Phase Reflectors as an In-Situ Passive, Unpowered, Wireless Structural Health Monitor for Lunar Excavation Vehicles

Frequency Selective Surfaces (FSSs) have been used for Structural Health Monitoring (SHM) for detection of normal/shear strain and temperature but must be applied as an external patch to the surface being inspected to monitor changes in the amplitude vs. frequency response of the FSS. This leads to several disadvantages. The first is needing an adhesive to affix the FSS to the surface, which degrades during large temperature excursions (common for in-space use). The second is moduli differences of the adhesive/FSS and the substrate being inspected. This leads to large strain mismatches under load, resulting in a correction factor to properly describe the strain state of the material being monitored. The same applies for monitoring temperature due to thermal expansion differences. The way that the FSS is installed can affect its performance. Ideally, the FSS could be completely integrated into the base material being monitored. Integration into the parent material itself is now possible by monitoring changes in the phase vs. frequency of the FSS. In this work, Texas Research Institute Austin and the Missouri University of Science and Technology will demonstrate using surface features of the parent material as an in situ FSS (phase reflectors) as a passive, unpowered, and wireless means to remotely monitor the state of the parent material. In Phase I, this will be examined for metallic materials, but the technology will be developed for a wide range of materials such as: metallic, composite, and dielectric (in this case lunar regolith) structures, so a final version of this sensing mechanism would allow: SHM that could provide varying levels of feedback on the health of the parent structure and direct remote inspection of lunar vehicles for engineering properties, such as, but not necessarily limited to: strain, stress, load verification, cracking, corrosion, erosion, disbonds/delaminations, pressure, impact damage, and damage from thermal cycling or impact damage.