What are the types of NDT? Complete knowledge of NDT

Date：2022-05-25 14:48:49Views：1028

Overview of nondestructive testing: NDT (non destructive testing) is the general name of all technical means to use acoustic, optical, magnetic and electrical characteristics to detect whether there are defects or non-uniformity in the tested object without damaging or affecting the service performance of the tested object, give information such as the size, location, nature and quantity of defects, and then determine the technical state of the tested object (such as qualified or not, remaining life, etc.).

Common nondestructive testing methods

Radiographic testing (RT), ultrasonic testing (UT), magnetic particle testing (MT) and liquid penetrant testing (PT). Other NDT methods: eddy current testing (ET), acoustic emission testing (at), thermal imaging / infrared (TIR), leakage test (LT), AC field measurement technology (acfmt), magnetic flux leakage testing (MFL), far-field testing (RFT), etc.

Application characteristics of nondestructive testing

1. The biggest feature of nondestructive testing without damaging the material and structure of the test piece is that it can be tested without damaging the material and structure of the test piece. Therefore, after the implementation of nondestructive testing, the inspection rate of the product can reach 100%. However, not all items and indicators that need to be tested can be subject to nondestructive testing, and nondestructive testing technology also has its own limitations. Some tests can only use destructive tests. Therefore, nondestructive testing can not replace destructive testing at present. In other words, for the evaluation of a workpiece, material, machinery and equipment, the results of nondestructive testing and destructive test must be compared and matched with each other in order to make an accurate evaluation.

2. Correctly select the time for nondestructive testing

In NDT, the time of NDT implementation must be correctly selected according to the purpose of NDT.

3. Correctly select the most appropriate nondestructive testing method. Since various testing methods have certain characteristics, in order to improve the reliability of testing results, appropriate nondestructive testing methods shall be selected according to the type, shape, position and orientation of possible defects according to the equipment material, manufacturing method, working medium, service conditions and failure mode.

4. Comprehensive application of various nondestructive testing methods. Any nondestructive testing method is not omnipotent. Each method has its own advantages and disadvantages. Several detection methods should be used as much as possible to learn from each other to ensure the safe operation of pressure equipment. In addition, in the application of nondestructive testing, we should also fully realize that the purpose of testing is not to unilaterally pursue the "high quality" with excessive requirements, but to focus on its economy on the premise of fully ensuring safety and appropriate risk rate. Only in this way can the application of nondestructive testing in pressure equipment achieve the expected purpose.

Common nondestructive testing methods

There are many nondestructive testing methods. According to the research and analysis of NASA, it can be divided into six categories and about 70 kinds. However, there are five common nondestructive testing methods in practical application, that is, what we call conventional nondestructive testing methods:

Conventional nondestructive testing methods

Visual testing (VT for short);

Ultrasonic testing (UT);

Radiographic testing (RT);

Magnetic particle testing (MT for short);

Penetrant testing (PT);

Eddy current testing (ET);

Acoustic emission (AE).

1. Visual inspection (VT)Visual inspection is the primary method in the first stage of nondestructive testing, which is less implemented in China but attaches great importance in the world. According to international practice, visual inspection shall be conducted first to confirm that it will not affect the subsequent inspection, and then four routine inspections shall be conducted. For example, for the PCN certification of bind, there are special VT1, 2 and 3 levels of assessment and special certificate requirements. After international training, its VT detection technology will be more professional and highly valued by international institutions.

VT is often used for visual inspection of welds. The welds themselves have process evaluation standards, which can be preliminarily inspected by visual inspection and direct measurement of dimensions. If unqualified appearance defects such as undercut are found, they must be polished or repaired first, and then other in-depth instrument inspection can be carried out. For example, there are many surfaces of weldments and castings, and more VT is done, while there are few forgings, and their inspection standards are basically consistent.

2. Ultrasonic testing (UT)

(1) Definition of ultrasonic testing: through the interaction between ultrasonic and test piece, study the reflected, transmitted and scattered waves, monitor the macro defects, measure the geometric characteristics, detect and characterize the changes of microstructure and mechanical properties, and then evaluate its specific application.

(2) Working principle of ultrasonic wave: it is mainly based on the propagation characteristics of ultrasonic wave in the test piece.

a. The ultrasonic wave is generated by the sound source, and a certain way is adopted to make the ultrasonic wave enter the test piece;

b. Ultrasonic wave propagates in the test piece and interacts with the test piece material and defects therein, so that its propagation direction or characteristics are changed;

c. The changed ultrasonic wave is received through the detection equipment and can be processed and analyzed;

d. According to the characteristics of the received ultrasonic wave, evaluate whether the test piece itself and its interior have defects and the characteristics of defects.

