仿生結構材料之機械性質模擬與分析──細胞狀結構與仿生珍珠母複合結構

Abstract

材料內部的微結構對於材料整體行為有著關鍵性的影響，生物大量運用此一概念，由演化的方式產生出符合其需求的結構材料；如存在於鳥嘴與骨頭疏質部中的多孔洞細胞狀結構(cellular structure)，使材料兼具高強度及低重量；又如珍珠母及骨頭密質部中的層狀複合結構(composite)，使其擁有極佳的韌性。而仿生技術以相同的概念，發展仿生結構材料(bioinspired structural material)，以求突破現今性質有限的人造材料，創作具有豐富性質的新興材料。 本研究取仿生結構材料中最具代表性的的細胞狀結構與仿生珍珠母複合結構作為研究模型，以有限元素法及快速傅立葉轉換配合微觀力學進行結構之機械性質的模擬與分析，並分別實作於Abaqus及Matlab，以設計仿生材料為目標，補足現階段所需之研究。 細胞狀結構中，本研究針對經典的蜂巢狀結構、Kagome結構及梯形結構，以相圖的方式呈現三個設計參數及其相對應的幾何參數，並探討階層式結構對於細胞狀結構的影響。在設計相圖中，使用者可於兩向等效楊氏係數、及體積密度三者中，選擇需求的數值並於圖中得知相對應的結構。而階層式結構則於研究中發現可以拓展有限的設計空間，將三種等向結構於設計空間(體積密度對上楊氏係數)中的三條設計直線，拓展為三條線內的所有範圍；且藉由兩尺度(two-scale)的方式，相對應的結構都能輕易的被找出來，顯示出階層式結構對於材料設計的潛力。 仿生珍珠母複合結構則以珍珠母之磚泥結構為啟發，探索更多可能存在的韌性機制。最後從五千多個案例中找出數百種具有韌性機制之結構，並將之分為具縱向細長軟材、橫向細長硬材及棋盤狀的三個群組，這三群結構內部都具有相同的微結構特徵、裂縫發展方式、應力應變曲線及應力分布趨勢，藉由這些共同點，微結構可能發展出良好的韌性。且於研究中發現這三種結構特徵也可以混合並同時應用於同種結構中，藉此能更進一步提升材料韌性的發展，可望往後作為設計具良好韌性之人造材料的基礎。

Microstructures play an important role in material properties; by evolution, it is widely applied in the Nature. For example, cellular structures in beak and cancellous bone form light and high strength materials; layered composite in nacre and osteon makes materials possessing high toughness. Inspired by the Nature, researchers use the same concept to create “bioinspired structural materials” to generate more artificial materials with rich properties. In this research, three types of cellular structures (honeycomb, trapezoid and Kagome) are analyzed to assist material design; in addition, a generalized nacre-inspired composite is modeled and analyzed to explore toughening mechanism. To get the effective properties of the microstructures, finite element method and Fast Fourier Transform (FFT) for micromechanics are introduced to be the modeling tool, complemented by Abaqus and Matlab respectively. In the result, three design phase graph for cellular structures are generated, and the effect of hierarchical cellular structures are explored to assist future material design. The design graph contain three design parameters (axis effective Young’s modulus, relative density) and two geometry parameters in one graph. Therefore, providing desire properties, user can get the corresponding geometry parameter. While hierarchical structures can fill the design space, which originally consisted of only three lines. In addition, by two-scale analysis, corresponding geometry parameters can be easily found, which show the potential of hierarchical structures design. Finally, three groups of microstructures possessing toughening mechanisms are discovered. Respectively, each group of microstructures possess the same structural features, growing patterns of microcracks, stress redistributed patterns and the trend of stress-strain curve. With these common features, structures may develop better toughness. Furthermore, these structural features can be mixed in the same time, and by mixing different groups of structures, material may possess much higher toughness.