Subsurface cavity defects with hidden characteristics disrupt the material continuity, and are prone to stress concentration and crack initiation under load, severely jeopardizing the material's service performance and service life. When the defect size is small, the amplitude of its reflected signal is small and often difficult to identify, easily resulting in missed detection. Therefore, the laser-electromagnetic ultrasonic hybrid technique is used to detect subsurface defects, analyzing the characteristics of signal amplitude variation and using them as the basis for determining the presence of defects.

The excitation principle of laser ultrasound under the ablation mechanism, the directivity of the surface wave acoustic field corresponding to the linear spot, the reception principle of the electromagnetic acoustic transducer, and the phenomenon of interference and superposition between the direct wave and the defect reflected signal forming a composite signal were theoretically analyzed. The interference superposition caused the signal amplitude to increase or decrease. A laser-electromagnetic ultrasonic detection experimental system for subsurface cavity defects was developed, in which the cylindrical focusing lens used to form the linear spot and the surface wave electromagnetic ultrasonic transducer were relatively fixed, forming the front-end detection module. Different from the scanning laser source method, this research focused on the variation pattern of composite signals when the receiving electromagnetic ultrasonic transducer traversed the defect. Scanning detection experiments were conducted on subsurface cavity defect specimens, obtaining scanning detection data for specimens containing defects with the same diameter but different burial depths and specimens containing defects with different diameters but the same burial depths, respectively. Among these, the phenomenon of interference and superposition between the direct wave signal and the defect reflected signal was concretely demonstrated in the A-scan signals. For the detection data, the maximum amplitude of the composite signal, its corresponding position, and the distribution pattern of the amplitude variation rate with respect to burial depth or defect diameter were analyzed.

The results demonstrated that, due to the distribution characteristics of surface wave energy in the depth direction, the position where the reflected signal was generated, and the curvature of the interface at the reflection position, for subsurface defects completely within one wavelength range of the surface wave, the position of the electromagnetic ultrasonic transducer corresponding to the occurrence of the maximum amplitude of the composite signal gradually shifted backward from −1 mm to 5 mm with increasing burial depth (Fig.5a), and shifted forward from 1 mm to −1 mm with increasing diameter (Fig.7a). The maximum amplitude decreased with increasing burial depth (Fig.5b), and increased with increasing defect diameter (Fig.7b). For defects with a diameter of 2 mm, when the burial depth was less than 3 mm, the amplitude variation rate of the composite signal exceeded 2 (Fig.5c); for defects with a burial depth of 2 mm, when the defect diameter was ≥ 2 mm, the amplitude variation rate of the composite signal surpassed 4 (Fig.7c). These findings indicated that buried defects distributed at burial depths less than half the wavelength had composite signals with large amplitude variation rates.

The study reveals that during the scanning detection process, the amplitude of the composite signal exhibits a pattern of first decreasing, then increasing to a peak value, and then decreasing to a trough. Using this pattern as the basis for identifying subsurface defects, compared to the ultrasonic reflection-based method, it breaks through the problem of defect determination caused by low signal-to-noise ratio and small defect signal amplitude, and has high defect recognition rate and enhanced defect detection capability. This research provides valuable insights for the non-contact detection of subsurface cavity defects and contributes to advancing the application of laser-electromagnetic ultrasonic hybrid detection technology in engineering.