## 5 Conclusions  
A feasibility study of the coplanar capacitive sensing technique which has certain advantages for NDT applications was explored through a numerical simulation and experimental tests. Numerical finite element (FE) analysis was conducted to demonstrate the validity of the proposed defect detection technique. The 3D FE modeling has demonstrated that the electric field distribution of a selected electrode pair penetrate into dielectric materials and can be used to detect the presence of defects in the form of air voids and crack. The preliminary investigation into the appliance of capacitive sensing for the detection of hidden holes within the specimen has been successfully shown. Experimental results on the Plexiglas plate containing subsurface holes of various depths gave a sign of the response of the technique. A good qualitative agreement was observed between the experimental and simulations result. The results obtained by the coplanar capacitive method illustrate that the technique is noncontact, noninvasive, and only requires single-sided access to the specimen. The couplant such as gel or water is not required, and there is no need for specific surface preparation. In addition, there are no radiation exposure problems. Therefore, this technique has the potential to be used in many NDT application areas where traditional methods may have difficulty.  
In practice, the condition is typically more complicated compared to the sample inspected during this work. Defect specifications will be unknown, and voids could also be poorly defined. Further development of the capacitive sensor is required to meet various practical needs and provide enhanced diagnostic information, e.g. systematic identification and characterization of defects. It should be emphasized that the depth of the penetration is mainly determined by the probe geometry, the electric properties of the specimen and the liftoff distance between the surface of the probe and the specimen, this will be further studied in future work.  
Conceptualization, methodology, formal analysis, software, experimental setup and data acquisition, validation, investigation, and writing--original draft preparation, F.A.-M.; methodology, formal analysis and experimental setup, S.S.; writing--review and editing, T.F., C.I.-C., and X.P.V.M.; supervision, X.P.V.M.; All authors have read and agreed to the published version of the manuscript.  
This research is funded by the Natural Sciences and Engineering Research Council of Canada (NSERC)--Discovery Grant Program, the Canadian Foundation for Innovation (CFI) and  
Figure 9: (**a**) Diagram of the Plexiglas specimen with three holes of different depths and (**b**) the capacitive image based on voltage values. In this image obtained from the scanning of the probe on the specimen, the striking feature is the red and yellow area, which suggests the successful detection of the defects. It can be seen that the colour of the defect-free area is green. (The reflection on the third rightmost defect is because of the mechanical adjustment.)  
Canada Research Chair Program. The support of the NSERC CREATE-oN DuTy! Program is also acknowledged.  
No institutional board review was necessary for this project.  
Data Availability Statement: Data sharing not applicable.  
The authors would like to thank and express their appreciation to the Eddyfi technologies and Vision Laboratory team at Laval University for supporting this research. They also would like to thank [PERSON] and [PERSON] from Eddyfi Technologies for their scientific advice.  
The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.