Numerical Research Of Forward And Inverse Methods Of Concrete Ultrasonic Tomography

Concrete is one of the most fundamental building materials in engineering today.However,the concrete structure,in addition to its own complex internal structure which contains many micro holes and cracks,will have many defects,such as holes and muds because of improper construction during architecture engineering.These defects would not only affect the durability of the concrete structure itself,but also have some damage to the safety of the whole building.Concrete ultrasonic tomography technology can reflect the structure of the inner tomography surface of the concrete structure by using the way of image,which makes it possible to quantitatively study the position,size and property of the defects.In the field of concrete crack detection,generally speaking,concrete ultrasonic tomography technology is the necessary means of development through the stage of nondestructive examination to the stage of nondestructive diagnosis.Aiming at solving this comprehensive subject,this paper makes a study of concrete ultrasonic tomography technology,starting with the theoretical foundation.And then,this paper goes further into the forward ray tracing method and the inversion algorithm.The main contents are as follows:To begin with,the domestic and international related literature was widely collected and organized.In order to discuss the proper tomography method used in concrete medium,the characteristics of the sound wave propagating in concrete medium,the basic methods of concrete ultrasonic testing and the mathematical basis of tomography were described briefly.The model,equation and system flow of the concrete ultrasonic tomography technology were introduced carefully.Then,according to the feature that the propagation path of ultrasonic wave in concrete structure is not a straight line,the ray tracing methods which is applicable to this case were studied.After analyzing the shortcomings of the traditional ray tracing methods,two kinds of high precision new ray tracing methods,called the shortest path ray tracing method and the linear travel-time interpolation ray tracing method,were pointed out.The principle,model and detailed steps of the two methods were introduced specifically.Comparing the ray tracing results of two methods through numerical simulation,this paper found that the results of the linear travel-time interpolation method had higher accuracy and less computation time.Therefore,the linear travel-time interpolation method was chosen to trace the rays in the next part of this paper.The last but not the least,after analyzing the characteristics of the inversion equation of tomography,it was found the equation can't be solved by classical methods.This paper carefully described the principles and procedures of several iterative inversion methods,including LSQR method,ART method,SIRT method and SART method.By analyzing the properties and disadvantages of these method,an improved SART method was proposed.In this method,the updating formula of SART was weighted with the result of simple back projection method,and the appropriate relaxation factor was added to improve the speed of algorithm and the accuracy of the reconstruction result.Using the numerical simulation to compare the results of SART method and the improved SART method,it was found that the results were in line with the expectation.The improved SART method enjoyed a higher convergence speed and better reconstruction result.