Based On Lfmcw L <sub> Cr </ Sub> Rinpoche Stress Testing

The working state of mechanical components is indicated by their inner stress situation. So, measuring inner stress efficiently is very important to improve the security, reliability and stability of products. In the past few years, ultrasonic detection technique, as one of the most potential experimental stress measurement methods, developed very fast due to its specific advantages like good non-destructive, high detection accuracy, and so on. As a specific ultrasonic longitudinal wave, the critically refracted longitudinal (LCR) wave becomes important in the field of ultrasonic stress measurement due to its specific mechanism of production and its sensitivity to the stress. However, there are some great inconveniences of the LCR wave stress measurement method. For instance, the change of LCR wave propagation velocity is too little to measure with popular timer, and the co-existence of shear waves and longitudinal waves leads to difficultly separate LCR wave from refracted waves.In this paper, firstly, the advantages and disadvantages of stress measurement with LCR wave using impulse wave exciting method are listed based on some experiments. And then a new ultrasonic exciting method, namely linear frequency modulated continuous wave (LFMCW), is used to excitated LCR wave. As a continuous wave, compared with impulse wave method, the LFMCW exciting method could provide more transmitting energy with lower voltage.In the method of stress measurement with LCR wave based on LFMCW, the propagation velocity of LCR wave is transformed to the frequency response of a sine wave, which is more flexible to measure. The LFMCW signal generator and frequency measurement are the two important parts, which influence the accuracy of stress measurement directly in this method. In this paper, firstly, simulation with Matlab is achieved to validate the feasibility of this method. Then a LFMCW signal generator with Direct Digital Synthesizer (DDS) is designed to excitated LCR wave. The mixer and filter are also designed to separate the beat frequency signal generated by LCR wave from all signals, and a frequency measurement algorithm based on parabola interpolation and least square method is used to measurement the frequency of the beat frequency signal generated by LCR wave.The results of experiment have proved that the new method, namely stress measurement with LCR wave based on LFMCW, is one of the most useful methods for further research on the stress measurement with LCR wave. It could resolve the problems like more energy requirement to the transmitting system due to attenuation of LCR wave, higher measurement requirement to the detection system due to the existence of many other ultrasonic waves, and so on.