(3) Advantages of ultrasonic testing:

a. It is 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. For example, for metal materials, thin-walled pipes and plates with a thickness of 1 ~ 2mm can be detected, and steel forgings with a length of several meters can also be detected;

c. Defect location is accurate;

d. The detection rate of area defects is high; e. It has high sensitivity and can detect defects with small internal size;

f. The detection has the advantages of low cost, fast speed, light equipment, no harm to human body and environment, and convenient on-site use.

(4) Limitations of ultrasonic testing:

a. The precise qualitative and quantitative analysis of the defects in the specimen still needs to be deeply studied;

b. It is difficult to carry out ultrasonic testing on specimens with complex or irregular shapes;

c. The location, orientation and shape of defects have a certain influence on the test results;

d. Material and grain size have great influence on the detection;

e. When the commonly used manual A-type pulse reflection method is used for detection, the result display is not intuitive, and there is no direct witness record for the detection result.

(5) Application scope of ultrasonic testing:

a. From the material of the test object, it can be used for metal, non-metal and composite materials;

b. From the manufacturing process of the test object, it can be used for forgings, castings, weldments, cemented parts, etc;

c. In terms of the shape of the detection object, it can be used for plate, bar, pipe, etc;

d. In terms of the size of the test object, the thickness can be as small as 1mm or as large as several meters;

e. From the defect position, it can be either surface defect or internal defect.

3. Radiography (RT)

It refers to the nondestructive testing method that uses X-ray or g-ray to penetrate the test piece and film as the equipment for recording information. This method is the most basic and widely used non-destructive testing method.

(1) Principle of radiographic inspection method: the ray can penetrate the material that cannot be penetrated by the naked eye to make the film sensitive. When X-ray or r-ray irradiates the film, the silver halide in the film emulsion layer can produce a latent image, just like ordinary light. Due to the different absorption coefficients of substances with different densities, the ray energy irradiating all parts of the film will also be different, The defect can be identified according to the blackness difference of the negative processed in the darkroom.

(2) Characteristics of radiographic method: the advantages and limitations of radiographic method are summarized as follows:

a. The visual image of defects can be obtained, which is qualitative and accurate, and the quantification of length, width and size is also accurate;

b. The test results are directly recorded and can be kept for a long time;

c. The detection rate of volumetric defects (porosity, slag inclusion, tungsten inclusion, burn through, undercut, weld bead, pit, etc.) is very high. For area defects (incomplete penetration, incomplete fusion, crack, etc.), if the camera angle is not appropriate, it is easy to miss the inspection;

d. It is suitable to inspect thin workpieces rather than thick workpieces, because the inspection of thick workpieces requires high-energy ray equipment, and its inspection sensitivity will decrease with the increase of thickness;

e. It is suitable to inspect butt welds, but not fillet welds, plates, bars, forgings, etc;

f. It is difficult to determine the position and size (height) of the defect in the thickness direction of the workpiece;

g. High detection cost and slow speed;

h. It has radiation biological effects, can kill biological cells, damage biological tissues and endanger the normal function of biological organs.

In general, RT is characterized by more accurate characterization, visual images for long-term preservation, relatively high overall cost, and radiation is harmful to human body, so the inspection speed will be slow.

Nondestructive testing X-ray machine

Industrial inspection X-ray machines used in the industrial sector are usually industrial nondestructive testing X-ray machines (loss free testing). Such portable X-ray machines can detect various industrial components, electronic components and circuits. For example, the detection of socket plug, rubber internal line connection, diode internal welding, etc. Industrial inspection X-ray machines such as bji-xz and bji-uc are X-ray machines that can be connected to computers for image processing. Such industrial inspection portable X-ray machines provide excellent solutions for the field of factory home appliance maintenance.

4. Magnetic particle testing (MT)

(1). Principle of magnetic particle testing: after ferromagnetic materials and workpieces are magnetized, due to the existence of discontinuity, the magnetic field lines on the surface and near the surface of the workpieces are locally distorted, resulting in magnetic leakage field. The magnetic particles applied on the surface of the workpieces are adsorbed to form visually visible magnetic marks under appropriate light, so as to show

Location, shape and size of discontinuities.

