An experimental study on non-destructive detection of gypsum component by using ultrasonic tomography

摘要

Ultrasonic computer tomography is an advanced new technology for non-destructive detection which can identify the velocity abnormalities with high resolution based on the theory of acoustic travel time. The images of tomography for field data depend on many factors such as the survey system, the complexity of the observed component, and the algorithms of forward modeling and inversion, which is usually different from the results by using numerical modeling methods. Therefore, the laboratory tests are very important to evaluate the application effect of tomography. We firstly carried on a series of tomography experiments by observing the travel time for intact gypsum, and then performed the same experiments for the gypsum component with designed flaws. Two different images of tomography can be obtained from the observed travel time of X- and Y-direction source-reception systems by using Moser's curved ray-tracing method and Gauss-Newton inversion method. Regardless of X- and Y-direction survey systems, the results of tomography for intact gypsum show that the sound speed of gypsum component is about 2500 m/s with small randomly disturbance in the magnitude. However, the results for the gypsum with embedded flaw show that an abnormal area with obvious low velocity. The center position of low velocity abnormality is corresponding to the place of the designed flaw. But the shape of image in X-direction source-reception system is stretched and distorted in X-direction, the same in Y-direction survey system. In order to identify the shape of designed flaw with higher resolution, we executed the same computer tomography by using the travel time both in X- and Y-direction source-reception systems. The image of both X- and Y-direction is well agreed with the shape of designed flaw in the gypsum. The experiment tests show that the results of using single X- or Y-direction survey system can detect the approximate position of embedded flaw, however, the results of using both X- and Y-direction systems can provide more reliable and accurate image. Making use of the characteristic analysis of the tomography image for gypsum, we can estimate the factors such as the survey system and inversion parameters for other construction components non-destructive detection.