Non-destructive testing is the use of acoustic, optical, magnetic and electrical properties, in the absence of damage or does not affect the use of the object under the premise of the performance of the object to be inspected, the detection of the object to be inspected for the presence of defects or inhomogeneities, giving the size, location, nature and number of defects and other information, and then determine the technical state of the object to be inspected (eg, qualified or unqualified, the remaining life, etc.) of the general term of all technical means.
Commonly used non-destructive testing methods: Ultrasonic Testing (UT), Magnetic Particle Testing (MT), Liquid Penetration Testing (PT) and X-ray Testing (RT).
Magnetic particle testing
First of all, let's understand the principle of magnetic particle inspection.
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 of the local distortion, and generate a leakage field, adsorption of magnetic powder applied to the surface of the workpiece, forming a visually visible magnetic trace in the appropriate light, thus showing the location, shape and size of the discontinuity.
The applicability and limitations of magnetic particle inspection are:
1, magnetic particle flaw detection is suitable for detecting discontinuities on the surface and near surface of ferromagnetic materials that are very small in size and have extremely narrow gaps that are difficult to see visually.
2, magnetic particle inspection can be a variety of cases of parts detection, but also a variety of types of parts to be detected.
3,Cracks, inclusions, hairline, white spots, folding, cold segregation and loose and other defects can be found.
4, magnetic particle inspection can not detect austenitic stainless steel materials and welds welded with austenitic stainless steel welding electrodes, and can not detect copper, aluminum, magnesium, titanium and other non-magnetic materials. For the surface of shallow scratches, buried deeper holes and with the workpiece surface angle less than 20 ° delamination and folding is difficult to find.
Liquid penetration detection
The basic principle of liquid penetration detection, the surface of the part is coated with fluorescent dyes or coloring dyes, in a period of time under the action of the capillary, the penetrating liquid can penetrate into the surface of the open defects;
After removing the excess penetrating liquid on the surface of the parts, and then apply the developer on the surface of the parts, again, under the action of the capillary, the developer will attract the defects retained in the penetrating liquid, the penetrating liquid seeps back into the developer, under a certain light source (ultraviolet light or white light), the defects at the penetrating liquid traces were reality, (yellow-green fluorescence or bright red), thus detecting the defects of the shape and distribution of the state.
The advantages of penetration detection are:
1, can detect a variety of materials;
2, with high sensitivity;
3, display intuitive, easy to operate, low detection costs.
And the disadvantages of penetration testing are:
1, not suitable for checking the workpiece made of porous loose material and rough surface workpiece;
2, penetration testing can only detect the surface distribution of defects, it is difficult to determine the actual depth of defects, and therefore it is difficult to make a quantitative evaluation of defects. The detection results are also affected by the operator.
X-ray Inspection
The last, ray detection, because X-rays through the irradiated object will have loss, different thicknesses of different substances on their absorption rate is different, and the negative is placed on the other side of the irradiated object, because of the intensity of the ray is different and produce the corresponding graphic, film evaluators can be based on the image to determine whether there are defects within the object as well as the nature of the defects.
Applicability and limitations of radiographic inspection:
1,It is more sensitive to detecting volumetric defects, and it is easier to characterize the defects.
2, the ray negative is easy to retain, there is traceability.
3, Visualize the shape and type of defects.
4, the disadvantages can not locate the buried depth of defects, while the detection of limited thickness, the negative needs to be sent specifically to wash, and a certain amount of harm to the human body, the cost is higher.
In short, ultrasonic, X-ray flaw detection is suitable for detecting internal defects; where ultrasonic for more than 5mm, and the shape of the regular parts, X-ray can not locate the buried depth of defects, radiation. Magnetic particle and penetration flaw detection is suitable for detecting defects on the surface of parts; among them, magnetic particle flaw detection is limited to detecting magnetic materials, and penetration flaw detection is limited to detecting open defects on the surface.
