Morphological advantages of angelwing shells in mechanical boring

Abstract

Rock boring is not only essential to infrastructure construction, subsurface resource recovery, and scientific drilling, but also a behavior that many creatures including rock-boring bivalves are born with and critical to their survival. This study investigates the boring mechanics of Cyrtopleura costata (commonly named angelwing clams) with a focus on their morphological advantages in rock cutting. The 3D morphology of an angelwing clam is obtained through photographic and X-ray computed tomography, based on which, mathematical models fully characterizing the shape of the shell as well as the positions and orientations of the surface denticles, are established in this study. These models allow quantitatively analyzing the rock-boring mechanics of angelwing clams under various locomotions. The results show that the logarithmic spiral-shaped cross-section of the angelwing shell is optimized for minimized energy in boring without sacrificing much dwelling space underneath the shell for the mollusk body. In addition, the back and side rake angles of denticles govern their cutting efficiency. Most denticles on the angelwing shell involved in advancing boring have back rake angles of 0–20° and side rake angles of 30–40°. Complementary discrete element method (DEM) simulations of a single denticle scratching rock confirm that a positive back rake angle is the most energy efficient in boring compared to a zero-degree or a negative back rake angle, and the side rake angles of angelwing denticles could be a compromise between the cutting efficiency and denticle durability. These results inspire the designs of novel drill bits and drilling methods.