Multiscale Mechanical Design Of High-Performance Structures Inspired By "Spear And Shield"-type Biological Competition

In nature,the "spear and shield"-type survival competition between predators and preys is the driving force to improve the mechanical properties of biological structure materials.The natural law of "survival of fittest" makes the biological structural materi-als such as the exoskeleton of mantis shrimp and nacre of seashell possess lightweight,high strength,high toughness and high impact-resistance.The excellent mechanical properties of biomaterials are closely associated with multiscale and multilayer struc-tures and interfaces.Learning from nature,people can reveal the internal relationships between structures,interfaces and mechanical properties in biomaterials using multi-scale mechanical methods,and provide new mechanical design schemes for the manu-factures of high-performance bioinspired structural materials.Different characteristics of structures and interfaces in biomaterials result in rich toughening strategies,such as crack deflection,crack bridging and interfacial delamination.However,previous stud-ies usually focus on single toughening strategy,while there are multiple toughening strategies in same biomaterial.The interactions between hybrid toughening strategies and the structural characteristics remain unclear.Therefore,we develop multiscale me-chanical methods to analyze the competition and cooperation among various toughening strategies in biological structural materials.We provide parametric schemes in terms of structural geometries and interfacial properties to achieve optimal strength,toughness and impact-resitance for bioinspied structural materials.The main works of this paper are as follows::Firstly,We develop a multiscal mechanical model for graphene-based artificial nacres.We analyse the competitive relationship between the interlayer and intralayer properties of graphenes due to crosslink.The results reveal the emergence of an opti-mal crosslink density at which the maximum strength and toughness are achieved.In addition,the increase of the crosslink density intensifies the nonuniformity of the shear stress.Finally,we propose a mechanical design scheme to simultaneously optimize the strength and toughness by tuning crosslink density.Secondly,we study the competition and cooperation between intralayer crack prop-agation and interlayer interfacial delamination in nactreous structure under low-velocity impact.The cooperation lead to a maximal energy dissipation in the presence of the op-timal interfacial strength.The optimal interfacial strength decreases with increasing impact velocity.The impact-resistance of nacreous structure under different impact velocities can be enhanced by tuning the interfacial strength.Thirdly,we propose a bioinspired prestressing strategy to achieve optimal impact-resistance of nacreous structure.We reveal that the competition between the prestress-promoted tablet sliding and prestress-weakened structural integrality results in opti-mized impact resistance.The critical prestress phenomenon is proved by drop tower tests on 3D-printed nacreous specimens under different pre-tensioning forces.Further-more,the bioinspired prestressing strategy was applied to a nacre-inspired separator to improve the impact tolerance of lithium batteries.Fourthly,inspired by the survival war between the mantis shrimps and abalones,we design a discontinuous fibrous Bouligand(DFB)architecture,a combination of Bouli-gand and nacreous staggered structures.Fracture mechanics analyses demonstrate that the hybrid toughening mechanisms of crack twisting and crack bridging mode arising from DFB architecture enable excellent fracture resistance with crack orientation in-sensitivity.We further illustrate that the optimized fracture energy can be achieved by tuning fracture energy of crack bridging,pitch angles,fiber lengths,and twist angles dis-tribution in DFB composites.Finally,we apply the proposed mechanical design scheme to synthesis the bioinspired structural materials with high strength,high toughness and high impact resistance.Fifthly,we focus on the influence of the structural features such as mixed fracture mode and interfacial properties on the evolution process of three-dimensional crack morphology.The phase field fracture method is developed to simulate the crack twist propagation in the mixed mode ?+? fracture process.We analyse the synergistic toughening effects between crack torsion and interfacial delamination in the layered structure under mix mode ?+? fracture.We reveal the evolution process of crack twisting and interfacial delamination in structure with twisted interface.