STRUCTURE EVALUATION OF MATERIALS BY ELECTRICAL METHODS

A theoretical and practical framework of different problems has been d iscussed to explain advantages and limitations of techniques based on electrical sensing methods which are consistent to implement structure evaluation of materials. The term ''structure evaluation'' is general ly used here to mean extracting the quantitative information of intere st of a material or an object to be examined from a physical measureme nt, or more typically from a set of physical measurements, that themse lves may be only indirectly related to the quantitative information de sired. The paper emphasizes the point that the structure evaluation is a typical inverse problem and as such is made more difficult, since i nverse problems are characteristically ill-conditioned, that is, small errors in the measurement typically lead to large errors in the solut ion. Use of physically motivated a priori information to diminishing s uch ill-conditioning is discussed. Features of updated electrical sens ing methods, particularly those used for nondestructive testing (NDT) are considered. Potential advantages of the electrical methods compare d to other NDT techniques are that they offer inexpensive, safe, nonha rmful, and fast testing. Some novel achievements including excitation with a number of complex electric field excitation patterns on the obj ect together with computer-assisted information acquisition have consi derably extended both the potential and the scope of the electrical me thods which will still be recognized, In order to present an up-to-dat e perspective, some conceptual and methodologic aspects have been cons idered for two structure evaluation problems, i.e., for generating a) algorithms for determining structural parameters of materials, such as density, moisture content, polymerization degree, etc., from dielectr ic spectra; b) tomographic cross-sectional maps of electrical paramete rs of the object to be examined.