What are the methods of nondestructive testing technology? Electronic product appraisal organization

Date：2022-04-21 13:46:53Views：1422

Nondestructive testing is a method to inspect and test the structure, properties and state of the internal and surface of the test piece by means of physical or chemical methods and with the help of advanced technology and equipment without damaging the test piece. In order to help you have an in-depth understanding, the following contents are sorted out by the core testing network, and the NDT Technical methods are summarized as follows:

(1) Visual testing (VT for short);

(2) Remote ultrasonic testing (lrut);

(3) Radiographic testing (RT);

(4) Magnetic particle testing (MT for short);

(5) Penetrant testing (PT);

(6) Eddy current testing (ECT);

(7) Acoustic emission (AET).

(8) Magnetic flux leakage test (MFL)

(9) Laser detection (LM)

(10) Thermal / infrared thermal imaging test (IRT)

(11) Vibration analysis (VA)

(12) Ultrasonic testing (UT)

(13) Leak detection (LT)

1、 Visual inspection (VT)

The earliest type of nondestructive testing is visual testing. It uses low-power equipment (including pipe mirror and fiber mirror) to monitor defects. Fast, inexpensive and direct visual testing can be used as an initial tool to identify asset and infrastructure problems, from cracks to corrosion. However, when trying to identify many different types of material failures as early as possible to safely repair or replace equipment, visual testing is not enough. When the sight is blocked or the defect is small or internal, the vision examination will fail. In fact, various shortcomings of visual inspection lead to the necessity of other forms of nondestructive testing.

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、 Remote ultrasonic testing (lrut)

Remote UT is an ultrasonic testing method specially used for pipelines. The ultrasonic transducer or coil is built into a ring traveling along the pipe. The transducer emits waves to provide an image of the interior of the tube wall. Irregularities and thickness changes can change the waves and show themselves to technicians. This method does not require a liquid coupling agent between the transducer and the surface.

3、 Radiographic testing (RT)

Radiographic testing has entered the public imagination through X-ray machines. The method uses radiation to penetrate objects and recording media. A darker area on the recording medium indicates that more radiation passes through the area of the object, indicating cracks, voids or density changes. X-rays are usually used for thinner materials. Gamma rays are thicker. Film or computer sensors can be used as recording media. Radiographic testing requires a lot of equipment and expertise, as well as safety precautions to prevent excessive exposure to radiation.

Neutron radiography tests use concentrated neutron rays instead of X-rays or X-rays γ Rays penetrate objects. Linear or electron accelerators must be used to generate these neutron beams. Neutrons pass through metals, but not through most organic materials. When used in conjunction with standard radiography, this will provide a more detailed image of the interior of the object. This technology is only used in a laboratory environment.

4、 Magnetic particle testing (MT)

Magnetic particle test uses the movement of indicator particles to prove the discontinuity inside ferromagnetic materials. The tested parts must be coated with dyed magnetic particles in the form of dry powder or liquid suspension. The magnet induces the electromagnetic field into the material to be tested. The magnetic field makes the magnetic particles move to any discontinuous point transverse to the direction of the magnetic field, so as to intuitively show the defects.

Magnetic particle testing is a wide range of subjects, which can use a variety of methods to induce magnetic field. Magnetic particle testing requires a lot of setup and cleaning work, so it cannot be easily used on site.

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 workpieces are locally distorted, resulting in magnetic leakage field. The magnetic particles applied on the surface of workpieces are adsorbed to form visually visible magnetic marks under appropriate light, so as to show the position, shape and size of discontinuity of B310 magnetic particle flaw detector.

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)

Liquid penetrant testing can visually show cracks or other defects connected to the surface of the material. Liquid penetrants are mainly used for nonporous materials because porous materials mask signs of defects. This test method applies or immerses the material in the indicator solution. The fluid flows into an opening on the surface of the material. When the liquid remaining on the surface is removed, the liquid returns from the crack. Any place where the liquid reappears shows defects; The more liquid, the bigger the defect.

If there is no channel connecting the defect to the surface, the liquid will not enter. Therefore, other methods must be used to detect closed voids or honeycomb structures. The surface of the material must also be clean because oil and other residues do not interfere with the ability of the liquid to enter the crack. In addition, liquid penetrant requires a lot of equipment, setting and cleaning to deal with the liquid itself. Although this technique can be used effectively, it is usually slower and more troublesome than other NDT methods.

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 defects after 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 (ECT)

Eddy current testing uses a magnetic field to form an image of a conductive material. Changes in material properties can produce discontinuities in the field, similar to the way rocks create vortices in streams. These changes provide signs of corrosion, cracks, voids, honeycomb, delamination and thickness loss.

Eddy current technology is regularly used in the industry because of its portability, speed and accuracy. One of the most critical uses of eddy current testing is in the power generation industry. Eddy current technology has proven to be effective and economical for inspecting heat exchanger and cooler tubes. Hand held eddy current equipment allows local inspection, thus reducing the downtime required to perform the inspection.

The latest innovation of eddy current testing is eddy current array (ECA) technology, which is very suitable for surface and near surface mapping in many industries such as aerospace, railway, manufacturing, oil and natural gas. ECA is an extremely fast, cost-effective and easy-to-use technology that provides highly accurate results.

Although eddy current technology can penetrate thin non-conductive coatings, such as zinc on galvanized steel, its use is limited to conductive materials. In addition, eddy currents may be difficult in the case of complex geometry or large area. Although these limits the range of eddy current equipment, it is still an efficient tool within its parameter range.

