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Title: Micro- and nano-mechanics of natural and bioinspired materials
Authors: Fu, Jimin
Degree: Ph.D.
Issue Date: 2019
Abstract: As a stealth hand, ecological systems have shaped all lives in the earth to achieve extreme performance in adapting to the severe environment. How nature can build a variety of materials with such novel structures and unique functions, and what are the relationships between these structures and functions? Experimental characterization and theoretical modeling expose that the unique properties of these natural materials rely on the complex structures ranging from macro- to nano-scales. In addition to the efforts devoted to understanding the mechanisms behind these fancy structures and functions, a growing effort were devoted to developing and fabricating materials with the novel properties by mimicking those found in nature. These bioinspired materials commonly include organic and inorganic components that need perfect assembly from macro- to nano-scales. In the thesis, the attentions are focused on abundant mechanical phenomena existing in biological and bioinspired systems, especially, tribological (wear, adhesion and contact) behaviors and fracture behaviors under impact loading. The objectives are to explicate the mechanisms behind the abundant mechanical phenomena. Although the investigations are found in the specific biological and bioinspired systems, the mechanical principles to be revealed can be broadly used as the guidelines of related engineering applications. Chapter 1 is about the fundamental knowledge concerning the mechanical phenomena in natural and bioinspired materials, and then a literature review on the related field in recent years to reveal and apply some mechanical, especially, tribological mechanisms and fracture-modes-controlled mechanisms in biological systems is presented in Chapter 2. In Chapter 3, the experimental methods including characterization and synthesis methods adopted in the studies are discussed, followed by brief introduction of molecular dynamic (MD) simulation method. Subsequently, prominent wear resistance of the teeth of black carp is studied in Chapter 4. In this chapter, the mechanism of the high wear resistance of the black carp teeth occlusal surface (OS) is found. Nano-scratching test and comparative X-ray diffraction analysis on the enameloid, the outermost layer of the teeth, indicate that hydroxyapatite (HAp) crystallites show c-axis preferential orientation in the vicinity of OS. The prominent wear resistance of the teeth is attributed to the c-axis preferential orientation of HAp near the OS since the (001) surface of HAp crystal, which is perpendicular to the c-axis, exhibits much better wear resistance compared to the other surfaces as demonstrated by the MD simulation. The following theoretical modeling indicates that scratching on the (001) surface of HAp tends to cause 'rubbing mode' failure rather than 'cutting mode' failure. The finding shows great promise of utilizing preferential orientation of crystals to increase the wear resistance of materials.
In Chapter 5, the excellent anti-fouling performance of the Sonneratia apetala leaves is demonstrated to be attributed to their ridge-like surface morphology. Inspired by the superior antifouling performance of the leaves of mangrove tree S. apetala, to combat biofouling by using surface with ridge-like morphology is proposed. Settlement tests with tubeworm larvae on polymeric replicas of S. apetala leaves confirm that the ridge-like surface morphology can effectively prevent biofouling. A contact mechanics-based model is then established to quantify the dependence of tubeworm settlement on the structural features of the ridge-like morphology, giving rise to theoretical guidelines to optimize the morphology for better antifouling performance. By following the obtained guidelines, a synthetic surface with ridge-like morphology is developed, exhibiting antifouling performance comparable to that of the S. apetala replica. The following theoretical modeling indicates that shape ridge can reduce the fouler attachment probability and then show good anti-fouling performance. In Chapter 6, inspired by toughening mechanisms adopted by various natural materials such as nacre, bones, and teeth, consisting of brittle mineral platelets embedded in a soft, ductile organic matrix, the fracture behavior of bio-inspired graphene-based structure under hypervelocity impact is studied through MD simulation. By optimizing the geometrical structures of the inclusions in the graphene membrane, the controlled fracture behavior can be achieved. Results show that the inclusions embedded in the graphene-based material can change crack path. Based on the simulation results, several means are proposed to improve anti-crack performance by using the shielding effect of the inclusions in the graphene-based material: (1) choosing a harder inclusion, (2) increasing the radius of inclusion, RI. Through the comprehensive studies on the novel mechanical properties of the typical natural materials, the relationships between their structures and functions are analyzed. The theoretical guidelines should be beneficial for the development of the corresponding bioinspired materials. In summary, the investigations present in this thesis not only decode the mechanical mechanisms accounting for the fancy mechanical properties of these natural material but also provide practical strategies and guidelines for the design of bioinspired materials and structures with better performance.
Subjects: Hong Kong Polytechnic University -- Dissertations
Pages: xxiii, 123 pages : color illustrations
Appears in Collections:Thesis

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