S. Gratkowski

Eddy Current Testing of Thick Aluminum Plates with Hidden Cracks

In this paper we present theoretical analysis which gives the possibility to determine optimum excitation frequencies in eddy current examination of thick aluminum plates. A computer controlled non‐destructive testing system and a probe with a magnetoresistive sensor were utilized in laboratory tests. Experiments with specimens containing hidden cracks were carried out for a wide spectrum of frequencies. The optimum operating frequencies achieved from the experiments are in a very good agreement with those obtained by using theoretical analysis. Application of the optimum frequencies makes it possible to detect shallow cracks located on the reverse side of the specimens. Detection of a 15 % crack in a 10 mm thick plate made of aluminum and a 20 % crack in a 20 mm thick plate has been confirmed.

New infinite elements for a finite element analysis of 2D scattering problems

Two new formulations of infinite elements are proposed. These elements in combination with the conventional finite elements yield coupled finite element formulations for 2D scattering problems. The first formulation works for any shape of the finite element region, while the second one has been developed for a rectangular outer boundary. In comparison with the previous infinite elements, the present infinite elements are constructed in such a way that simple expressions have been obtained for their element matrices. As a result of this, no formula for numerical quadrature over the infinite region is necessary and the infinite elements can easily be incorporated into standard finite element codes. Two numerical examples are given.

Identification of flaws using eddy current testing

The eddy current method of non‐destructive testing uses an alternating current excitation to induce secondary currents in a specimen under test. Flaws within the specimen affect the induced currents, causing changes in the impedance of a test coil. In this paper we present a method for obtaining a solution of inverse problems, in which the parameters of defects are unknown and the excitation function and the eddy current system response are given. The method is based on the use of artificial neural networks, which are trained using measurements. Illustrative examples are given.

Application of magnetoresistive sensors for eddy current non-destructive testing of materials

The idea of applying the magnetoresistive sensors (MR) for eddy current non-destructive testing (EC NDT) is presented. The probe for EC NDT with MR [1, 2] is described. Results of thick aluminum and steel plates testing are presented.

Infinite elements for axisymmetric electrical field problems with open boundaries

The paper presents infinite elements for axisymmetric electrical field problems with open boundaries. The formulation of the elements is so simple that closed‐form expressions for the infinite element matrix are obtained. In order to test the infinite elements, a simple problem, for which an analytical solution exists, is analysed.

Special Finite Element Shape Functions For Axisymmetric Magnetfc Problems

New infinite elements which can be used for a solution of axisymmetric magnetic problems with open boundaries are presented. It is supposed that the new elements are being developed for the vector potential A(r,z) and the modified vector potential rA formulation. The elements can represent any type of decay towards infinity, and the element matrix when the decay function is 1/ rho /sup n/, n>or=1, rho /sup 2/=r/sup 2/+z/sup 2/ is given explicity. Standard finite elements for the rA formulation yield very inaccurate results near r=0. The introduction, close to the symmetry axis, of so-called axial elements with special shape functions is proposed. The formulation is so simple that closed-form expressions for the element matrix are obtained. Numerical results are presented for a problem having an analytical solution. >

Simplified Numerical Analysis of ECT Probe - Eddy Current Benchmark Problem 3

In this paper a third eddy current benchmark problem is considered. The objective of the benchmark is to determine optimal operating frequency and size of the pancake coil designated for testing tubes made of Inconel. It can be achieved by maximization of the change in impedance of the coil due to a flaw. Approximation functions of the probe (coil) characteristic were developed and used in order to reduce number of required calculations. It results in significant speed up of the optimization process. An optimal testing frequency and size of the probe were achieved as a final result of the calculation.

High Sensitivity Eddy Current Probe for Inspection of Low Conductivity Materials

In this paper authors propose an eddy current probe, which is suitable for inspection of low conductivity structures. The probe consists of an excitation coil and two differentially connected search coils. The detection circuit was designed in this way that a resonance for an operating frequency was obtained. It creates possibility to achieve a high sensitivity of the measuring system. The transducer was used to examine heterogeneities of low conductivity specimens. The test specimens consist of various plexiglass elements placed into a tank filled with drinking water or water solution of NaCl. Results of measurements confirm usability of the sensor and the whole measuring system.

Choice of the Distance between the Pole-Pieces of the Electromagnet Yoke in a Magnetic Method of Material Testing

With magnetic methods of testing, a magnetic field is induced inside the object being tested and any resulting changes of magnetic flux in the region of interest are observed. In this paper we consider the magnetic field excitation by using an electromagnet yoke, operated by DC and with adjustable distance between the pole‐pieces of the yoke. The sensitivity of detection for any portion of a component being tested varies with the distance between the pole pieces and, hence, the choice of the distance is of great importance. Simplified theoretical analysis as well as results of measurements for steel plates are given.

Computational Results of the Eddy Current Benchmark Problem 3

In this paper a third eddy current benchmark problem is considered. The objective of the eddy current benchmark problem is to determine optimal operating frequency and size of the pancake coil designated for testing tubes made of Inconel. The problem considered in this paper is a true three‐dimensional one and should be modeled using numerical methods, e.g. finite element method, however, because the solution times involved are very large, we present a simplified analytical approach. Moreover, analytical solutions are useful for determining the efficiency of numerical methods and for a qualitative presentation of the role of various system parameters.