Overview and characteristics of five conventional flaw detection methods

    There are many methods of industrial nondestructive testing. At present, there are five most commonly used methods at home and abroad, which are often called the five conventional flaw detection methods. This article will first introduce the five conventional flaw detection methods and their characteristics, and select the flaw detection method that is more suitable for automobile maintenance in combination with the specific conditions and needs in automobile maintenance.

　　1. Overview of the five conventional flaw detection methods

　　The five conventional methods refer to X-ray flaw detection, ultrasonic flaw detection, magnetic particle flaw detection, eddy current flaw detection and penetrant flaw detection.

　　1. Radiographic flaw detection method

　　Radiographic flaw detection is a method of flaw detection that utilizes the penetrating and linear properties of rays. Although these rays cannot be directly detected by the naked eye like visible light, they can sensitize photographic film and can also be received by special receivers. Commonly used rays for flaw detection are x-rays and gamma rays emitted by isotopes, which are called x-ray flaw detection and gamma ray flaw detection, respectively. When these rays pass through a substance, the greater the density of the substance, the more the intensity of the rays is weakened, that is, the greater the intensity of the rays that can penetrate the substance. At this time, if photographic film is used for reception, the sensitivity of the film is small; if an instrument is used for reception, the signal obtained is weak. Therefore, when using radiation to illuminate the parts to be inspected, if there are defects such as pores and slag inclusions inside, the density of the material through which the radiation passes through the defective path is much smaller than that through the path without defects, and its intensity is less weakened, that is, the intensity of the transmission is greater. If it is received by film, the sensitivity is greater, and the plane projection of the defect perpendicular to the direction of the radiation can be reflected from the film; if other receivers are used, the plane projection of the defect perpendicular to the direction of the radiation and the amount of radiation transmission can also be reflected by instruments . It can be seen that under normal circumstances, it is not easy to find cracks by radiation inspection, or in other words, radiation inspection is insensitive to cracks. Therefore, radiation inspection is most sensitive to volume defects such as pores, slag inclusions, and incomplete penetration. That is, radiation inspection is suitable for volume defect detection, but not for area defect detection.

　　2. Ultrasonic flaw detection method

　　The frequency range of sound waves that people's ears can directly receive is usually 20Hz to 20kHz, that is, audio. The frequency below 20Hz is called infrasound, and the frequency above 20kHz is called ultrasound. In industry, several megahertz ultrasound is often used for flaw detection. The higher the frequency of ultrasound, the stronger the linearity of propagation, and the easier it is to propagate in solids. It is also easy to reflect when encountering the interface formed by two different media, so it can be used for flaw detection. Usually, the ultrasonic probe is in good contact with the surface of the workpiece to be detected. The probe can effectively transmit ultrasound to the workpiece, and can receive the ultrasound reflected from the interface, and convert it into an electrical signal at the same time, and then transmit it to the instrument for processing. According to the speed and time of ultrasound propagation in the medium, the location of the defect can be known. The larger the defect, the larger the reflection surface, and the greater the reflected energy. Therefore, the size of each defect can be found according to the size of the reflected energy. Commonly used flaw detection waveforms include longitudinal waves, transverse waves, surface waves, etc. The former two are suitable for detecting internal defects, and the latter is suitable for detecting surface defects, but the surface conditions are high.

　　3. Magnetic particle testing method

　　Magnetic particle inspection is a magnetic inspection method based on the principle of magnetic flux leakage. When magnetic lines of force pass through ferromagnetic materials and their products, leakage magnetic fields will be generated at their discontinuities, forming magnetic poles. At this time, if dry magnetic powder is sprinkled or magnetic suspension is poured, the magnetic poles will absorb the magnetic powder, producing obvious magnetic traces that can be directly observed with the naked eye. Therefore, the magnetic traces can be used to display the defects of ferromagnetic materials and their products. Magnetic particle inspection can detect tiny defects that are exposed on the surface and cannot be directly observed with the naked eye or with the help of a magnifying glass. It can also detect near-surface defects that are not exposed on the surface but buried a few millimeters below the surface. Although this method can also detect volume defects such as pores, inclusions, and incomplete penetration, it is more sensitive to area defects and is more suitable for inspecting cracks caused by quenching, rolling, forging, casting, welding, electroplating, grinding, fatigue, etc.

    There are many ways to display defects in magnetic flaw detection, some use magnetic particles, and some do not. The method that uses magnetic particles is called magnetic particle detection. It is one of the most commonly used methods because it is intuitive, easy to operate, and people are happy to use it. The method that does not use magnetic particles is usually called leakage magnetic flaw detection. It often uses induction coils, magnetic sensitive tubes, Hall elements, etc. to reflect defects. It is more hygienic than magnetic particle detection, but not as intuitive as the former. Since magnetic flaw detection currently mainly uses magnetic particles to display defects, people sometimes directly call magnetic particle detection magnetic flaw detection, and its equipment is called magnetic flaw detection equipment.

