Through transmission ultrasonic NDT without coupling fluids

The difficulties associated with through transmission airborne NDT are discussed using an acoustic simulation model. The model, based on a combination of finite element and linear systems theory, is used to optimise a separate transmit-receive transducer arrangement operating under high energy, narrowband conditions. A series of composite transducers are then evaluated theoretically and tested for airborne operation at frequencies between 250 kHz and 1.5 MHz, using excitation voltages down to 50 V. Through transmission airborne signatures are demonstrated on a number of solid samples, including glass, aluminium and carbon-fibre composite plates ranging from 3-50 mm in thickness