Ultrasonic non-destructive testing using pulse compression

In the testing of highly absorbent materials, it is necessary to use high transmitted power to obtain echoes with an acceptable signal-to-noise ratio from deep defects. However, the maximum peak power which can be used is limited by the construction problems of the probes and the physical properties of the crystal materials. Using longer pulses to transmit more energy could improve the detection, but would reduce the resolution of the system. Pulse compression techniques which overcome the conflict between resolution and pulse duration, provide a possible solution to the above problem. The method involves the transmission of a long coded pulse and the processing of the received echo to obtain a relatively narrow pulse, thus preserving resolution. After a study of the principles of pulse compression, various practical schemes were investigated, and the linear frequency-modulated pulse compression systems were found to be most economical to implement. Upon being received, the pulse may be compressed by means of a dispersive ultrasonic delay line, and simple Gaussian shape filter may be employed to reduce the resulting sidelobes. Theoretical studies on the dispersive modes of propagation of elastic waves in narrow metallic strips were then made, and demonstrated the feasibility of using a metallic strip as the dispersive delay device, provided that equalisers are introduced to compensate for the inherent time delay non-linearities in the strip. Design problems associated with the piezoelectric bar transducers for use with the line were also investigated. Based on the above studies, a pulse compression testing system consisting of a transmitting unit, a pair of wide-band transmitting and receiving transducers and a receiving unit, has been constructed. The transmitting unit comprises a linear frequency-modulated oscillator and timing circuits; the receiving unit incorporates equalisers, a weighting filter and an aluminium strip delay line. The operating system achieves a time bandwidth product of 80 and a sidelobe level of -25 dB. Practical tests were carried out and test results are reported. Finally, the power and limitations of the testing system are discussed.