Based On Lfm Pulse Wave L < Sub > Cr < / Sub > Stress Wave Detection Research

Ultrasonic stress measurement is becoming an important way in nondestructivestress testing due to its specific advantages such as high resolution, rapid respond andperfect maneuverability. As a specific ultrasonic longitudinal wave, the criticallyrefracted longitudinal (LCR) wave travels within an effective depth underneath thesurface and is sensitive to the stress. The stress measurement with LCRhas highpotential to predict stress level in engineering field.Usually, the continuous and impulse ultrasonic signals are both available in stressdetection. In this paper, firstly, the conventional approaches are listed basedabove-mentioned ultrasonic signal, then the principle is analyzed and the error sourcesare shown. Based on the analysis, a new ultrasonic signal, namely linear frequencymodulated (LFM) impulse signal, is used to detect stress. The LFM impulse signalprovides more transmitting energy with lower voltage and LCRwave is easier separatedfrom refracted waves easier than conventional signals.Based on the sound elasticity principle, velocity change of LCRwave caused bystress is transformed to time delay estimation of ultrasonic wave propagation. In thispaper, advantages of LFM pulse signal to improve timing are presented. Thesecharacteristics are simulated with MATLAB. A PSpice (Personal Simulation Programwith Integrated Circuit Emphasis) model for PZT is proposed with OrCAD to verifythat the LFM-style LCRwave that is radiated with LFM impulse is available. Then aLFM impulse signal generator with Direct Digital Synthesizer (DDS) is designed toradiate LCRwave. Matched filtering technique is employed to separate the LCRwave,and cross-correlation time delay estimation algorithm is used to timing delay ofultrasonic wave propagation.The results of experiment have proved that the method, namely stress measurementwith LCRwave based on LFM impulse, is valuable to improve the sensitivity andaccuracy in stress measurement with LCRwave.