Study On The Extraction Of Useful Segment With Hilbert Envelope For Stress Measurement By LCR-ultrasonic

With the development of science and technology, ultrasonic stress measurement isbecoming an important way in material properties and stress analysis, prediction of thefatigue, detection of weld, test of heat treatment and surface treatment, measurementof residual stress, etc.As a specific ultrasonic longitudinal wave, LCR(the critically refracted longitudinal)wave is sensitive to the stress and travels along the surface of the material. Researchersand scientists are more and more interested in the ultrasonic stress measurement basedon LCRwave. A mathematical model of the LCR-wave measurement system is discussedbased on the sound elasticity theory.Usually, time delay estimation of ultrasonic wave propagation is used to reflect thevelocity change of LCRwave caused by stress. After analysis of various delay estimationalgorithm, generalized cross-correlation algorithm is used to timing delay. Thealgorithm is very easy in program, it has high accuracy and an outstanding anti-noiseability. And then, a technology of envelope signal auto-extraction is used to improve theintegration and intellectualization of the LCR-wave measurement system in this paper.The envelope of signal is presented according to the Hilbert transform; the useful signalis extracted by screening the peak of envelope.Finally, the LCR-wave measurement system is designed. DDS (Direct DigitalSynthesizer) is used to generate a LFM (Linear Frequency Modulated) pulse source tostimulate the LFM ultrasonic. Echo signal is collected automatically with the Labviewand Oscilloscope, processed with the technology of envelope signal auto-extraction,calculated with the generalized cross-correlation algorithm. According to the dataanalysis, the technology of envelope signal auto-extraction is very useful to improve theprecision of the LCR-wave measurement system.