Conducting NDT for Fastener Cracks

Fasteners play a crucial role in engineering fields such as machinery, construction, bridges, and oil extraction, but they often suffer from defects during use, including cracks, corrosion, pits, and human-induced damage. Among these, cracks are particularly hazardous, posing a serious threat to structural safety. Crack detection aims to determine the presence, location, and extent of cracks in mechanical structures. With the advancement of technology, non-destructive testing (NDT) techniques have developed rapidly, leading to improvements in crack detection technology.

Traditional crack detection methods are divided into conventional and unconventional categories. Conventional methods include eddy current testing, penetrant testing, magnetic particle inspection, radiographic testing, and ultrasonic testing, which are widely used across various engineering fields. For fasteners, ultrasonic testing and eddy current testing are more commonly used.

Ultrasonic testing is suitable for metal plates, pipes, rods, and concrete structures, while eddy current testing is often used for conductive materials. However, eddy current testing is susceptible to interference and requires special signal processing techniques. Lamb wave detection, despite its strong penetration and high sensitivity, has blind spots and difficulties in characterizing defects. Magnetic particle inspection and fluorescent penetrant inspection are frequently used for fasteners, with high efficiency, but they are labor and resource-intensive and can be influenced by human factors, leading to missed detections.

Unconventional detection methods include acoustic emission, infrared thermography, and laser holographic testing. Acoustic emission technology is suitable for pressure-bearing equipment and rotating machinery, with the advantage of dynamic testing, but it is sensitive to material and environmental noise, with limited positioning accuracy. Infrared thermography is non-contact, safe, and reliable, but it is affected by surface conditions and background radiation interference, making it difficult to determine the shape, size, and location of defects. Laser holographic testing is highly sensitive but not suitable for on-site detection and has limitations in detecting deeply buried defects.

Modern crack detection technologies are based on signal processing and electromagnetic (eddy current) pulse NDT. Wavelet analysis, a representative method of signal processing, can determine the occurrence time and location of cracks through time and spatial domain response analysis, but it needs to be combined with other methods to improve recognition accuracy.

Electromagnetic (eddy current) pulse NDT combines multiple functions, such as pulsed eddy current testing, thermal imaging technology, and electromagnetic acoustic transducer (EMAT) dual-probe testing, to achieve quantitative crack detection. Pulsed eddy current technology detects cracks by analyzing time-domain transient response signals but is susceptible to lift-off effects and probe detection capabilities. Thermal imaging technology eliminates the lift-off effect and improves image quality.

Current research hotspots in crack detection include statistical identification methods considering uncertainty and the detection of microcracks in fasteners. Statistical identification methods are based on probability and statistical theory, dealing with system identification issues, mainly applied in system identification and pattern recognition. Microcrack detection methods like Industrial Computed Tomography (ICT) technology and laser ultrasound pitch-catch methods with laser-assisted heating have certain effects but have limitations; for example, ICT technology is influenced by image grayscale values, and the laser ultrasound pitch-catch method is complex to operate and cannot be used in harsh environments.

With the development of the social economy, the requirements for fastener crack detection are increasingly stringent, demanding real-time online, high sensitivity, simple operation, resistance to external interference, and the ability to work in harsh environments. It is necessary to quickly and accurately detect crack locations, sizes, widths, depths, and trends, with results that can be displayed and analyzed in images. Therefore, the detection of multiple and microcracks in fasteners is a current research focus, requiring rapid localization, precise quantification, improved detection accuracy, and reliability to achieve efficient, intuitive, and accurate crack detection.

In summary, although extensive research has been conducted on identifying crack damage in fasteners, current methods are still limited to traditional detection technologies. Considering costs, environmental conditions, and human factors, the detection of cracks in fasteners needs to move towards rapid localization, precise quantification, and improved accuracy and reliability to meet the practical needs of engineering.