Research on LFM signals in steel materials

Linear frequency signal (LFM) signal is first used in radar fields due to its remarkable properties, such as wide bandwidth, high energy and high signal-to noise ration. LFM excitation has recently been introduced in medical ultrasonic scanning, and much exciting results were obtained by research staffs. However, there is little study aiming to inspect solid materials utilizing coded excitation. In the present study, LFM signal were distinguished with pulsed compression method even merged noise. LFM signal with large time-bandwidth product can be designed theoretically, but the limited bandwidth of a transducer will lead to energy loss. Several coded signals were used to excite transducer, then the received signals´ amplitude and spectrum were analyzed. The results show that the amplitude will decrease when the frequency exceed the transducer´s bandwidth. In order to investigate the transmitting law in solid materials, the waveform and spectrum were analyzed after LFM signals penetrating steel materials with different thickness. Through the analysis, it is found that wave at high frequency band attenuates much quickly than wave at low frequency band. The results of investigations of the field structure in a steel plate are provided. The results of measured pressure field distributions before and after compression were compared with the recorded signals using short pulse excitation. The characteristics of waveform and spectrum were analyzed with LFM coded excitation. The SNR of LFM signal at output side will be improved with pulsed compression method. At the same time, the range resolution increase after matched filter. The sound field on the surface of a steel plate was measured using several LFM coded signal excavating. This paper shows the feasibility of inspecting steel material with LFM signals.