Mechanical Engineering, University of Mississippi, 229 Carrier Hall, Oxford, MS 38677, United States.
Geotechnical and Structures Laboratory, US Army ERDC, 3909 Halls Ferry Rd., Vicksburg, MS 39180, United States.
J Mech Behav Biomed Mater. 2017 May;69:395-403. doi: 10.1016/j.jmbbm.2017.02.003. Epub 2017 Feb 7.
The Mississippi Alligator gar (Atractosteus spatula) possesses a flexible exoskeleton armor consisting of overlapping ganoid scales used for predatory protection. Each scale is a two-phase biomineralized composite containing bio-modified hydroxyapatite (hard) minerals and collagen (soft) fibers. The protective layer consists of a stiff outer ganoine layer, a characteristic "sawtooth" pattern at the interface with the compliant bone inner layer. The garfish scale exhibits a decreasing elastic modulus from the external to the internal layers. Scanning electron microscopy (SEM) images of the cross-section revealed a two-layered structure. Elastic moduli, measured from nanoindentation experiments, were correlated to structural changes across each layer. The "material" symmetry of this materially and geometrically nonlinear biomineralized composite is unknown. Therefore, to be able to determine the stiffness tensor requires the use of finite element analysis (FEA). The gar fish scale was computationally modeled using the representative volume element (RVE) based approach. As a result, the unknown symmetry induced by the architecture and material layering require the use of complex FEA boundary conditions. The simulation was conducted in the pure uniaxial strain regimes of tension and shear, which necessitated the mathematical determination so appropriate surface loading conditions could be applied. This paper provides the results from a highly-resolved mesoscale RVE model based on iso-strain boundary conditions (ISBC) to determine the elastic stiffness tensor for the composite system. By assuming isotropic behavior in individual elements, the results for the RVE reveal the fish scale has an "orthotropic symmetry" with slight local strain variations occurring at the sawtooth interface.
密西西比雀鳝（Atractosteus spatula）拥有灵活的外骨骼盔甲，由重叠的硬鳞组成，用于捕食保护。每个鳞片都是一种两相生物矿化复合材料，包含生物改性羟磷灰石（硬）矿物和胶原（软）纤维。保护层由坚硬的外鳞层和与柔软的内层骨骼交界处的特征“锯齿状”图案组成。雀鳝鳞片的弹性模量从外部到内部逐渐减小。横截面的扫描电子显微镜 (SEM) 图像显示出双层结构。从纳米压痕实验测量的弹性模量与每层的结构变化相关。这种具有材料和几何非线性的生物矿化复合材料的“材料”对称性是未知的。因此，要确定刚度张量，需要使用有限元分析 (FEA)。使用基于代表性体积元 (RVE) 的方法对雀鳝鳞片进行了计算建模。因此，由于结构和材料分层引起的未知对称性需要使用复杂的 FEA 边界条件。模拟在纯单轴应变的拉伸和剪切状态下进行，这需要数学确定适当的表面加载条件。本文提供了基于等应变边界条件 (ISBC) 的高分辨率细观 RVE 模型的结果，以确定复合材料系统的弹性刚度张量。通过假设各向同性行为在单个元素中，RVE 的结果表明鳞片具有“正交各向异性对称性”，在锯齿状界面处会发生轻微的局部应变变化。
