What is nondestructive testing?
Nondestructive testing is the general term of all technical means that use the characteristics of sound, light, magnetism and electricity to detect whether there are defects or nonuniformity in the tested object, give the information of the size, location, nature and quantity of defects, and then determine the technical state of the tested object (such as qualified or not, residual life, etc.) without damaging or affecting the service performance of the tested object.
4 Nondestructive testing methods
Common nondestructive testing methods: ultrasonic testing (UT), magnetic particle testing (MT), liquid penetrant testing (PT) and X-ray testing (RT).
UT (ultrasonic testing) is one of the nondestructive testing methods in industry.
When an ultrasonic wave enters an object and encounters a defect, part of the sound wave will be reflected.
The transmitter and receiver can analyze the reflected wave, so that the defect can be measured extremely accurately, and the location and size of the internal defect can be displayed, and the thickness of the material can be measured.
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 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 portable, safe to operate, and easy to realize automatic inspection.
It is not easy to check the workpiece with complex shape, and the surface to be inspected is required to have a certain degree of finish, and coupling agent is required to fill the gap between the probe and the surface to be inspected, so as to ensure full acoustic coupling.
Magnetic particle inspection
First, let’s understand the principle of magnetic particle testing.
After ferromagnetic materials and workpieces are magnetized, due to the existence of discontinuity, the magnetic lines of force on the surface and near the surface of the workpieces are locally distorted, resulting in magnetic leakage field, which adsorbs the magnetic particles applied on the surface of the workpieces, forming magnetic marks that are visually visible under appropriate light, thus showing the position, shape and size of discontinuity.
The applicability and limitations of magnetic particle testing are:
1. Magnetic particle flaw detection is applicable to detect the discontinuities on the surface and near surface of ferromagnetic materials with small size and extremely narrow gap, which are difficult to be seen visually.
2. Magnetic particle testing can be used to detect parts and components under various conditions, as well as various types of parts.
3. Cracks, inclusions, hairlines, 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 holes buried 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 action of capillary for a period of time;
After removing the excess penetrant on the surface of the parts, the developer is applied on the surface of the parts.
Similarly, under the action of capillary, the developer will attract the penetrant retained in the defect, and the penetrant will seep back into the developer.
Under a certain light source (ultraviolet light or white light), the trace of penetrant at the defect will be displayed (yellow green fluorescence or bright red), so as to detect the morphology and distribution of defects.
The advantages of penetrant testing are:
1. It can detect all kinds of materials;
2. High sensitivity;
3. The display is intuitive, the operation is convenient, and the detection cost is low.
The disadvantages of penetrant testing are:
1. It is not suitable for checking workpieces made of porous and 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 a quantitative evaluation of defects.
The detection results are also greatly affected by the operator.
Finally, radiographic testing is because X-rays will be lost after passing through the irradiated object.
The absorption rates of substances with different thicknesses are different.
When the negative is placed on the other side of the irradiated object, corresponding graphics will be generated due to different ray intensities.
The film evaluator can judge whether there are defects inside the object and the nature of the defects according to the images.
Applicability and limitations of radiographic testing:
1. It is sensitive to detect volumetric defects, and it is easier to characterize defects.
2. Radiographs are easy to retain and traceable.
3. Visually display the shape and type of defects.
4. Disadvantages: it cannot locate the buried depth of defects, and the detection thickness is limited.
Negatives need to be specially sent for washing, which is harmful to human body, and the cost is high.
In a word, ultrasonic and X-ray flaw detection are applicable to detect internal defects;
Among them, ultrasonic wave is applicable to parts with regular shape above 5mm.
X-ray cannot locate the buried depth of defects, and there is radiation.
Magnetic particle and penetrant testing are applicable to the surface defects of tested parts;
Among them, magnetic particle inspection is limited to the detection of magnetic materials, and penetrant inspection is limited to the detection of surface opening defects.