ELECTRICAL RESISTANCE TOMOGRAPHY
Electrical Resistance Tomography (ERT) is a relatively new method in the field of structural health monitoring. ERT is a non-intrusive invasive method enabling mapping of the spatial representation of the electrical conductivity of the monitored object on the basis of measuring the electrical voltage on its boundaries that appear due to the flowing current (see Figure 1). It is, therefore, obvious that the measured object must be sufficiently conductive to allow safe electrical voltage to excite sufficient electrical current. An electrical conductivity of carbon fibers and percolation conductivity of carbon nanotubes are those materials suitable for resistance tomography usability allowing to use it in-situ. As well it is evident, that resistance tomography is able to visualize those defects, which affect the electrical conductivity of the material. Impact detection and assessment, delamination propagation, and debonding detection fall into the area of defects, which are affecting the electrical conductivity and therefore theoretically detectable by the ERT.
Figure 1. Schematic ERT set-up to a 2D body using the adjacent stimulation and measurement pattern.
The conductivity map reconstruction (see Figure 3) is an ill-posed (poorly determined) inverse problem which has to be regularised. A regularisation offers many ways how to tailor the image reconstruction to the considered area. For example in the case of carbon fiber reinforced polymer (CFRP) composites (see Figure 2), which suffer from anisotropy of conductivity, can be expected improvement by the incorporation of a priory information about anisotropy of searched conductivity. Therefore, the first contribution of the Ph.D. programme will be detection capability assessment of ERT in the area of CFRP composites with and without a priori information.
Conductivity map reconstruction is also affected by the position and shape of electrodes. Another intended contribution of the Ph.D. programme lay in the area of the electrode position and shape optimisation. An idea of optimisation is to incorporate a priory information about searched conductivity change by the help of the electrode configuration. A constraint resulting from permitted locations for installation of electrodes, given by structure of the body, is also incorporated into optimisation.
Figure 2. Photo of CFRP specimen with two defects.
Figure 3. Conductivity change map of two defects (X – the true position of the defects; O – centre of 'gravity').
► Design of the measurement instruments for the electrical resistance tomography
► Regularisation of the inverse problem in the electrical resistance tomography