Piping Valves [Weekly Topic] 11-11: What are the different types of non-destructive testing for pipelines? (Participation comes with rewards)
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This post was last edited by Benbenxuan on 2011-2-11 09:24. Please discuss: What are the various methods of non-destructive testing for pipelines, what are the characteristics of each method, and what are their advantages and disadvantages? Participation comes with rewards; those who engage in in-depth discussions will be awarded extra bonuses! (Please provide real-life examples on site to describe their advantages and disadvantages. Moderators, please give priority scoring to those who provide such examples. – Benbenxuan)
Radiographic testing, ultrasonic testing, magnetic particle testing, liquid penetrant testing, and eddy current testing
Ultrasonic flaw detection – Main advantages
① Strong penetration ability; detection depth can reach several meters； ②It has high sensitivity, enabling the detection of reflectors with a reflective capacity comparable to that of air gaps with a diameter of a few tenths of a millimeter ； ③It is relatively accurate in determining the orientation, size, shape, and properties of the internal reflector ； ④It is only necessary to approach the object under inspection from one side ； ⑤The results of defect inspection can be provided immediately; ⑥ Safe to operate, and the equipment is lightweight. Main disadvantages: ① It must be operated carefully by experienced personnel ； Ultrasonic flaw detection vehicle ② It is difficult to inspect rough, irregularly shaped, small, thin, or heterogeneous materials ； ③It remains difficult to provide highly accurate qualitative and quantitative characterization of the identified defects. Radiographic testing: Radiography can visually display the size and shape of internal defects in workpieces, thereby facilitating the determination of the nature of these defects. The radiographic films serve as original records of the inspection, which can be studied by multiple parties and preserved for long periods. However, this method incurs high costs for equipment such as X-ray film, and the inspection speed is slow; it is suitable only for detecting volumetric defects such as pores, inclusions, shrinkages, and porosity, but it is not effective at identifying cracks with very small gaps, defects such as lack of fusion, or internal stratification defects in forgings and shaped materials like pipes and rods. Furthermore, radiation is harmful to the human body, so appropriate protective measures need to be taken.
① Strong penetration ability; detection depth can reach several meters； ②It has high sensitivity, enabling the detection of reflectors with a reflective capacity comparable to that of air gaps with a diameter of a few tenths of a millimeter ； ③It is relatively accurate in determining the orientation, size, shape, and properties of the internal reflector ； ④It is only necessary to approach the object under inspection from one side ； ⑤The results of defect inspection can be provided immediately; ⑥ Safe to operate, and the equipment is lightweight. Main disadvantages: ① It must be operated carefully by experienced personnel ； Ultrasonic flaw detection vehicle ② It is difficult to inspect rough, irregularly shaped, small, thin, or heterogeneous materials ； ③It remains difficult to provide highly accurate qualitative and quantitative characterization of the identified defects. Radiographic testing: Radiography can visually display the size and shape of internal defects in workpieces, thereby facilitating the determination of the nature of these defects. The radiographic films serve as original records of the inspection, which can be studied by multiple parties and preserved for long periods. However, this method incurs high costs for equipment such as X-ray film, and the inspection speed is slow; it is suitable only for detecting volumetric defects such as pores, inclusions, shrinkages, and porosity, but it is not effective at identifying cracks with very small gaps, defects such as lack of fusion, or internal stratification defects in forgings and shaped materials like pipes and rods. Furthermore, radiation is harmful to the human body, so appropriate protective measures need to be taken.
Common non-destructive testing methods include: X-ray testing, ultrasonic testing, magnetic particle testing, penetrant testing, eddy current testing, gamma ray testing, fluorescent testing, and dye penetrant testing.
Non-destructive testing is a testing method used to examine the surface and internal quality of components being inspected, without damaging the condition of the workpiece or raw material. Common non-destructive testing methods include: X-ray testing, ultrasonic testing, magnetic particle testing, penetrant testing, eddy current testing, gamma ray testing, fluorescent testing, and dye penetrant testing. Magnetic particle inspection equipment is simple to use, easy to operate, allows for rapid testing, and possesses high sensitivity for detecting defects on the surface or near the surface of ferromagnetic materials such as nickel, cobalt and their alloys, carbon steel, and certain alloy steels ； It is suitable for inspecting thin-walled parts or surface cracks in welds, and it can also reveal underpenetration defects of certain depth and size ； However, it is difficult to detect pores, inclusions, and defects hidden deep within the weld. When ultrasonic waves propagate through a medium, they exhibit the property of reflection at different interfaces. When such waves encounter defects whose size is equal to or greater than the wavelength of the ultrasonic waves, the waves are reflected back from those defects, and the flaw detector can display these reflected waves ； When the size of the defect is even smaller than the wavelength, the sound waves will bypass the defect and not reflect ； The wave has good directivity; the higher the frequency, the better the directivity. It radiates into the medium in a very narrow beam, making it easy to determine the location of defects ； Ultrasonic waves possess high energy transmission capabilities; for example, the energy transmitted by an ultrasonic wave with a frequency of 1 MHz (100 hertz) is one million times that of an acoustic wave with the same amplitude but a frequency of 1000 Hz. ) I3 g$ B$ }1 E d z9 A- r Ultrasonic flaw detection offers advantages over X-ray flaw detection, including higher sensitivity, shorter inspection times, lower costs, greater flexibility and convenience, higher efficiency, and no harm to the human body ； The disadvantages are that it requires a smooth working surface, experienced inspectors to identify different types of defects, and there is no visual indication of the defects ； Ultrasonic flaw detection is suitable for inspecting parts with greater thickness.
