Five nondestructive testing methods commonly used in industry professional nondestructive testing organization

Date：2022-06-17 15:45:15Views：755

1. Ultrasonic testing technology

In the elastic medium (such as solid, liquid and gas), the longitudinal wave excited by the wave source whose frequency is 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 the internal and surface defects of materials. It can also be used for thickness measurement and other purposes.

The power oscillation excites high-frequency sound wave, which is reflected, scattered and attenuated when it is incident on the component. The ultrasonic wave is received and converted into an electrical signal by the probe, and then amplified for display. The location, 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 can be divided into longitudinal wave, transverse wave, surface wave and plate wave. Longitudinal wave is widely used in ultrasonic testing because it is easy to generate and receive. 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, which can be used to detect surface defects with complex shapes. The plate wave can detect the thin plate.

Ultrasonic flaw detection system is composed 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 metal to be tested and then receiving the echoes reflected from the defects. It is also divided into vertical flaw detection method and oblique angle flaw detection method. Vertical flaw detection method is mainly used to detect castings, forgings, plates and composite materials. The 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 discontinuity defects in components, provide three-dimensional position information of discontinuities, and provide data that can be used to evaluate defects. For example, inspect the weld defects, transmission shaft, high-strength bolt and material interlayer defects.

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 The operators are required to have a high level of knowledge and professional skills.

(2) Ultrasonic thickness measurement technology

The ultrasonic wave is used to detect the thickness of the material, and the inspection speed is fast. The thickness can be directly displayed by digital ultrasonic thickness gauge.

High temperature piezoelectric thickness gauge and high temperature couplant shall be used under high temperature. The high temperature thickness gauge shall be used within the indicated 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 commonly used rays, namely γ X-rays and X-rays.

X-ray -- when a high-speed electron stream strikes 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, traveling at the speed of light.

The ray has a very strong penetration ability. It irradiates from one side of the material. The ray penetrates the material to make the film on the other side sensitive to light, showing 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, etc. of castings, and discontinuity defects of welds.

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 two radiographic inspection techniques are compared as follows:

X-ray detection technology - the instrument is large in size, not easy to carry, with high penetration. It is used for thicker materials (steel members 120mm), without attenuation, and can adjust the intensity of the ray source. It is harmful to human body and requires 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, and the intensity of the ray source is not adjustable. It is very harmful to human body and does not need 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 the oil, rust and paint on the metal surface and wait for drying.

② Apply penetrant: after about 5 minutes, remove the penetrant from 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 applicable to detecting surface defects of various materials and components of various shapes. The device is simple, easy to carry, easy to operate, intuitive detection effect, low cost, and is used for surface open defects. Only make qualitative judgment on defects, and make rough estimation on 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, location 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, shape and possible types of defects of the components, and operate according to the regulations.

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

④ Inspection: if non fluorescent magnetic particles are used, natural light shall be used to observe the aggregation state of magnetic particles and determine the location and size of defects. If fluorescent magnetic particles are used, ultraviolet radiation shall be used for inspection in a dark room.

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

5. Eddy current testing technology

The basic principle of eddy current testing is to use electromagnetic induction to detect the defects of conductive materials. The eddy current detection probe or coil uses alternating current, and its alternating magnetic field induces eddy current in the tested parts. The defects of the parts cause changes in the intensity and distribution of eddy current, which are displayed on the cathode ray tube or instrument, and the defects are determined according to the changes of the tested eddy current.

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