Defect detection method for curved composite materials based on phased array ultrasonic focusing

Ultrasonic phased-array inspection of curved carbon fibre reinforced polymer (CFRP)-steel structures is complicated by refraction at the convex water-CFRP interface and strong acoustic-impedance mismatch at the CFRP-steel interface. This study develops a curved-interface focused phased-array ultrasonic testing method for quantitative defect evaluation in CFRP-steel composite cylinders. A refracted delay law accounts for the water path, convex interface and equivalent propagation path in CFRP to synchronise beam arrival at the target region. A normalised sound field model compares focusing performance across excitation frequencies, identifying 2.5 MHz as providing a compact focal zone with sufficient interfacial sound pressure. Finite element simulations and water-immersion phased-array experiments examine interfacial debonding and internal CFRP defects. For debonding, the peak amplitude of the focused interface echo, extracted after multi-focal-position processing, is calibrated against debonding lengths from 2 to 12 mm. For internal defects, echo position serves as a depth-sensitive feature and is calibrated against defect thicknesses from 0.5 to 2.5 mm. The results show improved energy concentration near the target region and provide quantitative features for both defect types in curved CFRP-steel structures.