Anastassios Skarlatos

Electromagnetic Modeling of a Damaged Ferromagnetic Metal Tube by a Volume Integral Equation Formulation

We propose a volume integral equation formulation for eddy-current nondestructive evaluation of ferromagnetic tubes affected by volumetric defects. We solve the system of integral equations (introduced into the model by application of Greens theorem) by the method of moments for both fictitious electric and magnetic currents within the defects, and calculate variations of impedance of the probes by the reciprocity theorem. We have made thorough comparisons of our results with finite-element-method simulations and with experimental data as well.

Impedance Calculation of a Bobbin Coil in a Conductive Tube With Eccentric Walls

The complex impedance of an air-cored coil in a conductive tube with eccentric inner and outer cylindrical surfaces is calculated. The analytic expressions for the induced fields and the impedance variation due to the eddy-current flow inside the tube wall are derived using a second-order potential approach. The addition theorem of Bessel functions is employed to perform the transition between the local coordinate systems that conform to the boundaries of the structure. Although the model can be used for any coil shape and orientation, we focus our study on the configuration of a bobbin coil with axis parallel to the axes of the tube surfaces, but not necessarily coinciding with either of them. The results of the presented analysis are verified by a finite-element-method (FEM) solution.

Solution of radiation and scattering problems in complex environments using a hybrid finite integration technique - uniform theory of diffraction approach

The finite integration technique (FIT) is combined with the uniform geometrical theory of diffraction (UTD) for the solution of radiation and scattering problems in complex environments. The presented hybrid formulation is capable of handling large objects allowing in the same time a precise modeling of important geometrical details and material inhomogeneities. The part of the structure which contains the details of the geometrical model and the material inhomogeneities is discretized and solved using the FIT discretization scheme, whereas the influence of the large scatterers to the total solution is resolved by means of UTD. In contrast with other finite-difference-based hybridizations, in the presented formulation the case of the strong coupling between the two subproblems without simplifications is considered. This is accomplished by introducing an appropriate boundary operator derived by the equivalence principle. The resulting equation system possesses a complex, nearly dense system matrix, which is difficult to handle even using iterative solvers. To overcome this difficulty the system of equations is solved using a two-step procedure, i.e., the FIT equation and the boundary condition are treated separately.

Analytical Treatment of Eddy-Current Induction in a Conducting Half-Space With a Cylindrical Hole Parallel to the Surface

The analytical solution for the eddy-current induction in a conducting half-space containing an infinitely long cylindrical hole parallel to the interface is derived by means of the second-order vector potential and the domain truncation technique. The eddy currents are induced by a cylindrical coil which can be placed either in the nonconductive region above the half-space or inside the hole or in both locations simultaneously. Analytical expressions for the coil impedance are derived in both cases. The problem is treated as a coupled scattering problem and the derived analytical expressions permit a clear physical interpretation of the different terms, thus enhancing the understanding of underlying physics. The validity of the solution is demonstrated by comparisons to results obtained with the finite element method.

Coupling of finite integration technique and ray tracing

We present a hybrid formulation which combines the finite integration technique (FIT) with a ray tracing technique. Ray tracing allows the treatment of complicated structures with very large objects. A typical application of ray tracing techniques is the simulation of indoor propagation. The presented hybrid technique has therefore a rather broad field of applications in contrast to other hybrid methods. A critical point in the implementation of the hybrid method is the modeling of the sources for the ray tracing part. The sources used by these techniques are considered usually as point sources with a given directivity. Applying the Franz integral representation for the far fields, the field distribution over the boundary surface can be substituted by a single point source. This approach can improve the performance of the method dramatically.

Semi-analytical calculation of the low-frequency electromagnetic scattering from a near-surface spherical inclusion in a conducting half-space

The low-frequency electromagnetic scattering problem of a near-surface hollow spherical inclusion is solved by means of a modal approach. The primary field is excited using a current-carrying coil. The presented solution addresses the full coupling between the two interfaces of the geometry in a rigorous way. There is a direct physical interpretation of the different occurring terms in the formal solution, which provides a deeper understanding of the underlying phenomena. The calculation is very fast, which makes the proposed model suitable for use with parametric inversion algorithms.

Start vector generation for implicit Newmark time integration of the wave equation

Different extrapolation strategies are presented for the construction of starting values in implicit Newmark time-stepping schemes used for the solution of the electromagnetic wave equation. The implicit character of this schemes implies the solution of a system of linear equations in each time step. The choice of a good start value derived from the extrapolation of the equation system is shown to improve the convergence of the iterative conjugate gradient solver which reduces the total simulation time

A hybrid technique combining the FDTD method and the time domain UTD

Abstracr: A hybrid technique that couples the FDTD with the TD-UTD is presented. The total problem is divided into two sub-problems: an interior which is solved using the FDTD and an exterior where the TD-UTD is applied. The transition between the two problems is achieved by means of the Stratton-Chu integral representation. The coupling is realised in time domain. Time Domain (TD) methods have become particularly interesting during the last years due of wideband excitation very effectively. TD methods which are based on rigorous numerical schemes, like FDTD and MOT (marching-on-time), although very useful for many classes of problems, become inefficient when, they are applied to large structures. This obstacle can be overcome by hybridizing methods of this class with asymptotic techniques. There are many hybrid formulations reported in the literature but most of them are either implemented in Frequency Domain (FD) or they are combining rigorous TD methods with asymptotic techniques in FD. The transition from one domain to the other is accomplished via an inverse Fourier Transform which however results to a decreased physical insight into the overall process. It would be therefore advantageous to develop a hybrid method with all its steps realized in TD. Such a method combining FDTD and TD Physical Optics is presented in [l]. to the importance of the analysis of transient phenomena as well as their ability to treat problem In this paper we present a hybrid technique that combines the Finite Integration Technique FIT (an alternative formulation of the FDTD) [2] and the Time-Domain Uniform Geometrical Theory of Diffraction [3]. The presented method can be applied to very complicated structures including both small and large structures as well as inhomogeneous materials. In contrary to other hybrid methods, the presented method is implemented purely in time domain which makes the whole procedure physically consistent by means that skipping from one domain to another is avoided. This fact in conjunction with the ray concept used by UTD, offers a good understanding of the physical mechanisms involved in the problem.

Impedance of an induction coil accounting for the end-effect in eddy-current inspection of steam generator tubes

A fast model for the eddy-current signal calculation of thin cracks at the vicinity of a conducting tube edge is presented. The model is based on the surface integral equation formalism using a dedicated Greens function. The approach followed for the construction of the Greens kernel and the primary field is the Truncated Region Eigenfunction Expansion (TREE) method.

ECT-Signal Calculation of Cracks Near Fastener Holes Using an Integral Equation Formalism With Dedicated Green’s Kernel

Simulation of flaw detection by eddy-current testing requires the calculation of the eddy-current signal due to the presence of the flaw. In this paper, the surface integral method (SIM) is applied for the calculation of the crack signal in the workpieces composed of a conducting plate with a borehole. The adopted approach makes use of Greens operator that takes into account the effect of the borehole, and thus, the integral formalism is restricted to the crack surface only. The truncated region eigenfunction expansion method is followed for the construction of Greens operator. Comparisons of the computed crack signal with numerical simulations carried out using a hybrid volume integral method-SIM approach and experimental results show excellent agreement.