Non-destructive testing is to use the characteristics of sound, light, magnetism, and electricity to detect whether there are defects or inhomogeneity in the inspected object without damaging or affecting the performance of the inspected object, and to give the size, location, The general term for all technical means to determine the technical state of the object under inspection (such as qualified or not, remaining life, etc.).
Commonly used non-destructive testing methods are ultrasonic testing (UT), magnetic particle testing (MT), liquid penetrant testing (PT), and X-ray testing (RT).
Magnetic particle inspection
First of all, let’s understand the principle of magnetic particle testing. After the ferromagnetic material and the workpiece are magnetized, due to the existence of discontinuity, the magnetic field lines on the surface and near the surface of the workpiece are locally distorted, and a leakage magnetic field is generated, which adsorbs the magnetic powder applied on the surface of the workpiece and forms a magnetic field that is visible under suitable illumination. traces, thereby showing the location, shape, and size of the discontinuity.
The applicability and limitations of magnetic particle testing are:
1.Magnetic particle inspection is suitable for detecting discontinuities on the surface and near-surface of ferromagnetic materials that are very small and the gap is extremely narrow, which is difficult to see visually.
2.Magnetic particle inspection can detect parts under various conditions, and can also detect various types of parts.
3.Defects such as cracks, inclusions, hairlines, white spots, folds, cold insulation, and looseness can be found.
4.Magnetic particle testing cannot detect austenitic stainless steel materials and welds welded with austenitic stainless steel electrodes, nor can it detect non-magnetic materials such as copper, aluminum, magnesium, and titanium. It is difficult to find shallow scratches on the surface, deep buried holes, and delaminations and folds with an angle less than 20° from the surface of the workpiece.
Liquid Penetration Testing
The basic principle of liquid penetration testing is that after the surface of the part is coated with fluorescent dyes or colored dyes, under the action of a capillary for a while, the permeate can penetrate the surface opening defects; The developer is applied on the surface of the part. Similarly, under the action of the capillary, the developer will attract the permeate remaining in the defect, and the permeate will seep back into the developer. Under a certain light source (ultraviolet light or white light), the defect is The traces of the infiltrated liquid are realized, (yellow-green fluorescence or bright red), to detect the morphology and distribution of defects.
The advantages of penetration testing are:
1.Various materials can be detected;
2.It has high sensitivity;
3.The display is intuitive, the operation is convenient, and the detection cost is low.
The disadvantages of penetration testing are:
1.It is not suitable for inspecting workpieces made of porous materials and workpieces with rough surfaces;
2.Penetration testing can only detect the surface distribution of defects, and it is difficult to determine the actual depth of defects, so it is difficult to quantitatively evaluate defects. The detection result is also greatly influenced by the operator.
X-ray inspection
The last one, ray detection, is because X-rays will be lost after passing through the irradiated object, and different materials with different thicknesses have different absorption rates for them, and the negative film is placed on the other side of the irradiated object. The corresponding graphics are generated, and the film reviewers can judge whether there is a defect inside the object and the nature of the defect according to the image.
Applicability and limitations of radiographic testing:
1.It is more sensitive to the detection of volume-type defects, and it is easier to characterize the defects.
2.The ray film is easy to retain and has traceability.
3.Visually display the shape and type of defects.
4.The disadvantage is that it cannot locate the buried depth of the defect. At the same time, the detection thickness is limited. The negative film needs to be specially sent for washing, and it is harmful to the human body and the cost is high.
All in all, ultrasonic and X-ray flaw detection are suitable for flaw detection of internal defects; among them, ultrasonic waves are suitable for parts with a size of more than 5mm and regular shapes. X-rays cannot locate the buried depth of defects, and there is radiation. Magnetic particle and penetrant testing are suitable for detecting surface defects of parts; magnetic particle testing is limited to detecting magnetic materials, and penetrating testing is limited to detecting surface opening defects.
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