Estimated Based On The Cross-correlation Delay L <sub> Cr </ Sub> Wave Tangential Stress Detection System

It is important to measure the stress distribution on material or component, not only for studying the material properties and force condition, but also for predicting the fatigue status for component, examining the effect from heat treatment and surface treatment, monitoring the cutting processing technology, evaluating the result about the process of stress relieving etc. In engineering field, especially in machinery, aerospace, transportation, defense and nuclear industry, the nondestructive detection technique for stress with ultrasonic has developed into a very important subject.In this paper, the essential characteristics of Critically Refracted Longitudinal wave (L_CR wave) are described based on the propagation properties of ultrasonic wave firstly. There is different from the other ultrasonic waves. As a special longitudinal wave, the L_CR wave propagates in material under the surface within small depth, and it is the most sensitive to normal stress when the propagation direction of L_CR wave is parallel to the action direction of normal stress. However, there are many factors to block the development and application of the nondestructive detection technique for stress with L_CR wave, for instance, the variance of propagation velocity of L_CR wave is too small to measure with conventional way, and it is difficult to extract the L_CR wave from the refracting waves effectively. Because there are shear waves and longitudinal waves coming from the refraction. It becomes the research emphasis to overcome the influencing factors that mentioned on above in this paper. A new way for stress measurement with L_CR wave is introduced based on sound elasticity principle and the high accuracy time measurement algorithm with cross-correlation time delay estimation in this paper. A exciting mode is selected to generate L_CR wave with pulse wave.A complete experiment system based on the working principle that measured the stress with L_CR wave is set up. In the system, the fixing device for ultrasonic transducer was designed firstly, at the same time, the driving circuit is developed excite transmitting probe. Secondly, the program for automatic data acquisition to achieve sampling from received signal with the data acquisition function of digital oscilloscope is built. Then, the wavelet filter is used to perform the digital signal processing . The high accuracy time measurement algorithm based on cross-correlation time delay estimation is finish.Serial tests have been completed with the experiment system, several results based on experimental data are present in detail. It is shown that the way not only has a good feasibility and operability, but improved the sensitivity and accuracy in stress measurement with L_CR wave.