Crack detection and judgment of angle bar
The angle bar in the transmission line tower needs the processing technology of opening angle, closing angle, bending and so on. In the production process, due to improper processing, the angle bar has cracks, and it is detected by ultrasonic according to the standard provisions. When the straight probe is used for detection, the detection direction of the straight probe sound beam is single, which is easy to form missed detection; when the oblique probe is used for detection of the diagonal steel edge, the ultrasonic wave produces the phenomenon of end angle reflection, and it is difficult to judge the existence of cracks. How to detect and judge the crack of angle bar effectively is of great significance to ensure the product quality.
In the steel structure engineering, the crack is a kind of harmful defect, which has the expansibility. The length of the crack needs to be dealt with regardless of its size. Cracks are divided into surface cracks and internal cracks according to their positions. Magnetic particle testing is used for surface cracks and ultrasonic testing is used for internal cracks. How to detect crack effectively. It also needs specific structure and treatment.
First, analyze the structural shape and stress of angle bar
(1) According to the analysis of the structural shape of the angle bar in Figure 1 and the processing drawing of the angle bar in Figure 2, it is difficult to determine whether there is a crack because of the end angle reflection formed when the angle bar is tested by ultrasonic. There are two structural shapes of angle bar, the shape is relatively regular, so it is easy to detect and judge the cracks when ultrasonic testing is used.
(2) According to the stress analysis diagram of the angle bar in Fig. 3, it can be seen that during the processing of the angle bar, the stress at 1 and 2 places is the largest, and the possibility of crack is the largest. Key test judgment shall be made.
Reasonable testing method is the basis of crack detection, so it is necessary to fully understand the principle, advantages and limitations of magnetic particle and ultrasonic testing.
The sound source generates the ultrasonic wave, and the ultrasonic wave enters the workpiece in a certain way. The ultrasonic wave propagates in the workpiece and interacts with the workpiece material and its defects, so that its propagation direction or characteristics are changed. The changed ultrasonic wave is received by the testing equipment, and can be processed and analyzed. According to the characteristics of the received ultrasonic, evaluate whether the workpiece itself and its internal defects and the characteristics of defects.
(2) Advantages of ultrasonic testing
It is suitable for non-destructive testing of metal, non-metal and review materials. With strong penetrability, it can detect internal defects of workpieces within a large thickness range. The defect location is more accurate. The detection rate of area defect is high. With high sensitivity, it can detect the defects with small internal dimension of workpiece.
(3) Limitations of ultrasonic testing
The precise qualitative and quantitative analysis of defects in workpieces still needs to be further studied. It is difficult to carry out ultrasonic inspection for workpieces with complex or irregular shapes. The location, orientation and shape of defects have some influence on the test results. Therefore, the sound wave of angle probe is incident into the workpiece. When the sound wave meets the defect, it is reflected to the detection instrument. The defect is located by the instrument. When the sound wave is perpendicular to the maximum area of the defect, the reflected wave amplitude is the largest. Otherwise, when the sound wave is parallel to the maximum area of the defect, the reflected wave amplitude is the smallest, so it is easy to form a missed inspection.
Magnetic particle testing
After the ferromagnetic material or workpiece is magnetized properly, the magnetic field line at the surface or near surface defects will be partially distorted, and the magnetic pole will be formed on the surface of the workpiece to generate the magnetic leakage field. The magnetic particle applied on the surface of the workpiece will be absorbed, forming the defect magnetic powder image (magnetic trace) that can be seen visually under the appropriate observation conditions. When the magnetic field line is perpendicular to the defect, the leakage magnetic field is the strongest, which is also the most favorable for the defect If it is parallel to the defect, there is no leakage magnetic field and the defect cannot be detected, so it is necessary to conduct magnetic particle testing. The workpiece should be tested in all directions to find out the perpendicular direction between the magnetic field line and the defect, and then judge the defect.
(2) Advantages of magnetic particle testing
It can show the location, shape, size and severity of defects intuitively, and determine the nature of defects by experience. The sensitivity has nothing to do with the size of the workpiece, and the detection sensitivity of linear surface defects is very high. It is almost unlimited by the size and shape of the workpiece. It can not only detect the tiny defects of ferromagnetic materials open to the surface, but also detect the near surface defects without opening to the surface in many cases.