Non-destructive testing is a testing method used to examine the surface and internal quality of components being inspected, without damaging the condition of the workpiece or raw material. Common non-destructive testing methods include penetrant testing (PT), which includes fluorescent and colored penetrant testing; magnetic particle testing (MT); radiographic testing (RT); and ultrasonic testing (UT). Penetrant testing can only detect surface-opening defects ； Magnetic particle testing can detect surface and near-surface defects in ferromagnetic materials, while radiographic testing and ultrasonic testing are mainly used to detect internal defects. Generally, radiographic testing is more sensitive to volumetric defects such as pores, inclusions, and lack of penetration, whereas ultrasonic testing is more sensitive to planar defects such as cracks. However, both methods are limited by the equipment available. Radiographic testing becomes difficult when the wall thickness exceeds 100 mm, while ultrasonic testing can be carried out more easily in such cases. However, the ultrasonic flaw detectors commonly used in China are all pulse-type, which presents certain difficulties in both the qualitative and quantitative assessment of defects. During the manufacturing of pressure vessels, ultrasonic testing is primarily used to detect internal defects in the metal plates; radiographic testing is employed to detect internal defects in butt welds, while penetrant testing is used to inspect surface defects in fillet welds. Comparison table of defect detection capabilities of different inspection methods! Defect type, Inspection method: Planar, Point-like, Strip-like, Linear (surface), Circular (surface); Cracks, Lack of fusion, Under-welding, Porosity, Inclusions, Surface cracks. RT: O or △ ※ ※ UT: ※ ○ ○ MT: ※ O or △ PT: O or △. Note: ※ = Good ； ○ Better ； △ Difficulties
As far as I know, there are two types: 1. Rays: they emit radiation that affects the human body. It has film, and it is excellent at detecting defects such as pores and inclusions that exceed the specified limits. The British tend to favor this method ； 2. Ultrasound: It emits no radiation to the human body. Without negative films, it is highly effective at detecting defects such as cracks that exceed specified limits. Europeans are more optimistic about this method. Their characteristics are as follows: X- and γ-rays are sensitive to volumetric defects; however, they have difficulty detecting linear defects, especially tiny lack-of-penetration (incomplete fusion) or microcracks in thick plates. On the other hand, ultrasonic waves are sensitive to linear defects, but it is difficult to accurately quantify point-like defects ； Radiographic inspection does not have high requirements regarding the surface of the workpiece; it evaluates the quality of welding based on the film used. Its advantages are that it provides an intuitive view and makes it easy to conduct qualitative assessments and keep records, but it is difficult to determine dimensions in the depth direction. Ultrasonic testing, on the other hand, has stricter requirements for the surface to be inspected; it assesses defects by examining the waveforms displayed on a fluorescent screen. Its advantage is that it allows for easy determination of depth, but it is not intuitive and makes it difficult to keep records. Qualitative assessments require comprehensive evaluation, and the inspectors need to be highly skilled and responsible ； Radiation is harmful to the human body, so protection is necessary; it also requires a large amount of film and chemicals, resulting in high inspection costs. In contrast, ultrasound is harmless to the human body and has lower inspection costs ； Radiography can detect coarse-grained materials (such as austenitic welds), while ultrasonic testing is difficult for such materials.
