Impact of Shell Opening of a Model Razor Clam on the Evolution of Force Chains in Granular Media

The razor clams alternately inflate the shelled body and the muscular foot when burrowing down to the ground. It is found from previous numerical simulations that inflation of the shelled body not only forms a firm anchor for the foot penetration, but also reduces penetration resistance; on the other hand, further foot penetration relaxes the shell anchorage, which may compromise the burrowing effectiveness and efficiency. This study utilizes a photoelasticity-based technology, augmented with image processing, to validate the numerical findings. A simplified penetrator model composed of an expandable rectangular shell and a protrusible triangular foot is designed and incorporated into a transparent cell containing more than four thousand photoelastic disks. Sequential images are then taken during the model penetration, which include an initial foot penetration, followed by shell expansion and then another foot penetration. An image processing algorithm is developed to detect the evolution of grain contact forces (orientation and magnitude of contact forces) during the shell expansion and foot penetration of the model. Results from this study confirm the existence of the mutual influence between shell expansion pressure and foot penetration resistance: that is, increasing one causes reduction of the other and vice versa.