This paper explains in detail the five commonly used NDT technologies, and the test institutions summarize and share them

Date：2021-08-26 14:25:58Views：1952

With the research and development of some major nondestructive testing instruments included in the national development special plan, China's nondestructive testing technology has developed on a much higher platform than ever before. Nondestructive testing refers to the method of detecting the internal defects, properties and states of the tested structure by using various physical fields on the premise of not damaging the tested object. This paper collects and arranges some relevant knowledge of nondestructive testing. It is expected that this paper will have great reference value to all readers.

1. Ultrasonic testing technology

In elastic media (such as solid, liquid and gas), the longitudinal wave excited by the wave source with a frequency less than 20Hz is an infrasonic wave, 20 ~ 20000hz is an acoustic wave, and greater than 20000hz is an ultrasonic wave. Because ultrasonic wave can penetrate most materials, it can be used to detect defects inside and on the surface of materials. It can also be used for other purposes such as measuring thickness.

The power oscillation excites the high-frequency sound wave, which is reflected, scattered and attenuated when it encounters a defect after incident on the component. The ultrasonic wave is received and converted into an electrical signal by the probe, and then amplified and displayed. The position, size and nature of the defect are judged according to the wave type, and the hazard degree of the defect is determined by the corresponding judgment standards and specifications.

(1) Ultrasonic flaw detection technology

① Basic principles

Ultrasonic wave is divided into longitudinal wave, transverse wave, surface wave and plate wave. Longitudinal wave is widely used in ultrasonic flaw detection, because it is easy to generate and receive longitudinal wave. Shear wave is mostly used for ultrasonic flaw detection of welds. Surface wave propagates along the metal surface and is very sensitive to surface defects to detect surface defects with complex shapes. The plate wave can detect the thin plate.

Ultrasonic flaw detection system consists of ultrasonic flaw detector and probe. Generally, coupling agent is used. The metal surface in contact with the probe shall be polished to form a smooth and clean surface.

② Ultrasonic flaw detection method

The most widely used method is pulse reflection method. A method of judging defects by transmitting ultrasonic waves into the tested metal and then receiving the echo reflected from the defect. It is also divided into vertical flaw detection method and oblique angle flaw detection method. Vertical flaw detection method is mainly used for the detection of castings, forgings, plates and composite materials. Oblique angle flaw detection method is mainly used to detect internal defects such as welds and pipe fittings.

③ Application characteristics of ultrasonic flaw detection technology

Ultrasonic flaw detection technology is widely used to detect discontinuous defects in components, provide three-dimensional position information of discontinuities, and give data that can be used to evaluate defects. For example, detect the defects of weld, transmission shaft, high-strength bolt and material interlayer.

Its main features are:

① A wide range of material types and thicknesses.

② It can provide the size, depth, location and nature of defects and make accurate judgment.

③ No damage to personnel and materials.

④ Easy to carry, low detection cost, flexible and timely operation.

⑤ Operators are required to have a high level of knowledge and professional skills.

(2) Ultrasonic thickness measurement technology

Using ultrasonic to detect the thickness of materials, the inspection speed is fast. The thickness can be directly displayed by digital ultrasonic thickness gauge.

High temperature piezoelectric thickness gauge shall be used under high temperature, and high temperature coupling agent shall be used. The high temperature thickness gauge shall be used within the indicated service temperature range. It is not suitable for the measurement of coarse grain materials such as stainless steel castings.

2. Radiographic inspection technology

(1) Basic principles

Radiographic inspection technology is a commonly used and important inspection technology to detect the internal defects of materials. There are two types of rays commonly used, namely γ X-rays and X-rays.

X-ray -- when a high-speed electron stream hits some solid surfaces (targets), it produces special rays (electromagnetic wave frequency 3) × 1016-20hz, wavelength 10-6 ~ 10-100cm).

γ Radiation - radioisotopes (such as 60Co) can emit electromagnetic waves with very short wavelength, i.e γ Rays, reaching the speed of light.

The ray has a strong penetration ability. It irradiates from one side of the material. The ray penetrates the material, sensitizes the film on the other side, and shows the detected defects. It can also be converted into visible light to display the detected defects with TV camera. X-ray computed tomography can determine the location and spatial size of defects.