This post was last edited by diwu0205 on February 11, 2011, at 11:00. The five common methods refer to radiographic testing, ultrasonic testing, magnetic particle testing, eddy current testing, and penetrant testing. 1. Radiographic testing method
Radiographic testing is a method that utilizes the penetrability and rectilinearity of radiation for flaw detection. Although these rays cannot be directly perceived by the naked eye like visible light, they can expose photographic film and can also be detected using special receivers. Radiations commonly used for flaw detection include X-rays and gamma rays emitted by isotopes; these are referred to as X-ray inspection and gamma-ray inspection, respectively. When these rays pass through (irradiate) a material, the greater the density of that material, the more the intensity of the rays is reduced; in other words, the weaker the intensity of the rays that can penetrate the material. At this time, if a photographic film is used for reception, the sensitivity of the film will be low ； If a device is used to receive it, the signal obtained will be weak. Therefore, when rays are used to irradiate the components to be inspected, if there are defects such as pores or inclusions inside them, the path taken by the rays through these defective areas has a much lower material density compared to the paths without defects; as a result, the intensity of the rays is reduced less, meaning that more intensity passes through. When a film is used to capture this radiation, the amount of light exposure on the film increases, allowing the planar projection of the defects perpendicular to the direction of the rays to be detected on the film ； Using other receivers, it is also possible to use the instrument to show the planar projection of the defect perpendicular to the direction of the ray, as well as the amount of radiation that passes through it. It can be seen that, under normal circumstances, radiographic inspection is not effective at detecting cracks; in other words, radiographic inspection is insensitive to cracks. Therefore, radiographic testing is most sensitive to volumetric defects such as pores, slag inclusions, and lack of penetration. That is, radiographic testing is suitable for detecting volumetric defects, but not suitable for detecting planar defects. 2. Ultrasonic testing method: The frequency range of sound waves that human ears can directly perceive is typically from 20 Hz to 20 kHz; this is known as the audio frequency range. Waves with frequencies below 20 Hz are called infrasound, while those with frequencies above 20 kHz are called ultrasound. In industry, ultrasonic waves in the range of several megahertz are commonly used for flaw detection. Ultrasonic waves have a high frequency, which gives them strong linear propagation properties; they can travel easily through solids, and they are prone to reflection when encountering an interface between two different media. This property allows them to be used for flaw detection. Typically, an ultrasonic probe is brought into good contact with the surface of the workpiece to be inspected; this allows the probe to effectively transmit ultrasonic waves toward the workpiece, as well as receive the ultrasonic waves reflected from any defects present. These reflected waves are then converted into electrical signals, which are sent to the instrument for further processing. The location of the defect can be determined based on the speed at which ultrasound travels through the medium (commonly referred to as the sound speed) and the time it takes to travel. The larger the defect, the larger the reflecting surface, and the greater the reflected energy. Therefore, the size of each defect (equivalent size) can be determined based on the amount of reflected energy. Commonly used ultrasonic testing waveforms include longitudinal waves, transverse waves, and surface waves. The first two are suitable for detecting internal defects, while the latter is appropriate for detecting surface defects; however, it has high requirements regarding surface conditions. 3. Magnetic particle testing method
Magnetic particle testing is a magnetic flaw detection method based on the principle of magnetic flux leakage. When magnetic flux lines pass through ferromagnetic materials and their products, a leakage magnetic field is generated at their (magnetic) discontinuities, forming magnetic poles. At this point, by sprinkling dry magnetic powder or pouring a magnetic suspension, the magnetic poles will attract the powder, resulting in visible magnetic marks that can be observed with the naked eye. Therefore, this magnetic mark can be used to reveal the defects in ferromagnetic materials and their products. Magnetic particle testing can detect tiny defects that are exposed on the surface and cannot be seen directly with the naked eye or even with the help of a magnifying glass. It can also detect near-surface defects that are not exposed but are buried several millimeters beneath the surface. Although this method can also detect volume defects such as pores, inclusions, and lack of penetration, it is more sensitive to surface defects and is thus more suitable for detecting cracks caused by quenching, rolling, forging, casting, welding, electroplating, grinding, fatigue, etc. There are various methods for detecting defects in magnetic particle inspection, some of which use magnetic particles while others do not. The method that uses magnetic powder for detection is called magnetic particle testing; it is one of the most commonly used methods due to its intuitive display, simple operation, and ease of use. It does not require the use of magnetic powder; it is commonly referred to as magnetic flux leakage testing. Defects are detected using induction coils, magnetosensitive tubes, Hall elements, etc. This method is cleaner than magnetic powder testing, but it is less intuitive than the former. Since magnetic particle testing currently uses magnetic particles to reveal defects, it is sometimes referred to simply as magnetic testing, with its equipment being called magnetic testing equipment. 4. Eddy current testing method Eddy current testing involves using an alternating magnetic field generated by alternating current to act on the conductive material to be inspected, thereby inducing eddy currents. If there are defects in the material, they will interfere with the eddy currents generated, thereby creating interference signals. By detecting the interference signals using an eddy current tester, one can determine the condition of the defects. There are many factors that affect eddy currents; in other words, eddy currents carry a wealth of signals, and these signals are related to various properties of the material. Separating the useful signals from among all these signals is a challenge for researchers working on eddy current phenomena. Some progress has been made over the years, and certain problems can be solved under specific conditions, but this is still far from meeting the requirements in practical applications, and further development is needed. A notable feature of eddy current testing is that it works on conductive materials, not necessarily ferromagnetic ones, but its effectiveness is lower for ferromagnetic materials. Secondly, the surface finish, flatness, and edges of the workpiece to be inspected have a significant impact on eddy current testing; therefore, eddy current testing is often used for inspecting non-ferromagnetic workpieces such as copper pipes, which have regular shapes and smooth surfaces. 