鱷魚是一種適應力強的活化石，演化出天擇優化後的骨甲。此骨甲由各自獨立的骨板所構成，骨板與骨板間以鋸齒狀的邊緣相互扣鎖，並由非礦化的膠原蛋白纖維連結，在提升防禦能力的同時兼顧可撓曲性。鱷魚骨板內部為複合夾層構造，由多孔性的內部和較緻密的外層共同組成，兼具抗彎剛度及能量吸收能力。此研究從材料科學觀點出發，探討美洲短吻鱷骨板在多尺度下的結構與其機械性能。X光斷層掃描影像顯示骨板內部的神經血管系統，形成複雜的三維孔洞結構。透過光學顯微鏡和掃描式電子顯微鏡的觀測可得知，骨板是由位於上半部的編織骨和底部的層狀骨所複合而成。利用奈米壓痕量測系統可測得骨板局部位置的硬度與彈性係數，巨觀的機械性質則由壓縮試驗獲得。結果顯示，骨板是一種非均質材料，在不同區域具有不同的礦物質含量及多孔性，外層較硬且較緻密的編織骨以漸次變化與底部較韌的層狀骨進行結合，形成獨特的功能梯度複合材料。本研究提出並印證三種鱷魚骨板的防禦機制：一、鋸齒狀邊緣扣鎖和非礦化的膠原蛋白纖維提供適量可撓曲性；二、多孔性的內部構造能吸收能量，使外殼不易碎裂；三、非均勻分布的微結構和機械性質則有助於外力之再分配並提升抗衝擊之能力。鱷魚骨板的最佳化結構和機械性能可望提供新型盔甲和先進複合材料設計與製造之靈感。

Alligator is a well-adapted living fossil covered with a dorsal armor. This dermal shield consists of bony plates, called osteoderms, interconnected by sutures and non-mineralized collagen fibers, providing a dual function of protection and flexibility. Osteoderm features a sandwich structure, combining an inner porous core and an outer dense cortex, to offer enhancements for bending stiffness and energy absorbance. In this study, hierarchical structure and mechanical behaviors of the American alligator (Alligator mississippiensis) osteoderm were investigated. Micro-computed tomography was applied to reveal the complex 3-dimesional neurovascular network. Through the observation under optical and scanning electron microscopes, the osteoderm was found to consist of woven bone in the dorsal region and lamellar-zonal bone in ventral region. Nanoindentation and compressive tests were performed to evaluate the mechanical properties of osteoderms. The varying mineral contents and porosity resulted in a graded mechanical property: from a hard and stiff dorsal cortex gradually transform to a more compliant ventral base. Three protective mechanisms were proposed and observed for alligator osteoderms: (1) flexibility provided by sutures and non-mineralized collagen fibers; (2) energy absorption under compression contributed from the interior cellular foams; (3) non-uniform microstructure and graded mechanical properties offer load re-distribution and impact resistance. The inspirations from alligator osteoderms may lead to the optimized design of novel synthetic armors and advanced composites.

Contents
List of Tables V
Figure Caption VI
Chapter I Introduction 1
1.1 Background 1
1.2 Motivations and Goals 4
Chapter II Literature Review 7
2.1 Biological Materials 7
2.1.1 Overview 7
2.1.2 Basic Building Blocks of Biological Materials 11
2.1.3 Cellular Solids 16
2.1.4 Sandwich-structured Composite 18
2.2 Bone 18
2.2.1 Hierarchical Structure 19
2.2.2 Mechanical Behavior 19
2.3 Natural Armors 21
2.3.1 Inflexible Protective Shields 21
2.3.2 Flexible Dermal Armors 24
Chapter III Experimental Procedure 57
3.1 Sample Preparation and Treatment 57
3.2 Compositional Analysis 58
3.2.1 Ash Content Measurement 58
3.2.2 X-ray Diffraction 58
3.2.3 Electron Probe Microanalysis 59
3.2.4 Energy-Dispersive Spectroscopy 59
3.3 Structural Characterization 60
3.3.1 Macroscopic Observation 60
3.3.2 Micro-Computed Tomography 60
3.3.3 Optical Microscopy and Stereoscopy 61
3.3.4 Scanning Electron Microscopy 61
3.4 Mechanical Testing 62
3.4.1 Nanoindentation 62
3.4.2 Compressive Testing 63
3.4.3 Flexibility Demonstration 65
3.4.4 Whole Osteoderm Compression 65
3.5 Weibull Analysis 65
Chapter IV Results and Discussion 70
4.1 Overview: Hierarchical Structure 70
4.2 Macroscopic Observation 70
4.3 Mineral Content Measurement and Elemental Analysis 71
4.4 Micro-CT Imaging 73
4.5 Microstructural Characterization 75
4.5.1 Optical Microscopy 75
4.5.2 Scanning Electron Microscopy 77
4.6 Mechanical Behavior 79
4.6.1 Nano-mechanical Evaluation 79
4.6.2 Compressive Mechanical Behavior 82
4.7 Deformation Mechanisms 87
Chapter V Conclusions 104
Chapter VI Future Work 107
6.1 Fracture Toughness Measurement 107
6.1.1 Fracture Resistance Curve (R-curve) 109
6.1.2 Crack Propagation and Toughening Mechanisms 110
6.2 Bio-inspired Synthesis 111
6.2.1 Hybrid Organic/Inorganic Multi-layer Composites 112
6.2.2 Functionally Graded Ceramics 114
6.2.3 3D-printed Models and Novel Designs 116
References 128

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