How to Detect Cracks in Fasteners Non-Destructively?

This article summarizes the current status, advantages and shortcomings, as well as the hotspots and development directions of the existing crack detection technologies based on the existing fastener crack detection methods, from the aspects of wavelet analysis and electromagnetic pulse nondestructive testing.

Cracks In Fasteners

Fasteners are currently widely used in engineering fields such as machinery, construction, bridges and oil production.

As the basic unit of large structural components, many fasteners are subject to defects such as cracks, corrosion, pits, and human damage in operation.

The proportion and hazard of crack defects are very large and seriously threaten the safety and reliability of existing structures and institutions.

Crack detection is to detect and evaluate the mechanical structure to determine whether there is a crack, and then 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 firstly introduces traditional crack detection methods.

On this basis, it summarizes the modern NDT methods based on wavelet analysis and electromagnetic (eddy current) pulses, and points out the hotspot and direction of the development of crack detection methods for fasteners.

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, radiation testing and ultrasonic testing; unconventional testing methods include acoustic emission, infrared testing and laser holographic testing.

(1) Routine testing methods 

At present, simple crack detection in engineering fields such as machinery, construction and oil production generally uses conventional testing methods.

Different testing methods are used for different institutions.

For example, ultrasonic testing is mainly used in the inspection of metal plates, pipes and bars, castings, forgings and welds, as well as in the inspection of concrete structures such as bridges and housing constructions.

X-ray testing is mainly used for the inspection of castings and welds in machinery, weapons, shipbuilding, electronics, aerospace, petrochemical and other fields.

Magnetic particle testing is mainly used in the inspection of metal castings, forgings and welds.

Penetration testing is mainly used for the inspection of castings, forgings, weldments, powder metallurgy parts and ceramic, plastic and glass products of non-ferrous and ferrous metals.

Eddy current detection is mainly used for flaw detection and material sorting of conductive pipes, rods and wires.

For the crack detection of fasteners, ultrasonic testing and eddy current detection can be used.

For example, in the experimental study on the best eddy current detection parameters for small cracks of fasteners, the best detection parameter segment with a linear relationship between small crack eddy current detection parameters and phase signal was obtained, which has an important guiding role in improving the accuracy of small crack detection in bars and the selection of eddy current detection parameters for external fasteners.

However, the eddy current detection has more interfering factors, which requires special signal processing techniques.

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 spots occur, blockages occur, and close cracks cannot be found.

It is difficult to qualitatively and quantitatively characterize the defects found.

For most fasteners, magnetic particle testing and fluorescent flaw detection methods are used.

The relative testing efficiency is relatively high, but it consumes manpower and material resources and damages human health.

At the same time, due to human factors, missed inspections often occur.

(2) Non-conventional testing methods 

When testing fasteners for cracks, if conventional testing methods cannot 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-bearing equipment.

It has achieved ideal results in the safety assessment of pressure vessels and pressure-bearing pipelines.

It has also been vigorously developed in the crack detection of aerospace, composite materials, etc.

For rotating machinery crack diagnosis, there has been a certain degree of development mainly in the detection of rotating shafts, gear fatigue cracks 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 nondestructive 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 testing 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.

It is 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 because the detection sensitivity is related to the thermal emissivity, it is interfered by the surface of the test piece and background radiation.

Affected by the size and buried depth of the defect, the resolution of the original specimen is poor, and the shape, size and position of the defect cannot be accurately measured.

The interpretation of test results is more complicated and requires reference standards.

Testing operators need to be trained.

3) Laser holographic detection.

It is mainly used for honeycomb structure, composite material inspection, solid rocket motor shell, insulation layer, coating layer and propellant grain interface defect detection, printed circuit board solder joint quality inspection and pressure vessel fatigue crack detection, 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 it needs to take strict vibration isolation measures, so it is not conducive to on-site testing and has certain limitations.

2. New modern crack detection technologies

With the rapid development of science and technology, the requirements for crack detection in engineering fields such as machinery, construction and oil production have become higher and higher, thus many new technologies for crack detection have emerged.

Crack detection methods based on signal processing and electromagnetic (eddy current) pulse nondestructive testing are the 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 time-frequency-domain methods, which are Fourier transform, short-time Fourier transform, WignerVille distribution, Hilbert-Huang transform (HHT) and blind source separation etc.

Among them, wavelet analysis is the most representative method.

Crack identification methods using wavelet analysis directly can be divided into the following two categories:

Analysis method based on time domain response.

It includes the method of using the singular points of the decomposition map in the time domain, 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.

The spatial response based analysis method.

It is to use the spatial position of the spatial axis instead of the time axis of the time domain response signal to perform wavelet analysis with the spatial domain response as the input.

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 detection, eddy current imaging, eddy current array and pulsed eddy current detection to form modern new technologies for electromagnetic inspection.

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 etc. from the National University of Defense Technology 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 for pulsed eddy current testing.

The change in the form of the electric dipole in the contribution of its own electric field to the total electric field inside the conductor 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 YNG laser beams similar to Gaussian beam shape into the surface of the metal sheet, using pulsed eddy current and electromagnetic acoustic transducer detection technology, through the sudden change of the ultrasonic waveform or the sudden increase of frequency components in the waveform when the laser irradiates the crack to identify the crack .

3. Hot spots for 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 the statistical identification method considering the influence of uncertainty, and the other is the identification of fastener micro-cracks.

The crack damage detection will have many uncertainties, so it is proposed to use the statistical inference method to deal with the system identification problem.

With the rapid development of damage identification research, the research of damage identification methods based on probabilistic statistical theory has been deepening.

The current main research application areas of this method are system identification and pattern recognition.

Nowadays, there have been methods to detect microcracks of fasteners, such as microcrack detection based on ICT technology and laser ultrasonic casting based on laser-assisted heating to identify microcracks, but all of them have 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.

Therefore, it affects the image quality, makes image acquisition difficult, and also puts forward higher requirements for image post-processing.

Moreover, when extracting the microcracks with VG Studio MAX software, it is uncertain to extract the spatial range containing all the microcracks.

The limitation of the laser-assisted heating based on laser ultrasonic projection trapping method to identify microcracks is that the operation is complicated and cannot be detected under harsh environment, so it still needs to be developed.

With the continuous development of the social economy, the requirements for the detection methods of fastener cracks are getting higher and higher.

It must meet the requirements of real-time online detection, high sensitivity, simple operation and resistance to external interference, and be able to work in harsh external environments;

Quickly and accurately detect the location, size, width, depth and development trend of the crack;

The test results can be displayed in images and can be analyzed;

It integrates fast detection speed, high efficiency, and intuitive results.

4. Conclusion

A lot of research work has been carried out on fastener crack damage identification, but the current damage identification methods or indicators are limited to traditional detection methods.

Considering the cost of testing equipment, the environment in which it is used, and human factors, the detection of multiple cracks and microcracks in fasteners is a hot topic of research today.

To achieve rapid positioning, accurate quantification, improve detection accuracy and reliability, and achieve good and fast crack detection, these are the development directions of fastener crack detection.

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