###### 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. coplanar sensor; capacitive sensing; NDT techniques; FE modeling; defects +
Footnote †: This paper is an extended version of the paper presented in Structural Health Monitoring and Non-destructive Testing 2020.  
testing allows the inspection of large areas in a fast and contactless manner, but is limited to relatively shallow defects and requires advanced signal/image processing for defect characterization. Eddy current testing (ECT) is effective for certain types of defects (cracks, corrosion, etc.) under several layers of materials, but is limited to conductive materials and is not well suited for the detection of delaminations in composites. It has recently been shown that an alternative electromagnetic method based on a capacitive sensor can be used as an NDT method [2]. For nonconducting materials, NDT methods are not as well developed as compared to those for metallic materials. One approach to evaluate nonconducting materials is to characterize their dielectric properties. The dielectric strength and the dielectric constant are the dielectric properties of nonconducting or low conductivity materials. The voltage that a material can withstand before an electrical breakdown happens is called \"dielectric strength\" and a measure of the material's capability of storing electric energy is called \"dielectric constant\" or \"permittivity\".There exist alternative approaches that are based on the characterization of materials permittivity such as microwave techniques and resonant testing. However, these techniques require expensive equipment and/or complex operating procedures. Capacitive sensing on the contrary is a straightforward and inexpensive approach [2]. The coplanar capacitive technique was first introduced in 2006 [4]. This novel approach, usually referred to as Capacitive Imaging (CI), is an electromagnetic NDT method that uses arrays of electrodes to generate an electric field distribution within the specimen. The electric field distribution can penetrate within dielectric materials and changes in response to the structure of the material under test leading to a variation in the output voltage [5; 6]. The capacitive technique has shown great potential to inspect a wide range of materials and structures from insulators to conductors [5]. CI technique offers a possible way to overcome some of the restrictions imposed by existing NDT methods. For instance, the technique works in a volume averaging manner and therefore the scattering issue with the ultrasonic method is absent [7]. The affordable cost, straightforward response, noninvasive, no ionizing radiation involved and flexibility in the design of the electrodes provide CI with a great potential to be applied in a extensive range of applications. Furthermore, the coplanar structure allows one-sided inspection [8], which is especially useful when access to both sides of the specimen is limited [9].