How to select NDT methods? Comparison of four NDT methods

Date：2022-10-26 16:11:44 Views：1219

Non destructive testing refers to a test method to inspect the surface and internal quality of the inspected parts without damaging the working conditions of the workpiece or raw materials. It is a test method to inspect the surface and internal quality of components. It is the general term of all technical means to detect whether there are defects or nonuniformities in the inspected object by using acoustic, optical, magnetic, electrical and other characteristics without damaging or affecting the service performance of the inspected object, and to give the size, location, nature, quantity and other information of defects, so as to determine the technical state (such as whether the inspected object is qualified, remaining life, etc.) of the inspected object.

Common nondestructive testing methods: ultrasonic testing (UT), magnetic particle testing (MT), liquid penetrant testing (PT) and X-ray testing (RT).

Ultrasonic testing method

Ultrasonic testing, referred to as UT for short, is the most widely used, most frequently used and rapidly developed nondestructive testing technology in industrial nondestructive testing. It can be used in many aspects such as quality control, raw material inspection, and process improvement in product manufacturing. It is also an indispensable means in equipment maintenance.

Magnetic particle testing

Now let's understand the principle of magnetic particle testing.After the ferromagnetic materials and workpieces are magnetized, due to the existence of discontinuities, the magnetic lines of force on the surface and near the surface of the workpieces are partially distorted, resulting in a magnetic leakage field. The magnetic particles applied on the surface of the workpieces are adsorbed, forming magnetic marks visible under appropriate light, thus showing the position, shape and size of the discontinuities.

Applicability and limitations of magnetic particle testing include:

1. Magnetic particle flaw detection is applicable to detecting the discontinuities on the surface and near surface of ferromagnetic materials that are very small, and the gap is very narrow.

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

3. Defects such as cracks, inclusions, hairlines, white spots, folds, cold shuts 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 copper, aluminum, magnesium, titanium and other non-magnetic materials. 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: after the part surface is coated with fluorescent dye or dye, under the capillary action for a period of time, the penetrant can penetrate into the surface opening defects. After removing the excess penetrant on the part surface, apply the imaging agent on the part surface;

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 trace of penetrant at the defect will be realized (yellow green fluorescence or bright red), so as to detect the morphology and distribution of the defect.

The advantages of penetrant testing include:

1. Various materials can be tested;

2. 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 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 result is also greatly affected by the operator.

X-ray testing

The last one, radiographic testing, is because there will be losses when X-rays pass through the irradiated objects, and different materials with different thicknesses have different absorption rates for them;

When the negative film is placed on the other side of the object to be irradiated, corresponding figures will be generated due to different ray intensities, and the film reviewer can judge whether there are defects and the nature of defects in the object according to the image.

Applicability and limitations of radiographic testing:

1. It is sensitive to detect volumetric defects, and it is easy to determine the nature of defects.

2. Radiographs are easy to retain and traceable.

3. Visually display the shape and type of defects.

4. Disadvantages: the buried depth of the defect 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 summary, ultrasonic and X-ray flaw detection are applicable to detecting internal defects; Among them, ultrasonic wave is applicable to parts with regular shape of more than 5mm. X-ray can not locate the buried depth of defects, so there is radiation. Magnetic particle and penetrant flaw detection are applicable to detecting surface defects of components; Among them, magnetic particle inspection is only used to detect magnetic materials, and penetrant inspection is only used to detect surface opening defects. Our company has a team of professional engineers and industry elites, and has built three standardized laboratories with an area of more than 1800 square meters, which can undertake a variety of test projects such as electronic component test verification, IC authenticity identification, product design material selection, failure analysis, functional testing, factory incoming material inspection and tape weaving.