Comparative Study on Ultrasonic C-Scan Imaging of Composite Lap Joints Using Piezoelectric Transducer: Pulse-Echo and Pitch-Catch Configurations

This article compares low-frequency (LF) Lamb waves in a pitch-catch configuration and high-frequency (HF) bulk waves in a pulse-echo mode for ultrasonic C-scan imaging of composite lap joints. The pulse-echo measurement is conducted by using a delay line transducer. On the other hand, the ultrasonic guided waves are measured in a pitch-catch configuration by using a PZT disk moved by a positioning system over the scan area. In both methods, the signals are acquired in a C-scan manner over the adhesive joint area. Applying signal processing techniques, including frequency-wavenumber filtering and bandpass filtering, along with different feature extraction, the images of the scan area are obtained. For this study, two different types of composite lap joints are used. The first joint consists of composite plates reinforced with woven composite fabric, with defects in the form of missing adhesive. In the second joint, quasi-isotropic composite plates are employed, and disbonds are used to simulate defects. The Fisher discriminant criterion is used to assess the capability of each method in accurately distinguishing defects from the pristine condition. This article concludes with a discussion of the results and effects of using both techniques.