| In modern military conflicts and wars,due to the advancing lethality of weapons and equipment,the requirements for the ballistic resistance of individual protective equipment are becoming increasingly stringent.Lightweight and excellent protective performance are the goals that have been pursued in the development and improvement of protective equipment.With the development and progress of the advanced materials industry,high-performance fiber-reinforced composite materials have gradually replaced traditional metal and ceramic materials due to their advantages of low density,superior energy absorption and strong designability,and have become the mainstream materials for the structural components of individual protective equipment,which are widely used in the manufacture of bulletproof helmets and bulletproof plates,etc.High-performance fiber-reinforced composites are composed of matrix materials and reinforcing fibers.Fiber properties,matrix material properties,and interface bonding properties between fibers and matrix materials are the key factors affecting the properties of fiber-reinforced composites.At present,in the field of ballistic protection materials,commonly used high-performance fibers include aramid fiber,carbon fiber,ultra-high molecular weight polyethylene fiber（UHMWPE）,etc.Compared with other fibers,UHMWPE fiber has the characteristics of low density,high strength,high modulus,high penetration resistance and high energy absorption.Therefore,as a reinforcing fiber,UHMWPE fiber can not only enhance protective performance,but also improve the comfort of human wear with lower equipment weight.However,the smooth surface and chemical inertness of UHMWPE fiber pose challenges to its rapid development and wide application in the field of fiber-reinforced composites.As far as matrix materials,epoxy resins have been widely used in fiber-reinforced composites due to their good adhesion,high strength and stiffness.Nevertheless,its high stiffness can lead to drawbacks such as brittleness and limited impact resistance.Nanoparticle hybrid modification presents a reliable method for improving the toughness of epoxy resins,while the poor dispersion of nanoparticles in resins has become a major pain point in resin modification.So,boosting the dispersion of nanoparticles within the resin is imperative to create robust modified resins that can elevate the performance of composites.Furthermore,optimizing the structural design of fiber-reinforced composites proves to be an effective method for improving the mechanical properties of high-performance fiber-reinforced composites.After countless years of evolution driven by natural selection,some organisms have evolved unique fibrous strengthening mechanisms and lightweight and high-strength structural characteristics.The material and structural characteristics of these organisms can provide inspirations and blueprints for the innovative design of fiber-reinforced composite materials.On the one hand,some organisms can prepare perfect materials that are lightweight,high-strength,and impact-resistant through simple composition and gentle synthesis processes.The tips of spider fangs can achieve remarkable hardening through the cross-linking between keratin and metal ions,and this hardening mechanism can be applied in the surface modification of UHMWPE fiber.On the other hand,some organisms can mineralize and synthesize minerals from simple biomass materials as templates.This synthesis idea can be applied to the fabrication of nanofibers with multi-level structure that combine the advantages of organic polymer nanofibers and inorganic nanoparticles.These nanofibers can further be employed as nanofillers to modify epoxy resin.In addition to the outstanding performances of biological components,the lightweight and high-strength structures found in organisms also serve as favorable models for material design.The hollow and porous sandwich structure of biological spines,like porcupine and hedgehog,endows spines with striking impact resistance and bending capabilities,and these biological structures can inspire the design of lightweight and high-strength structures for fiber-reinforced composites.Therefore,based on a deep understanding of the failure mechanisms of fiber-reinforced composites,this dissertation delves into the factors influencing the performance of UHMWPE fiber-reinforced composites（i.e.,fiber,matrix,interface and structure）.By using various biomimetic design strategies,combined with experimental verification and theoretical analysis,a series of studies have been conducted to enhance the mechanical and protective properties of UHMWPE fiber-reinforced resin composites.The aim is to improve the static and dynamic mechanical performances as well as ballistic resistance of the composites,providing theoretical guidance for the design of high-performance ballistic protection equipment for soldiers.The main research contents of this dissertation are as follows:（1）In order to overcome the problem of weak interface