5. Penetrant testing method: Penetrant testing is a method that utilizes capillary action for inspection. For parts with smooth and clean surfaces, a colored (usually red) or fluorescent liquid with high permeability is applied to the surface of the part to be inspected. If there are micro-cracks on the surface that cannot be seen with the naked eye, due to the high penetrability of this liquid, it will penetrate along these cracks to their roots. Then the penetrant on the surface is washed off, and a contrasting developer (usually white) is applied. After being left for a while, due to the narrow nature of the cracks, capillary action plays a significant role; the penetrant that has seeped into the cracks rises to the surface and spreads, resulting in thicker red lines on the white background. This reveals the shape of the cracks at the surface, which is why this method is often referred to as colored flaw detection. If a fluorescent liquid is used as the penetrant, the liquid that rises to the surface due to capillary action will emit fluorescence under ultraviolet light. This makes it easier to visualize the shape of any surface cracks. Therefore, this type of penetrant testing is often referred to as fluorescent testing. This flaw detection method can also be used for detecting flaws on metal and non-metal surfaces. The flaw detection liquid used has a strong odor and is often somewhat toxic. In addition to these five conventional methods, new non-destructive testing methods such as infrared and acoustic emission have emerged in recent years
Radiographic testing is a method that utilizes the penetrability and rectilinearity of radiation for flaw detection. Although these rays cannot be directly perceived by the naked eye like visible light, they can expose photographic film and can also be detected using special receivers. Radiations commonly used for flaw detection include X-rays and gamma rays emitted by isotopes; these are referred to as X-ray inspection and gamma-ray inspection, respectively. When these rays pass through (irradiate) a material, the greater the density of that material, the more the intensity of the rays is reduced; in other words, the weaker the intensity of the rays that can penetrate the material. At this time, if a photographic film is used for reception, the sensitivity of the film will be low ； If a device is used to receive it, the signal obtained will be weak. Therefore, when rays are used to irradiate the components to be inspected, if there are defects such as pores or inclusions inside them, the path taken by the rays through these defective areas has a much lower material density compared to the paths without defects; as a result, the intensity of the rays is reduced less, meaning that more intensity passes through. When a film is used to capture this radiation, the amount of light exposure on the film increases, allowing the planar projection of the defects perpendicular to the direction of the rays to be detected on the film ； Using other receivers, it is also possible to use the instrument to show the planar projection of the defect perpendicular to the direction of the ray, as well as the amount of radiation that passes through it. It can be seen that, under normal circumstances, radiographic inspection is not effective at detecting cracks; in other words, radiographic inspection is insensitive to cracks. Therefore, radiographic testing is most sensitive to volumetric defects such as pores, slag inclusions, and lack of penetration. That is, radiographic testing is suitable for detecting volumetric defects, but not suitable for detecting planar defects. 2. Ultrasonic testing method: The frequency range of sound waves that human ears can directly perceive is typically from 20 Hz to 20 kHz; this is known as the audio frequency range. Waves with frequencies below 20 Hz are called infrasound, while those with frequencies above 20 kHz are called ultrasound. In industry, ultrasonic waves in the range of several megahertz are commonly used for flaw detection. Ultrasonic waves have a high frequency, which gives them strong linear propagation properties; they can travel easily through solids, and they are prone to reflection when encountering an interface between two different media. This property allows them to be used for flaw detection. Typically, an ultrasonic probe is brought into good contact with the surface of the workpiece to be inspected; this allows the probe to effectively transmit ultrasonic waves toward the workpiece, as well as receive the ultrasonic waves reflected from any defects present. These reflected waves are then converted into electrical signals, which are sent to the instrument for further processing. The location of the defect can be determined based on the speed at which ultrasound travels through the medium (commonly referred to as the sound speed) and the time it takes to travel. The larger the defect, the larger the reflecting surface, and the greater the reflected energy. Therefore, the size of each defect (equivalent size) can be determined based on the amount of reflected energy. Commonly used ultrasonic testing waveforms include longitudinal waves, transverse waves, and surface waves. The first two are suitable for detecting internal defects, while the latter is appropriate for detecting surface defects; however, it has high requirements regarding surface conditions. 3. Magnetic particle testing method
Magnetic particle testing is a magnetic flaw detection method based on the principle of magnetic flux leakage. When magnetic flux lines pass through ferromagnetic materials and their products, a leakage magnetic field is generated at their (magnetic) discontinuities, forming magnetic poles. At this point, by sprinkling dry magnetic powder or pouring a magnetic suspension, the magnetic poles will attract the powder, resulting in visible magnetic marks that can be observed with the naked eye. Therefore, this magnetic mark can be used to reveal the defects in ferromagnetic materials and their products. Magnetic particle testing can detect tiny defects that are exposed on the surface and cannot be seen directly with the naked eye or even with the help of a magnifying glass. It can also detect near-surface defects that are not exposed but are buried several millimeters beneath the surface. Although this method can also detect volume defects such as pores, inclusions, and lack of penetration, it is more sensitive to surface defects and is thus more suitable for detecting cracks caused by quenching, rolling, forging, casting, welding, electroplating, grinding, fatigue, etc. There are various methods for detecting defects in magnetic particle inspection, some of which use magnetic particles while others do not. The method that uses magnetic powder for detection is called magnetic particle testing; it is one of the most commonly used methods due to its intuitive display, simple operation, and ease of use. It does not require the use of magnetic powder; it is commonly referred to as magnetic flux leakage testing. Defects are detected using induction coils, magnetosensitive tubes, Hall elements, etc. This method is cleaner than magnetic powder testing, but it is less intuitive than the former. Since magnetic particle testing currently uses magnetic particles to reveal defects, it is sometimes referred to simply as magnetic testing, with its equipment being called magnetic testing equipment. 