Air-coupled ultrasonic inspection of composite materials for aerospace applications

Inspection of carbon fiber reinforced polymers (CFRP) using air-coupled (AC) ultrasonic measurements is challenging due to the large acoustic mismatch of the material to air. The acquired signal-to-noise ratio (SNR) of measurement process is not enough to obtain the required sensitivity of inspection. Pulse compression (PuC) is applied together with other signal processing tools to overcome the problem of low SNR of the measurement process. In this work, a linear chirp signal is used to implement PuC for the AC ultrasonic inspection of a CFRP sample for the delamination identifications. It is shown that the defects are characterized with a good lateral and spatial resolution in C-Scan images obtained by exploiting both the time amplitude and phase parameters of the retrieved impulse response. The required sensitivity of inspection is achieved by optimizing time duration and bandwidth of the applied chirp signal in accordance with the measurement setup.