properties between the fibers and the matrixes caused by the inertness of UHMWPE fiber,a modification method of bio-inspired coating,i.e.,Cu2+chelated chitosan biomimetic coating has been designed to coat and modify the surface of UHMWPE fiber,which is inspired by the hardening mechanism of cross-linking between metal ions and polysaccharide/protein in spider fangs.The surface morphology and mechanical properties of the modified fibers have been analyzed.Subsequently,the UHMWPE fiber resin matrix composites have been further prepared,and the interlaminar shear strength,interlaminar toughness,as well as the performance enhancement mechanisms of composites have been investigated and explored.It has been found that the surface of the modified UHMWPE fiber transforms from inert to active,resulting in an 18.89%increase in tensile strength.Moreover,bio-inspired UHMWPE fiber composites exhibit a 37.72%enhancement in interlaminar shear strength and a 135.90%improvement in fracture toughness,respectively.（2）To enhance and strengthen epoxy resin,a strategy of nanomaterial hybrid modification has been adopted.By mimicking biomineralization process,nanocellulose fibers have been subjected to biomimetic mineralization modification,allowing hydroxyapatite nanoparticles to grow in situ on their surfaces.By leveraging the advantages of nanocellulose fibers and hydroxyapatite nanoparticles,the modified nanocellulose fibers with hierarchical structures have been fabricated.This bio-inspired approach addresses the poor dispersion of individual nanoparticles in resin,and improve the entanglement capability of nanocellulose fibers within the resin.The properties of the mineralized nanocellulose fibers have been tested,and these fibers have been applied to the hybrid modification of epoxy resins.Further,the properties of the modified resins have been explored to determine the the most appropriate addition ratio,and to reveal the mechanisms behind the performance enhancement of mineralized nanocellulose fibers-modified resins.It has been found that biomimetic mineralized nanocellulose fibers exhibit hierarchical structures,with rougher surfaces and increased surface area,as well as improved dispersion in the resins.Moreover,the tensile strength of the resins modified with mineralized nanocellulose fibers is increased by more than 2 times,and the flexural strength is increased by 62.92%under the enhanced toughness.（3）On one hand,the UHMWPE fiber modified resin composites have been designed by using modified resins as the matrixes,and their flexural strength and interlaminar fracture toughness have been analyzed to investigate the inherent reasons and key influencing factors of the performance enhancement.It has been found that bio-inspired mineralized nanocellulose fibers can not only enhance the toughened of the resins,but also improve the interfacial bonding ability between the resin and UHMWPE fiber.Moreover,the flexural strength and Mode II interlaminar fracture toughness of the modified resin-based UHMWPE fiber-reinforced composites are increased by 63.16%and 350%,respectively.On the other hand,inspired by the multi-level structure of porcupine spines and hedgehog spines known for their“tough on the outside,soft on the inside”characteristic,a lightweight nanocellulose fiber aerogel has been prepared based on the porous foam structure of the inner core.The aerogel has been then mineralized and modified to improve its mechanical properties.Next,the bio-inspired aerogel-core UHMWPE fiber composites have been constructed by using mineralized aerogel as the core material,and their bending properties and interlaminar fracture toughness between the panel and the core material have been explored.It has been found that the flexural strength of the bio-inspired aerogel-core composites is increased by 83.11%,and the interlaminar fracture toughness between the panel and the core material far exceeds that between fibers,reaching up to 7 times as much.（4）The dynamic impact performance of bio-inspired UHMWPE fiber-reinforced composites prepared under the three biomimetic design strategies mentioned above have been explored.Firstly,the dynamic mechanical property tests have been conducted to analyze the dynamic compression and dynamic shear properties of the three composites under high strain rates,and to further elucidate their failure mechanisms.Additionally,the ballistic impact tests have been carried out to provide comprehensive analyses of the ballistic performance exhibited by the three composites.Finally,the mechanisms of improving the ballistic performance of the three composites have been analyzed,and their application scenarios and design strategies have also been clarified.In summary,the bio-inspired preparation strategies in this dissertation can improve the protective performance of UHMWPE fiber reinforced composites,and provide new solutions and ideas for the applications and designs of UHMWPE fiber reinforced composites. |
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