Research On Numerical Methods To Electromagnetic Scatting Problems Of Microwave Imaging For Reinforced-concrete Structure

The non-destructive testing of reinforced-concrete is an important mean for the concrete quality monitoring, diagnosis and assessment. Microwave imaging technology-solving the inverse problem makes the internal structure of reinforced concrete observations possible. Microwave imaging requires not only mathematical algorithms in the inverse problem giving the support and improvement, but also giving the systematic exposition in the inverse problem theoretically, that is, the existence of solutions and uniqueness, etc. must be expressed. Moreover, the inverse problem iterative solving process often requires iterative calculation of electromagnetic scattering-the direct problem middle data. Therefore, the study of the direct problem is very necessary.In this paper, through comparing various algorithms, Finite-Difference Time-Domain method (FDTD) for electric waves is selected for calculation which is applicable to this subject. According to the field divided requirements of FDTD algorithm, two-dimensional model of electromagnetic scattering is established. Grid subdivision for the model has been done which should satisfy the Courant stability condition and numerical dispersion requirements. Using Yee's differential outside the absorbing boundary-Perfectly Matched Layer (PML), and in the PML-layer using index differential equations for unit calculation. Moreover, the calculation mean of the variation conductivity in the PML was given. According to the principle of equivalence, the incident plane wave was laded to the total field boundary with 0°angle. The algorithm programming has done to the established model, and scattering results of reinforced-concrete in different locations have been got. Through the figure of electric field strength-location relationship, the location of steel bar in the concrete was clearly reflected, and results are in line with the actual model.The electromagnetic scattering of reinforced-concrete models were also calculated with commercial finite element software ANSYS. By comparing with FDTD finite difference method, in light of the subject actually requiring, it was obtained that FDTD method in two-dimensional model calculation is more satisfied to the microwave imaging (inverse problem) requirements. The energy distribution of high-frequency plane electromagnetic wave scatting in medium are studied by using the powerful post-processing function of ANSYS, and the phenomenon of energy regional distribution is observed by experiment. Finally, the results of FDTD algorithm are compared with the ANSYS simulation results, and the two data curves match well. This illuminates the correctness of FDTD in reinforced-concrete electromagnetic scattering calculation.