This article provides a summary of the current state of crack detection technologies, including their advantages and limitations, as well as important topics and future development directions. It is based on existing fastener crack detection methods, focusing on wavelet analysis and non-destructive electromagnetic pulse testing.
Fasteners are widely used in various engineering fields such as machinery, construction, bridges and oil production. As a basic component of large structures, fasteners are susceptible to various defects, including cracks, corrosion, holes, and human-induced damage during operation.
Crack defects pose a significant threat to the safety and reliability of structures and institutions, making crack detection an essential aspect of structural assessment. Crack detection involves identifying and evaluating cracks in mechanical structures to determine their location and extent.
With the advancement of modern machine manufacturing, electronics and computer technology, non-destructive testing (NDT) has improved significantly, leading to the development of advanced crack detection techniques.
This article provides an overview of traditional crack detection methods and focuses on modern NDT methods based on wavelet and electromagnetic pulse (eddy current) analysis. Additionally, it highlights current topics and future development directions in crack detection techniques for fasteners.
1. Traditional crack detection method
There are numerous traditional crack detection techniques, which can be categorized into two groups: conventional and unconventional detection methods.
Conventional detection methods encompass eddy current testing, penetrant testing, magnetic particle testing, radiation testing, and ultrasonic testing. Unconventional detection methods, on the other hand, include acoustic emission testing, infrared testing, and laser holographic testing.
(1) Routine testing methods
Currently, conventional testing methods are widely used for simple crack detection in engineering fields such as machinery, construction and oil production. The methods used vary depending on the institution.
For example, ultrasonic testing is primarily used to inspect metal plates, pipes, bars, castings, forgings, and welds, as well as concrete structures such as bridges and housing constructions. X-ray testing is mainly used to inspect castings and welds in industries such as machinery, weapons, shipbuilding, electronics, aerospace, petrochemicals and others. Magnetic particle testing is mainly used for the inspection of metal castings, forgings and welds. Penetration testing is mainly used for the inspection of castings, forgings, weldments, powder metallurgy parts, and ferrous and non-ferrous metal products made of ceramics, plastics, and glass. Eddy current detection is mainly used for flaw detection and material classification of pipes, rods and conductive wires.
For detection of cracks in fasteners, ultrasonic testing and eddy current detection can be used. For example, an experimental study found the best eddy current detection parameters for small cracks in fasteners. The results of the study showed that the best detection parameters had a linear relationship between the small crack eddy current detection parameters and the phase signal, which is important for improving the accuracy of detecting small cracks in bars and selecting the Eddy current detection parameters for external fixators. However, eddy current detection has more interference factors and requires special signal processing techniques.
Another method used for crack detection is the Lamb wave propagation power spectrum structure crack detection method, which is known for its strong penetration ability, high sensitivity, speed and convenience. However, it has limitations, such as blind spots and blockages, which can result in missed closed cracks. The method is also challenging to characterize the defects found quantitatively and qualitatively.
For most fasteners, magnetic particle testing and fluorescent flaw detection methods are used. These methods are relatively efficient, but they consume labor and material resources and can cause harm to human health. Additionally, missed inspections often occur due to human factors.
(2) Unconventional testing methods
When conventional testing methods cannot detect cracks in fasteners, unconventional testing methods can be employed as an alternative.
Three commonly used unconventional crack detection methods include:
1) Acoustic emission technology.
Acoustic emission technology is widely recognized as the most advanced method for detecting cracks in pressure-bearing equipment. It has been successfully used in the safety assessment of pressure vessels and pipelines, as well as in the detection of cracks in aerospace and composite materials. In the field of crack diagnosis in rotating machinery, it has mainly been used to detect cracks in rotating shafts, gears and bearings.
One of the main advantages of Acoustic Emission is that it is a dynamic detection method, using the energy emitted by the object being tested, rather than external non-destructive testing equipment such as ultrasonic or radiographic testing. This makes it highly sensitive to defects and capable of detecting and evaluating the state of active defects throughout the structure.
However, there are also some disadvantages to consider. Acoustic emission detection is greatly affected by the material being tested and can be disrupted by electrical and mechanical noise in the test environment. Furthermore, detection accuracy may be limited by low positioning accuracy, and the information obtained from crack identification is often limited.
2) Infrared detection.
Infrared non-destructive testing (NDT) technology is widely used in a variety of industries, including power equipment, petrochemical equipment, mechanical processing, fire detection, crop analysis, and defect detection in materials and components.
One of the main advantages of infrared NDT is that it is a non-contact testing technology that is safe, reliable, harmless to humans and highly sensitive. It has a wide detection range, fast speed and does not affect the object being tested. It also has high spatial resolution over long distances.
However, there are also some disadvantages to consider. Infrared detection sensitivity depends on the thermal emissivity of the test piece and can be affected by surface interference and background radiation. The resolution of the original sample is poor, making it difficult to accurately measure the shape, size and position of defects, especially when they are small or deeply buried.
Furthermore, interpreting test results is complex and requires benchmarks, and operators need to be trained to use the technology effectively.
3) Laser holographic detection.
Laser holographic detection is mainly used for the inspection of various structures such as honeycomb structures, composite materials, solid rocket engine casings, insulation layers, cladding layers and propellant grain interfaces for defect detection. It is also used to evaluate the quality of solder joints on printed circuit boards and detect fatigue cracks in pressure vessels.
