Study on Travel Time Measurement with LCR Wave in Shear Stress Evaluation

In this paper, an experimental system of stress evaluation is introduced by using a special type of ultrasonic called critically refracted longitudinal (L_CR) wave. Although the strain inside the object may affect the propagation velocity of the L_CR waves in the direction of the stress field, actually it is difficult to measure the velocity change corresponding to the stress variation for its negligible alteration. The more effective way tends to measure the wave´s travel time in a fixed distance, which could be obtained through a particular method called zero crossing detection. In other words, collecting travel time data to further gain the linear relationship between time and stress is the essential problem in stress evaluation. The whole system is made up of several basic parts that work harmoniously under the control of MCU. These components include a drive unit, receiving unit and time measuring unit using high precision TDC (time to digital converter) as the time interval measuring chip, which are discussed in detail in this article