Parameter Design of Linear Frequency Modulated Excitation Waveform for Ultrasonic Nondestructive Testing of Metallic Materials

Linear frequency modulated (LFM) pulses are used as excitation sources for the ultrasonic nondestructive testing of metallic materials. The detection resolution relies on both the bandwidth of the LFM pulse and the amplitude spectrum of the transducer. A bandwidth selection method to analyze the effect of the LFM bandwidth on the inspection results was proposed, as well as to select an optimized LFM bandwidth corresponding to different transducers. Meanwhile, an ultrasonic through- transmission technique to test four different pairs of transducers excited by LFM signals with different bandwidth was used. Experimental results show that the time resolution significantly improves with an increase in the LFM bandwidth. Optimum parameter is achieved when the LFM bandwidth is close to the \(-\)12 dB bandwidth \(B_{-12\hbox {dB}}\) of the transducer, and it then remains roughly stationary. The results were confirmed by carrying out ultrasonic LFM time-of flight diffraction (LFM-TOFD) testing for a steel plate with three buried defects employing a pair of 10 MHz transducers. The waveforms of the echoes extracted from these defects become narrower with an increase in the exciting bandwidth. In addition, the diffracted waves from the upper and lower tips of the 2 mm defect can be distinguished when the LFM bandwidth is close to the \(-\)12 dB bandwidth of the 10 MHz transducer. The proposed bandwidth selection method can enhance the efficiency and precision of the actual ultrasonic LFM detection and is suitable for different ultrasonic testing systems, transducers and inspection requirements.