(2). Applicability and limitations of magnetic particle testing:

a. Magnetic particle flaw detection is applicable to detect the discontinuities on the surface and near surface of ferromagnetic materials with very small size and very narrow gap (for example, 0.1mm long and micron wide cracks can be detected), which are difficult to be seen by visual inspection.

b. Magnetic particle testing can detect raw materials, semi-finished products, finished products and parts in service, as well as plates, profiles, pipes, bars, weldments, steel castings and forged steel parts.

c. Cracks, inclusions, hair lines, white spots, folds, cold shuts, looseness and other defects can be found.

d. 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, deeply buried holes and delaminations and folds with an included angle of less than 20 ° with the workpiece surface.

5. Penetrant testing (PT)

(1). Basic principle of liquid penetrant testing: after the part surface is coated with penetrant containing fluorescent dye or colored dye, under the action of capillary, the penetrant can penetrate into the surface opening defect for a period of time; After the excess penetrant on the part surface is removed, the developer is applied on the part surface. 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 realized (yellow green fluorescence or bright red), so as to detect the morphology and distribution of the defect.

(2). Advantages of penetrant testing:

a. It can detect all kinds of materials, metal and non-metallic materials; Magnetic and non-magnetic materials; Welding, forging, rolling and other processing methods;

b. High sensitivity (0.1 can be found) μ M wide defect)

c. It has the advantages of intuitive display, convenient operation and low detection cost.

(3). Disadvantages and limitations of penetrant testing:

a. It can only detect the defects of surface openings;

b. It is not suitable for checking workpieces made of porous and loose materials and workpieces with rough surface;

c. 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 results are also greatly affected by the operator.

6. Eddy current testing (ET)

(1). Basic principle of eddy current testing: place the coil with alternating current on the metal plate to be tested or sleeve it outside the metal pipe to be tested (see Figure). At this time, an alternating magnetic field will be generated in and near the coil, resulting in a vortex like induced alternating current in the test piece, which is called eddy current. The distribution and size of eddy current depend not only on the shape and size of the coil, the size and frequency of AC current, but also on the conductivity, permeability, shape and size of the test piece, the distance from the coil and whether there are cracks on the surface. Therefore, under the condition of keeping other factors relatively unchanged, measuring the change of magnetic field caused by eddy current with a detection coil can deduce the size and phase change of eddy current in the test piece, and then obtain the information about the change of conductivity, defect, material condition and other physical quantities (such as shape, size, etc.) or the existence of defects. However, because eddy current is alternating current and has skin effect, the detected information can only reflect the situation on the surface or near the surface of the specimen.

(2). Application: different types of coils can be used according to the shape of the test piece and the purpose of testing. There are usually three types of coils: through coil, probe coil and plug-in coil. The through coil is used to detect pipes, bars and wires. Its inner diameter is slightly larger than the inspected object. When in use, the inspected object passes through the coil at a certain speed, and defects such as cracks, inclusions and pits can be found. The probe coil is suitable for local detection of the test piece. When applied, the coil is placed on the metal plate, pipe or other parts to check the fatigue cracks on the inner cylinder of aircraft landing brace and turbine engine blades. Plug in coil, also known as internal probe, is placed in the hole of pipe or part for inner wall detection. It can be used to check the corrosion degree of inner wall of various pipes. In order to improve the detection sensitivity, most of the probe and plug-in coils are equipped with magnetic cores. Eddy current method is mainly used for the rapid detection of metal pipes, rods and lines on the production line, as well as the flaw detection of large quantities of parts such as bearing steel balls and steam valves (at this time, in addition to eddy current instruments, automatic loading, unloading and transmission mechanical devices must be equipped), material sorting and hardness measurement. It can also be used to measure the thickness of coating and film.

(3) Advantages and disadvantages: during eddy current testing, the coil does not need to be in direct contact with the tested object, and can carry out high-speed testing, which is easy to realize automation, but it is not suitable for parts with complex shape, and can only detect the surface and near surface defects of conductive materials, and the test results are easy to be disturbed by the material itself and other factors.

(7) Acoustic emission AE is a new nondestructive testing method, which is tested by the sound emitted by crack expansion inside the material. It is mainly used to detect the defects of equipment and devices in use, that is, the development of defects, so as to judge their good performance.

Unconventional nondestructive testing methods

Acoustic emission (AE);

Eddy current testing (ET)

Leak testing (LT);

Time of flight diffraction (TOFD);

Guided wave testing;There are many, many

This article is introduced here. I hope it will be helpful to you! Nondestructive testing is an indispensable and effective tool for industrial development. To a certain extent, it reflects the industrial development level of a country, and its importance has been recognized.