4. Eddy current testing method Eddy current testing involves using an alternating magnetic field generated by alternating current to act on the conductive material to be inspected, thereby inducing eddy currents. If there are defects in the material, they will interfere with the eddy currents generated, thereby creating interference signals. By detecting the interference signals using an eddy current tester, one can determine the condition of the defects. There are many factors that affect eddy currents; in other words, eddy currents carry a wealth of signals, and these signals are related to various properties of the material. Separating the useful signals from among all these signals is a challenge for researchers working on eddy current phenomena. Some progress has been made over the years, and certain problems can be solved under specific conditions, but this is still far from meeting the requirements in practical applications, and further development is needed. A notable feature of eddy current testing is that it works on conductive materials, not necessarily ferromagnetic ones, but its effectiveness is lower for ferromagnetic materials. Secondly, the surface finish, flatness, and edges of the workpiece to be inspected have a significant impact on eddy current testing; therefore, eddy current testing is often used for inspecting non-ferromagnetic workpieces such as copper pipes, which have regular shapes and smooth surfaces. 5. Penetrant testing method: Penetrant testing is a method that utilizes capillary action for inspection. For parts with smooth and clean surfaces, a colored (usually red) or fluorescent liquid with high permeability is applied to the surface of the part to be inspected. If there are micro-cracks on the surface that cannot be seen with the naked eye, due to the high penetrability of this liquid, it will penetrate along these cracks to their roots. Then the penetrant on the surface is washed off, and a contrasting developer (usually white) is applied. After being left for a while, due to the narrow nature of the cracks, capillary action plays a significant role; the penetrant that has seeped into the cracks rises to the surface and spreads, resulting in thicker red lines on the white background. This reveals the shape of the cracks at the surface, which is why this method is often referred to as colored flaw detection. If a fluorescent liquid is used as the penetrant, the liquid that rises to the surface due to capillary action will emit fluorescence under ultraviolet light. This makes it easier to visualize the shape of any surface cracks. Therefore, this type of penetrant testing is often referred to as fluorescent testing. This flaw detection method can also be used for detecting flaws on metal and non-metal surfaces. The flaw detection liquid used has a strong odor and is often somewhat toxic. In addition to these five conventional methods, new non-destructive testing methods such as infrared and acoustic emission have emerged in recent years
I would like to ask whether penetration testing such as ammonia penetration can be used as a substitute for non-destructive testing
This post was last edited by sooker on 2011-2-11 at 10:54.
A. Non-destructive testing methods include: X-ray testing, ultrasonic testing, magnetic particle testing, penetrant testing, eddy current testing, gamma ray testing, fluorescent testing, dye penetrant testing, etc.
B. Advantages, disadvantages, and characteristics
1. Ray-based testing methods
Ray-based testing is a method that utilizes the penetrability and rectilinear propagation of rays for inspection. Although these rays from ultrasonic flaw detectors cannot be seen directly with the naked eye like visible light, they can expose photographic film and can also be detected using special receivers. Radiations commonly used for flaw detection include X-rays and gamma rays emitted by isotopes; these are referred to as X-ray inspection and gamma-ray inspection, respectively. When these rays pass through (irradiate) a material, the greater the density of that material, the more the intensity of the rays is reduced; in other words, the weaker the intensity of the rays that can penetrate the material. At this time, if a photographic film is used for reception, the sensitivity of the film will be low ； If a device is used to receive it, the signal obtained will be weak. Therefore, when an ultrasonic flaw detector uses rays to irradiate the components to be inspected, if there are defects such as pores or inclusions inside them, the density of the material through which the rays pass along the paths with defects is much lower than that along the paths without defects; as a result, the intensity of the rays is reduced less, meaning the intensity that passes through is greater. If a film is used to capture this radiation, the amount of light exposure on the film will be higher, allowing the planar projection of the defects perpendicular to the direction of the rays to be revealed on the film ； Using other receivers, it is also possible to use the instrument to show the planar projection of the defect perpendicular to the direction of the ray, as well as the amount of radiation that passes through it. It can be seen that, under normal circumstances, radiographic inspection is not effective at detecting cracks; in other words, radiographic inspection is insensitive to cracks. Therefore, radiographic testing is most sensitive to volumetric defects such as pores, slag inclusions, and lack of penetration. That is, radiographic testing is suitable for detecting volumetric defects, but not suitable for detecting planar defects. 2. Ultrasonic testing method The frequency range of sound waves that can be directly detected by human ears is usually between 20Hz and 20kHz, which is the audio frequency range. Waves with frequencies below 20 Hz are called infrasound, while those with frequencies above 20 kHz in ultrasonic flaw detectors are called ultrasound. In industry, ultrasonic waves in the range of several megahertz are commonly used for flaw detection. Ultrasonic waves have a high frequency; thus, they propagate in a highly linear manner. They also travel easily through solids and are prone to reflection when encountering interfaces formed by two different media. This makes them suitable for flaw detection. Typically, the ultrasonic probe makes good contact with the surface of the workpiece to be inspected. This allows the probe to effectively transmit ultrasonic waves toward the workpiece and receive the ultrasonic waves reflected from any defect interfaces; these waves are then converted into electrical signals. The ultrasonic flaw detector transmits these signals to the instrument for further processing. The location of the defect can be determined based on the speed at which ultrasound travels through the medium (commonly referred to as the sound speed) and the time it takes to travel. The larger the defect, the larger the reflecting surface, and the greater the reflected energy. Therefore, the size of each defect (equivalent size) can be determined based on the amount of reflected energy. Commonly used ultrasonic testing waveforms include longitudinal waves, transverse waves, and surface waves. The first two are suitable for detecting internal defects, while ultrasonic flaw detectors using surface waves are appropriate for detecting surface defects; however, they require stringent surface conditions. 3. Magnetic particle testing method
