多级结构贝壳材料力学性能的跨尺度理论表征和实验观测

Many biological materials have hierarchical structures from microscopic to macroscopic scales and because of their superior macro-mechanical properties, which closely rely on their hierarchical microstructures (trans-scale mechanical behaviors), these materials become a more and more important part of biomimetic material sources for advanced material design. In this project, we consider shell materials with hierarchical structures and study their trans-scale mechanical behaviors through both experimental observation and model characterization. For experimental observation: first, the microstructure evolution and the mechanical properties of nanoparticle mineral platelets are studied through the real-time and in vivo observation using transmission electron microscope, atomic force microscope and nanoindenter. Then, the strength and toughness mechanism of the interfacial adhesion between mineral platelets and organic matters are studied through the real-time and in vivo observation using scanning electron microscope. Finally, the tension, compression and bending tests on the shell materials with notch are carried on systematically to find the dependency of the macro-mechanical properties on the mineral platelet/organic matter interfacial properties. For model characterization, using the trans-scale mechanics theory which considering both strain gradient effect and surface/interface effect, the trans-scale mechanical behavior of the hierarchical structural shell materials is characterized (such behavior ranges from the platelets of nanoparticle/organic matter in micron scale, the micron scale platelet/organic matter interface, to the shell materials in macron scale). Researches in this project can help to systematically understand the correlations between the macro-mechanical properties and the nano-/micro-structures of biological hierarchical materials.

许多生物材料具有自微观到宏观的多级结构特征，其优异的宏观力学性能密切地依赖于其多级微结构特征（跨尺度力学行为），越来越成为先进材料设计的重要仿生来源。本项目针对多级结构的贝壳材料，从模型表征和实验观测入手研究该类材料的跨尺度力学行为。在实验观测方面：首先，通过透射电镜、原子力显微镜和纳米压痕仪的实时在位观测，研究纳米颗粒矿物小板的微结构演化与力学性能；其次，通过扫描电镜的实时在位观测，研究矿物小板与有机质界面结合强韧机制；最后，通过含缺口的贝壳材料的宏观拉压弯实验系统地研究宏观力学性能对矿物小板/有机质界面性能的依赖关系。在模型表征方面，采用跨尺度力学理论（同时考虑应变梯度和表界面效应），对多级结构贝壳材料（由纳米颗粒/有机质的微米尺度板，微米尺度板/有机质界面，到宏观贝壳材料）的跨尺度力学行为进行表征。通过该课题的研究为系统认识生物多级材料宏观力学性能与纳微结构的关联机制建立基础。

生物多级微纳米结构材料具有自微观到宏观的多级结构特征，其优异的宏观力学性能密切地依赖于其多级微结构特征（跨尺度力学行为），越来越成为先进材料设计的重要仿生来源。本项目针对多级结构贝壳材料，从模型表征和实验观测入手研究该类材料的跨尺度力学行为。首先，系统地开展了从浅压痕到深压痕的硬度测试实验，并采用跨尺度力学理论（同时考虑应变梯度和表/界面效应）表征了其纳/宏观压入硬度的尺度效应，得到的应变梯度特征尺度参量在微米量级，贝壳材料第一级结构纳米颗粒的尺寸与实验观测结果相符；其次，实时在位地观察了裂纹起源、演化、发展的全过程，捕捉到了裂尖分叉与裂纹偏转机制的内在联系，进一步阐明了增韧机制；精细地观测了该类材料的微裂纹萌生、演化及汇合的破坏机制，并提出了有效力学表征模型，由此得到的能量释放率与实验相符；最后，采用应变梯度理论描述有机质层，同时在矿物小板与有机质层的界面处采用内聚力模型，得到了该类材料的跨尺度计算模型，通过在有效范围内调控特征尺度参量与有机质层厚度的比值，使得材料的微结构尺度从微观过渡到宏观，由该模型得到的应力应变关系与实验结果符合较好，优于传统理论模型。通过该项目的研究为系统认识生物多级材料宏观力学性能与纳微结构的关联机制建立了基础，为仿生材料的设计和力学性能评判提供了科学依据。