A capacitive sensor-based technology which can conduct to understand non-destructive evaluation of building surfaces. The novelty of this sensor is that it can generate a real-time 2D subsurface image which can be used to understand structure beneath the top surface. Finite Element Analysis (FEA) simulations are done to understand the best sensor head configuration that gives optimum results. Hardware and software components are custom-built to facilitate real-time imaging capability. The sensor is validated by laboratory tests, which revealed the ability of the proposed capacitive sensing technology to see through common building materials like wood and concrete. The 2D image generated by the sensor is found to be useful in understanding the subsurface structure beneath the top surface.
Detection of corrosion under paint (CUP), even when the surface to be inspected is underwater, via single sided, non-capacitance imaging.
The major practical advantage of this approach is that it is non invasive, non and only requires single sided access to the object being examined.
The basic configuration is to “open up” a standard parallel-plate capacitor so that both electrodes are now in the same plane i.e., co-planar capacitor, as shown in Figure 1. An AC voltage is then applied, and an electric field distribution is established.
Co-planer copper electrodes are simple to fabricate via standard printed circuit board (PCB) techniques. Care has to be taken to ensure that the obverse side of PCB had a continuous, grounded copper coating so that the radiated electric field emanate predominantly in the “forwards” direction.
There are practical issues which arise when implementing this imaging technique on conducting as opposed to insulating, dielectric objects and whilst the general theoretical description has elements common to both of these applications, the situation of surface features on conductors which are in turn covered by an insulator – i.e situation of corrosion under paint (CUP).
In principle, the capacitance imaging technique can be applied to inspecting materials which have a wide range of conductivity. Also, as with other forms of AC impedance imaging, the driving frequency is also a very important factor.
Baseline measurements on defect-bearing metal surfaces covered by insulating layers
For baseline measurement purposes and to test the validity of the models, the first test sample was an aluminium plate, containing four separate machined 20 mm square flat-bottomed holes of different depths (2 mm, 4 mm, 6 mm and 8 mm).
A capacitive imaging scan was performed over an area of (200 x 50) mm, with the probe being kept at a constant stand-off distance of 1 mm from the metal surface containing the holes. (The aluminium plate was also kept electrically isolated during the whole process). The resulting image is shown in Figure 8(a). The holes are clearly imaged and the depth of hole is related to the contrast intensity of the image.
The scan was then repeated with the same surface now being coated with a 5 mm layer of insulating foam. (Figure 8 (b)). The increase of the stand-off distance reduces the contrast resolution of the image but the insulation layer itself presents no major impediment to imaging and he holes were still readily detected, with again clear differences between holes of different depths. If this insulation layer (5mm) were to be much thinner like a coating(s) of paint (20-50)μm then the image contrast would be of the order presented in the smallest stand-off distance result.
For a fixed frequency and geometry of probe, the spatial resolution of the imaging of defects is mostly sensitive to variations in the stand-off distance between the probe and the surface of the painted object, as illustrated in Figure 9 below.
Scans of surface corrosion under paint
Figure 10 (a) shows a steel plate which was immersed into brine (seawater) to half its depth and held at a high temperature for 10 days to accelerate aging
A capacitive imaging scan of this surface readily detected the main areas of where rust was present, as shown in Figure 10 (b). This is almost certainly due to the change in electrical impedance properties of the oxide layers and a small contribution from the fact that the topographic profile and surface height changes slightly too.
A second sample, which differed slightly from the first in as much as the unexposed surface was galvanized, was also treated to the same accelerated aging process over half its surface and the results are presented in Figure 11. The results are similar to the previous example but Figure 11 (c) shows the result of a scan taken of the corroded surface through a 5mm layer of insulation and the hidden corrosion is still readily detectible and identifiable.