What are the methods of non-destructive testing? What’s the difference?

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Non-destructive testing refers to the collective term for all technical methods that utilize properties such as sound, light, magnetism, and electricity to detect the presence of defects or irregularities in a test object, without damaging or affecting its functional performance. These methods provide information on the size, location, nature, and quantity of defects, thereby enabling determination of the technical condition of the object in question (such as whether it is acceptable or not, and its remaining service life). Common non-destructive testing methods include ultrasonic testing (UT), magnetic particle testing (MT), liquid penetrant testing (PT), and X-ray testing (RT). UT (Ultrasonic Testing) is one of the non-destructive testing methods used in industry. When ultrasonic waves enter an object and encounter defects, part of the sound waves get reflected. By analyzing these reflected waves, the transmitter and receiver can detect defects with high precision, and determine the location and size of internal defects as well as measure the thickness of the material. Advantages of ultrasonic testing: 1. It has a high penetration capacity; for example, the effective detection depth in steel can exceed 1 meter. 2. It offers high sensitivity for detecting planar defects such as cracks and delaminations, and it is also possible to determine the depth and relative size of these defects ； 3. The equipment is lightweight, safe to operate, and facilitates automated inspection. Disadvantages: It is difficult to inspect workpieces with complex shapes; the surface to be inspected must have a certain level of smoothness, and a coupling agent is required to fill the gap between the probe and the surface being inspected in order to ensure adequate acoustic coupling. Magnetic particle testing: First, let’s understand the principle of magnetic particle testing. After 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 workpiece’s surface, forming magnetic traces that are visible under appropriate lighting, thereby revealing the location, shape, and size of the discontinuities. The applicability and limitations of magnetic particle testing are as follows: 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, making them difficult to see visually. 2. Magnetic particle inspection can be used to examine components under various conditions, as well as a variety of parts. 3. Defects such as cracks, inclusions, grain boundaries, white spots, folds, cold shuts, and porosity can be detected. 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. 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. Liquid penetrant testing: The basic principle of liquid penetrant testing is that after a surface is coated with a fluid containing fluorescent or colored dyes, under the effect of capillary action over a period of time, the penetrant can penetrate into any surface defects ； After removing the excess penetrant from the surface of the part, a developer is applied to the part’s surface. Similarly, under the action of capillarity, the developer attracts the penetrant remaining in the defects; the penetrant then returns into the developer. Under a certain light source (ultraviolet light or white light), the traces of penetrant at the defects become visible (in yellow-green fluorescence or bright red), thereby allowing the shape and distribution of the defects to be detected. The advantages of penetrant testing are: 1. It can detect various materials ； 2. It has high sensitivity ； 3. It features an intuitive display, easy operation, and low testing costs. The disadvantages of penetrant testing are: 1. It is not suitable for inspecting workpieces made of porous and loose materials, as well as those with rough surfaces ； 2. Penetrant testing can only detect the surface distribution of defects, and it is difficult to determine the actual depth of these defects; as a result, it is hard to provide a quantitative assessment of them. The detection results are also greatly affected by the operator. X-ray inspection: The last method is X-ray inspection. This works because X-rays experience attenuation as they pass through the object being inspected; different materials of varying thicknesses have different absorption rates for these rays. By placing a film on the other side of the object, patterns are formed due to the differences in ray intensity. Inspectors can then use these images to determine whether there are any defects inside the object and what type of defects they are. Applicability and limitations of radiographic inspection: 1. It is relatively sensitive to detecting volumetric defects, and it is easy to determine the nature of such defects. 2. Radiographic films are easy to preserve and provide traceability. 3. Visually display the shape and type of defects. 4. The disadvantages include an inability to determine the depth at which defects are located, a limited detection range for thickness, the need to send the film to a specialized facility for processing, potential harm to human health, and high costs. In summary, ultrasonic and X-ray testing are suitable for detecting internal defects ； Among them, ultrasound is suitable for components larger than 5 mm with regular shapes; X-rays cannot determine the depth at which defects are located, and they involve radiation. Magnetic particle and penetrant testing are suitable for detecting surface defects in components ； Among them, magnetic particle testing is limited to detecting magnetic materials, while penetrant testing is limited to detecting surface open defects.