Description: Frequency selective surfaces (FSSs) are periodic arrays of conductive elements or patches that cause a particular reflection or transmission response when illuminated with high frequency electromagnetic energy. These arrays have been used as high frequency filters and in radar, stealth and advanced antenna applications, and more recently, as sensors. In particular, FSS-based sensing has found a home as a next-generation structural health monitoring (SHM) approach. FSS sensors are inherently wireless and passive, and are interrogated remotely via microwave energy. These sensors can be embedded in layered dielectric (non-conducting) structures during manufacture or installed during the service lifetime on the surface (conductive or dielectric). Microwaves penetrate through dielectrics, so in the case of layered structures, FSS sensors can be placed on materials/layers of interest that may be covered by additional dielectrics (such as reentry heat tiles covered with insulation). Multiple sensing parameters can be concurrently sensed through proper sensor design and interrogation, as is illustrated below through a strain and temperature sensor. This Phase I effort will focus on creating a design for a field deployable prototype that can be ruggedized for use in space environments.

Benefits: FSS sensors can be applied to any of the current or any future launch vehicles used by NASA. The sensors can be applied under Thermal Protection Systems, high temperature reusable surface insuslaiton, advanced flexible reusable insulation, and under composite overwrap on pressure vessels. FSS sensors can be applied to habitat areas on structural components that are not readily accessible. The FSS sensors can be used to monitor strain loads, detect impact damage, and monitor temperatures in crewed environments.

FSS sensors can be applied to large infrastructure assets including bridges and dams and other hard to inspect large area structures. FSS sensors can provide instantaneous readouts of the strain state of a structure. FSS sensors can also be applied in the petrochemical field, being placed on pipes and tanks monitoring for any sign of induced strain or a temperature change that would be indicative of a potential leak.