Large-field detection of Metal/CFRP hybrid composites based on air-coupled laser ultrasound

Metal/carbon fiber reinforced polymer (CFRP) hybrid composites are widely used in the aeronautical and aerospace industry due to their excellent mechanical properties. However, traditional nondestructive testing methods are difficult to achieve fast, non-contact, large-field, high resolution, and high contrast detection at the same time, due to the large differences in thickness, density, acoustic impedance, and thermal diffusion between metal and fiber layers. Here, an air-coupled laser ultrasound (ACLU) method was presented for large-field detection of internal defects on the non-homogeneous metal/CFRP interface. Simulation analysis was conducted to analyze the propagation characteristics of laser ultrasound in Al/CFRP hybrid composites without defect and with defect. Test results for ACLU system display that the maximum imaging field of view is 300 mm × 75 mm at the frame rate of 1.5 s/frame. Under a 0.3 mm metal layer, the detecting resolution of the system can still reach up to 200 μm. The system was applied to detect three types of crack and disbond defects of metal/CFRP hybrid composites, and making a side-by-side comparison with phased array ultrasonic testing (PAUT) and radiographic testing (RT). The C-scan results show that the constructed system obtains a high signal-to-noise ratio (SNR) of 23 dB, which improves 4 times and 20 times compared with PAUT and RT, respectively. It indicates that the developed ACLU system hold a great prospect in non-contact large-field inspection of metal/CFRP hybrid composites.