Title Authors Presenting Author Presentation Type Date and Time Abstract Reference # Multiscale Design of a Natural Articulating Armor: the Chiton Ischnochiton Ruber a Multiscale Structural, Optical and Mechanical Study of a Highly- Translucent Natural Armor: Placuna Placenta

A common feature of the outer layer of biological exoskeletons is structural anisotropy, which has been hypothesized to direct crack propagation, stress and energy dissipation into the underlying more ductile layers, as well as reduce interfacial stresses and mitigate delamination. This study applied focused ion beam (FIB) annular milling to the outer ganoine layer of the mineralized scales of the armored fish Polypterus senegalus to fabricate micropillars (1μm in diameter, 3μm in height) aligned at various angles θ = 0°, 45° and 90° to the surface normal. Uniaxial micro-compression experiments are able to elucidate mechanical anisotropy more rigorously than multiaxial indentation. The ganoine is composed of ~95% (v/v) rod-like hydroxyapatite (HAP) nanocrystals (~40nm in width, ~220nm in length) embedded in organic, with the long axis oriented parallel to the surface normal. Uniaxial microcompression of the micropillars using a flat-punch diamond indenter (diameter ~ 10μm) showed significant orientation-dependent elastic modulus, E, (0° (51±2MPa) > 90° (40±1MPa) > 45° (36±2GPa)), yield stress, σY, (0° ≈ 90° (1.0±0.1GPa) > 45° (0.8±0.1GPa)) (n ≥ 10 pillars for each case), and various crack propagation pathways observed by scanning electron microscopy An anisotropic elastic-plastic finite element model (FEM) was constructed to capture the nanostructures of ganoine and predict the experimentally measured nanomechanical behavior of the ganoine. Comparison between experiments and theory provided direct support that cracking took place within the organic material, and the oblique deformation and shear localization resulted in reduced E and σY upon off-axis compression (45°). A unique post-yield strain hardening was observed at 45°, which was due to the prism-to-prism interlocking, and can lead to higher energy dissipation during crack propagation. Therefore, this study provided direct experimental proof of the importance of structural anisotropy in the mechanical behavior of ganoine for threat protection and damage localization of P. senegalus. December 12, 2011 (Monday): Oral Session 7: Biomimetic and Bioinspired Materials 7:50 pm 9:10