Mihai Rebican

Shape reconstruction of multiple cracks from ECT signals by means of a stochastic method

This paper presents an approach to reconstruct multiple cracks from eddy current testing signals by means of a stochastic method such as tabu search. The size, number, and locations of cracks are considered unknown in the inversion process, whereas only the orientation of cracks is supposed to be revealed in advance. Eddy current inspections of a stainless steel plate with closely located EDM slits are performed, and the inversions of measured signals are carried out. The results agree well with the true ones, which validates the proposed algorithm

Fast simulation of ECT signal due to a conductive crack of arbitrary width

This paper proposes a strategy for the fast simulation of eddy current testing signals due to a conductive crack of arbitrary width. To cope with a crack of width less than that selected for establishing the database, which is necessary in the fast-forward analysis scheme proposed by authors, a new finite element is introduced to treat the case when the crack boundary is contained in the element. By using such a new element, the fast-forward analysis scheme becomes suitable for the reconstruction of both the shape and width of a planar crack. It is verified that such a multiple material element is efficient for an ECT sensor inducing eddy current parallel with the crack surface. For the case with perpendicular eddy current component, however, the approach is not valid because of a scalar potential jump at the crack surface. Finally, the reason of such a singularity is investigated through numerical simulations

Three-Dimensional Simulation of Remote Field ECT using the Ar Method and a New Formula for Signal Calculation

ABSTRACT In this article, a three-dimensional (3D) edge-element finite element method (FEM) code of the Ar method is upgraded to enable the 3D simulation of remote field eddy current testing (RFECT) problem by introducing a new formula for the pickup signal calculation. The validity of the proposed formulation and the corresponding implementation are verified by comparing numerical results of the 3D code with the measured signals and those of an axisymmetric code. The numerical results show that, although both the conventional formulation and the proposed method are for a nonmagnetic tube, the upgraded code has a more satisfactory simulation accuracy for a tube of ferromagnetic material. It is also proven that the upgraded Ar-method code is very efficient in the 3D simulation of the RFECT problem and even a normal PC can accomplish the calculations.

Investigation of numerical precision of 3-D RFECT signal simulations

This paper evaluates the numerical precision of a three-dimensional A/sub r/ method with edge-based finite elements in the simulation of remote field eddy-current testing of a ferromagnetic tube. The method is employed with a suitable formula for the electromotive force signal calculation in magnetic materials. Using a supercomputer, analyses of the mesh division effect on numerical precision are carried out to find out the proper mesh conditions for signal simulations of 3-D defects. Good agreements of the simulated signals with experimental ones are obtained using this method with an acceptable computational resource. In this way, the 3-D simulation of remote field eddy-current testing signals for a magnetic tube has become possible in practical applications.

Numerical model for eddy-current testing of ferromagnetic steel parts

The paper presents an efficient numerical method to simulate eddy-current testing (ECT) of ferromagnetic steel parts. The method is based on the nonlinear integral equation for quasi-stationary electromagnetic field with gauge condition based on tree/cotree decomposition. The eddy-current density in the conductive domain and the magnetization in the ferromagnetic domain are discretized with edge elements and vectorial volume elements, respectively. A simple, but very effective magnetization model of steel in a low field is also presented. The algorithm convergence for solving the forward ECT nonlinear problem with time-periodic boundary conditions is proved.

B‐H characteristic extraction using devices with non‐uniform field

The paper describes an efficient method to extract the B‐H nonlinear characteristic from the experimental flux‐current Φ‐I data obtained using a non‐uniform magnetic field device. Both functions are monotonically piecewise linear approximated with the same number of breakpoints. The method was successfully applied to characterize the ribbon core material of a fluxset magnetic field sector. In this case the hysteresis loop and the lumped magnetic circuit were extracted. Comparison with experimental results validates the proposed method.

Three-dimensional inversion of volumetric defects profiles from electromagnetic nondestructive testing signals by means of stochastic methods with the aid of parallel computation

This study applies five stochastic inversion methods to the reconstruction of three-dimensional volumetric defect regions from eddy current testing signals, and evaluates their efficiencies in relation to this problem. The five stochastic methods considered here are iterative local search, tabu search, simple genetic algorithm, parameter-free genetic algorithms, and simulated annealing. Since stochastic methods require evaluation of many solution candidates, simulations here are performed on a supercomputer aided by parallel computation to avoid too lengthy computational times. Three-dimensional volumetric defects in a thin flat conductive plate are considered. The results of this study demonstrate that the stochastic methods applied to eddy current inversion problems are highly compatible with parallel computation, and that computational time can be significantly shortened with the use of parallel computation. Among the five algorithms adopted in this study, tabu search and simulated annealing provided good results whereas genetic algorithms had very poor performances.

Diagnosis of real cracks from eddy current testing signals using parallel computation

Abstract The paper presents a novel approach for three-dimensional diagnosis of partially conductive cracks from two-dimensional eddy current testing signals by means of the tabu search stochastic method. A new testing probe driving uniformly distributed eddy currents is employed for the inspection. Three spatial components of the perturbation electromagnetic field due to partially conductive cracks are sensed as the response signals in order to enhance information level of the inspection. The signals are simulated by a fast forward FEM-BEM solver using a database. Cracks are modeled as defects with a complex shape and homogeneous conductivity. The length, depth, width and conductivity of a detected crack are unknown in the inversion process. Numerical results of the three-dimensional reconstruction of partially conductive cracks from simulated two-dimensional signals are presented and discussed in the paper. Artificial white noise is added to the simulated signals and robustness of the algorithm is tested. Moreover, parallel computation is employed to reduce the computation time.

Advanced PSO algorithms and local search strategies for NDT-ECT inverse problems

This paper studies the efficiency of some advanced Particle Swarm Optimization (PSO) algorithms, such as Standard PSO and Quantum-behaved PSO, to solve a nondestructive eddy current (NDT-ECT) inverse problem. The inverse problem is formulated as an optimization problem, aiming to minimize the distance between the true 2D ECT signal, produced by the real crack and the simulated signal, produced by a potential solution (crack). To increase the efficiency of the PSO based methods, enhancements based on problem specific local search strategies are also tested in order to speed up the optimization process. The inversion schemes are compared in the three-dimensional reconstruction of partially conductive cracks, which have a uniform conductivity smaller than the conductivity of the base material and a cuboid shape.

Advanced procedure for non-destructive diagnosis of real cracks from eddy current testing signals

The paper presents an advanced procedure for tree-dimensional diagnosis of real cracks from two-dimensional simulated eddy current testing response signals by means of the tabu search stochastic method. A new testing probe driving uniformly distributed eddy currents is employed for the inspection. All three spatial components of the perturbation electromagnetic field are acquired during two-dimensional scan of the probe. The ECT signals due to partially conductive cracks are simulated by a fast forward FEM-BEM solver using a database. Two crack models are proposed for the inversion. The first one has a cuboid shape and the later one reflects a more complex geometry. Both the cracks models consider uniform distribution of the partial conductivity. Numerical results of the three-dimensional reconstruction of partially conductive cracks from simulated two-dimensional signals are presented and discussed in the paper.