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Recently, frequency selective surfaces (FSSs) have been shown to have potential as embedded structural health monitoring (SHM) sensors. An FSS is an array of metallic elements that provides a filtering response to incident electromagnetic energy. This filtering response, caused by inductive and capacitive coupling between the elements, depends on the FSS geometry (that is, element shape, spacing, and dimensions) and local (to the FSS) environment (that is, material properties, temperature, and so on). Consequently, this dependency can be utilized for SHM sensing. Previous work has shown that FSS-based sensors can be used to detect delaminations and disbonds in layered media because of the effect of the delamination on the capacitance of the FSS. Additionally, it was found that this sensing capability is limited to the region local to the FSS, limiting the FSS’s sensing ability in larger structures. Thus, this investigation proposes the use of multiple FSS layers located throughout a layered structure to provide wider sensing coverage and allow different regions of the structure to be independently monitored. To accomplish this, a series of simulations and measurements were conducted on a set of two FSS sensors integrated into a layered dielectric structure. Each sensor was designed to have different resonant frequencies, allowing the two sensors to be individually monitored. Using full wave simulation, it was found that the frequency response of a given FSS sensor is more sensitive to local delaminations, while FSS sensors located elsewhere in the structure remain essentially unaffected. This indicates that multiple FSS sensors can be used to provide broad sensing coverage of a layered structure. Simulated and measured results were in good agreement with respect to the effect of delaminations on resonant frequency, but resonant depth was found to be less consistent between measurement and simulation. As such, the use of resonant depth for delamination monitoring may not be a reliable sensing metric.

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