牙本质多级结构分层的协同增韧机制研究

Dentin is a biocomposite forming the bulk of the tooth. The complex hierarchical structure leads to unique toughening mechanisms, which enable dentin to fulfill the physiological functions effectively. The experimental efforts were devoted to the investigation of the relationship between structure and mechanical properties of dentin, whereas quantitative understanding of the toughening mechanisms associated with hierarchical structure is not achieved. This research proposal is focused on the development of multiscale fracture mechanics model for dentin to quantitatively explore the relationship between structural hierarchy and toughening mechanisms. Firstly, a large deformation, viscoplastic damage model for protein is developed and the nanocomposite model of dentin is proposed. Based on the developed models, the toughening mechanisms of nanostructured dentin are analyzed. Then, a plasticity model of dentin is developed by accounting for the plastic deformation mechanisms of dentin at the microscopic scale. The computational simulations are performed and the intrinsic toughening mechanisms are analyzed. In addition, the model characterizing crack growth in dentin is proposed by taking into account the difference in mechanical properties between intertubular dentin and peritubular dentin, and the unique spatial distribution of dentin tubules. With this model, the extrinsic toughening mechanisms are analyzed, and the toughening mechanisms caused by the microstructure of dentin are summarized. We also conduct experiments of crack growth in dentin to validate the developed models. Finally, based on the toughening mechanisms of nanostructure and microstructure, a multiscale fracture model delineating the toughening mechanisms by structural hierarchy is developed, and design principles of bioinspired materials with high toughness are proposed.

牙本质是一种生物复合材料，构成了牙齿的主体部分；其复杂的分层结构导致了独特的增韧机制，从而保证了牙本质有效地完成生理功能。目前关于牙本质的结构与力学性质关系的研究还停留在实验阶段，缺乏定量地理解分层结构引起的增韧机制。本项目拟建立牙本质多尺度断裂力学模型，定量研究分层结构与增韧机制的关系。首先，建立蛋白质大变形粘塑性损伤本构模型，发展牙本质纳米尺度的复合材料模型，分析纳米结构的增韧机制。然后，考虑牙本质的微观塑性变形机理，建立塑性力学模型，通过计算模拟，分析内部增韧机制。另外，考虑管间牙本质和管周牙本质的力学性质差异，以及牙本质小管的独特空间分布，建立牙本质的微观裂纹扩展模型，分析外部增韧机制，并总结微米尺度结构的增韧机制。开展裂纹扩展实验，验证模型的有效性。最后，基于纳米结构和微米结构的增韧机制，建立牙本质分层结构增韧的多尺度断裂力学模型，并根据此模型提出仿生高韧度材料设计原理。

牙本质是一种具有多尺度分层结构的生物材料，表现出增强的抵抗断裂的能力。探索牙本质的分层结构与断裂力学性质的关系能够为高性能仿生材料设计提供指导。本项目系统研究了牙本质的分层结构引起的多尺度增韧机理。我们通过考虑牙本质在纳米尺度、亚微观尺度、微观尺度和宏观尺度上的结构特征以及基本的力学性质，建立了多尺度断裂力学模型，开展了牙本质在不同尺度上断裂的数值模拟。亚微观尺度的数值模拟表明矿化胶原纤维内部矿物质晶体的相对滑移产生的塑性变形、纤维外基底的塑性耗散和纤维与纤维外基底间界面脱粘机理协同作用，控制着牙本质亚微观尺度的增韧。纤维外基底的低屈服应变和强应变硬化行为能够引起增强的能量耗散，激发矿化胶原纤维的塑性变形，同时产生弥散型的界面损伤行为，从而减缓了牙本质的断裂，提高了断裂韧度。我们进一步发现牙本质亚微观尺度的断裂行为依赖于纤维外基底的应变率敏感性和纤维与纤维外基底间界面的性质。对于强界面，矿化胶原纤维的塑性耗散是主要的增韧来源，低应变率敏感性的纤维外基底能够促进牙本质增韧。对于韧性界面，牙本质的断裂韧度随着纤维外基底应变率敏感度的提高而增大。对于弱界面，界面脱粘是牙本质主要的增韧来源，纤维外基底的应变率敏感性对牙本质断裂影响很小。微观尺度的数值模拟发现了牙本质的两种断裂机理：主裂纹前端管周牙本质萌生微裂纹、主裂纹沿着管周与管间牙本质间界面偏斜扩展。我们揭示了管周与管间牙本质间界面的强度是控制牙本质两种断裂机理转化的关键因素，而界面韧度对牙本质的断裂影响很小。根据亚微观和微观尺度的数值模拟结果，我们建立了描述牙本质非弹性变形的宏观本构理论，并发展了数值模拟算法，计算了牙本质裂纹端部场。结果发现由微裂纹引起的塑性体膨胀变形能够扩大裂纹端部的塑性区尺寸，从而潜在地提高牙本质的断裂韧度。