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  • 1
    Electronic Resource
    Electronic Resource
    Eddy-current probe impedance due to a volumetric flaw (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 1107-1114 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Eddy current induced in a metal by a coil carrying an alternating current may be perturbed by the presence of any macroscopic defects in the material, such as cracks, surface indentations, or inclusions. In eddy-current nondestructive evaluation, defects are commonly sensed by a change of the coil impedance resulting from perturbations in the electromagnetic field. This paper describes theoretical predictions of eddy-current probe responses for surface cracks with finite opening. The theory expresses the electromagnetic field scattered by a three-dimensional flaw as a volume integral with a dyadic kernel. Probe signals are found by first solving an integral equation for the field at the flaw. The field equation is approximated by a discrete form using the moment method and a numerical solution found using conjugate gradients. The change in probe impedance due to a flaw is calculated from the flaw field. Predictions of the theory are compared with experimental impedances due to eddy-current interaction with a rectangular surface breaking slot. Good agreement is found between predictions and the measurements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349615
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Low-frequency perturbation theory in eddy-current non-destructive evaluation (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 4090-4100 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A method is presented by which series solutions for the impedance change in an eddy-current test probe due to closed cracks in a non-magnetic, conducting half-space can be derived at low frequency. The series solution is applicable for flaws whose dimensions are much smaller than the electromagnetic skin-depth. The problem is formulated using an approach in which the flaw is represented by an equivalent distribution of current dipoles. The electric field scattered by the flaw is then written as an integral, over the flaw, of the product of the dipole density distribution and an appropriate Green's function. Terms in the series expansion for the dipole density are calculated by solving the integral equation at each order in the chosen small parameter, using perturbation theory and a dual integral equation method. The impedance change due to the crack is then calculated from the dipole distribution using the reciprocity theorem. Example solutions are given for semi-circular surface-breaking cracks and for long, uniformly deep surface-breaking cracks. Results are compared with other analytical solutions and the predictions of an independent numerical scheme, and very good agreement is observed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363280
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Eddy-current interaction with an ideal crack. I. The forward problem (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 8128-8137 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The impedance of an eddy-current probe changes when the current it induces in an electrical conductor is perturbed by a flaw such as a crack. In predicting the probe signals, it is expedient to introduce idealizations about the nature of the flaw. Eddy-current interaction is considered with an ideal crack having a negligible opening and acting as a impenetrable barrier to electric current. The barrier gives rise to a discontinuity in the electromagnetic field that has been calculated by finding an equivalent electrical source distribution that produces the same effect. The choice of source is between a current dipole layer or a magnetic dipole layer; either will give the required jump in the electric field at the crack. Here a current dipole layer is used. The strength of the equivalent source distribution has been found by solving a boundary integral equation with a singular kernel. From the solution, the probe impedance due to the crack has been evaluated. Although analytical solutions are possible for special cases, numerical approximations are needed for cracks of arbitrary shape. Following a moment method scheme, numerical predictions have been made for both rectangular and semielliptical ideal cracks. These predictions have been compared with experiments performed on narrow slots used to simulate ideal cracks. Good agreement has been found between the calculations and the measurements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.356511
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  • 4
    Electronic Resource
    Electronic Resource
    Analysis of eddy-current interaction with a surface-breaking crack (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 4853-4856 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The change in electromagnetic impedance of a conductor due to the presence of a long, perpendicular surface-breaking crack in a normally incident, uniform electric field is calculated in closed form in the high-frequency limit. At high frequencies, where the skin depth is much smaller than the depth of the crack, the fields near the edge and corners of the crack are effectively decoupled. This means that the solution may be formulated as the sum of contributions from the corners, faces, and edge of the crack. Simple analytical expressions for the electric field are found and used to calculate the impedance due to the crack in the high-frequency limit without resorting to numerical methods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357259
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  • 5
    Electronic Resource
    Electronic Resource
    Theory of thin-skin eddy-current interaction with surface cracks (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 82 (1997), S. 4590-4603 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Eddy-current non-destructive evaluation is commonly performed at relatively high frequencies at which the skin depths are significantly smaller than the dimensions of a typical crack. A thin-skin analysis of eddy currents is presented in which the electromagnetic fields on the crack faces are described in terms of a potential which obeys a two-dimensional Laplace equation. Solutions of this equation for defects in both magnetic and non-magnetic materials are determined by applying thin-skin boundary conditions at the crack perimeter. The impedance change of an eddy-current coil due to the defect is then calculated by numerical evaluation of one-dimensional integrals over the line of the crack mouth, the impedance integrals having been derived with the aid of a reciprocity relationship. Theoretical predictions are compared with experimental data for long, uniformly deep slots in aluminium and mild steel and good agreement between theory and experiment is obtained. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.366196
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    Paper (German National Licenses)
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  • 6
    Electronic Resource
    Electronic Resource
    Time domain half-space dyadic Green's functions for eddy-current calculations (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 6494-6500 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The field due to an impulsive current dipole embedded in a half-space conductor adjoining a nonconducting half space is given by an exact solution of the quasistatic field equations. This solution has been used to construct a half-space dyadic Green's function containing a term for an unbounded conductor plus terms representing the field reflected at the interface between conducting and nonconducting regions. The resulting kernel can be used in the formulation of time-dependent scattering problems to express the electric field in a conductor as an integral over an electric dipole distribution. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.371634
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  • 7
    Electronic Resource
    Electronic Resource
    Eddy current calculations using half-space Green's functions (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 61 (1987), S. 833-839 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A simple scalar representation is used to describe the electromagnetic field in the quasi-static limit for an arbitrary time-harmonic source current above an imperfectly conducting half-space. Solutions are given in terms of half-space scalar and dyadic Green's functions. The general results are then used to derive analytical expressions for the fields arising from circular filaments and extended sources whose axes of symmetry are parallel to the surface of the conductor. These tangent coil solutions have applications in the theory of inductive sensors, particularly for eddy current nondestructive testing.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.338131
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