Study On The Strengthening And Toughening Design Of The Structure And Interface Of Nacre-like Composites And Its Optimization Mechanisms

With the continuous development of our science,technology and industrial level,the manufacturing industry has become more and more demanding in terms of comprehensive performance of materials.Especially in the aerospace,defense and other high-tech fields,not only require materials with light weight,high strength/toughness and other characteristics but also require them to be able to have good impact resistance,oxidation resistance and creep resistance under high temperature,high speed,high load and other complex working conditions.This puts forward higher and more demanding requirements for the design and preparation of engineering materials.Compared with man-made materials,natural biological materials have many distinct advantages after millions of years evolution.Especially nacre-like composites,as one of the most studied and imitated natural materials,is a highly regular "brick-and-mortar" structural material formed by the alternating arrangement of hard phases(e.g.,mineral aragonite,95 wt.%)and a small amount of soft phases(e.g.,biopolymer,5 wt.%),as well as a lightweight material with excellent tensile strength and fracture toughness.It provides a rich source of inspiration for the design and development of high-performance bionic composites.Inspired by the mineral bridge between hard phase layers of natural nacre,this paper investigates the mechanical properties and optimization strategies of GO-based bionic "brick-mortar" structural composites with interface modification by CNTs.After selectively combining CNTs with GO sheets of different sizes,four biomimetic composites with the same structure but different distributions of strong,medium and weak interfaces were obtained.A comparative study on the mechanical property tests and the finite element simulation results of the samples revealed that the difference in the distributions of different strength interfaces within the material/structure have a significant impact on their localized interfacial failures and subsequent crack extensions,making the deflection of cracks more inclined along the weak interface.Further analysis showed that the targeted and focused optimization of dangerous interfaces with low interfacial bond strength and high stress levels within the material is an effective way to improve the mechanical properties of bionic "brick-mortar" structural composites.The results of the above study will provide useful guidance for the interface optimization and lightweight strengthening and toughness design of composites.In fact,the excellent mechanical properties of natural nacre are closely related not only to its unique interfacial bonding properties but also to the special hierarchical structure with staggered overlapping layers between its soft and hard phases.In Chapter 3,we prepared six sets of "brick-and-mortar" bionic composites with the same dimensions but different overlap lengths of the hard phase layers using carbon fiber prepreg as the hard phase and thermoplastic polyurethane(TPU)as the soft phase.It was concluded that the overlap length of the hard phase layers in the "brick-and-mortar" structure has an important influence on the distribution pattern of shear stress between the soft phases.It was found that increasing the overlap length of the hard phase layer in the "brick-and-mortar" structure could improve the interfacial bonding strength between the soft and hard phases,and thus effectively improve the overall mechanical properties of the samples.When the overlap length of hard phase is shorter than the critical value,the interfacial bonding strength between soft and hard phases is less than the fracture strength of hard phase "brick",and the material as a whole shows interfacial debonding damage.When the overlap length of hard phase is longer than the critical value,the interfacial bonding strength between soft and hard phases is greater than the fracture strength of hard phase "brick",and the material as a whole shows the fracture damage of the hard phase layers.What is more,it is found that the shear-lag effect on the overall mechanical properties of the material is always present in the "brick-and-mortar" structured bionic composites.And the longer the overlap length of the hard phase layers,the more serious the shear-lag effect is,and the lower the efficiency of load transfer in the“brick-and-mortar” structural materials.Therefore,in order to better mitigate the shear-lag effect on the overall mechanical properties of the materials,a specific characteristic length should be selected as the overlap length of the hard phase layers under the premise of selecting a higher strength hard phase layers as much as possible,which can most effectively improve the overall strength/toughness of the “brick-and-mortar” structural bionic composites.The above results provide researchers with a strategy to improve the structure optimization of "brick-and-mortar" bionic composites and contribute useful guidance for the design and development of bionic materials.In summary,this paper designs and prepares "brick-and-mortar" structural bionic composites with different interfacial/structural properties based on carbon based materials such as graphene oxide,carbon nanotubes and carbon fiber prepreg.Combining the mechanical properties,internal microstructure characterization and finite element simulations of the experimental samples,the deep-seated reasons for the light weight,high strength and high toughness of the natural nacre were analyzed.And thus obtained the optimization mechanisms to improve the overall mechanical properties of these bionic composites.The above results provide a reference value for the interface/structure optimization design of bionic composites,and also provide useful guiding strategies for researchers to prepare higher performance "brick-and-mortar" structured bionic composites.