Magnetic particle testing is a magnetic flaw detection method based on the principle of magnetic flux leakage. When magnetic flux lines pass through ferromagnetic materials and their products, a leakage magnetic field is generated at their (magnetic) discontinuities, forming magnetic poles. At this point, by sprinkling dry magnetic powder or pouring a magnetic suspension, the magnetic poles will attract the powder, resulting in visible magnetic marks that can be observed with the naked eye. Therefore, this magnetic mark can be used to reveal the defects in ferromagnetic materials and their products. Magnetic particle testing can detect tiny defects that are exposed on the surface and cannot be seen directly with the naked eye or even with the help of a magnifying glass. It can also detect near-surface defects that are not exposed but are buried several millimeters beneath the surface. Although this method can also detect volume defects such as pores, inclusions, and lack of penetration, it is more sensitive to surface defects and is thus more suitable for detecting cracks caused by quenching, rolling, forging, casting, welding, electroplating, grinding, fatigue, etc. There are various methods for detecting defects in magnetic particle inspection, some of which use magnetic particles while others do not. The method that uses magnetic powder for detection is called magnetic particle testing; it is one of the most commonly used methods due to its intuitive display, simple operation, and ease of use. It does not require the use of magnetic powder; it is commonly referred to as magnetic flux leakage testing. Defects are detected using induction coils, magnetosensitive tubes, Hall elements, etc. This method is cleaner than magnetic powder testing, but it is less intuitive than the former. Since magnetic particle testing currently uses magnetic particles to reveal defects, it is sometimes referred to simply as magnetic testing, with its equipment being called magnetic testing equipment. 4. Penetrant testing method Penetrant testing is a method that utilizes capillary action for inspection. For parts with a smooth and clean surface, a colored ultrasonic flaw detector (usually red) or a fluorescent, highly penetrative liquid is applied to the surface of the part to be inspected. If there are micro-cracks on the surface that cannot be seen with the naked eye, due to the high penetrability of this liquid, it will penetrate along these cracks to their roots. The penetrating fluid on the surface is then washed away, and the ultrasonic flaw detector is coated with a contrast-rich display fluid (usually white). After being left for a while, due to the narrow nature of the cracks, capillary action plays a significant role; the penetrant that has seeped into the cracks rises to the surface and spreads, resulting in thicker red lines on the white background. This reveals the shape of the cracks at the surface, which is why this method is often referred to as colored flaw detection. If the penetrant is a fluorescent liquid that rises to the surface due to capillary action, it will emit fluorescence under ultraviolet light, allowing the ultrasonic flaw detector to more clearly show the shape of the cracks on the surface; therefore, penetrant testing in this case is often referred to as fluorescent testing. This flaw detection method can also be used for detecting flaws on metal and non-metal surfaces. The flaw detection liquid used has a strong odor and is often somewhat toxic. 5. Eddy current testing method Eddy current testing involves using an alternating magnetic field generated by alternating current to act on the conductive material to be inspected, thereby inducing eddy currents. If there are defects in the material, an ultrasonic flaw detector will interfere with the eddy currents generated, thereby creating interference signals. By detecting the interference signals using an eddy current tester, one can determine the condition of the defects. There are many factors that affect eddy currents; in other words, eddy currents carry a wealth of signals, and these signals are related to various properties of the material. Separating the useful signals from among all these signals is a challenge for researchers working on eddy current phenomena. Some progress has been made over the years, and certain problems can be solved under specific conditions, but this is still far from meeting the requirements in practical applications, and further development is needed. A notable feature of eddy current ultrasonic flaw detectors is that they can work on conductive materials, not necessarily ferromagnetic ones, but their effectiveness is lower for ferromagnetic materials. Secondly, the surface finish, flatness, and edges of the workpiece to be inspected have a significant impact on eddy current testing; therefore, eddy current testing is often used for inspecting non-ferromagnetic workpieces such as copper pipes, which have regular shapes and smooth surfaces. 6. Gamma rays are electromagnetic waves emitted by radioactive isotopes during natural fission. The main radiation sources used in non-destructive testing include 60Co (cobalt), 137Cs (cesium), 192Ir (iridium), 170Tm (thulium), etc. Gamma rays have a shorter wavelength than X-rays, so they have a greater ability to penetrate materials. Moreover, their devices are compact, making them suitable for use in field inspections. High-frequency sound waves can be used for metal flaw detection because they have short wavelengths, allowing them to be reflected effectively even when they encounter small defects ； High-frequency sound waves have good distance resolution and high resolution for detecting defects ； High-frequency sound waves have good directivity and can form narrow beams. 7. Fluorescent inspection method: This is a technique in which a penetrant containing fluorescent dyes is introduced into the tiny cracks on the surface of the workpiece. After cleaning, an adsorbent is applied to cause the fluorescent liquid contained within the defects to emerge onto the surface; under ultraviolet light, yellow-green fluorescent spots or stripes appear, thereby allowing the detection and identification of defects. Fluorescent inspection should be classified as liquid penetrant inspection. Fluorescent inspection is used to detect defects on the surface of parts. It can test magnetic and non-magnetic metal materials, as well as non-metallic materials. Principle of flaw detection: Utilizing the property of fluorescent substances to emit light under ultraviolet illumination, these substances are applied to the surface of parts, and fluorescence is used to detect surface defects in those parts. 8. Coloring inspection method: The coloring inspection test is a non-destructive testing method used to detect surface defects such as cracks that are not visible to the naked eye. Some kind of liquid, with a colorant applied to the surface of the material. The coloring agent penetrates into the damaged areas. After leaving it for a while, rinse off the coloring agent from the surface. Apply a developer to the cleaned surface; the damaged areas become clearly visible as the coloring agent penetrates into them. These points are called indicator points. The acceptance criteria will specify which indication points are unacceptable (damaged) or acceptable. Color flaw detection testing is mainly used for the weld layer, weld spots, and heat-affected zone.
