The strength-toughness tradeoff in biological materials such as nacre and bone is essentially due to their stacked microstructures formed by hard and soft phases. In some of these materials, purely soft phase acts as interface layers linking hard phases (platelets), while in some others, hard-phase bridges exist in the soft phase to form a hybrid interface. In order to disclose the selection mechanism of such different interface structures in biological materials, a novel shear-lag model with an interface consisting of alternatively distributed elasto-plastic (soft) and brittle-elastic (hard) segments is proposed. Using this model, solutions of tensile stress and tensile displacement in hard platelets and shear stresses in soft and hard interfacial segments are analytically achieved. Effects of the hybrid interface on the effective mechanical performances of the composite are analyzed, the results of which are well consistent with the existing experimental observations in biocomposites and bio-inspired composites. The most important finding is that the fracture strain of the soft phase has a decisive effect on the selection of a purely soft-phase interface or a hybrid interface of hard and soft phases in stacked biological materials in order to realize a tradeoff between strength and toughness. When the failure strain of the soft phase is relatively small, such as nacre, the purely soft-phase interface is too weak to transfer enough load to the platelet, and hard bridges are necessarily required to reinforce the interface and guarantee an efficient load transfer. When the soft phase has a sufficiently large failure strain, such as bone, the purely soft-phase interface is tough enough to sustain a large shear deformation, realizing an efficient load transfer and adequate utilization of all constituents, while an additional hard bridge is not conducive to the composite toughness due to its reducing effect on the interfacial shear deformation. The results not only help people gain a deeper understanding of the secrets behind the construction of different interfaces in biological materials, but also provide useful guidance for interface optimization design in strong and tough artificial materials.

中文翻译：

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堆叠生物材料界面矿物桥的选择机制以实现强度-韧性权衡


珍珠质和骨骼等生物材料的强度与韧性权衡本质上是由于它们由硬相和软相形成的堆叠微观结构。在其中一些材料中，纯软相充当连接硬相（片状）的界面层，而在另一些材料中，硬相桥存在于软相中以形成混合界面。为了揭示生物材料中这种不同界面结构的选择机制，提出了一种新颖的剪切滞后模型，其界面由交替分布的弹塑性（软）和脆弹性（硬）段组成。使用该模型，可以分析得出硬片中的拉伸应力和拉伸位移以及软硬界面段中的剪切应力。分析了杂化界面对复合材料有效力学性能的影响，结果与生物复合材料和仿生复合材料现有的实验观察结果非常一致。最重要的发现是，软相的断裂应变对于堆叠生物材料中纯软相界面或硬软相混合界面的选择具有决定性影响，以实现强度和韧性之间的权衡。当软相的破坏应变相对较小时，例如珍珠质，纯软相界面太弱，无法将足够的载荷传递到片晶上，因此必须需要硬桥来加固界面并保证有效的载荷传递。 当软相具有足够大的失效应变（例如骨骼）时，纯软相界面足够坚韧，足以承受大的剪切变形，实现有效的载荷传递和所有成分的充分利用，而额外的硬桥则不需要由于其减少界面剪切变形的作用，有利于复合材料的韧性。研究结果不仅帮助人们更深入地了解生物材料中不同界面构建背后的秘密，而且为强韧人造材料的界面优化设计提供了有益的指导。