Abstract
Nondestructive evaluation of various conductive objects through metal enclosures is investigated by using ELF/VLF magnetic induction fields in detailed simulations. ELF/VLF magnetic fields (<30 kHz) have a unique ability to penetrate highly conductive or permeable shields. Using a magnetic dipole source antenna, objects hidden inside a metal enclosure are imaged via examining distortions to the field outside the enclosure. The field distortion is parametrically studied by varying the size, conductivity, and permeability of the hidden objects. Furthermore, the importance of the conductivity of the enclosure itself is investigated using both low (106 S/m) and high (108 S/m) conductivity metallic shields. It is shown that the responses are quite sensitive to the object and shield parameters; both qualitative and quantitative properties of the field distortions are described in detail. The simulation results suggest that properties of hidden conductive or permeable objects, over a relatively wide range of parameters (both geometry and material), can be inferred nondestructively using ELF/VLF magnetic induction fields.
Highlights
ELF/VLF magnetic fields have been demonstrated to be a viable source for imaging conductive objects that are obscured by metal shields [7,8]
The methods aresurface highlyarea to volume ratio, because a large number of grid cells are required in the free space region computationally intensive and not ideally suited for thin shell shielded enclosures with a inside the shield
To examine the relative effects of the shield, the gain G is expressed by G = 20 ∗
Summary
ELF/VLF magnetic fields have been demonstrated to be a viable source for imaging conductive objects that are obscured by metal shields [7,8]. We parametrically investigate the sensitivity of nondestructive ELF/VLF testing using simulations of objects with different material properties and geometries. Analytical approximations can be utilized to provide a high-level understanding of the important features that are present problem. ELF/VLF magnetic fields as a nondestructive modality will in in the general reThus,athis section approach. A magnetic shielding effect is vide a high-level understanding the important features3.that are present in the problem. As a guide interpreting induced dipole moment is investigated as a in function permeability, radius results from more sophisticated simulations
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