(1) magnetic particle method

This method has the advantages of simple equipment, reliable detection and convenient operation, but it is only suitable for the detection of defects on the surface and near surface of ferromagnetic materials. Its principle is that the use of ferromagnetic materials under the action of electromagnetic fields can produce the phenomenon of magnetization, parts to be tested under the effect of electromagnetic field, due to its surface or near surface (within a few millimeters) flaws, the lines of magnetic force had to bypass defects, magnetic leakage or gathered to form local magnetic adsorption absorption magnetic powder, which shows the location of the defects, shape and orientation. Figure 1 shows the principle of magnetic particle detection.

When USING magnetic particle method, we must pay attention to the selection of magnetization method, so that the direction of magnetic force line is as vertical as possible or at a certain Angle through the orientation of the defect, in order to obtain the best detection effect, and at the same time, attention should be paid to demagnetization after detection.

(2) the osmosis

The microcracks of 1υm width on the surface of the parts made of any material and any shape can be detected by penetration method. This method is simple and convenient to detect. The principle and process is that the permeant is coated on the surface of the cleaned part, and the permeant enters the defect through the capillary action of the surface defect. In this case, the position and shape of the defect can be shown by the penetration agent in the defect with the characteristics of color or fluorescence under ultraviolet irradiation.

③ Ultrasonic method

The main characteristics of this method are strong penetration ability, high sensitivity, wide range of application, not limited by materials, light equipment, easy to use, can be detected on site, but only suitable for the detection of internal defects of parts. The principle is that when the ultrasonic wave generated by the piezoelectric effect of some substances (quartz, barium titanate, etc.) propagates in the medium, it encounters the interface of different media (internal cracks, slag inclusion, shrinkage hole and other defects), which will produce reflection, refraction and other characteristics. The reflection generated by the ultrasonic wave at the defect can be transferred by the detection instrument.

The refraction wave is displayed on the fluorescent screen to determine the location, size and nature of the defects inside the part.

(4) ray method

The most important feature of this method is that it is easier to determine the shape, size and nature of the defects on the photographic film, and the film can be stored for future reference for a long time. However, the investment and cost of testing equipment are relatively high, and corresponding safety measures against radiation are required. It is only used for the detection of important parts or when the defects cannot be determined by ultrasonic testing. Its principle is that the use of radiation (X - ray) irradiation, so that it through the parts, if encountered defects (cracks, pores, loose or slag, etc.), the ray is easier to pass through the characteristics. In this way, more energy is transmitted from the defect of the measured part than from other places. When these rays illuminate the film, after photosensitivity and development, the formation of different blackness (contrast), so as to analyze and judge the shape, size and location of parts defects.

It must be pointed out that when detecting and classifying parts, we must also pay attention to the corresponding special tests combined with the special requirements of parts, such as the balance test of high-speed movement, the elastic test of elastic parts and the sealing test of seals, etc., only in this way can we make a comprehensive, accurate identification and correct classification of the technical state of parts.