Department of Mechanical Engineering, McGill University, Montreal, Que., Canada.
Acta Biomater. 2010 Oct;6(10):4081-9. doi: 10.1016/j.actbio.2010.04.008. Epub 2010 Apr 18.
Mineralized biological materials such as nacre or bone achieve remarkable combinations of stiffness and toughness by way of staggered arrangements of stiff components (nanoscale or microscale fibers or tablets) bonded by softer materials. Under applied stress these components slide on one another, generating inelastic deformations and toughness on the macroscale. This mechanism is prominent in nacre, a remarkable material which is now serving as a model for biomimetic materials. In order to better identify which type of nacre should serve as a biomimetic model, the toughness of nacre from four different mollusk species was determined in this study. Nacre from the pearl oyster was found to be toughest, and for the first time remarkable deformation and fracture patterns were observed using in situ optical and atomic force microscopy. Under stress, stair-like deformation bands deformed at an angle to the loading direction, forming a dense, tree-like network. This marks a clear difference from the now well-documented "columnar" failure mode, in which deformation bands are perpendicular to the loading direction. Analytical and numerical models reveal the conditions for the transition between the columnar and stair failure modes, namely large or random overlap between inclusions and local shear stress generated by inhomogeneities in the material. "Stair" failure promotes spreading of non-linear deformation and energy dissipation, which translates into a greater toughness overall. A similar mechanism may also occur in bone, which has a microstructure which is in many ways similar to sheet nacre.
矿化生物材料，如珍珠层或骨骼，通过硬组件（纳米或微纤维或片材）的交错排列，并结合较软的材料，实现了显著的刚度和韧性的结合。在施加的应力下，这些组件相互滑动，在宏观尺度上产生非弹性变形和韧性。这种机制在珍珠层中很明显，珍珠层是一种很出色的材料，现在正作为仿生材料的模型。为了更好地确定哪种珍珠层应该作为仿生模型，本研究测定了来自四种不同软体动物的珍珠层的韧性。珍珠层来自珍珠贝，被发现是最坚韧的，并且首次使用原位光学和原子力显微镜观察到了显著的变形和断裂模式。在应力下，阶梯状的变形带以与加载方向成一定角度变形，形成密集的、树状的网络。这与现在已经有充分记录的“柱状”失效模式有明显的区别，在柱状失效模式中，变形带垂直于加载方向。分析和数值模型揭示了柱状和阶梯失效模式之间转变的条件，即包含物之间的大或随机重叠以及材料不均匀性产生的局部剪切应力。“阶梯”失效促进了非线性变形和能量耗散的扩展，这转化为整体更大的韧性。类似的机制也可能发生在骨骼中，骨骼的微观结构在许多方面与片状珍珠层相似。
