Fasteners are currently widely used in engineering fields such as machinery, construction, bridges, and oil production. As the basic unit of large-scale structural parts, many fasteners will have defects such as cracks, corrosion, pits, and man-made damage during work, and crack defects account for a very large proportion and harmfulness, which seriously threatens the existing structure and The safety and reliability of the organization.
Crack detection is to detect and evaluate the mechanical structure to determine whether there is a crack, and then to determine the location and extent of the crack. With the rapid development of modern machinery manufacturing, electronic technology and computer technology, non-destructive testing technology has been greatly developed, and crack detection technology has also been rapidly developed. This article first introduces traditional crack detection methods, and on this basis, summarizes modern non-destructive detection methods based on wavelet analysis and electromagnetic (eddy current) pulses, and points out the hot spots and directions of the development of fastener crack detection methods.
1. Traditional crack detection method
There are many traditional crack detection methods, which can be divided into two categories: conventional detection and unconventional detection. Conventional testing methods include eddy current testing, penetrant testing, magnetic particle testing, radiographic testing and ultrasonic testing; unconventional testing methods include acoustic emission, infrared testing and laser holographic testing.
(1) Routine testing methods
At present, the general simple crack detection in engineering fields such as machinery, construction and oil production all use conventional detection methods. Different inspection methods are adopted for different institutions. For example, ultrasonic inspection is mainly used for the inspection of metal plates, pipes and bars, castings, forgings and welds, as well as bridges, housing constructions and other concrete structures; radiographic inspections are mainly used for machinery, Detection of castings and welds in the fields of weapons, shipbuilding, electronics, aerospace, petrochemicals, etc.; magnetic particle testing is mainly used for metal castings, forgings and welds; magnetic particle testing is mainly used for metal castings, forgings and welds Penetration testing is mainly used for non-ferrous and ferrous metal castings, forgings, welding parts, powder metallurgy parts and ceramics, plastics and glass products; eddy current testing is mainly used for flaw detection and testing of conductive pipes, bars, and wires. Material sorting. For fastener crack detection, ultrasonic testing and eddy current testing can be used. For example, in the experimental research on the best eddy current detection parameters for small cracks in fasteners, the best detection parameter section in which the eddy current detection parameters of small cracks and the phase signal are linear has been obtained, which can improve the detection accuracy of small cracks in bars and the external type The selection of fastener eddy current testing parameters has an important guiding role. However, eddy current detection has many interference factors and requires special signal processing technology. In addition, there is a Lamb wave propagation energy spectrum structure crack detection method, which has the characteristics of strong penetrating ability, high sensitivity, fast and convenient, but sometimes blind areas occur, blockages occur, and short-distance cracks cannot be found. It is difficult to qualitatively and quantitatively characterize the defects found. For most fasteners, magnetic particle detection and fluorescent flaw detection methods are used. The detection efficiency is relatively high, but it consumes manpower and material resources and damages people's health. At the same time, due to human factors, there are often missed inspections.
(2) Unconventional detection methods
When testing fasteners for cracks, if conventional testing methods fail to achieve the required purpose, unconventional testing methods can be considered. Here are three commonly used unconventional crack detection methods.
1) Acoustic emission technology. This technology is the most mature in the crack detection of pressure equipment. It has achieved ideal results in the safety assessment of pressure vessels and pressure pipelines. It has also been vigorously developed in the crack detection of aerospace, composite materials, etc. For the crack diagnosis of rotating machinery, there has been a certain degree of development mainly in the detection of fatigue cracks in rotating shafts, gears and bearing cracks. The advantage of acoustic emission is that it is a dynamic detection method. The energy detected by acoustic emission comes from the object under test itself, rather than provided by non-destructive testing equipment like ultrasonic or radiographic testing. Acoustic emission detection is very sensitive to defects and can detect and evaluate the active defect status in the structure as a whole. The disadvantage is that the detection is greatly affected by the material; the detection room is affected by electrical noise and mechanical noise; the positioning accuracy is not high, and the identification of cracks can only give limited information.
2) Infrared detection. Mainly used in power equipment, petrochemical equipment, mechanical processing process detection, fire detection, crop varieties, and non-destructive detection of defects in materials and components. The advantage of infrared non-destructive testing technology is that it is a non-contact testing technology with high long-distance spatial resolution, safe and reliable, harmless to the human body, high sensitivity, wide detection range, fast speed, and no impact on the object being tested. The disadvantage of infrared detection is that the detection sensitivity is related to the thermal emissivity, so it is interfered by the surface of the test piece and background radiation, and is affected by the size and buried depth of the defect. The resolution of the original test piece is poor, and the shape and size of the defect cannot be accurately measured. And the location, the interpretation of the test results is more complicated, a reference standard is required, and the test operator needs to be trained.
3) Laser holographic detection. Mainly used for honeycomb structure, composite material inspection, solid rocket motor shell, insulation layer, coating layer and propellant grain interface defect inspection, printed circuit board solder joint quality inspection and pressure vessel fatigue crack inspection, etc. Its advantages are convenient detection, high sensitivity, no special requirements for the tested object, and quantitative analysis of defects. The disadvantage is that the deeply buried debonding defects can only be detected when the debonding area is quite large. In addition, laser holographic detection is mostly carried out in a dark room, and strict vibration isolation measures are required, which is not conducive to on-site detection and has certain limitations.