This method offers several advantages, including ease of use, high sensitivity, no special requirements for the tested object, and the ability to perform quantitative defect analysis.
However, one of its disadvantages is that deeply buried debonding defects can only be detected when the debonding area is substantial.
Furthermore, laser holographic detection typically requires a dark environment and strict vibration isolation measures, making it less suitable for on-site testing and having certain limitations.
2. New modern crack detection technologies
With advancements in science and technology, there has been an increasing demand for more advanced crack detection methods in various engineering fields such as machinery, construction and petroleum production. This has led to the emergence of new technologies for crack detection.
Signal processing and non-destructive electromagnetic pulse (eddy current) testing are two of the commonly used and effective new technologies for crack detection. These methods offer efficient and reliable solutions for crack identification in various applications.
(1) Crack detection method based on wavelet analysis
With the advancement of signal processing technology, various crack detection methods based on signal processing have emerged, including time domain, frequency domain and time frequency domain methods such as Fourier transform, short Fourier transform term, Wigner-Ville distribution. , Hilbert-Huang transform (HHT) and blind source separation.
Of these methods, wavelet analysis is the most widely used and representative.
Crack identification methods using wavelet analysis can be divided into two categories:
① Time domain response analysis method:
This method includes using the singular points of the time domain decomposition map, the change of wavelet coefficients, and the energy change after wavelet decomposition. The objective of this method is to identify the moment when crack damage occurs.
② Spatial response-based analysis method:
This method uses the spatial position of the spatial axis rather than the time axis of the time-domain response signal for wavelet analysis, with the spatial-domain response as input. This method allows you to determine the location of the crack.
Although the wavelet method itself can only determine the time when damage occurs or the location of damage, the former has more applications. To identify small cracks, wavelet analysis must be combined with other methods.
(2) Non-destructive electromagnetic pulse (eddy current) test
Electromagnetic technology combines multiple functions such as ultrasonic detection, eddy current imaging, eddy current array and pulsed eddy current detection to form new advanced technologies for electromagnetic inspection.
Common crack detection technologies include pulsed eddy current testing, pulsed eddy current thermal imaging, dual-probe non-destructive testing using pulsed eddy current and electromagnetic acoustic transducer (EMAT), and metallic magnetic memory testing technology.
Pulsed eddy current testing involves exciting a coil with a pulse current, analyzing the time-domain transient response signal induced by the detection probe, and quantitatively detecting cracks by selecting the peak value, zero-crossing time, and signal peak.
Research by Yang Binfeng and others at the National University of Defense Technology showed that pulsed eddy current can quantitatively detect cracks of different depths with just one scan. Some researchers use harmonic coils as an alternative technology for pulsed eddy current testing.
However, the peak value of the pulsed eddy current signal is easily affected by other factors, such as the lifting effect, and the detection ability of the pulsed eddy current probe can also affect crack detection.
Pulsed eddy current imaging instruments use coils as inspection sensors, while some use Hall sensors. In recent years, superquantum interference instruments have begun to be used in the field of non-destructive inspection.
Pulsed eddy current thermal imaging technology eliminates the lift-off effect in other detections and ensures accurate imaging results. Some researchers use YNG laser beams with Gaussian beam shape on the surface of metal sheets, using pulsed eddy current and electromagnetic acoustic transducer detection technology. They identify cracks by detecting a sudden change in the ultrasonic waveform or a sudden increase in frequency components when the laser beam irradiates the crack.
3. Hot spots for crack research
Currently, research on fastener crack detection is limited to traditional detection methods. In order to advance detection technology and solve practical application problems, the focus of crack damage identification is mainly on two aspects: statistical identification methods that consider the impact of uncertainty and the identification of microcracks in fasteners.
The uncertainty of crack damage detection requires the use of statistical inference methods to solve the system identification problem. With the advancement of damage identification research, the study of damage identification methods based on probabilistic statistical theory has deepened, with the current main areas of application being system identification and pattern recognition.
Currently, there are methods to detect microcracks in fasteners, such as microcrack detection based on ICT technology and ultrasonic laser casting based on laser-assisted heating, but both have limitations. For example, microcrack detection based on TIC technology may have difficulty distinguishing details if the gray value in the collected image is not very different from the background gray value, affecting image quality and making image acquisition difficult. Furthermore, it is uncertain to extract the spatial strip containing all the microcracks when using VG Studio MAX software. The limitation of the laser-assisted heating method based on ultrasonic laser projection is that it is complicated to operate and cannot be used in harsh environments, so it still needs further development.
As the social economy continues to develop, the requirements for fastener crack detection methods become increasingly demanding. These methods must meet the needs of real-time online detection, be highly sensitive, simple to operate and resistant to external interference, and be able to function in harsh environments. They must also quickly and accurately detect the location, size, width, depth and development trend of cracks, display test results in images that can be analyzed, and offer fast detection speed, high efficiency and intuitive results.
4. Conclusion
Extensive research has been conducted on identifying crack damage in fasteners, but current damage identification methods and indicators are limited to traditional detection methods. Given 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 current area of research interest.
The development direction of fastener crack detection is to achieve rapid positioning, accurate quantification, improve detection accuracy and reliability, and achieve rapid and effective crack detection.
