Methodology And Experiment Research On Concrete Ultrasonic Computerized Tomography

There exist inevitably some macroscopical injuries and defects in concrete structure in the process of building and using, because of the reasons in design, construction, loads and materials so on. These defects should affect the safety and durability of structures seriously. Ultrasonic computerized tomography for concrete introduced recently, might reflect the inner defects of concrete by images in tomography. That is to be inevitable development tendency in ultrasonic test for concrete from non-destructive test to non-destructive diagnosis. At present, the theoretical research in this field is still on its early stage. Under the supports of the program of National Natural Science Foundation"method study on ultrasonic computerized tomography for concrete"(No. 50678066), this dissertation has made a relatively deeply and systematically researches from several aspects of theoretical basis, inversion algorithm, inversion strategy, incomplete rays inversion and procession of inversion images. The main content is as follows.(1) The domestic and international literature concerned was extensively collected and summarized, and the mathematical basis of computerized tomography was described briefly. The relative theories and methods on ultrasonic computerized tomography for concrete were systematically summarized. These works have established a good basis for future research.(2) According to the ill-posed nature of ultrasonic computerized tomography for concrete, a inversion algorithm is put forward, which combined the weighted generalized inverses based nature weight matrix with regularization methods of Yang Wencai. The algorithm meets the characteristics of ultrasonic computerized tomography for concrete. The inversion images of NWGI algorithm, conventional SIRT algorithm and CGLS algorithm are compared in terms of numerical models and concrete specimen models. The results show that NWGI algorithm is more excellent than conventional algorithm and has higher resolution and reliability. The efficiency of this algorithm is verified through two groups with a total of 8 concrete specimens which simulated different inner defects. The images results indicate that NWGI algorithm and its inversion images can reflect the location and distribution of inner defects in concrete specimens very well.(3) In order to obtain the real solution coincident with practical case, the inversion strategy of ultrasonic computerized tomography for concrete was studied. ①Qualitative inversion method was proposed to realize computer processing of defect identification by acoustic shadow overlapping method. The results of numerical models and concrete specimen models show that qualitative inversion can identify the location and distribution of regions in which interior sound velocity is high, low or normal approximately.②Based on the prior information obtained by qualitative inversion, the two stages inversion method was put forward. That is, the first stage is that distinguishing the high-quality element and non- high-quality element, and determining the initial iteration value of element sound velocity reasonably. The second stage is that treating differentially the constraint weights of the high-quality element and non- high-quality element, and then quantitatively inversing the final sound velocity distribution. The results of two concrete specimens indicate that the two stages inversion method can inhibit artifacts effectively and has better defect identification effect than normal one stage inversion.③The self- adaptability grid technique was proposed to improve the mathematical property of ray path matrix. The grids which traversed by a few or few rays were merged, and then the updated grids had equilibrium length of ray traversing relatively. That might self-adapt the geometric distribution of ray paths and improve the stability of ultrasonic computerized tomography for concrete from origin. The results of numerical models show that the computerized tomography used self- adaptability grid technique has strong anti-noise ability and capability of defect identification compared with the conventional stationary grids.(4) Adopting the uniform experiment design distributes the incomplete rays, and the feasibility of ultrasonic computerized tomography for concrete by incomplete rays distribution was studied. The results of numerical models show that on the premise of ensuring quantity and representativeness of rays, the defect identification capability of the computerized tomography by incomplete rays distribution can reach or be superior to the computerized tomography by complete rays distribution. This has brought new thinking to the research of computerized tomography. The experiment design methods of distributing incomplete rays were researched as followed.①The stochastic evolution and optimizztion algorithm was improved by means of balancing depth search and width search. The calculation time can be saved about 30～70%.②The fractional steps experiment design method was proposed. With decreasing the search space by the " break up the whole into parts " method, the calculation time of this method can be saved about 40～60% compared with the conventional one step experiment design method. The substitution model of RBF neural network was made respectively by 3 different test functions. The error analyses indicate that the fractional steps experiment design method is very practical for the substitution model needed large scale test sample points.(5) The image processing method of ultrasonic computerized tomography for concrete was studied also.①The median filtering and clustering analysis based on SOFM neural network were introduced to the post-procession of inversion images. The application examples show that both of the two methods can inhibit artifacts effectively, explain the defects and improve the resolution, reliability and readability of tomography images.②Image fusion technology based on wavelet transform was used to fuse information of tomography images in different inversion parameters. The application examples show that this method can integrate information of fused images effectively and make the tomography images fused have less background noise and better defect identification effect.