Abstract
It was recently demonstrated that a coplanar capacitive sensor could be applied to the evaluation of materials without the disadvantages associated with the other techniques. This technique effectively detects changes in the dielectric properties of the materials due to, for instance, imperfections or variations in the internal structure, by moving a set of simple electrodes on the surface of the specimen. An AC voltage is applied to one or more electrodes and signals are detected by others. This is a promising inspection method for imaging the interior structure of the numerous materials, without the necessity to be in contact with the surface of the sample. In this paper, finite element (FE) modeling was employed to simulate the electric field distribution from a coplanar capacitive sensor and the way it interacts with a nonconducting sample. Physical experiments with a prototype capacitive sensor were also performed on a Plexiglas sample with subsurface defects, to assess the imaging performance of the sensor. A good qualitative agreement was observed between the numerical simulation and experimental result.
Highlights
Accepted: 16 April 2021There are several methods for evaluating the integrity of materials, and an important category of them is nondestructive evaluation (NDE) or nondestructive testing (NDT)methods
The coplanar electrodes can be scanned over the surface of a large mechanical structure to investigate the physical properties of the specimen and obtain information. Such as some other NDT methods, the capacitive technique is sensitive to liftoff, which refers to the air gap between the surface of the coplanar electrodes and specimen which causes a decrease in the capacitance between the electrodes and a reduction in output voltage
This technique is different from the other NDT method such as eddy currents, where the variation of frequency leads to a change in the depth of penetration while the sensing depth of capacitive sensors mainly depends on the probe geometry and properties of the specimen [18]
Summary
There are several methods for evaluating the integrity of materials, and an important category of them is nondestructive evaluation (NDE) or nondestructive testing (NDT). These features of coplanar capacitive sensor make it an attractive option for applications in NDT [6], material characterization [8], and imaging [12] The principle of this technique involves placing two (or more) electrodes over the surface of the sample and applying an AC voltage between them. Such as some other NDT methods, the capacitive technique is sensitive to liftoff, which refers to the air gap between the surface of the coplanar electrodes and specimen which causes a decrease in the capacitance between the electrodes and a reduction in output voltage It reduces the extent of penetration of the electric field into the sample which leads to a decrease in the depth of penetration [9]. These results approved by physical experiments and It was illustrated that the electric field strength and distribution changed with respect to the defect depth in agreement with experimental data
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