How to detect the crack of auto parts?

2020-01-09 09:41:29

In the process of automobile repair, it is necessary to inspect the cracks of important automobile parts. If it is not found in time, it may cause parts fracture and serious accidents.

The methods to inspect the cracks of the parts include magnetic flaw detection, fluorescent flaw detection and aerosol flaw detection. For the cast parts such as water-cooled cylinder block and cylinder head, cracks are often found by water pressure test. For some hidden cracks on the surface of shaft parts, the method of immersion knocking can also be used to find cracks.

(1) Magnetic flaw detection.

The magnetic flaw detection method has the advantages of simple equipment, accurate measurement, rapid and so on, which is widely used in automobile repair enterprises.

The principle of magnetic flaw detection is: when the magnetic line of force passes the inspected part, the part is magnetized. If there is a crack on the surface of the part, the magnetic field line at the crack will be interrupted because the crack does not conduct magnetism, and the magnetic field line will be scattered and form a magnetic pole. At this time, the surface of the part is sprinkled with magnetic iron powder or liquid iron powder, and the iron powder is magnetized and adsorbed on the crack, thus showing the location and size of the crack. When the direction of the crack is parallel to the direction of the magnetic field, the number of magnetic lines cut off by the crack is small, the magnetic poles will not occur on both sides of the crack, and the iron powder cannot be adsorbed. Therefore, when using magnetic flaw detection, the crack must be perpendicular to the magnetic field direction. Therefore, in the inspection, it is necessary to estimate the possible position and direction of the cracks, and adopt different magnetization methods: the transverse cracks should make the parts magnetized longitudinally, and the longitudinal cracks should make the parts magnetized transversely (or ring magnetization).

Longitudinal magnetization is to place the inspected parts between the two poles of the horseshoe electromagnet. When the coil winding is energized with current, the electromagnet will generate magnetic flux, and form a closed magnetic circuit through the parts, which will generate the longitudinal magnetic field parallel to the axis of the parts, so that the transverse cracks can be found.

The transverse magnetization is that the current passes through the part directly, so the circular magnetic force line is generated on the peripheral surface of the part. When the crack is parallel to the axis direction of the part, the magnetic pole can be formed and the magnetic iron powder can be absorbed, so the hidden damage can be found.

For the cracks with both directions of magnetization at a certain angle, it is better to use the combined magnetization method. The magnetizing current can be DC or AC, mainly low-voltage and high-intensity current, so that strong magnetic field can be obtained without electric shock.

AC magnetic flaw detection is widely used. Because it only needs step-down transformer, the equipment is simple. But it has skin effect. It can only test the cracks on or near the surface, which is suitable for fatigue crack detection.

The shape of the part has a great influence on the distribution uniformity of the magnetic field lines. If the long axis with uniform diameter is magnetized longitudinally, the electromagnetic induction at both ends of the axis is much larger than that in the middle, so it is not easy to find hidden damage in the middle. Therefore, the magnetization test should be carried out for the long axis one by one.

For parts with irregular shape, the magnetic field line is very uneven when magnetizing. Therefore, when checking the longitudinal crack of the crankshaft, a strong current (about 4000 a) is needed for circular magnetization, while when checking the radial crack, a sectional longitudinal magnetization is needed.

After the magnetization inspection of the parts, some remanence will be left, so demagnetization must be carried out. Otherwise, the parts will attract iron scraps in use, resulting in abrasive wear. A simple demagnetization method is to slowly withdraw the part from the alternating magnetic field, or directly apply alternating current to the part and gradually reduce the current intensity to zero. However, when using alternating current demagnetization, it is only effective on the surface of the parts. Therefore, for the parts with direct current electromagnetization, it is better to use direct current demagnetization. When DC is applied to the parts for demagnetization, the polarity of the magnetic field shall be constantly changed, and the current shall be gradually reduced to zero.

2-5 μ m iron oxide powder (Fe3O4) can be used as the iron powder for magnetic flaw detection. Iron powder can be used dry, but it is more sensitive to prepare iron oxide suspension, that is, 20-30g iron oxide powder is added into one liter transformer oil or low viscosity diesel oil or kerosene.

(2) Fluorescent inspection.

Fluorescent inspection is a kind of inspection method which uses the principle of ultraviolet radiation to make fluorescent materials glow to show the surface defects of parts.

The molecules of fluorescent substances can absorb and emit light energy. When they are exposed to ultraviolet light, each molecule absorbs a certain amount of light energy. If a molecule absorbs more light energy than it normally does, it can emit a certain amount of light energy to restore its equilibrium state, which is the visible fluorescence. The fluorescent material at the crack can emit bright light, so the crack can be easily found.

In order to test the defects on the surface of the parts, a layer of fluorescent emulsion with good permeability is coated on the surface of the parts, which can penetrate into the fine cracks. After a period of time, the fluorescent solution on the surface of the parts is washed away, but there is still a fluorescent solution in the defects, which glows under the irradiation of ultraviolet light, so that the position, shape and size of the defects can be determined.

The inspection procedure of fluorescent flaw detection is as follows:

1) Before the flaw detection, the oil stain and rust spot on the surface of the parts shall be removed, cleaned and dried under the temperature of 20-40 ℃, and the water is easy to penetrate after evaporation.

2) During the penetration treatment, small parts can be immersed in the fluorescent solution for 10-20 minutes, and large parts can be coated with a brush, and then the penetrant will run out. Wash with water, usually with water pressure of 1.5pa, water temperature of 20-40 ℃, and then dry at low temperature; after 1-2min under 85 ℃.

3) During the imaging processing, a layer of magnesium oxide (dry powder) imaging powder is coated on the surface of the part, which has good absorption performance, so that the penetrant immersed in the crack can be absorbed out and diffused for a certain width