Advanced Structural Health Monitoring

Frequency selective surfaces (FSSs) are periodic arrays of conductive elements/patches that cause a particular reflection response when illuminated with high frequency electromagnetic energy. nbsp;These arrays arenbsp;used for high frequency filters and in antenna applications.nbsp; We propose to use FSSs as multi-functional sensors. nbsp;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.nbsp; 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.nbsp; 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. nbsp;Phase I work included:nbsp; 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.