Frequency selective surfaces (FSSs) are periodic arrays of conductive elements/patches that cause a particular reflection response when illuminated with high frequency electromagnetic energy. These arrays are used for high frequency filters and in antenna applications. We propose to use FSSs as multi-functional sensors. FSS sensors are unpowered, low-profile (thin), wireless and passive, and are interrogated remotely via low power microwave energy. These sensors can be embedded in non-conductive structures or surface mounted on conductive or non-conductive surfaces. They can be surface mounted at any point in the service life of a structure.
Microwaves penetrate through dielectrics, so FSS sensors can be interrogated through non-conductive materials such as paint, insulation, fiberglass, Kevlar etc. Multiple sensing parameters can be concurrently sensed through proper sensor design and interrogation, as was illustrated in Phase I via both simulation and experimental measurement of a prototype strain and temperature sensor. Phase I work included: completing a study on the effect of FSS dimensions and illumination footprint on achievable resolution; miniaturization was applied and shown to improve sensor response and resolution; modeling correctly predicted sensor ability to measure temperature and strain; FSS sensors and tensile test samples were constructed and bonded together; and thermal and mechanical testing were completed to empirically confirm capabilities and limitations.
In Phase II we plan to improve sensor materials and designs; address measuring ranges of strains and temperatures on varying surfaces; optimize FSS bonding methods; build and test a complete strain/temperature measurement prototype; test the system on structures (both visually available and covered by nonconductive materials) undergoing various loads and temperature changes; quantify capabilities and limitations; deliver the system to NASA Langley; and develop a commercialization plan.
A low profile (thin), un-powered, passive, wireless sensor that can be interrogated non-contact through coatings to monitor the underlying structures strain and shape change would have many applications. Some examples include NASA’s Human Exploration Operations programs such as: crew transportation systems; ISS support; Orion crew vehicle; deep space habitation; and Advanced Exploration Systems could all benefit from the proposed strain and temperature measuring system – especially for monitoring strain of structures through various outer coverings/coatings. The system could also be used to measure and assess deformation in multi-layer polymers, Nextel, ceramic fabrics etc such as those used in Whipple bumpers. It could also be used to assess creep/strain damage to pressure vessels through Kevlar and/or fiberglass composite overwrap. The system could also be applied to inspection of more Earthbound applications within the Safety, Security and Mission Services/Construction & Environmental Compliance and Restoration programs. It could monitor composite, elastomer, polymeric, ceramic and civil materials for degradation.
The United States faces a backlog of infrastructure inspections. FSS sensors provide a cost effective tool for providing long term monitoring of strains and deformations in structures such as bridges and dams. Because the sensors can be applied in a non-zero strain environment, the FSS sensors can be embedded on legacy infrastructure and can therefore provide the ability to monitor loads from that point on. According to the FHWA, there are 54,560 Structurally Deficient Bridges in the United States, plus another 47,619 bridges rated structurally poor. The system we propose to develop in this Phase II effort would allow bridge structures to be monitored over time, even if paint, fiberglass repairs, concrete or other patches are added to the structures. FSS sensors can be applied to architectural structures in areas that are prone to natural disasters. During these events the buildings may experience unnatural strain loading resulting in microcracking or deformations on structural components that are hidden from view. Using the remote sensing ability of this system can allow architectural engineers a glimpse into the any unseen damage the building may have suffered. FSS sensors can be incorporated into aerospace structures such as the fuselage or the wing skin. The quick, non-contact scanning of permanently placed FSS sensors would allow lifetime strain monitoring of the aircraft. This would assist in driving more efficient maintenance schedules.