　　4. Eddy current flaw detection method

　　Eddy current flaw detection is the process of inducing eddy currents by applying the alternating magnetic field generated by alternating current to the conductive material to be inspected. If there are defects in the material, it will interfere with the generated eddy currents, that is, form interference signals. By detecting the interference signals with an eddy current flaw detector, the condition of the defects can be known. There are many factors that affect eddy currents, that is, eddy currents carry a wealth of signals, which are related to many factors of the material. How to separate the useful signals from the many signals is a difficult problem for eddy current researchers. Some progress has been made over the years, and some problems can be solved under certain conditions, but it is still far from meeting the requirements of the site and needs to be vigorously developed.

　　The remarkable feature of eddy current testing is that it can work on conductive materials, not necessarily ferromagnetic materials, but the effect on ferromagnetic materials is poor. Secondly, the surface finish, flatness, and edge of the workpiece to be inspected have a great influence on eddy current testing, so eddy current testing is often used for non-ferromagnetic workpieces such as copper tubes with regular shapes and smooth surfaces.

　　5. Penetrant testing method

　　Penetrant testing is a method of flaw detection using capillary phenomena. For parts with smooth and clean surfaces, a colored or fluorescent, highly permeable liquid is applied to the surface of the parts to be inspected. If there are microcracks on the surface that cannot be directly detected by the naked eye, the liquid will penetrate along the cracks to their roots due to its strong permeability. Then wash off the penetrant on the surface and apply a display liquid with a large contrast. After leaving it for a while, because the cracks are very narrow and the capillary phenomenon is significant, the penetrant that originally penetrated into the cracks will rise to the surface and spread, showing a thicker red line on the white substrate, thus showing the shape of the crack exposed on the surface, so it is often called color flaw detection. If the penetrant is a fluorescent liquid, the liquid that rises to the surface by capillary phenomenon will emit fluorescence under the irradiation of ultraviolet light, thus better showing the shape of the crack exposed on the surface, so the penetrant flaw detection at this time is often directly called fluorescent flaw detection. This flaw detection method can also be used for metal and non-metal surface flaw detection. The flaw detection liquid used has a strong odor and is often toxic.

　　In addition to the above five conventional methods, infrared and acoustic emission have been developed in recent years.

And some new flaw detection methods.

　　2. Specific conditions and requirements for flaw detection in automobile maintenance

　　During the manufacturing process, the automobile has gone through a series of flaw detections, and only when all the inspections are intact can it be shipped as a qualified product. After the automobile reaches the hands of the user, some parts are often subjected to alternating stress during operation. Under the action of long-term alternating stress, the originally intact parts will also produce fatigue cracks. This fatigue crack generally starts from the surface of the parts, and then gradually develops from the outside to the inside, that is, it belongs to the surface crack. Some rotating parts have surface cracks under the action of overheating or alternating stress, and it is possible that a dense covering layer is produced on the surface due to rotation and grinding, covering the cracks and turning them into near-surface cracks that are not exposed on the surface. The initial surface cracks are generally very small and difficult to observe with the naked eye or with the help of a magnifying glass, and it is impossible to observe near-surface cracks. Parts with such initial tiny cracks will not break immediately, but they already have hidden dangers. Therefore, the main task of flaw detection in automobile maintenance is to detect whether there are extremely fine surface and near-surface cracks on its parts to eliminate the potential safety hazards of the car during driving; secondly, the surface condition of each part after operation is not as good as when it is new from the factory, but varies according to the operation conditions; thirdly, the size of the appearance of each part to be inspected in automobile maintenance is different, that is, there are many varieties and small quantities; in addition, its working site is generally not as good as the conditions of the manufacturing plant; at the same time, the construction period is generally required to be more urgent. Therefore, we can only combine these specific conditions and needs in maintenance to select a flaw detection method that is more suitable for automobile maintenance.

　　3. Application of flaw detection in automobile maintenance

　　The parts to be inspected in automobile maintenance are mainly made of steel materials, and the purpose of flaw detection is mainly to detect whether there are surface and near-surface cracks. Through the comparison of the above-mentioned flaw detection methods, it can be seen that magnetic particle flaw detection has a relatively high sensitivity for surface and near-surface flaw detection of ferromagnetic parts, is non-toxic, has low requirements for the shape, surface, technical requirements and investment requirements of parts, and is intuitive and convenient. Therefore, among the non-destructive flaw detection methods for automobile maintenance, magnetic particle flaw detection is currently the best method.

　　In fact, magnetic particle inspection is also the main method used for automobile parts in automobile manufacturing plants. Based on a large number of magnetic particle inspections, corresponding magnetic particle inspection standards have been formulated for some automobile parts, such as crankshafts, camshafts, connecting rods, valves, piston pins, and oil nozzles. In automobile maintenance, magnetic particle inspection of parts can refer to these standards to increase the reliability of inspection. However, other inspection methods are rarely used in automobile parts inspection, and there is no corresponding inspection standard.