High-resolution ultrasonic non-destructive testing
The use of ultra-short pulse wideband ultrasonic transducers in Non-Destructive-Testing (NDT) has been investigated both theoretically and experimentally. It is demonstrated that the resolution of pulse-echo NDT is affected by diffraction effects which also complicate the interpretation of echo signals. These diffraction effects are interpreted in terms of the plane- and edge-wave model of *transducer fields. Improverents can be obtained by the use of non-uniformly excited transducers of two basic types: the first, the plane-wave-only (PWO) source; is more strongly excited at its centre than towards the rim, where the excitation is gradually reduced to zero in order to remove the edge wave. The second type, an edge-wave-only (EWO) source, is more strongly excited at its rim than in the centre, thereby effectively removing the plane wave. Computer modelling of pressure waveforms in the field of PWO and EWO sources has been carried out using an extension to the impulse response method. Experimental point-pressure waveform measurements in the field of a prototype EWO transducer, made using a miniature ultrasonic probei are in reasonable agreement with the calculated results. Detailed calculations are made of the transmit-receive mode (pulse-echo) responses arising from solid targets of various size in a flu- id medium interrogated by uniformly and non-uniformly excited sources. The theoretically predicted results are in good agreement with experimentally measured results obtained using a conventional transducer and an equivalent prototype EWO transducer. The effects of target size, field position and material on both the amplitude and shape of the echo responses are investigated. The structure of the responses is explained in terms of the plane and edge waves radiated by the source. Implications for the use of techniques to both size (Distance, Gain, Size curves) and characterise (ultrasonic spectroscopy) defects are examined. The applications of new, nonuniformly excited transducers in high-resolution NDT and ultrasonic imaging are evaluated.