(2) Technology application and characteristics

Radiographic testing is mainly used to inspect shrinkage cavity, air hole, non-metallic slag inclusion, weld discontinuity defects, etc.

It is characterized by intuitive defect detection, long-term preservation of negatives, wide range of applicable materials, low cost, and lower professional ability and experience level of operators than ultrasonic testing.

The comparison of the two radiographic inspection techniques is as follows:

X-ray detection technology - the instrument is large, inconvenient to carry, high penetration, used for thicker materials (steel members 120mm), does not decay, can adjust the intensity of ray source, is harmful to human body, and needs power supply.

γ Radiographic testing technology - the instrument is small in size, easy to carry and has strong penetration. It is used for thick wall materials (up to 300mm for steel members), attenuation, non adjustable intensity of ray source, great harm to human body and no power supply.

3. Penetrant testing technology

Penetrant testing technology is to apply penetrant to the clean surface of the inspected parts. If there are open defects on the surface, the penetrant will penetrate into the defects, remove the excess penetrant on the surface, and then apply developer. The defects will show traces. Observe with natural light or ultraviolet light to judge the type and size of defects.

(1) Basic operation method

① Cleaning: remove oil stains, rust spots and coatings on the metal surface and wait for drying.

② Apply penetrant: after about 5 minutes, remove the penetrant on the surface with water or solvent.

③ Imaging: spray the developer on the metal surface, and the defects will be displayed soon after drying. If fluorescent developer is used, observe the defect under ultraviolet irradiation.

④ Remove the developer from the surface: note that some penetrants may contain chloride and cannot be used for austenitic stainless steel.

(2) Scope of application

Penetrant flaw detection is suitable for detecting surface defects of various materials and components of various shapes. The utility model has the advantages of simple equipment, portability, simple operation, easy learning, intuitive detection effect and low cost. It is used for surface open defects. Only make qualitative judgment on defects and make a rough estimate of the depth of defects based on experience.

4. Magnetic particle testing technology

The basic principle of magnetic particle flaw detection technology is to place ferromagnetic materials (iron, cobalt and nickel) in a strong magnetic field to magnetize them. If there are defects on the surface or near the surface, some magnetic lines of force will overflow to form a magnetic leakage field, which will adsorb the magnetic particles applied on the surface. The defective parts of the magnetic particles show traces of defects, reflecting the orientation, position and size of the defects.

Operation process:

① Pretreatment to remove oil, paint and rust on the metal surface.

② Magnetization: select the magnetization method according to the size and shape of the component and the possible types of defects, and operate according to the regulations.

③ Apply magnetic powder and apply magnetic powder or magnetic suspension to the magnetized component.

④ Inspection: if non fluorescent magnetic particles are used, observe the aggregation state of magnetic particles with natural light to determine the location and size of defects. If fluorescent magnetic powder is used, it shall be inspected by ultraviolet irradiation in a dark room.

⑤ After treatment, demagnetize and remove magnetic particles after inspection.

5. Eddy current flaw detection technology

The basic principle of eddy current testing is to detect the defects of conductive materials by electromagnetic induction. The eddy current detection probe or coil uses alternating current, and its alternating magnetic field induces eddy current of the tested component. The defect of the component causes the change of eddy current intensity and distribution, which is displayed on the cathode ray tube or instrument, and the defect is determined according to the change of test eddy current.

Eddy current flaw detection technology is mainly used for flaw detection of surface and near surface defects of conductors (steel, non-ferrous metals, graphite), inspection of corrosion, deformation, thickness measurement, material delamination, etc. The depth dimension of the defect can be provided. Check the defects of boilers, condensers, furnace tubes, pipelines and other equipment used in power plants, atomic energy, chemical industry and chemical fertilizer industry, such as cracks, corrosion, deformation, etc. Eddy current testing technology is adopted, which has fast detection speed and high accuracy. Quantitative inspection can be carried out, and its thickness error is ± 0.05mm. It can also realize automatic detection and recording, automation and computer data processing. However, it is difficult to use for components with complex shapes.

In China, nondestructive testing technology has been integrated into the national overall economic development goal. The emergence of new materials, new manufacturing technology and new processing methods has posed a challenge to the traditional nondestructive testing technology, while the emergence of new sensor technology, cloud computing and big data has posed a challenge to the traditional nondestructive testing theory itself. 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.