Capacitive imaging technique for non-destructive evaluation (NDE)

Preview

PDF WRAP_THESIS_Yin_2011.pdf - Requires a PDF viewer. Download (10MB)

Request Changes to record.

Abstract

This thesis describes the development and characterization of a novel NDE methodthe
Capacitive Imaging (CI) technique. The CI technique employs a pair of (or
multiple) electrodes to form a co-planar capacitor, and uses the fringing quasi-static
electric field established across the electrodes to investigate specimens of interest. In
general, the CI probe is sensitive to surface and hidden defects in insulating materials,
and surface features on conducting materials. The CI technique is advantageous for its
non-contact and non-invasive nature, and the capacitive coupling allows the CI
technique to work on a wide variety of material properties.
The theoretical background to the CI technique has been developed. It is shown that in
the frequency range of operation (10 kHz to 1 MHz), the quasi-static approximation is
valid and the Maxwell’s Equations describing the general electromagnetic phenomena
can be simplified. The practical implementation of the CI system is based on this
analysis, and it is shown that the CI technique has features that can complement
techniques such as eddy current methods that are already established in NDE.
The design principles of the CI probes that are required for an optimum imaging
performance have been determined, by considering the key measures of the
performance including the depth of penetration, the measurement sensitivity, the
imaging resolution and the signal to noise ratio (SNR). It has been shown that the
operation frequency is not an influential factor - the performance of the CI probe is
determined primarily by the geometry of the probe (e.g. size/shape of the electrodes,
separation between electrodes, guard electrodes etc.). Symmetric CI probes with
triangular-shaped electrodes were identified as a good general purpose design. Finite
Element (FE) models were constructed both in 2D and 3D in COMSOLTM to predict
the electric field distributions from CI probes. Effects of thickness of specimen, liftoff
distance and relative permittivity value etc were examined using the 2D models.
The sensitivity distributions of different CI probes were obtained from the 3D models
and were used to characterize the imaging ability of the given CI probes.
The fundamental concepts of the CI technique have been experimentally validated in
a series of scans where the defects were successfully imaged in insulating (Perspex)
and conducting (e.g. Aluminium, Steel and carbon fibre composite) specimens. The
detection of corrosion under insulation (CUI) has also been demonstrated. The
imaging abilities were assessed by investigating various standard specimens under
different situations. The CI technique was then successfully applied to various
practical specimens, including glass fibre laminated composites and sandwich
structures, laminated carbon fibre composites, corroded steel plate and pipe, and
concrete specimens. Further measurements were also conducted using modified CI
probes, to demonstrate the wide range of applications of the CI technique.

Request changes or add full text files to a record

Repository staff actions (login required)

View Item