11.5 Wednesday, Jan. 4 Microstructure and mechanics of the tiled and actuating exoskeleton of the helmet urchin, Podophora atrata CHEN, T*; BOYCE, M; ORTIZ, C; Massachusetts Institute of Technology email@example.com
The helmet urchin, Podophora atrata possesses an unusual reduction in spines which forms a smooth tiling of mm-sized, flattened aboral plates, each articulating with the underlying test via a ball-and-socket joint. The microstructure is a porous network of single-crystal magnesium calcite with a few percent of intercalated organic. With high resolution X-ray microcomputed tomography, 3D microstructural characterization revealed a gradient in volume porosity from 10% at the joint to 50% at the outer surface. The galleried mesh (avg. pore size ~ 15 microns) was modeled using 3D elastic finite element analysis that consisted of a microstructurally-based volume element with periodic boundary conditions and a heat flow algorithm was used to model the anisotropic directionality of the unit element’s local coordinates with respect to the global coordinates of the aboral plate. Loading configurations on the unit elements resulted in an orthotropic effective mechanical behavior with the stiffness in the plane transverse to the long axis of the microstructure (E1, E2) approximately half the stiffness in the axial direction (E3). E3 was found to decrease linearly from 0.87 of the solid elastic modulus (Es) to 0.34 of Es as the volume fraction decreases from 0.88 to 0.46. E1 and E2 also decrease linearly from 0.49 of Es to 0.18 of Es. Simulation of blunt indentation showed that the graded porosity of the microstructure exhibits a lower effective stiffness than the solid material but serves to increase the strains near the exterior surface of the aboral spine while reducing the strains near the joint. This open-pore structure and trabeculae alignment results in a directional strengthening due to inhomogeneous deformations in the porous structure and provides resistance against blunt impacts and containment of penetration into the surface of the plate.