Force-chain evolution in granular packings under a razor-clam inspired penetration

The Atlantic Razor clam facilitates downward burrowing in soils by changing the shapes of the shell and the foot. Here we investigate how the shell opening facilitates the soil burrowing of razor clams using a clam-inspired penetrator and photoelasticity. The penetrator has an expandable ‘shell’ and a protrusible ‘foot’, and penetrates dry granular materials by modeling the sequential shell opening and foot extension of razor clams. The soil-penetrator interactions are intuitively visualized through the evolution of force chains in granular materials. Results imply that the ‘shell’ opening facilitates the ‘foot’ extension by forming a penetration anchor through compressing the surrounding granules; it also relaxes the stresses of granules around the ‘foot’, leading to a reduction of soil resistance applied on the ‘foot’. However, the following ‘foot’ extension causes disappearance of the stress relaxation, resulting in a temporary resistance reduction; it also tends to weaken the penetration anchor. Findings from this study shed lights on developing a ‘living’ clam-inspired burrowing robot for underground explorations.