Objective Metallic materials are extensively used in scientific instruments and modern industrial equipment. Various challenges are encountered throughout the processes of fabrication, manufacturing, and long-term utilization of metallic materials, among which the generation and accumulation of small defects significantly affect their applications. These small defects, which cause structural performance degradation due to corrosion, fatigue, and stress concentration, can progress gradually and ultimately result in quality issues or safety hazards in the equipment. Hence, precise and efficient detection of such small defects is imperative. Although several common detection methods have been investigated, such as piezoelectric detection, they still exhibit limitations in applications, such as non-contact or simultaneous detections of multiple points. Therefore, more advanced technologies are warranted to address these challenges. The method proposed in this study is based on multi-channel fiber interferometric sensing and laser ultrasonic inspection, which offers significant innovation and application prospects. This approach integrates multi-channel fiber interferometers and laser ultrasonic techniques, thereby overcoming the limitations of conventional methods and providing precise, high-resolution detection capabilities. Methods The Fizeau fiber interferometer utilizes the same single fiber as both the sample and reference arms. Therefore, by constructing a multi-channel Fizeau fiber interferometer, information can be acquired simultaneously from multiple points in an extremely small scale. This technology processes and analyzes the propagation characteristics and frequency properties of multipoint signals simultaneously, thus enabling the rapid detection of elastic waves along the sample surface without necessitating probe scanning. We conduct defect detection on aluminum alloy plates with circular hole defects experimentally and assess the result based on a comparison with aluminum plates without defects. By analyzing echo signals from the boundaries and defect reflections, we successfully reconstruct the optical paths and accurately determine the boundaries and positions of the defects. In this study, we not only generate and localize individual defects but also conduct multipoint detection simultaneously at different locations, thus significantly improving the detection efficiency and precision. Compared with conventional scanning methods, this technique based on a Fizeau fiber interferometer provides much better real-time performance and accuracy, thus enabling more precise detections of small defects inside the samples. Additionally, by comparing samples with and without defects, we successfully differentiate and validate the echo signals from the plate boundaries and defect reflections. Results and Discussions The multi-channel Fizeau fiber interferometer constructed for the experiment (Fig. 2) enables simultaneous and high-resolution detections. The inner diameter of the multi-channel fiber probe (Fig. 3) is much smaller than those of common piezoelectric ultrasonic transducers, with a spatial interval between consecutive detection points of less than 0. 24 mm. Additionally, the lateral resolution is determined by the precision of the stepper motor, which can be 10 mu m at the minimum. This multi-channel signal-processing technique facilitates the capture of more detailed features of small defects with a much higher resolution. Based on a multiparameter analysis of the signals, parameters characterizing the size of the metal sample and the propagation characteristics of internal elastic surface waves can be obtained. During the inspection of the defect-free aluminum plate, echo signals are detected and then analyzed (Fig. 8). To inspect the aluminum plate with circular hole defects, an experiment is conducted in three sets comprising six channels per set, thus resulting in the measurement of 18-channel elastic wave signals (Fig. 10). The specific size and boundary positions of the circular hole defects can be identified and delineated based on the actual parameters of the reflected signals. This provides a practical new approach for the quality inspection of metallic materials. Conclusions In this study, a laser ultrasonic non-destructive testing method based on multi-channel fiber interferometry is proposed that utilizes multiple Fizeau fiber interferometers to construct a multi-channel elastic surface wave detection system. Under laser pulse excitation, non-contact excitation and detection are achieved using multi-channel fiber probes. Compared with the conventional ultrasonic probe detection system, this multi-channel fiber interferometric detection system enables simultaneous multipoint measurements of elastic surface waves with a much higher resolution. It can obtain more information simultaneously at much smaller scales. By analyzing the frequency and propagation characteristics of elastic surface wave signals, the elastic wave velocity within the sample can be detected rapidly without necessitating probe scanning. Based on a multiparameter analysis of the multi-channel signals, not only the size of the metal plate samples is obtained but also the morphology of the circular hole defect boundary within the plate is delineated. This system can detect elastic surface wave signals at different positions simultaneously. Furthermore, it offers extremely high temporal and spatial resolutions, thus providing a new technological reference for the laser ultrasonic non-destructive testing of small defects in metals.
