Micro-buckling in the nanocomposite structure of biological materials
Micro-buckling in the nanocomposite structure of biological materials
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DOI：
10.1016/j.jmps.2012.05.003
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发表时间：
2012-10
影响因子：
5.3
通讯作者：
Yewang Su;B. Ji;K. Hwang;Yonggang Huang
中科院分区：
文献类型：
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作者：
Yewang Su;B. Ji;K. Hwang;Yonggang Huang
纳米复合结构是由硬矿物和软蛋白质组成的生物材料的基本组成部分，矿物晶体在蛋白质基质中交错排列。这种特殊的矿物排列被认为对生物材料在压缩载荷下的结构稳定性至关重要，但其潜在的机制尚不清楚。在本研究中，我们明确考虑了矿物晶体的交错排列，以及在屈曲变形过程中矿物之间的配合，对纳米复合材料结构的屈曲强度进行了解析分析。确定了纳米结构的两种局部屈曲模态，即对称模态和反对称模态。结果表明，对称模式多发生在大长径比和大体积分数下，而反对称模式发生在小长径比和小体积分数下。此外，我们发现，由于矿物在交错排列的帮助下相互协调，这两种模式的屈曲强度接近Rosen模型给出的大长径比下理想连续纤维增强复合材料的屈曲强度，对矿物尖端之间存在的“间隙”状缺陷不敏感。此外，我们还发现了随着长径比的增加，屈曲模式从局部屈曲向全局屈曲转变的机制，这归因于屈曲强度与长径比的双相依赖。即当展弦比较小时，局部屈曲强度小于全局屈曲强度，从而主导了纳米复合材料的屈曲行为；当长径比较大时，局部屈曲强度高于整体屈曲强度，整体屈曲支配屈曲行为。我们还发现，层次化结构可以有效地提高生物纳米复合材料的屈曲强度，特别是这种结构设计可以使生物纳米复合材料避免局部屈曲，从而通过层次化设计在宏观尺度上实现整体屈曲。这些特征对于生物材料的机械功能非常重要，例如骨、牙齿和珍珠，它们经常承受较大的压缩载荷。
Nanocomposite structure, consisting of hard mineral and soft protein, is the elementary building block of biological materials, where the mineral crystals are arranged in a staggered manner in protein matrix. This special alignment of mineral is supposed to be crucial to the structural stability of the biological materials under compressive load, but the underlying mechanism is not yet clear. In this study, we performed analytical analysis on the buckling strength of the nanocomposite structure by explicitly considering the staggered alignment of the mineral crystals, as well as the coordination among the minerals during the buckling deformation. Two local buckling modes of the nanostructure were identified, i.e., the symmetric mode and anti-symmetric mode. We showed that the symmetric mode often happens at large aspect ratio and large volume fraction of mineral, while the anti-symmetric happens at small aspect ratio and small volume fraction. In addition, we showed that because of the coordination of minerals with the help of their staggered alignment, the buckling strength of these two modes approached to that of the ideally continuous fiber reinforced composites at large aspect ratio given by Rosen's model, insensitive to the existing “gap”-like flaws between mineral tips. Furthermore, we identified a mechanism of buckling mode transition from local to global buckling with increase of aspect ratio, which was attributed to the biphasic dependence of the buckling strength on the aspect ratio. That is, for small aspect ratio, the local buckling strength is smaller than that of global buckling so that it dominates the buckling behavior of the nanocomposite; for comparatively larger aspect ratio, the local buckling strength is higher than that of global buckling so that the global buckling dominates the buckling behavior. We also found that the hierarchical structure can effectively enhance the buckling strength, particularly, this structural design enables biological nanocomposites to avoid local buckling so as to achieve global buckling at macroscopic scales through hierarchical design. These features are remarkably important for the mechanical functions of biological materials, such as bone, teeth and nacre, which often sustain large compressive load.