regarding certification. Thanks to international-level training, their VT testing techniques are quite professional and are highly valued by international organizations. VT is often used for visual inspection of welds. There are standard criteria for evaluating weld quality, and preliminary inspections can be carried out through visual inspection and direct measurement of dimensions. Any defective appearance issues such as undercutting must be addressed by grinding or trimming before further in-depth instrumental testing is conducted. For example, VT is frequently performed on the surfaces of welded parts and castings; however, it is rarely used for forgings. Moreover, their inspection standards are essentially the same. 7. Time-of-Flight Diffraction ultrasonic testing (TOFD): During testing, one or multiple pairs of wide-beam transducers are used; each pair is symmetrically positioned relative to the weld. The sound beams cover the inspection area, and when they encounter defects, reflected waves and diffracted waves are generated. The probe receives both reflected and diffracted waves; by measuring the propagation time of the diffracted waves and using trigonometric equations, it determines the size and location of the defect. 7.1 Characteristics of TOFD testing: 7.1.1 The inspection is carried out using a pair of wide-beam, longitudinal wave probes; the frequency of these probes is higher than that of the probes used in pulse-echo method (PE), and the probes are positioned symmetrically with respect to the weld. 7.1.2 When the sound beam propagates through a weld and encounters a defect, the defect generates reflected waves, while diffraction waves are produced at both ends of the defect; these diffraction waves are 20–30 dB lower in amplitude than the reflected waves. The receiving probe has extremely high sensitivity and detects the diffracted waves. 7.1.3 Based on the precise measurement of the transmission time of diffracted waves and simple trigonometric equations, a computer is used to measure the size and location of defects. 7.1.4 TOFD inspection does not rely on measuring the echo height of defects; rather, it determines the size and location of defects by accurately measuring the travel time of the diffracted waves. The measurement accuracy is 1 mm for natural cracks, and 0.1 mm for artificially created reflectors (under laboratory conditions). 7.2 Limitations of TOFD 7.2.1 Blind zones exist near the outer and inner surfaces. 7.2.2 It is difficult to interpret; currently, there is no experience in China regarding the classification of defects, and other testing methods are often required as a supplement. 7.2.3 Some benign defects, such as pores, cold laps, and internal lack of fusion, are exaggerated. 7.2.4 In actual testing, the error in the height direction of defects is small, while the error in the length direction is larger (measured using the 6dB method). 8. Phased Array Ultrasonic Testing (PAUT): Phased array technology is a type of ultrasonic technology that utilizes an array of miniature transducers to generate ultrasonic beams. By establishing focusing rules, electronic devices control the transmission and reception timing of each array element, thereby producing multiple ultrasonic beams. By controlling the excitation and reception timing of the array, it is possible to regulate parameters such as beam angle, focal depth, and focal spot size, enabling rapid scanning and imaging of workpieces. 8.1 Advantages of phased array technology 8.1.1 Multiple two-dimensional imaging combinations, and even three-dimensional simulations, make the ultrasound examination results more intuitive and facilitate evaluation. 8.1.2 During scanning, the probe can utilize longitudinal waves, shear waves, and other waveforms for defect detection without needing to move back and forth; this results in fast and efficient detection, and it is particularly advantageous for detecting defects that are directional in nature. 8.1.3 Multi-angle movement detection of the array facilitates optimal angular detection and quantification of defects by the sound beam. 8.1.4 The detection has high resolution and high accuracy. 8.1.5 Volume defect cross-sectional views at each location are displayed through fan-swept sectional images, similar to metallographic sections used for quantitative characterization of defects; the result analysis is intuitive and precise. 8.1.6 Phased arrays have the additional advantage over conventional ultrasound of allowing the operator to closely examine the entire A-scan data, thereby enabling more reliable defect characterization. 8.1.7 The probe is small in size, allowing for flexible detection. 8.1.8 Advantages such as a large scanning range and variable angles make it highly suitable for inspecting complex components and hard-to-reach areas. 8.1.9 Virtually no consumables are required, resulting in low detection costs. 8.1.10 The phased array uses S-scanning, which allows for the use of ultrasonic waves at multiple angles simultaneously; it is as if multiple probes operating at different angles are being used at the same time. Therefore, the phased array does not require zigzag scanning – it is sufficient to move the probe along the weld, resulting in higher inspection efficiency. Suitable for automated production and mass production. 8.1.11 Phased arrays can have a focusing function, whereas conventional ultrasound generally does not (except for focused probes); therefore, the sensitivity and resolution of phased array detection are higher than those of conventional ultrasound detection. 8.1.12 Phased array detection allows for B-scan, D-scan, S-scanning, and C-scanning simultaneously; by means of modeling, a three-dimensional image can be created, making the detection of defects very intuitive – even those who are not familiar with NDT can understand it. In contrast, conventional ultrasonic testing can only identify defects based on waveforms. 8.1.13 Ultrasonic phased arrays can inspect complex workpieces; for example, they can examine the root portions of turbine blades. Conventional ultrasonic testing has significant blind areas due to the single beam angle of the probe, which leads to missed inspections. And phased arrays can enable rapid and intuitive detection. 8.2 Disadvantages and limitations of phased array ultrasonic testing 8.2.1 Suitable for detecting internal defects in components with a thickness of over 5 mm and regular shapes. 8.2.2 The defect display is not intuitive, making it difficult to determine the nature of the defects. 8.2.3 It requires relatively high skills from the operators.