2. New technology of modern crack detection
With the rapid development of science and technology, engineering fields such as machinery, construction, and oil production have higher and higher requirements for crack detection. Therefore, many new crack detection technologies have emerged. Crack detection methods based on signal processing and electromagnetic (eddy current) pulse non-destructive testing are new technologies commonly used in modern times.
(1) Crack detection method based on wavelet analysis
With the development of signal processing technology, crack detection methods based on signal processing have emerged, including time domain, frequency domain and frequency domain methods, including Fourier transform, short-time Fourier transform, WignerVille distribution, and Hilbert -Huang transform (HHT), blind source separation, etc. Among them, the wavelet analysis method is the most representative. The crack identification methods directly using wavelet analysis can be divided into the following two types:
1) Analysis method based on time domain response. Including the method of using the singular points of the time-domain decomposition map, the method of using the change of wavelet coefficients, and the method of using the energy change after wavelet decomposition. The analysis method based on time domain response aims to find the moment when crack damage occurs.
2) Analysis method based on spatial response. It is to replace the time axis of the time domain response signal with the spatial coordinate axis of the spatial position, and use the spatial domain response as the input for wavelet analysis. Based on the spatial domain response analysis method, the location of the crack can be determined. The wavelet method itself can only judge the time when the damage occurs or where the damage occurs, and the former has more applications. If you want to identify small cracks, you need to combine wavelet with other methods to detect cracks.
(2) Electromagnetic (eddy current) pulse nondestructive testing
Electromagnetic technology combines many functions such as ultrasonic testing, eddy current imaging, array eddy current and pulsed eddy current testing to form a new modern electromagnetic testing technology. The common crack detection technologies include pulsed eddy current testing, pulsed eddy current thermal imaging technology, pulsed eddy current and electromagnetic acoustic transducer (EMAT) dual-probe nondestructive testing and metal magnetic memory testing technology.
Pulse eddy current uses a pulse current to excite the coil, analyze the time-domain transient response signal induced by the detection probe, and select the peak value, zero-crossing time and peak time of the signal to quantitatively detect the crack. Yang Binfeng of the National University of Defense Technology and others have used experiments to prove that pulsed eddy current can quantitatively detect cracks of different depths on the test piece with only one scan; some researchers use the alternative technology of harmonic coils to perform pulsed eddy current detection, and use its own electric field to conduct The change in the form of the electric dipole contributed by the total electric field is higher than the change on the conductor measured by the magnetic field sensor, and the distribution density of the electric dipole in the crack area is found to detect the crack.
The disadvantage of pulsed eddy current is that the peak value of pulsed eddy current signal is easily affected by other factors (such as lift-off effect), and the detection ability of pulsed eddy current probe will affect the detection of cracks.
Pulsed eddy current imaging instruments all use coils as inspection sensors. Some people use Hall sensors as inspection sensors. In recent years, super quantum interference instruments have begun to be applied to the field of non-destructive inspection. The use of pulsed eddy current thermal imaging technology eliminates the lift-off effect in other detections and avoids the distortion of imaging results.
Some researchers use a YNG laser similar to a Gaussian beam to penetrate the surface of the metal sheet, using pulsed eddy current and electromagnetic acoustic transducer detection technology, to identify the crack by the sudden change of the ultrasonic waveform or the sudden increase in the frequency component of the waveform when the laser irradiates the crack. .
3. Hot spots of crack research
At present, the research on fastener crack detection only stays on traditional detection methods. In order to develop detection technology and solve practical application problems, the hot spots of crack damage identification are mainly concentrated in the following two aspects: One is to consider uncertainty The statistical identification method of influence, the second is the identification of fastener microcracks.
There will be many uncertainties in crack damage detection, so a statistical inference method is proposed to deal with the system identification problem. With the rapid development of damage identification research, the research on damage identification methods based on probability and statistics theory has continued to deepen. At present, the main research application fields of this method are system identification and pattern recognition.
There are methods for detecting micro-cracks in fasteners, such as micro-crack detection based on ICT technology and laser-assisted heating-based laser ultrasonic trapping method to identify micro-cracks, but they all have their limitations. For example, the limitation of micro-crack detection based on ICT technology is that the gray value in the collected image is different from the background gray value. If the gray value is not much different from the background gray value, the details are more difficult to distinguish. Image quality makes image acquisition difficult, and at the same time puts forward higher requirements for image post-processing. Moreover, when the VG Studio MAX software is used to extract the micro-cracks, it is necessary to extract the space that contains all the micro-cracks, which is uncertain. Based on laser-assisted heating, the limitation of identifying micro-cracks is that the operation is more complicated and cannot be detected in harsh environments, so it has yet to be developed.
With the continuous development of society and economy, the requirements for fastener crack detection methods are becoming higher and higher. It must meet the requirements of real-time online detection, high sensitivity, simple operation, and resistance to external interference. It can be used in harsh external environments. Work; quickly and accurately detect the location, size, width, depth and development trend of the crack; the detection result can be displayed in image mode and can be analyzed; it integrates fast detection speed, high efficiency, and intuitive results.