Summary of common NDT methods for NDT

Date：2022-07-13 14:55:23Views：819

In short, nondestructive testing is to use the characteristics of sound, light, magnetism and electricity inside the inspection machine to detect whether there are defects or nonuniformity in the inspected object without damaging or affecting the service performance of the inspected object, give the size, location, nature and quantity of defects and other information, and then determine the technical state of the inspected object (such as qualified or not, residual life, etc.), This is a kind of testing method that does not damage or affect the future performance or use of materials or workpieces. The following mainly introduces the summary of common NDT methods.

1. PT for penetrant testing

Penetrant testing is mainly applicable to nondestructive testing of surface opening defects. Such as cracks, folds, pores, cold shuts and porosity, it is not limited by the material structure and chemical composition. It can check not only metal materials, but also non porous materials such as plastics, ceramics and glass.

The penetration display is intuitive and easy to judge. The operation method is fast and simple. Defects in any direction can be detected through operation, but it also has certain limitations. It can only detect open surface defects, and it can not effectively detect defects blocked by pollutants or closed openings after mechanical treatment (polishing, grinding, etc.). It is also not suitable for checking workpieces made of porous loose materials and workpieces with rough surfaces, The best observation time of the imaging agent is 8-10 minutes, and the effective retention time is 30-45 minutes. In general, it cannot be used at the same time with magnetic particle testing. The magnetic suspension applied by its magnetic particle will block the opening of the defect. In case of special requirements, penetrant testing can be done first and then magnetic particle testing, but its detection rate will be very low, which has no practical significance.

2. Magnetic particle testing MT

Magnetic particle testing is mainly used to detect the surface and near surface defects of carbon steel, alloy structural steel, precipitation hardening steel and electrical steel. Because discontinuous magnetic traces are accumulated on the surface of the inspected workpiece, it can visually display the shape, position and size of the discontinuity, and roughly determine its nature. The sensitivity of magnetic particle testing is also high, and the width of defects that can be detected can reach 0.1 μ m. Some discontinuities buried several millimeters or even tens of millimeters deep can also be detected.

Magnetic particle testing is almost not limited by the size and shape of the tested parts, and various magnetization technologies are used to inspect the defects of various parts. Its process is relatively simple, fast and low cost. However, it cannot inspect non ferromagnetic metals, such as aluminum, magnesium and copper, nor can it inspect non-metallic materials, such as rubber, plastic, glass, ceramics, etc. It also cannot inspect austenitic stainless steel. It is mainly used for the inspection of hull welds, diesel engine parts, steel forgings and steel castings.

Magnetic particle inspection is only applicable to ferromagnetic materials; Only surface and near surface defects can be detected; Have a strong ability to detect cracks.

3. Ultrasonic flaw detection UT

Ultrasonic flaw detection is widely used in industry, mainly used in forgings, rolled parts, welds, castings, etc. of various sizes, and is suitable for ferrous, non-ferrous and non-metallic materials and parts.

Ultrasonic is suitable for detecting planar defects, such as cracks, folds, interlayers, incomplete penetration, incomplete fusion, etc. As long as the ultrasonic beam is perpendicular to the crack plane, a high defect echo can be obtained. For spherical defects such as pores and slag inclusions, they are not sensitive enough and are lower than rays.

Advantages of ultrasonic testing: A. suitable for nondestructive testing of metal, non-metal and composite materials; b. It has strong penetration ability, and can detect the internal defects of the specimen in a large thickness range. c. The defect location is relatively accurate; d. The detection rate of area type defects is high; e. High sensitivity, which can detect defects with small internal size of the test piece; f. The detection cost is low, the speed is fast, the equipment is portable, harmless to human body and the environment, and it is convenient to use on site.

Ultrasonic testing is mainly used for the detection of internal defects, and has a high detection rate for area defects, such as non fusion, cracks and delamination. However, it is difficult to detect its qualitative, quantitative and complex shape, which requires coupling agents and reference standards, and the surface finish tested is required to be high. It is mainly used for the detection of full penetration welds of ferritic steel with a base metal thickness of 6-100mm on ships.

4. Radiographic inspection RT

X-ray flaw detection is one of the earliest and most common nondestructive testing methods.

Its principle is to identify the defects in the object according to the different attenuation degree of X-ray after penetrating the object, so the images with different blackness are generated on the negative film. The defect image is intuitive and easy to locate, qualitative and quantitative defects. Suitable for metal and non-metal materials.

Compared with ultrasonic testing, both radiographic testing and ultrasonic testing can detect the internal defects of materials or workpieces, and it mainly detects volumetric defects, that is, the workpieces are not deformed by pressure processing after forming, such as castings, welds, powder metallurgy parts, etc., which are widely used in the detection of welds and castings, especially the inspection of welds. Radiography is the most widely used and effective method. It can effectively detect defects such as pores, slag inclusions and looseness, but it is difficult to detect delaminations and cracks. And there should be thickness difference or density difference in the ray direction. It can visually observe the nature, shape, size and position of defects on the negative film, which is convenient for defect positioning, quantitative and qualitative. The negative film can be stored for a long time as a reliable basis for recording the test results. However, its ability to detect planar defects is poor, especially for the most dangerous defect in the workpiece - crack. If the relative angle between the defect orientation and the ray direction is inappropriate, the detection rate will decline significantly, or even cannot be detected at all. In addition, the cost is high and the operation process is complex. Corresponding protective measures must be taken for radiographic testing.

The above is the NDT related content compiled by the core testing team. I hope it will be helpful to you. Shenzhen Chuangxin Online Testing Technology Co., Ltd. is a well-known professional testing institution for electronic components in China, with 3 standardized laboratories covering an area of more than 1000 square meters. The scope of testing services covers: electronic component testing and verification, IC authenticity identification, product design and material selection, failure analysis, function testing, factory incoming material inspection, component X-ray testing, taping and other test items.