Non-destructive testing is the use of sound, light, magnetic and electrical properties, without damaging or affecting the use of the object performance, to detect the presence of defects or inhomogeneities in the object under inspection, to give the size, location, nature and number of defects and other information, and then determine the technical status of the object under inspection (such as qualified or unqualified, remaining life, etc.) the general term of all technical means.
Commonly used NDT methods: Ultrasonic Testing (UT), Magnetic Particle Testing (MT), Penetrant Testing (PT), X-ray Testing (RT), Eddy Current Testing (ET)
The Principle of Magnetic Particle Testing
After the magnetization of ferromagnetic materials and workpieces, due to the existence of discontinuities, the magnetic lines of force on the surface of the workpiece and near the surface are distorted locally, and a leakage magnetic field is generated, which adsorbs the magnetic powder applied to the surface of the workpiece and forms magnetic traces visually visible under suitable lighting, thus showing the location, shape and size of the discontinuities.
The Applicability and Limitations of Magnetic Particle Testing
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1. Magnetic particle testing is suitable for detecting the surface of ferromagnetic materials and near-surface size is very small, the gap is very narrow visually difficult to see the discontinuity.
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2. Magnetic particle testing can be detected in a variety of cases of parts, but also a variety of type parts for detection.
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3. Can find cracks, inclusions, hairline, white spots, folding, cold separation and loosening and other defects.
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4. Magnetic particle testing can not detect austenitic stainless steel materials and weld seams welded with austenitic stainless steel welding rod, but also can not detect copper, aluminum, magnesium, titanium and other non-magnetic materials. For the surface shallow scratches, buried deeper hole and with the workpiece surface angle of less than 20 ° delamination and folding is difficult to find.
Basic Principles of Penetration Testing
The test object is coated with a solution that contains a visible or fluorescent dye. Excess solution is then removed from the surface of the object but leaving it in surface breaking defects. A developer is then applied to draw the penetrant out of the defects. With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, thus allowing imperfections to be readily seen . With visible dyes, vivid color contrasts between the penetrant and developer make "bleedout" easy to see. The red indications below represent a number of defects in this component.
Advantages of Penetration Testing
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1. The possibility of detecting various materials.
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2. High sensitivity.
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3. Display intuitive, easy to operate, low cost of detection.
Disadvantages of Penetration Testing
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1. Not suitable for inspection of workpieces made of porous and loose materials and workpieces with rough surfaces.
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2. Penetration testing can only detect the surface distribution of defects, it is difficult to determine the actual depth of defects, and therefore difficult to make a quantitative evaluation of defects. The detection results are also affected by the operator's influence.
X-ray Testing
The X-rays pass through the irradiated object with loss, and different thicknesses of different substances absorb them at different rates, and the negative is placed on the other side of the irradiated object, which will produce a corresponding pattern due to the difference in the intensity of the rays, so that the image can be used to determine whether there are defects inside the object and the nature of the defects.
The Applicability and Limitations of X-ray Testing
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1. More sensitive to the detection of volumetric defects, it is easier to characterize the defects.
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2. Radiographic negatives are easy to retain and have traceability.
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3. Visual display of the shape and type of defects.
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4. The disadvantage can not locate the buried depth of the defect, while the detection thickness is limited, the negative needs to be specially sent to wash, and a certain harm to the human body, the cost is higher.
