Characterization of capacitive array for NDE applications

We have developed a capacitive array sensor which responds to the complex dielectric constant of an interrogated material. The sensor requires only single-sided access and operates in a differential mode for detection of discontinuities in the relative dielectric constant, ε_r, or in the absolute mode where the interest is in absolute quantities. The device in general is noncontacting but can be fabricated as an embedded sensor. Various proof-of-concept studies have been performed to explore possible applications of the device. In the differential mode, small surface features (notches) were detected in a conductor; in dielectrics, both surface and subsurface features were detected. The probe was sensitive to change in impedance caused by a curing epoxy, viewed through a graphite-epoxy composite panel. A study was performed to assess the use of the probe for noncontact characterization of sintering of ceramics. In this (absolute) mode, the effects of liftoff and dielectric constant must be separated. We propose a scheme based on the ability to multiplex the capacitive array probe to accomplish this. Preliminary investigation shows that the effect of parasitic capacitance between the probe and ground points in the environment must be suppressed.