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Hotspots of Fastener Non-Destructive Testing Technology (Part 2)

May. 26, 2020

2. Modern crack detection technology

With the rapid development of science and technology, the requirements for crack detection in engineering fields such as machinery, construction, and oil production are also becoming higher and higher, so many new crack detection technologies have emerged. Crack detection methods based on signal processing and electromagnetic (eddy current) pulse non-destructive testing is modern and commonly used new technologies.

(1) Crack detection method based on wavelet analysis

With the development of signal processing technology, crack detection methods based on signal processing have appeared, including time domain, frequency domain, and frequency domain methods, mainly including Fourier transform, short-time Fourier transform, Wigner-Ville distribution, Hilbert -Huang transform (HHT), blind source separation, etc. Among them, the method of wavelet analysis 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 point of the time domain decomposition graph, the method of using the wavelet coefficient change, 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 space coordinate axis of the space position and to use the space domain response as input for wavelet analysis. The location of the crack can be determined based on the spatial domain response analysis method. The wavelet method itself can only judge the time when damage occurs or the location where 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. Common crack detection technologies include pulse eddy current detection, pulse eddy current thermal imaging technology, pulsed eddy current and electromagnetic acoustic transducer (EMAT) dual-probe nondestructive detection, and metal magnetic memory detection technology.

Pulsed 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 the peak time of the signal to quantitatively detect the crack. Yang Jinfeng and others from the National University of Defense Technology have demonstrated that pulsed eddy current can quantitatively detect cracks at different depths on the test piece with only one scan; some researchers use alternative techniques of harmonic coils to perform pulsed eddy current detection, using their own electric field to the inside of conductor The change in the form of the electric dipole contribution of the total electric field is higher than the change in 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 the pulsed eddy current is that the peak value of the pulsed eddy current signal is easily affected by other factors (such as lift-off effect), and the detection ability of the 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, ultra-quantum interference instruments have been applied to the field of non-destructive testing. The use of pulsed eddy current thermal imaging technology eliminates the lift-off effect in other inspections and avoids distortion of imaging results.

Some researchers used a YNG laser with a Gaussian beam shape to shoot into the surface of a metal sheet, using pulse eddy current and electromagnetic sound transducer detection technology to identify the crack by the sudden change of the ultrasonic waveform or the sudden increase of the frequency component of the waveform when the laser-irradiated the crack.

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