A. Non-destructive testing methods include: X-ray testing, ultrasonic testing, magnetic particle testing, penetrant testing, eddy current testing, gamma ray testing, fluorescent testing, dye penetrant testing, etc.
B. Advantages, disadvantages, and characteristics
1. Ray-based testing methods
Ray-based testing is a method that utilizes the penetrability and rectilinear propagation of rays for inspection. Although these rays from ultrasonic flaw detectors cannot be seen directly with the naked eye like visible light, they can expose photographic film and can also be detected using special receivers. Radiations commonly used for flaw detection include X-rays and gamma rays emitted by isotopes; these are referred to as X-ray inspection and gamma-ray inspection, respectively. When these rays pass through (irradiate) a material, the greater the density of that material, the more the intensity of the rays is reduced; in other words, the weaker the intensity of the rays that can penetrate the material. At this time, if a photographic film is used for reception, the sensitivity of the film will be low ； If a device is used to receive it, the signal obtained will be weak. Therefore, when an ultrasonic flaw detector uses rays to irradiate the components to be inspected, if there are defects such as pores or inclusions inside them, the density of the material through which the rays pass along the paths with defects is much lower than that along the paths without defects; as a result, the intensity of the rays is reduced less, meaning the intensity that passes through is greater. If a film is used to capture this radiation, the amount of light exposure on the film will be higher, allowing the planar projection of the defects perpendicular to the direction of the rays to be revealed on the film ； Using other receivers, it is also possible to use the instrument to show the planar projection of the defect perpendicular to the direction of the ray, as well as the amount of radiation that passes through it. It can be seen that, under normal circumstances, radiographic inspection is not effective at detecting cracks; in other words, radiographic inspection is insensitive to cracks. Therefore, radiographic testing is most sensitive to volumetric defects such as pores, slag inclusions, and lack of penetration. That is, radiographic testing is suitable for detecting volumetric defects, but not suitable for detecting planar defects. 2. Ultrasonic testing method The frequency range of sound waves that can be directly detected by human ears is usually between 20Hz and 20kHz, which is the audio frequency range. Waves with frequencies below 20 Hz are called infrasound, while those with frequencies above 20 kHz in ultrasonic flaw detectors are called ultrasound. In industry, ultrasonic waves in the range of several megahertz are commonly used for flaw detection. Ultrasonic waves have a high frequency; thus, they propagate in a highly linear manner. They also travel easily through solids and are prone to reflection when encountering interfaces formed by two different media. This makes them suitable for flaw detection. Typically, the ultrasonic probe makes good contact with the surface of the workpiece to be inspected. This allows the probe to effectively transmit ultrasonic waves toward the workpiece and receive the ultrasonic waves reflected from any defect interfaces; these waves are then converted into electrical signals. The ultrasonic flaw detector transmits these signals to the instrument for further processing. The location of the defect can be determined based on the speed at which ultrasound travels through the medium (commonly referred to as the sound speed) and the time it takes to travel. The larger the defect, the larger the reflecting surface, and the greater the reflected energy. Therefore, the size of each defect (equivalent size) can be determined based on the amount of reflected energy. Commonly used ultrasonic testing waveforms include longitudinal waves, transverse waves, and surface waves. The first two are suitable for detecting internal defects, while ultrasonic flaw detectors using surface waves are appropriate for detecting surface defects; however, they require stringent surface conditions. 3. Magnetic particle testing method
Magnetic particle testing is a magnetic flaw detection method based on the principle of magnetic flux leakage. When magnetic flux lines pass through ferromagnetic materials and their products, a leakage magnetic field is generated at their (magnetic) discontinuities, forming magnetic poles. At this point, by sprinkling dry magnetic powder or pouring a magnetic suspension, the magnetic poles will attract the powder, resulting in visible magnetic marks that can be observed with the naked eye. Therefore, this magnetic mark can be used to reveal the defects in ferromagnetic materials and their products. Magnetic particle testing can detect tiny defects that are exposed on the surface and cannot be seen directly with the naked eye or even with the help of a magnifying glass. It can also detect near-surface defects that are not exposed but are buried several millimeters beneath the surface. Although this method can also detect volume defects such as pores, inclusions, and lack of penetration, it is more sensitive to surface defects and is thus more suitable for detecting cracks caused by quenching, rolling, forging, casting, welding, electroplating, grinding, fatigue, etc. There are various methods for detecting defects in magnetic particle inspection, some of which use magnetic particles while others do not. The method that uses magnetic powder for detection is called magnetic particle testing; it is one of the most commonly used methods due to its intuitive display, simple operation, and ease of use. It does not require the use of magnetic powder; it is commonly referred to as magnetic flux leakage testing. Defects are detected using induction coils, magnetosensitive tubes, Hall elements, etc. This method is cleaner than magnetic powder testing, but it is less intuitive than the former. Since magnetic particle testing currently uses magnetic particles to reveal defects, it is sometimes referred to simply as magnetic testing, with its equipment being called magnetic testing equipment. 