Using the principle of electromagnetic induction, non-destructive evaluation of certain properties of conductive materials and their workpieces by detecting changes in induced eddy currents in the detected workpiece, or non-destructive testing methods for detecting defects is called non-destructive testing. In industrial production, eddy current testing is one of the main means to control the quality of various metal materials and a few non-metals (such as graphite, carbon fiber composites, etc.) and their products. Compared with other non-destructive testing methods, eddy current testing is easier to automate, especially for tubes, rods and wires.
Eddy current testing
When the eddy current is placed in a changing magnetic field, the induced electric field acts on the free charge in the conductor due to the presence of a vortex induced electric field around the changing magnetic field, causing the electric charge to move and form a vortex.
Eddy current testing (abbreviation ET). Faraday's law of electromagnetic induction is known, in which alternating current is applied to the detection coil to produce an alternating magnetic field perpendicular to the workpiece. When the detecting coil is close to the workpiece to be inspected, the surface of the workpiece induces an eddy current while generating a magnetic field opposite to the direction of the original magnetic field, partially canceling the original magnetic field, resulting in a change in the resistance and inductance of the detecting coil. If there is a defect in the metal workpiece, the strength and distribution of the eddy current field will be changed to change the impedance of the coil, and the change can be detected to determine whether there is a defect.
With the development of microelectronics and computer technology and the adoption of various signal processing technologies, eddy current detection transducers, eddy current detection signal processing technology and eddy current testing instruments have made great progress.
Characteristics of eddy current testing
First, the advantages
1. When detecting, the coil does not need to touch the workpiece, and there is no need to couple the medium, so the detection speed is fast.
2. It has high detection sensitivity for defects on the surface of the workpiece or near surface, and has a good linear indication within a certain range, which can be used for quality management and control.
3. It can be tested in high temperature state, narrow area of ​​workpiece and deep hole wall (including pipe wall).
4. It can measure the thickness of metal coating or non-metal coating.
5. It can inspect non-metallic materials that can induce eddy currents, such as graphite.
6. The detection signal is an electrical signal, which can be digitized for storage, reproduction and data comparison and processing.
Second, shortcomings
1. The object must be a conductive material and is only suitable for detecting metal surface defects.
2. The detection depth and detection sensitivity are contradictory. When performing ET on a material, it should be considered comprehensively according to the material, surface state and inspection standard, and then the detection scheme and technical parameters are determined.
3. When the ET is used through the through coil, the specific position on the circumference where the defect is located cannot be determined.
4. Rotating probe type ET can be positioned, but the detection speed is slow.
Signal processing technology for eddy current testing
It is necessary to improve the signal-to-noise ratio and anti-interference ability of the detection signal to realize signal identification, analysis and diagnosis to obtain the best signal characteristics and detection results.
1. Signal feature quantity extraction
Commonly used feature quantity extraction methods include Fourier description, principal component analysis and wavelet transform.
Fourier description is a common method of extracting eigenvalues. The advantage is that it is not affected by the probe speed, and the impedance map can be reconstructed by the description method. The more the number of sampling points, the closer the reconstruction curve is to the original curve. However, this method is only sensitive to the shape of the curve, and is insensitive to the zero point and gain of the eddy current detector, and does not change with the rotation, translation, size change and starting point selection of the curve.
The method of depicting signal features using the eigenvalues ​​and eigenvectors of the test signal autocorrelation matrix is ​​called principal component analysis, which has a strong resolution for similar defects.
Wavelet transform is an advanced signal time-frequency analysis method. Multi-resolution analysis in wavelet transform is applied to the analysis of eddy current detection signals, and different wavelet coefficients are processed and reconstructed. The signal-to-noise ratio of the signal processed by the wavelet transform is greatly improved.
2, signal analysis
(1) Artificial neural network
The input vector of artificial neural network is the characteristic parameter of the signal. The correct selection and extraction of the characteristic parameters of the signal is the key to the success of neural network intelligent discrimination. The combined neural network model uses hierarchical discriminant method to reduce the dimension of network input variables from N2 to N. The network structure is greatly simplified, the training speed is fast, and the defect recognition rate and practical value are high.
Neural networks can achieve defect classification, have the advantage of high recognition accuracy, and are equally effective for incomplete and unclear data.
(2) Information fusion technology
Information fusion is a multi-level processing of detection, correlation, correlation, estimation and synthesis from different information sources, and obtains a unified best estimate of the measured object.
The eddy current scanning image is fused, the image is decomposed into multiple sub-band images, and the fusion algorithm is used in the conversion region to realize image fusion. Ka Bartels et al. combined the eddy current signal with the best signal-to-noise ratio method and used the spatial frequency compensation method to blur the high-frequency signal before the combination and the low-frequency signal became clear. Z Liu et al. use the maximum criterion to select the discrete wavelet transform coefficients of different signals, and select the largest absolute value of the coefficients to be fused as the combined conversion coefficients. Thus the fused signal can be reconstructed using an inverse wavelet transform based on these coefficients. Wavelet transform can effectively extract significant features in different proportions. In the process of merging signals, the useful features of all signals are preserved, so internal and surface defect information is enhanced.
