Air-coupled ultrasonic testing of metals using broadband pulses in through-transmission

A pair of air-coupled electrostatic ultrasonic transducers with polished metal backplates have been used to test metals using broadband pulses in non-contact through-transmission. A range of aluminium samples between 1.7 mm and 12.9 mm thick were successfully tested using entirely air-coupled non-contact bulk waves. Individual longitudinal reflections and mode-converted shear waves could be seen in the signals obtained through the thicker samples. The results demonstrate that it is possible to both generate and detect discrete broadband pulses in metals using air-coupled transducers, making entirely air-coupled through-thickness NDT of metals an achievable goal.