Abstract: The experiment reliability and defect quantification accuracy of air-coupled ultrasonic testing are highly affected by the bad SNR(signal to noise ratio) of the through transmitted signal because of the high damped medium property in air and giant mismatch of acoustic impedance in air-solid interface. To deal with these handicaps, the matching filter design and its implementation in air-coupled ultrasonic inspection based on the theory of phased Coded pulse compression are discussed. Influences of different sequences, bandwidth and cycles of Barker code on pulse compressed results are analyzed either. SNR of the through-transmitted signal and defect quantification accuracy of C-scan results on glass fiber reinforced polymer(GFRP) specimen are compared using single frequency excitation and pulse compression method, respectively. The results show that the smallest time width and peak amplitude of the main lobe are acquired using central frequency of the transducer as bandwidth of Barker code. Using phase coded pulse compression method, the SNR of the through-transmitted signal is increased about 12.4 dB; the quantification accuracy is increased about 12% compared to the single frequency excitation, and the edge of the defect is better highlighted. The results represent that the SNR of the through transmitted signal and defect quantification accuracy are apparently increased by using phase Coded pulse compression method in air-coupled ultrasonic testing.

Key words: phase coded;pulse compression;air-coupled ultrasound;non-destructive testing;signal processing;composites