3. Solution of eddy current inverse problem
The signal detected by the transducer implied information such as the location, shape, size and media properties of the defect. The media parameter (conductivity) or shape (defect) is reversed from the known signal, which is an inverse problem in the electromagnetic field theory.
In order to solve the eddy current inverse problem, we must first establish a mathematical model of defect recognition, such as artificial rules with regular shapes, natural defects with complex boundaries, single defects and multiple defects. In terms of media type, there are composite materials and surface magnetic properties of the tested parts. Models such as conductivity changes.
With the development of computer technology, various numerical solutions of defect models have also progressed. The finite element method, the moment method and the boundary element method appear.
Development and current status of eddy current testing technology
1824 Gabe vortex exists
1831 Faraday electromagnetic induction phenomenon
1873 Maxwell's equation electromagnetic field theory
1879 Hughes first applied to judge different metals and alloys
1926 Eddy Current Thickness Gauge
1935 Eddy Current Flaw Detector
1942 Automated testing
The perfection of the theory and practice of Foster impedance analysis in the 1950s
In the 1960s, China began research, mainly in the field of aerospace and other fields.
The US EM3300 and MIZ-20 are typical products using impedance flat panel display technology, while the TM-128 eddy current meter is the first eddy current flaw detector with a microcomputer with impedance plane display.
MFE-1 three-frequency eddy current meter is the first multi-frequency eddy current testing equipment developed in China. Subsequently, various types of multi-frequency eddy current detectors, such as EEC-35, EEC-36, EEC-38, EEC-39 and ET-355, ET-555, ET-556, etc., were successfully developed in China.
At present, China has obtained research results in finite element numerical simulation, far field eddy current probe performance index analysis and detection system development, and launched commercial far field eddy current testing instruments, among which ET-556H and EEC-39RFT have been used in chemical refining equipment. In-service inspection of steel heat exchange tubes and high-pressure heater tubes for power plants.
Application of eddy current testing in high-end fields of various industries
1. Aerospace, aviation
Eddy current testing technology has been widely used in the detection of metal components in the aerospace and aerospace fields. In order to ensure the flight safety of the aircraft, the relevant components must be inspected regularly. Eddy current technology is commonly used to detect aero-engine blade cracks, bolts, cracks in screw holes, multi-layer structures of aircraft, landing gear, hub and aluminum skin and other subsurface defects, and to detect defects in wing joint welds. Wait. Interference signals caused by probe sloshing and uneven material can be effectively suppressed during the detection. Metal magnetic memory testing technology can be used to diagnose the stress concentration of the above components or early damage.
2. Electricity, petrochemical
Eddy current testing technology is used in non-ferrous and ferrous metal pipelines (such as copper pipes, titanium pipes, stainless steel pipes, boilers, etc.) in power stations (fire power plants, nuclear power plants), petrochemicals (oil fields, oil refineries, chemical plants). Servicing and pre-service testing. Inter-crystal corrosion, thin wall thickness reduction and outer wall wear can be reliably detected, and the interference signals of the support plate and the tube plate can be effectively removed during the detection. In addition, the eddy current method is also used for the detection of steam turbine large shaft center hole, engine blade, pumping rod, drill collar, bolt, screw hole and other components; acoustic pulse detection technology can be used for rapid detection of various metal or non-metal pipelines; Magnetic memory technology is used for the diagnosis of early damage to ferromagnetic parts of in-service equipment.
3. Metallurgy and machinery
Eddy current testing technology is used for on-line and off-line inspection of various metal tubes, rods, wires and wires. In the process of flaw detection, it can simultaneously take into account long defects such as long pass injury and slow change and short defects (such as through holes); it can effectively suppress some interference signals (such as uneven material, sway, etc.) when the pipeline is online or offline. It has high sensitivity to the detection of defects in the inner and outer walls of metal pipes; it can also be used for mixing and sorting of mechanical parts, carburizing depth and heat treatment state evaluation, hardness measurement, etc.
4. Nuclear energy, military industry
Eddy current testing technology is used for the detection of metal fuel rods, titanium tubes, threaded tubes and other metal pipes; for military weapons, barrels, missile launchers, shell bases, shells, engine blades, wings, landing gear and wheels, etc. Pre-service and in-service testing; metal magnetic memory technology is used for early diagnosis of metal structures such as armored vehicles and ships; low-frequency electromagnetic fields and magnetic flux leakage technology are used for ferromagnetic materials such as decks and oil storage tanks and weld quality control.
Future research and development of eddy current testing technology includes: perfecting transducer design theory, developing better eddy current testing transducers; studying eddy current localization technology and 3D imaging technology for defect size and shape depth; researching and popularizing far field eddy current testing technology; further Research on the fatigue cracking, cracking, mechanical grinding wear and residual stress eddy current testing technology of metal materials. The application of this technology for non-destructive testing will be widely used.
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