Characterization of Rupturable Substrate Mechanical Properties Using Force-Based Metrics via a Robotic Leg

Rupturable substrates, such as surface crusts, play a crucial role in protecting soil from erosion. Understanding their mechanical properties – such as yield force, ductility, and resilience -- is essential for assessing their resistance to disturbances. To enable a fast, reliable method to determine these properties in-situ, we developed a direct-drive robotic leg that can accurately measure substrate resistive forces during penetration. Measurements from White Sands, NM showed that substrates with surface crusts exhibited significantly greater resistance to initial penetration than homogeneous dry granular media. We also observed two distinct rupture force responses from crusts at different dune types: Barchan (arid) dune crusts exhibited a sharp rupture failure upon penetration, while Parabolic (vegetated) dune crusts exhibited a smoother failure. These differences likely stem from variations in binding agents -- Barchan crusts were mostly abiotic (salt) and thus more brittle, whereas Parabolic crusts contain biological binding agents (e.g., EPS), resulting in larger compliance and resilience. Our findings revealed key force signatures of rupturable substrates, enabling the development of force-based metrics for legged robots to map terrain characteristics by walking.