Models for microwave nondestructive testing of materials

The need for testing of dielectric and lossy dielectric materials has renewed the interest in high frequency methods of testing. In particular, testing with microwaves and millimeter waves has received new attention because of their suitability to work with nonmetallic composite materials. This renewed interest presents new challenges in modeling and characterization of these complex materials. The common models used for high frequency applications such as the method of moments cannot, in general, be used for this purpose, primarily because they cannot take into account sources but, perhaps more importantly, because they have been developed specifically as ''far field'' models. On the other hand, finite elements, and combined finite elements-method of moments methods can, and are being used for accurate and detailed modeling of the testing environment. This paper presents some useful techniques, applicable to the test environment at all frequencies, but in particular in the microwave and millimeter wave domain. Anticipating both testing in resonant cavities and non resonant closed structures as well as scattering methods, two separate, broad techniques have been devised for this purpose. One method, suitable for resonant structures is based on evaluation of resonant frequencies of the system using an electric field formulation. The method can be used for modeling of lossless and lossy dielectrics as well as monitoring of production processes in microwave cavities. The second method is a scattering method suitable for modeling in open domains and in the vicinity of apertures. Both lossy and lossless dielectric materials can be modeled. The methods are described and representative results are given to demonstrate their utility in modeling microwave nondestructive testing processes.