The Four Most Common Nondestructive Testing Methods
Aug 13, 2025
Nondestructive testing (NDT) is a general term for all technical methods that utilize acoustic, optical, magnetic, and electrical properties to detect defects or unevenness in an object without damaging or affecting its performance. The methods provide information such as the size, location, nature, and quantity of the defects, thereby determining the object's technical status (such as acceptance or failure, remaining service life, etc.).
Common NDT methods include ultrasonic testing (UT), magnetic particle testing (MT), liquid penetrant testing (PT), and X-ray testing (RT).
Magnetic Particle Testing
First, let's understand the principles of magnetic particle testing. When ferromagnetic materials and workpieces are magnetized, the presence of discontinuities causes local distortion of magnetic field lines on and near the workpiece surface. This generates a leakage magnetic field, which attracts magnetic particles applied to the workpiece surface, forming magnetic traces that are visible under appropriate lighting, revealing the location, shape, and size of the discontinuity. The applicability and limitations of magnetic particle testing are as follows:
1. Magnetic particle testing is suitable for detecting small, narrow, and visually indiscontinuities on and near the surface of ferromagnetic materials.
2. Magnetic particle testing can be used to inspect components in a variety of situations and for a wide range of parts.
3. It can detect defects such as cracks, inclusions, hairline cracks, white spots, folds, cold shuts, and porosity. (Thanks for following Dingding Automatic Welding.)
4. Magnetic particle testing cannot inspect austenitic stainless steel or welds made with austenitic stainless steel electrodes, nor can it inspect non-magnetic materials such as copper, aluminum, magnesium, and titanium. It is difficult to detect shallow surface scratches, deep buried holes, and delamination and folds at angles less than 20° to the workpiece surface. Liquid Penetrant Testing
The basic principle of liquid penetrant testing is that after a fluorescent or colored dye is applied to a part's surface, capillary action allows the penetrant to penetrate into open surface defects.
After removing excess penetrant from the part's surface, a developer is applied. Similarly, capillary action attracts the penetrant retained in the defect, which then seeps back into the developer. Under a specific light source (ultraviolet light or white light), traces of the penetrant at the defect are visible (fluorescent yellow-green or bright red), thereby detecting the defect's morphology and distribution.
Advantages of penetrant testing include:
1. It can inspect a wide range of materials;
2. It has high sensitivity;
3. It offers intuitive display, is easy to operate, and has low testing costs.
Disadvantages of penetrant testing include:
1. It is not suitable for inspecting workpieces made of porous materials or with rough surfaces;
2. Penetrant testing can only detect the surface distribution of defects, making it difficult to determine their actual depth, making it difficult to quantitatively assess them. Inspection results are also significantly affected by operator input. X-ray testing
The last method, radiographic testing, is based on the fact that X-rays are attenuated after passing through the irradiated object. Different thicknesses and materials have different absorption rates for X-rays. When a film is placed on the other side of the irradiated object, the varying intensity of the radiation produces a corresponding pattern. The film reviewer can use this image to determine the presence and nature of internal defects.
Applicability and limitations of radiographic testing:
1. It is more sensitive to detecting volumetric defects and is easier to characterize.
2. Radiographic films are easily preserved and traceable.
3. It visually displays the shape and type of defects.
4. Disadvantages include the inability to locate the depth of defects, limited inspection thickness, the need for specialized film development, potential health hazards, and high costs.
In summary, ultrasonic and X-ray testing are suitable for detecting internal defects. Ultrasonic testing is suitable for components thicker than 5 mm and with regular shapes, while X-rays cannot locate the depth of defects and emit radiation. Magnetic particle and penetrant testing are suitable for detecting surface defects.
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