Kamat S, Su X, Ballarini R, Heuer A H
Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7204, USA.
Nature. 2000 Jun 29;405(6790):1036-40. doi: 10.1038/35016535.
Natural composite materials are renowned for their mechanical strength and toughness: despite being highly mineralized, with the organic component constituting not more than a few per cent of the composite material, the fracture toughness exceeds that of single crystals of the pure mineral by two to three orders of magnitude. The judicious placement of the organic matrix, relative to the mineral phase, and the hierarchical structural architecture extending over several distinct length scales both play crucial roles in the mechanical response of natural composites to external loads. Here we use transmission electron microscopy studies and beam bending experiments to show that the resistance of the shell of the conch Strombus gigas to catastrophic fracture can be understood quantitatively by invoking two energy-dissipating mechanisms: multiple microcracking in the outer layers at low mechanical loads, and crack bridging in the shell's tougher middle layers at higher loads. Both mechanisms are intimately associated with the so-called crossed lamellar microarchitecture of the shell, which provides for 'channel' cracking in the outer layers and uncracked structural features that bridge crack surfaces, thereby significantly increasing the work of fracture, and hence the toughness, of the material. Despite a high mineral content of about 99% (by volume) of aragonite, the shell of Strombus gigas can thus be considered a 'ceramic plywood' and can guide the biomimetic design of tough, lightweight structures.
尽管其矿化程度很高，有机成分在复合材料中所占比例不超过百分之几，但其断裂韧性比纯矿物的单晶高出两到三个数量级。有机基体相对于矿相的合理布局以及跨越几个不同长度尺度的分级结构体系，在天然复合材料对外加载荷的力学响应中都起着关键作用。在这里，我们通过透射电子显微镜研究和梁弯曲实验表明，海螺Strombus gigas外壳对灾难性断裂的抗性可以通过两种能量耗散机制来定量理解：在低机械载荷下外层的多重微裂纹，以及在较高载荷下外壳较坚韧的中间层中的裂纹桥接。这两种机制都与外壳所谓的交叉层状微结构密切相关，这种结构在外层提供了“通道”开裂，并在裂纹表面形成未开裂的结构特征，从而显著增加了断裂功，进而提高了材料的韧性。尽管Strombus gigas外壳的文石矿含量约为99%（按体积计），但它仍可被视为一种“陶瓷胶合板”，并可为坚韧、轻质结构的仿生设计提供指导。
