Main technical methods and functions of nondestructive testing

Date：2021-08-11 17:45:06Views：1360

NDT is the abbreviation of non-destructive testing. It refers to a testing method that does not damage or affect the future performance or use of materials or workpieces. Through the use of NDT, the defects existing in the interior and surface of materials or workpieces can be found, the geometric characteristics and dimensions of workpieces can be measured, and the internal composition, structure, physical properties and state of materials or workpieces can be measured. It is widely used in the detection of metal materials, non-metallic materials, composite materials and their products, as well as some electronic components.

Common NDT technologies include:

1. Radiographic inspection

Using x-rays or γ When the ray penetrates each part of the tested object, the intensity attenuation is different, and the defects of the tested object are detected. If the rays absorbed in different degrees are projected onto the X-ray film, the photos showing the thickness change and internal defects of the object can be obtained after development. If a fluorescent screen is used instead of film, the internal condition of the tested object can be observed directly.

2. Ultrasonic testing

The influence of the acoustic characteristics of the object itself or the defect on the ultrasonic propagation is used to detect the defect or some physical characteristics of the object. The commonly used ultrasonic frequency in ultrasonic testing is 0.5 ~ 5 MHz (MHz). The most commonly used ultrasonic testing is pulse flaw detection.

3. Acoustic emission test

The performance or structural integrity of the material is evaluated by receiving and analyzing the acoustic emission signal of the material. Acoustic emission (AE) is the phenomenon that stress waves are generated due to the rapid release of strain energy caused by crack propagation, plastic deformation or phase transformation in materials. The acoustic emission generated by the material under the action of external factors is received by the acoustic sensor, converted into an electrical signal, amplified and sent to the signal processor, so as to measure various characteristic parameters of the acoustic emission signal.

4. Penetrant flaw detection

Surface defects are detected by the permeability of some liquids to narrow gaps. The commonly used penetrant is the liquid containing colored dye or fluorescence.

5. Magnetic particle flaw detection

Defects on or near the surface of the object are detected by the accumulation of magnetic particles in the leakage magnetic field near the object defect. The detected object must be ferromagnetic. In addition, new nondestructive testing technologies such as neutron radiography, laser holography, ultrasonic holography, infrared testing and microwave testing have also been developed and applied.

6. Eddy current flaw detection

English and abbreviations of common NDT methods:

Ultrasonic testing UT

Magnetic particle testing MT

Computed tomography testing - CT

Visual testing - vt

Radiographic testing RT

Penetrant testing - Pt

Acoustic emission testing - at, AE

Eddy current testing - et

Leak testing - LT

Nondestructive testing has four major functions. The first is to ensure product quality, the second is to ensure the safety of use, the third is to improve the process, and the fourth is to reduce the cost.

First of all, in order to ensure the quality of products, especially the quality of parts, enterprises will detect parts through acoustic, optical, thermal, electromagnetic and other technical means. In the process of testing, parts cannot be damaged in any form, so nondestructive testing equipment will be used. For example, castings, welds and welding all belong to this category.

The second major direction is to ensure safe use. For example, after each flight mission, civil airliners need to carry out nondestructive testing on some parts by various means. After passing the testing, they can carry out the next flight mission. Another example is that the nuclear power plant will stop running every time, and the operation state of the whole equipment will be detected to prevent some defects. These defects are a long-term process. They can be safely used only if they are tracked and maintained through nondestructive testing.

Most non-destructive testing to ensure safe use is to detect the use of products and equipment in the process of use, and repair them in time.

The third is to improve the process. At present, many companies purchase NDT equipment to improve the production process. For example, in the automobile industry, the previous cars were made of iron or alloy, which would make each car very heavy. Now the automobile lightweight is very popular. Many parts begin to use some light metal materials such as aluminum alloy and magnesium alloy, but it is impossible to directly calculate the deviation from the actual after production, which requires nondestructive testing. The process transformation will go through a long-term experimental process, and then observe the improvement through nondestructive testing, before it can be put into actual production and manufacturing.

The last one is to reduce costs. Some batch processing plants, such as wheel hub manufacturing, will carry out nondestructive testing at the first time after casting, because if the testing is not carried out at this time, and some defects are suddenly found after other steps are almost processed, the wasted man hours, manpower and resources are huge, Therefore, many such enterprises will introduce on-line nondestructive testing equipment for batch testing in order to reduce the rework cost in the later stage.