Defect detection in aerospace thin-walled stiffener structures made of aluminium alloy using line source laser-excited ultrasonics

Aluminium alloy thin-walled stiffened structures, valued for their lightweight and high strength, are widely used in aerospace engineering. However, manufacturing and service-induced defects can severely degrade mechanical performance, making reliable non-destructive testing essential. Laser ultrasonic testing, with its non-contact operation, high spatial resolution and high sensitivity, offers great potential for aerospace components. A key challenge is the significant signal attenuation caused by slender stiffeners, which hinders the detection of deep and small defects. To address this, a line source laser ultrasonic method is proposed for defect detection in thin-walled stiffened structures. A three-dimensional finite element model is first established to investigate how laser spot geometry and size influence ultrasonic directivity, amplitude, frequency and defect detection capability. The propagation behaviour of line source laser ultrasound and its interaction with defects are then analysed. The proposed method is validated experimentally, demonstrating effective detection of defects as deep as 21 mm with a diameter of 0.5 mm. Notably, for deep and small defects in slender stiffener regions, the line source configuration markedly outperforms the point source, achieving an average signal-to-noise ratio improvement of 9.54 dB. This study extends laser ultrasonic testing to complex thin-walled stiffened structures, providing a valuable reference for aerospace defect detection.