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.

Lunar Transport Vehicles (LTV) will need to handle the extreme conditions at the Moon’s south pole so that they can be used by future astronaut crews to explore, transport scientific equipment, and collect samples of the lunar surface, much farther than they could on foot, enabling increased science returns. Currently there is no path for monitoring and inspection of lunar vehicles. These systems will be required to continually safely operate lunar vehicles. These gaps have been identified in the Space Technology Mission Directorate Strategic Framework under Advanced Materials. Proper engineering of the surface features (phase reflectors) can be used to create a sensing mechanism by observing the phase change vs. frequency, rather than the E-field magnitude vs. frequency. The technology needs to be proven out experimentally in a well-characterized environment to ensure that it behaves as expected. The technology would allow structural health monitoring 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. This effort is intended to demonstrate sensing capabilities, but many of the requirements that NASA requires for SHM/DRI can begin to be addressed within Phase I by ensuring that the produced designs provide a tangible path forward to the ultimate goal of mission insertion. This technology (an in-situ sensor system that utilizes the identical material as the parent material) has not been developed to this point and offers immense commercialization potential. Examples of use cases are: aircraft, spacecraft, ships, pressure vessels, and any structure that requires/benefits from nondestructive, direct remote inspection monitoring. There are two means to add material to a substrate such to apply the sensing mechanism, either pre- and post-manufacture. During manufacture, excess material can be used to produce parts, with the pattern required for in-situ phase reflection sensing CNC-machined into the surface. There are many different ways to achieve this, via either subtractive or additive machining processes depending on the material and its end use. Post-manufacture, additive manufacturing is required to apply the sensing material on the substrate, such as wire-fed AM and cold-spray AM. These techniques are being developed both for new manufacture and for repairs/modifications of existing structures. TRI Austin maintains relationships that allow commercial insertion within these technology areas (such as with EOS and with Boeing/Lockheed/AFRL as part of efforts to validate cold-spray repairs of aerospace components to extend inspection intervals within USAF’s Aircraft Structural Integrity Program [ASIP] aerospace management structure).

This is a mature technology (TRL 7+) — the realistic path in is usually NASA's Technology Transfer Program: licensing an existing NASA patent, or a Space Act Agreement to use NASA facilities/expertise directly. NASA also runs a startup licensing program with no upfront fee for companies formed to commercialize a specific NASA technology.

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