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 tube to be tested. 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. In addition to the size of the test piece, the shape of the coil and the distribution of the eddy current, it also depends on the size and the frequency of the eddy current and the size of the coil. 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: according to the shape of the test piece and the purpose of testing, different forms of coils can be used, usually including 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 be tested at high speed, 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 AET

Acoustic emission testing relies on a similar principle of ultrasonic testing, that is, the transmission of sound waves through solid objects. However, wave propagation and measurement are done in different ways. Waves are induced by a sharp force applied to an object, such as the impact of a hammer or other mechanical load. Changes in temperature and pressure will also cause appropriate fluctuations.

Acoustic emission test is not to listen to the changes of wave characteristics and the mapping of these characteristics, but to detect the physical movement of the medium itself. Changes or inconsistencies in the material of the object (e.g. voids) can be detected by motion differences detected by separate sensors. Although acoustic emission testing is effective for plastics and other materials, it is less common and equipment intensive than other nondestructive testing methods. This technology is most often found in laboratory environments.

This is a new nondestructive testing method, which is tested by the sound of crack expansion in 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.

8、 Magnetic flux leakage test (MFL)

Flux leakage is an effective field testing technology, which is mainly used to inspect large pipelines, pipelines and tank bottoms. Powerful magnets are used to fill materials with magnetic fields. The sensor detects magnetic field fluctuations caused by differences in material characteristics, such as corrosion, pitting, thickness loss or cracks. Using a magnet and a sensor moving along the length of the cylinder, the pipe can be scanned without removing the insulation. The tank floor must be scanned with a field generator arranged in series. This technology is suitable for ferrous materials and is an effective method to detect defects in large infrastructure.

9、 Laser detection (LM)

Three types of laser based NDT are dominant - profilometry, shear imaging and holographic testing. Profilometry uses a rotating laser to image the outer surface of the pipe to detect cracks, corrosion or pitting.

Shear imaging is a high-precision "front and back" method to detect material defects. The laser records images of the material before and after the application of stress, and infers the internal structure using the detected differences.

Holography uses a similar "before and after" method to infer micron scale defects. The two technologies differ in the equipment and software used to generate results. For larger surfaces, the shear method is preferred. Small hologram.

10、 Thermal / infrared thermal imaging test (IRT)

The thermal test uses captured infrared radiation emitted from the object to provide an image of the object surface. Thermal imaging can indicate corrosion, voids, foreign matter, or delamination. In order to make the infrared thermal imager have direct line of sight, the area to be scanned must be covered. Although the thermal test can be effective, the defects it detects can also be corrected by other methods, which require a much smaller setting range.

11、 Vibration analysis (VA)

Vibration analysis is good at testing the integrity of rotating parts, including turbines, gears, shafts and bearings. Three types of vibration analysis are usually used: accelerometer, velocity sensor and eddy current displacement sensor.

Accelerometers are sensitive to high speed, so they are most effective for high-speed applications. The speed sensor uses a magnet to generate an electric field from rotating parts, which can effectively measure parts moving at slow or medium speed.

Eddy current displacement sensors measure the physical movement of rotating parts on unwanted horizontal or vertical axes. They can detect changes in clearance or shaft movement, indicating the need for repair.

12、 Ultrasonic testing (UT)

Ultrasonic testing has been proved to be one of the most effective methods in modern nondestructive testing. This method works by introducing high-frequency sound waves into solid objects, usually metals or composites. The propagation of sound waves is affected by irregular factors, such as density changes, cracks, voids, honeycombs or foreign objects. By collecting and interpreting the returned sound waves, ultrasonic testing equipment can draw the internal diagrams of many solid objects. According to the requirements of the equipment and application used, waves reflected back or passing through the scanned material can be collected.

Ultrasonic testing relies on transducers to convert electrical energy into ultrasonic waves. Older methods use only one transducer at a time, while modern phased array ultrasonic testing (paut) equipment uses multiple transducers running in series. This technique greatly improves the inspection speed, coverage and specificity.

Recently, advanced paut instruments have added higher performance, including time of flight diffraction (TOFD) and total focusing method (TFM). These newer technologies are well suited to handle more complex inspections.

Because of its various advantages, ultrasonic equipment is widely used in the volume test of the whole industry. Paut provides fast and accurate readings with almost no setting required. The equipment itself can be light and convenient for on-site operation, but the strength is enough to cope with the harsh environment. The range of testing applications of ultrasonic covers makes the technology attractive to large organizations because it simplifies the procurement and training of company equipment.

Like all NDT methods, ultrasonic testing is not perfect for every application. Coarse grained materials such as iron will interfere with the propagation of waves. Strange geometries (including surfaces) sometimes make coverage difficult without defined advanced technologies or complete solutions. In addition, the probe quality will significantly affect the penetration depth and image quality.

13、 Leak detection (LT)

Leak testing is a non-destructive test that involves several methods of determining whether there is a leak in a sealed container. There are four common methods for detecting gas leaks, although they are somewhat similar. The pressure change test will pressurize or create a vacuum in a sealed container. Loss of pressure or vacuum indicates a leak. Bubble testing also relies on pressure indicators. Pressurize the part and immerse it in liquid. The presence of bubbles indicates the location of the leak.

Similar to the halogen diode and mass spectrometer tests, both use identification gas to detect the presence of leakage. Introduce halogen or helium (usually mixed with air) into a pressurized container. A halogen diode detector or mass spectrometer located outside the pressurized area will alert the technician to the presence of halogen or helium, indicating a leak.

Some bubble tests can be performed in the field using special equipment to create a sealing area on a large and / or flat surface. However, bubble testing and other leak testing methods are time-consuming and require cumbersome equipment and settings. They are best carried out in a laboratory environment.