(3) Limitations of magnetic particle testing
Only surface and near surface defects can be detected, and the detectable depth of near surface is generally 1-2mm. Only the length of the defect can be displayed, and the depth of the defect and its height cannot be determined. Wide and shallow defects are difficult to detect. The direction of the magnetization field should intersect the main plane of the defect, and the included angle should be 45 ° – 90 °. Sometimes, multiple magnetizations are needed from different directions.
According to the analysis of the structural shape (section) of angle bar and the working principle of magnetic powder and ultrasonic, when ultrasonic testing is adopted for the structural shape 2 of angle bar, the straight probe is used to detect the plane crack parallel to the angle bar, and the inclined probe is used to detect the plane crack of angle bar. The probe selection meets the standard requirements, finds out the maximum wave amplitude of the defect, and then judges the defect. For the angle bar structure shape 1, there are right angle and arc transition, and the stress is concentrated. When ultrasonic testing is used, the angle reflection is formed by the angle probe, so it is difficult to determine the wave amplitude. At this time, the comprehensive judgment of surface testing and ultrasonic testing is needed. Firstly, magnetic particle is used to detect whether there is 1-2mm crack on the surface of angle bar. If there is no surface crack, then ultrasonic detection is used to detect the internal crack.
Judgment of surface crack
According to the crack diagram in Fig. 4 and the stress analysis diagram in Fig. 5, the direction of the crack produced in bending is perpendicular to the length direction of the angle steel, while the crack direction in Fig. 4 is parallel to the length direction of the angle steel, and the crack is long and thin with sharp ends. Therefore, the crack is produced in the rolling process. After bending, the crack spreads and the crack characteristics are more obvious. If the direction of the crack is perpendicular to the length of the angle steel, the crack is produced in the bending process.
Internal crack determination
According to the long-term manual comparison of Fig. 6 and Fig. 7, we can see that: firstly, magnetic particle testing is used to check whether there is crack at 1-2mm of angle steel bend surface, and then ultrasonic testing is used to judge after eliminating the surface crack. According to the manual comparison of the test piece in Figure 6, the thickness of the angle steel is 18mm, and the angle between the angle probe and the edge of the angle steel is 35 °. There are two amplitudes at the place where the artificial defect of the angle steel is detected, as shown in Figure 7. From the ultrasonic amplitude diagram in Figure 7, it can be seen that the maximum amplitude of the gate is 12.5mm, and the horizontal distance is 25.5mm. The front edge of the probe is measured with a ruler, which is consistent with the display of the instrument. The other amplitude is the reflection amplitude of the end angle, so The highest amplitude is the defect amplitude. In the NDT of angle steel bending (1 position of angle steel structure shape), the wave amplitude shown in Figure 7 is the crack wave amplitude produced during the angle steel bending process.
Fig. 6 Manual comparison test piece with width of 1mm and depth of 2mm
Figure.7 Amplitude diagram of ultrasonic
For the detection and determination of the cracks in the angle steel of the transmission line, first understand the structure shape of the detection area, analyze the stress of the workpiece, determine the specific detection method, and finally make the correct determination of the cracks through the detection instrument, analysis of the crack characteristics and processing technology. The difficulty of angle steel inspection lies in the irregular shape of the edge of angle steel and the excessive arc. When the ultrasonic straight probe is used for inspection, the inspection direction is single and it is easy to be missed. When the angle probe is used for inspection, it is difficult to determine the defect due to the formation of end angle reflection. For this place, magnetic particle and ultrasonic testing have good results. Firstly, magnetic particle is used to detect whether there is surface defect on the edge of angle bar. If there is no surface defect, ultrasonic angle probe is used to detect. The crack amplitude is characterized by a defect wave and an end angle reflection wave amplitude, as shown in Figure 7. If there is no crack defect, the instrument shows only one end angle reflection amplitude.
Source: China Angle Bars Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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