Huang Wei, Zaheri Alireza, Jung Jae-Young, Espinosa Horacio D, Mckittrick Joanna
Materials Science and Engineering Program, University of California, San Diego, La Jolla, CA 92037, USA.
Theoretical and Applied Mechanics Program, Northwestern University, Evanston, IL 60062, USA.
Acta Biomater. 2017 Dec;64:1-14. doi: 10.1016/j.actbio.2017.09.043. Epub 2017 Sep 30.
Bighorn sheep (Ovis canadensis) rams hurl themselves at each other at speeds of ∼9 m/s (20 mph) to fight for dominance and mating rights. This necessitates impact resistance and energy absorption mechanisms, which stem from material-structure components in horns. In this study, the material hierarchical structure as well as correlations between the structure and mechanical properties are investigated. The major microstructural elements of horns are found as tubules and cell lamellae, which are oriented with (∼30⁰) angle with respect to each other. The cell lamellae contain keratin cells, in the shape of pancakes, possessing an average thickness of ∼2 µm and diameter of ∼20-30 µm. The morphology of keratin cells reveals the presence of keratin fibers and intermediate filaments with diameter of ∼200 nm and ∼12 nm, respectively, parallel to the cell surface. Quasi-static and high strain rate impact experiments, in different loading directions and hydration states, revealed a strong strain rate dependency for both dried and hydrated conditions. A strong anisotropy behavior was observed under impact for the dried state. The results show that the radial direction is the most preferable impact orientation because of its superior energy absorption. Detailed failure mechanisms under the aforementioned conditions are examined by bar impact recovery experiments. Shear banding, buckling of cell lamellae, and delamination in longitudinal and transverse direction were identified as the cause for strain softening under high strain rate impact. While collapse of tubules occurs in both quasi-static and impact tests, in radial and transverse directions, the former leads to more energy absorption and impact resistance.
Bighorn sheep (Ovis canadensis) horns show remarkable impact resistance and energy absorption when undergoing high speed impact during the intraspecific fights. The present work illustrates the hierarchical structure of bighorn sheep horn at different length scales and investigates the energy dissipation mechanisms under different strain rates, loading orientations and hydration states. These results demonstrate how horn dissipates large amounts of energy, thus provide a new path to fabricate energy absorbent and crashworthiness engineering materials.
大角羊（加拿大盘羊）公羊会以约9米/秒（20英里/小时）的速度相互撞击，以争夺主导权和交配权。这就需要抗冲击和能量吸收机制，这些机制源于角中的材料结构成分。在本研究中，对角的材料层次结构以及结构与力学性能之间的相关性进行了研究。发现角的主要微观结构元素是细管和细胞薄片，它们彼此之间呈约30°角排列。细胞薄片包含薄饼状的角蛋白细胞，平均厚度约为2微米，直径约为20 - 30微米。角蛋白细胞的形态显示存在分别与细胞表面平行的直径约为200纳米和约12纳米的角蛋白纤维和中间丝。在不同加载方向和水化状态下进行的准静态和高应变率冲击实验表明，干燥和水化条件下都有很强的应变率依赖性。在冲击下观察到干燥状态有很强的各向异性行为。结果表明，径向是最理想的冲击方向，因为它具有卓越的能量吸收能力。通过棒冲击恢复实验研究了上述条件下的详细失效机制。剪切带、细胞薄片的屈曲以及纵向和横向的分层被确定为高应变率冲击下应变软化的原因。虽然在准静态和冲击试验中，无论是径向还是横向，细管都会塌陷，但前者会导致更多的能量吸收和抗冲击能力。
大角羊（加拿大盘羊）的角在种内争斗中遭受高速冲击时表现出显著的抗冲击和能量吸收能力。本工作阐述了大角羊角在不同长度尺度下的层次结构，并研究了不同应变率、加载方向和水化状态下的能量耗散机制。这些结果展示了角如何耗散大量能量，从而为制造能量吸收和防撞工程材料提供了一条新途径。
