Nondestructive testing is the general term of all technical means that use acoustic, optical, magnetic and electrical characteristics to detect whether there are defects or unevenness in the tested object, give information such as the size, location, nature and quantity of defects, and then determine the technical status of the tested object (such as qualified or not, remaining life, etc.) without damaging or affecting the service performance of the tested object.

Common nondestructive 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 nondestructive testing methods in industry. When the ultrasonic wave enters the object and encounters a defect, part of the acoustic wave will be reflected. The transmitter and receiver can analyze the reflected wave, so that the defect can be detected abnormally accurately, and the position and size of the internal defect can be displayed, and the material thickness can be measured.

Advantages of ultrasonic testing:

1. The penetration capacity is large, for example, the effective detection depth in steel can reach more than 1m;

2. For plane defects such as cracks and interlayer, the flaw detection sensitivity is high, and the depth and relative size of defects can be measured;

3. The equipment is light, safe to operate and easy to realize automatic inspection.

Disadvantages:

For workpieces with complex shapes that are not easy to inspect, the surface to be inspected shall have a certain finish, and the gap between the probe and the surface to be inspected shall be filled with coupling agent to ensure sufficient acoustic coupling.

Magnetic particle testing

First, let's understand the principle of magnetic particle testing. After the ferromagnetic materials and workpieces are magnetized, due to the existence of discontinuity, the magnetic lines of force on the surface and near surface of the workpieces are locally distorted, resulting in magnetic leakage. The magnetic particles applied on the surface of the workpieces are adsorbed, forming magnetic marks that can be seen visually under appropriate light, thus showing the position, shape and size of discontinuity.

The applicability and limitations of magnetic particle testing include:

1. Magnetic particle flaw detection is applicable to the detection of discontinuities on the surface and near surface of ferromagnetic materials that are small in size, extremely narrow in gap and difficult to be seen visually.

2. Magnetic particle testing can be used to test parts and components under various conditions, as well as various types of parts.

3. Cracks, inclusions, hair lines, white spots, folds, cold shuts, looseness and other defects 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 of less than 20 ° with the workpiece surface.

Liquid penetrant testing

The basic principle of liquid penetrant testing is that after the part surface is coated with fluorescent or colored dyes, the penetrant can penetrate into the surface opening defects under the capillary action for a period of time; After the excess penetrant on the surface of the part is removed, the developer is applied on the surface of the part.

Similarly, under the action of capillary, the developer will attract the penetrant retained in the defect, and the penetrant will permeate back into the developer. Under a certain light source (ultraviolet light or white light), the penetrant trace at the defect will be displayed, (yellow green fluorescence or bright red), so as to detect the morphology and distribution status of the defect.

The advantages of penetrant testing include:

1. Can detect various materials;

2. With high sensitivity;

3. The display is intuitive, the operation is convenient, and the detection cost is low.

The disadvantages of penetrant testing include:1. It is not suitable for inspecting workpieces made of porous loose materials and workpieces with rough surfaces;

2. Penetrant 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 make quantitative evaluation of defects. The detection result is also greatly influenced by the operator.

X-ray inspection

The last one, radiographic testing, is because X-rays will be lost after passing through the irradiated object, and different substances with different thicknesses will have different absorption rates. When the negative film is placed on the other side of the irradiated object, it will produce corresponding patterns due to different radiation intensities, so that the film reviewer can judge whether there are defects inside the object and the nature of the defects according to the image.

Applicability and limitations of radiographic testing:

1. It is sensitive to detect volumetric defects and easy to identify defects.

2. X-ray films are easy to retain and traceable.

3. Visually display the shape and type of defects.

4. The disadvantage is that the buried depth of defects cannot be located, and the detection thickness is limited. The negative film needs to be specially washed, which is harmful to human body, and the cost is high.

In a word, ultrasonic and X-ray flaw detection are suitable for detecting internal defects; Among them, ultrasonic wave is applicable to parts with regular shape over 5mm. X-ray can not locate the buried depth of defects, and there is radiation. Magnetic particle and penetrant flaw detection are applicable to detect surface defects of components; Among them, magnetic particle inspection is only limited to detecting magnetic materials, and penetrant inspection is only limited to detecting surface opening defects.

Source: welding technology exchange station

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