Toughening mechanisms in bioinspired multilayered materials

Outstanding mechanical properties of biological multilayered materials are strongly influenced by nanoscale features in their structure. In this study, mechanical behaviour and toughening mechanisms of abalone nacre-inspired multilayered materials are explored. In nacre''s structure, the organic matrix, pillars and the roughness of the aragonite platelets play important roles in its overall mechanical performance. A micromechanical model for multilayered biological materials is proposed to simulate their mechanical deformation and toughening mechanisms. The fundamental hypothesis of the model is the inclusion of nanoscale pillars with near theoretical strength (σ_th ~ E/30). It is also assumed that pillars and asperities confine the organic matrix to the proximity of the platelets, and, hence, increase their stiffness, since it has been previously shown that the organic matrix behaves more stiffly in the proximity of mineral platelets. The modelling results are in excellent agreement with the available experimental data for abalone nacre. The results demonstrate that the aragonite platelets, pillars and organic matrix synergistically affect the stiffness of nacre, and the pillars significantly contribute to the mechanical performance of nacre. It is also shown that the roughness induced interactions between the organic matrix and aragonite platelet, represented in the model by asperity elements, play a key role in strength and toughness of abalone nacre. The highly nonlinear behaviour of the proposed multilayered material is the result of distributed deformation in the nacre-like structure due to the existence of nano-asperities and nanopillars with near theoretical strength. Finally, tensile toughness is studied as a function of the components in the microstructure of nacre.     

Structural Design Elements in Biological Materials: Application to Bioinspiration

Eight structural elements in biological materials are identified as the most common amongst a variety of animal taxa. These are proposed as a new paradigm in the field of biological materials science as they can serve as a toolbox for rationalizing the complex mechanical behavior of structural biological materials and for systematizing the development of bioinspired designs for structural applications. They are employed to improve the mechanical properties, namely strength, wear resistance, stiffness, flexibility, fracture toughness, and energy absorption of different biological materials for a variety of functions (e.g., body support, joint movement, impact protection, weight reduction). The structural elements identified are: fibrous, helical, gradient, layered, tubular, cellular, suture, and overlapping. For each of the structural design elements, critical design parameters are presented along with constitutive equations with a focus on mechanical properties. Additionally, example organisms from varying biological classes are presented for each case to display the wide variety of environments where each of these elements is present. Examples of current bioinspired materials are also introduced for each element.     

The quest for stiff,strong and tough hybrid materials: an exhaustive exploration

How to arrange soft materials with strong but brittle reinforcements to achieve attractive combinations of stiffness, strength and toughness is an ongoing and fascinating question in engineering and biological materials science. Recent advances in topology optimization and bioinspiration have brought interesting answers to this question, but they provide only small windows into the vast design space associated with this problem. Here, we take a more global approach in which we assess the mechanical performance of thousands of possible microstructures. This exhaustive exploration gives a global picture of structure–property relationships and guarantees that global optima can be found. Landscapes of optimum solutions for different combinations of desired properties can also be created, revealing the robustness of each of the solutions. Interestingly, while some of the major hybrid designs used in engineering are absent from the set of solutions, the microstructures emerging from this process are reminiscent of materials, such as bone, nacre or spider silk.     

A two‐scale failure model for heterogeneous materials: numerical implementation based on the finite element method

In the first part of this contribution, a brief theoretical revision of the mechanical and variational foundations of a Failure‐Oriented Multiscale Formulation devised for modeling failure in heterogeneous materials is described. The proposed model considers two well separated physical length scales, namely: (i) the macroscale where nucleation and evolution of a cohesive surface is considered as a medium to characterize the degradation phenomenon occurring at the lower length scale, and (ii) the microscale where some mechanical processes that lead to the material failure are taking place, such as strain localization, damage, shear band formation, and so on. These processes are modeled using the concept of Representative Volume Element (RVE). On the macroscale, the traction separation response, characterizing the mechanical behavior of the cohesive interface, is a result of the failure processes simulated in the microscale. The traction separation response is obtained by a particular homogenization technique applied on specific RVE sub‐domains. Standard, as well as, Non‐Standard boundary conditions are consistently derived in order to preserve objectivity of the homogenized response with respect to the micro‐cell size. In the second part of the paper, and as an original contribution, the detailed numerical implementation of the two‐scale model based on the finite element method is presented. Special attention is devoted to the topics, which are distinctive of the Failure‐Oriented Multiscale Formulation, such as: (i) the finite element technologies adopted in each scale along with their corresponding algorithmic expressions, (ii) the generalized treatment given to the kinematical boundary conditions in the RVE, and (iii) how these kinematical restrictions affect the capturing of macroscopic material instability modes and the posterior evolution of failure at the RVE level. Finally, a set of numerical simulations is performed in order to show the potentialities of the proposed methodology, as well as, to compare and validate the numerical solutions furnished by the two‐scale model with respect to a direct numerical simulation approach. Copyright © 2013 John Wiley & Sons, Ltd.     

多尺度材料模型研究及应用

在分别介绍宏观,介观,微观,原子和电子尺度材料模型研究的基础上,论述了多尺度材料模型（MMM）这一新兴的跨学科的前沿研究领域产生的前提,概念主其在材料科学,特别是在宏观形变及新断裂过程研究中的重要作用,综合分析了多种跨尺度关联方法的原理,技术方案及其应用,并探讨了当前多尺度研究的热点及进一步发展的方向。     

天然和仿生柔性生物结构的设计

自然界中生物材料表现出的力学性能与其结构设计形式紧密相关。柔性生物材料多为多级结构设计,其独特的功能梯度特征使其具备优异的变形能力及良好的断裂韧性。本文借鉴工程结构设计基本单元的思想提出柔性结构仿生元素理念,根据几何形态将结构仿生元素分为：线元素、梁元素、柱元素、板壳元素、薄膜元素及组合元素。根据系统论的观点建立仿生柔性结构设计体系,归纳总结出柔性仿生结构的设计准则,并基于鱼鳞梯度结构设计新型仿生功能梯度板。通过有限元的方法对功能梯度板归一化自然频率进行分析。结果表明,类鱼鳞功能梯度板具有柔韧性及刚度软化特性。阐述了仿生柔性结构的设计方法,包括模仿设计、组合设计及选择匹配设计。     

晶须增韧复合材料机理的研究

本文介绍了晶须增韧复合材料的机理 ,增韧机理主要包括 :裂纹桥联、裂纹偏转、拔出效应 ;讨论了界面性质、晶须性能和基质性质对机理的影响 ;并展望了今后的研究方向。     

Concurrent multiscale modeling of amorphous materials in 3D

A general three‐dimensional concurrent multiscale modeling approach is developed for amorphous materials. The material is first constructed as a tessellation of hexahedral amorphous cells. For regions of linear deformation, the number of degrees of freedom is reduced by computing the displacements of the vertices of the amorphous cells only instead of the atoms within. This is achieved by determining, a priori, the atom displacements within such pseudoamorphous cells associated with orthogonal deformation modes of the cell. Actual atom displacements are calculated using traditional molecular mechanics for regions of nonlinear deformation. Computational implementation of the coupling between pseudoamorphous cells and molecular mechanics regions and stiffness matrix formulation are elucidated. Multiscale simulations of nanoindentation on polymer and crystalline substrates show good agreement with pure molecular mechanics simulations. Copyright © 2012 John Wiley & Sons, Ltd.