Comparison of four conventional non-destructive testing methods

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Non-destructive testing refers to the general term for all technical methods that utilize properties of sound, light, magnetism, electricity, etc., to detect whether there are any defects or inhomogeneities in an object under test, without damaging or affecting its performance. These methods provide information regarding the size, location, nature, and quantity of the defects, thereby enabling a determination of the object’s technical condition (such as whether it is acceptable or what its remaining lifespan is). Common non-destructive testing methods include ultrasonic testing (UT), magnetic particle testing (MT), liquid penetrant testing (PT), and radiographic testing (RT). Let’s first 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 surface of the workpiece, forming magnetic traces that are visible under appropriate lighting, thereby indicating 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 testing can be used to inspect 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 shallow scratches, relatively deeply buried holes, and delaminations and folds with an angle of less than 20° relative to the workpiece surface are difficult to detect. The basic principle of liquid penetrant testing is that after a surface is coated with a fluorescent or colored dye, 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 that surface. Again, due to capillary action, the developer attracts the penetrant remaining in the defects; this penetrant then returns into the developer. Under a certain light source (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. The advantages of penetrant testing are: 1. It can detect various materials ； 2. It has high sensitivity ； 3. Intuitive display, easy operation, and low detection costs. The disadvantages of penetrant testing are: 1. It is not suitable for inspecting workpieces made of porous and porous materials, as well as workpieces 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 influenced by the operator. X-ray inspection: The last method is x-ray inspection. This works because X-rays suffer 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 testing: 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 to 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.