Comparative analysis of laser ultrasonic propagation imaging system with capacitance and piezoelectric air-coupled transducers

Air-coupled nondestructive testing is an attractive technique that allows easy scanning inspection capability due to its noncontact nature with the specimen. A piezoelectric air-coupled transducer has been combined with a laser ultrasonic propagation imaging system as a fixed noncontact sensor. However, the large impedance mismatch between air and a specimen and its narrowband frequency range has prevented it from being efficiently used in real-world applications as a laser-air-coupled transducer ultrasonic propagation imaging system. Therefore, instead of combination with a piezoelectric air-coupled transducer, a combination between a laser and a capacitance air-coupled transducer is proposed through a comparative analysis between the capacitance and piezoelectric air-coupled transducers as the ultrasonic detector of the ultrasonic propagation imaging system. Under the same conditions, a series of experiments was conducted to examine the damage sensitivity in metal and composite specimens that have a hidden crack and barely visible impact damages, respectively. This noncontact system was further improved using repeat-scanning technique to enhance the signal-to-noise ratio and to overcome high attenuation problems, especially in composite specimens.