Lighter, stiffer, stronger, and more durable dielectric composites are replacing metals in many applications. These composites require alternative testing approaches since traditional electromagnetic NDE methods applicable to metallic structures may not be able to inspect them. This is partly due to the relatively thick nature of these composites, attenuation and scattering caused by the various layers, their low electrical conductivity and the thin planar anomalies that commonly appear in these structures. On the other hand, microwave NDE techniques are well-suited for testing these structures because microwave signals can easily penetrate low-loss dielectric materials, such as glass-fiber reinforced polymer skins, foam and honeycomb, without suffering from significant signal attenuation. In some situations, microwave NDE techniques may be the unique solution or can be combined with other approaches to become a more comprehensive solution to a testing demand.
Microwave NDE sensors usually operate in one of two basic modes: far-field or near-field. Each of these offers certain advantages and drawbacks. Spatial resolution of the far-field technique is proportional to the operation wavelength while that of the near-field technique is proportional to the sensor’s sensing area. Both near-field and far-field techniques are subjects of current research at CNDE. For example, a microwave patch sensor that operates in the near-field mode has been developed for evaluation of radome structures and a metamaterial-superlens-based microwave NDE system working in the far-field mode is being developed for sub-wavelength resolution NDE of flawed dielectrics.