4. Penetrant testing method Penetrant testing is a method that utilizes capillary action for inspection. For parts with a smooth and clean surface, a colored ultrasonic flaw detector (usually red) or a fluorescent, highly penetrative liquid is applied to the surface of the part to be inspected. If there are micro-cracks on the surface that cannot be seen with the naked eye, due to the high penetrability of this liquid, it will penetrate along these cracks to their roots. The penetrating fluid on the surface is then washed away, and the ultrasonic flaw detector is coated with a contrast-rich display fluid (usually white). After being left for a while, due to the narrow nature of the cracks, capillary action plays a significant role; the penetrant that has seeped into the cracks rises to the surface and spreads, resulting in thicker red lines on the white background. This reveals the shape of the cracks at the surface, which is why this method is often referred to as colored flaw detection. If the penetrant is a fluorescent liquid that rises to the surface due to capillary action, it will emit fluorescence under ultraviolet light, allowing the ultrasonic flaw detector to more clearly show the shape of the cracks on the surface; therefore, penetrant testing in this case is often referred to as fluorescent testing. This flaw detection method can also be used for detecting flaws on metal and non-metal surfaces. The flaw detection liquid used has a strong odor and is often somewhat toxic. 5. Eddy current testing method Eddy current testing involves using an alternating magnetic field generated by alternating current to act on the conductive material to be inspected, thereby inducing eddy currents. If there are defects in the material, an ultrasonic flaw detector will interfere with the eddy currents generated, thereby creating interference signals. By detecting the interference signals using an eddy current tester, one can determine the condition of the defects. There are many factors that affect eddy currents; in other words, eddy currents carry a wealth of signals, and these signals are related to various properties of the material. Separating the useful signals from among all these signals is a challenge for researchers working on eddy current phenomena. Some progress has been made over the years, and certain problems can be solved under specific conditions, but this is still far from meeting the requirements in practical applications, and further development is needed. A notable feature of eddy current ultrasonic flaw detectors is that they can work on conductive materials, not necessarily ferromagnetic ones, but their effectiveness is lower for ferromagnetic materials. Secondly, the surface finish, flatness, and edges of the workpiece to be inspected have a significant impact on eddy current testing; therefore, eddy current testing is often used for inspecting non-ferromagnetic workpieces such as copper pipes, which have regular shapes and smooth surfaces. 6. Gamma rays are electromagnetic waves emitted by radioactive isotopes during natural fission. The main radiation sources used in non-destructive testing include 60Co (cobalt), 137Cs (cesium), 192Ir (iridium), 170Tm (thulium), etc. Gamma rays have a shorter wavelength than X-rays, so they have a greater ability to penetrate materials. Moreover, their devices are compact, making them suitable for use in field inspections. High-frequency sound waves can be used for metal flaw detection because they have short wavelengths, allowing them to be reflected effectively even when they encounter small defects ； High-frequency sound waves have good distance resolution and high resolution for detecting defects ； High-frequency sound waves have good directivity and can form narrow beams. 7. Fluorescent inspection method: This is a technique in which a penetrant containing fluorescent dyes is introduced into the tiny cracks on the surface of the workpiece. After cleaning, an adsorbent is applied to cause the fluorescent liquid contained within the defects to emerge onto the surface; under ultraviolet light, yellow-green fluorescent spots or stripes appear, thereby allowing the detection and identification of defects. Fluorescent inspection should be classified as liquid penetrant inspection. Fluorescent inspection is used to detect defects on the surface of parts. It can test magnetic and non-magnetic metal materials, as well as non-metallic materials. Principle of flaw detection: Utilizing the property of fluorescent substances to emit light under ultraviolet illumination, these substances are applied to the surface of parts, and fluorescence is used to detect surface defects in those parts. 8. Coloring inspection method: The coloring inspection test is a non-destructive testing method used to detect surface defects such as cracks that are not visible to the naked eye. Some kind of liquid, with a colorant applied to the surface of the material. The coloring agent penetrates into the damaged areas. After leaving it for a while, rinse off the coloring agent from the surface. Apply a developer to the cleaned surface; the damaged areas become clearly visible as the coloring agent penetrates into them. These points are called indicator points. The acceptance criteria will specify which indication points are unacceptable (damaged) or acceptable. Color flaw detection testing is mainly used for the weld layer, weld spots, and heat-affected zone.