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: ultrasonic testing (UT), magnetic particle testing (MT), liquid penetrant testing (PT) and X-ray testing (RT).
Ultrasonic testing
UT (Ultrasonic Testing) is one of the methods of non-destructive testing in the industry. When the ultrasonic wave enters the object and encounters a defect, a part of the sound wave will be reflected, and the transmitter and receiver can analyze the reflected wave, and the defect can be detected extremely accurately. And it can display the position and size of internal defects, and measure the thickness of materials.
Advantages of ultrasonic testing:
1. The penetration ability is large, for example, the effective detection depth in steel can reach more than 1 meter;
2. For flat defects such as cracks, interlayers, etc., the flaw detection sensitivity is high, and the depth and relative size of the defects can be determined;
3. The equipment is portable, safe to operate, and easy to realize automatic inspection.
Shortcoming:
It is not easy to inspect workpieces with complex shapes, and the surface to be inspected is required to have a certain degree of smoothness, and a couplant should be used to fill the gap between the probe and the surface to be inspected to ensure sufficient acoustic coupling.
Magnetic particle inspection
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, hair lines, 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 period of time, the permeate can penetrate into the surface opening defects; The developer is applied to the surface of the part.
Similarly, under the action of the capillary, the imaging agent will attract the permeate remaining in the defect, and the permeate will seep back into the imaging agent. Under a certain light source (ultraviolet light or white light), the trace of permeate at the defect is displayed. , (yellow-green fluorescence or bright red), so as 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 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 type, 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. Disadvantages: The burial depth of the defect cannot be located, and 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; among them, magnetic particle testing is limited to detecting magnetic materials, and penetrating testing is limited to detecting surface opening defects.