Mechanical Behaviors And Functional Applications Of Designs Via Integration Of Soft And Hard Matter/Structures

Integration of soft and hard matter is ubiquitous in biological materials and structures created by nature,and the integration across multiple length scales leads to excellent mechanical properties of the biological materials/structures.Based on the microstructures and hierarchical features of biomaterials,bioinspired materials are also designed to achieve improvement of mechanical performance or functional properties.Besides,in the design of newly emerging composite materials/structures/devices such as phononic crystals,metamaterials,metasurfaces and soft electronics,etc.,soft-hard integration is also utilized to obtain specific mechanical behaviors and functionalities and to enhance their adaptability to various environments.In this dissertation,the concept of soft-hard integrated design is comprehensively explained and its applications are envisioned by taking advantage of the merits of soft and hard materials/structures and their matual interaction.In particular,the mechanical behaviors of two composite systems,i.e.foam-core/solid-shell spherical structure and hard-particle/soft-matrix composite materials,are systematically studied via a combination of elasticity theory,finite element analysis(FEA)and experimental tests,and some engineering applications of these two soft-hard composite systems are also discussed and studied.The main contributions and conclusions of this dissertation are summarized as follows.(1)A novel foam-core/solid-shell spherical(FSS)structure is proposed,and its effective Young's modulus,critical buckling and deformation characteristics are analyzed under plate compression loading using both theory and finite element modeling.Analysis results show that the mechanical properties of FSS structures vary nonlinearly with geometric parameters,and the deformation patterns of FSS structures can be categorized into three modes,i.e.foam domninated mode,shell dominated mode and combined mode.Considering the deformation coupling between inner foam structure and outer solid shell,theoretical models are created to predict the effective Young' s moduli,critical buckling forces and geometric relations that decides deformation patterns.Good agreement between theoretical and FEA results is achieved.Further,the energy absorbing property of FSS structures is analyzed under compression and a new dimensionless index is proposed to evaluate their energy absorption ability.It is shown that the structures with combined or foam-dominated deformation modes have better energy absorbing capability than those with shell dominated deformation,which is due to multiple deformation mechanisms such as local buckling,curling and folding,etc.(2)Fabrication and experimental test of FSS structures are conducted.First,the feasibility of fabricating FSS structures using 3D printing techniques is discussed,and trial manufacture is conducted by employing fused deposition modeling(FDM)technique.However,due to the low-precision shortcoming of FDM technique,the printed structures perform much worse than prediction.Then,the work mainly focuses on the mechanical behaviors of foam-core/thin-shell amorphous carbon nanospheres made via ultrasonic spray pyrolysis(USP)process.In-situ TEM compression test and simplified FEA are carried out for three typical structures,i.e.hollow spherical shell,macroporous sphere and microporous sphere,obtained from USP.Results show that the load-displacement curve of macroporous sphere can be divided into three stages including elasticity,plateau and densification,thus leading to large deformation ability and high energy absorbing capability,which is superior to the brittle property of bulk amorphous cabon.Such performance is also attributed to multiple deformation mechanisms such as post-buckling deformation of thin outer shell,collapse of inner pore structures and bending/buckling of pore struts.Further paremater analysis using finite elemnt modeling indicates that both inner pore structure and outer shell have positive contributions to the large deformation and energy absorption properties of the hybrid structures,thus proving the advantage of our integrated structure design.(3)For the first time,a theoretical framework is established for the rotation of multiple randomly located particles with arbitrary shapes in a soft infinite matrix,and extensive FEA are conducted to verify the theory.Based on assumptions including rigid inclusion,elastic matrix and perfect bonding between particles and matrix,the interaction between multiple arbitrarily shaped particles are decribed by using supervision principle and complex function approach in plane elasticity,and the effect of interaction on the rotation of particles is also studied.Geometric nonlinearity is considered in the established theory according to the large deformation feature of soft matrix.Besides,a simplified and explicit solution for single arbitrarily shaped particle is given,which amends the classical solution for single elliptical particle from Muskheshvili.Finite element modeling and analysis are perform for different shapes,numbers,arrangements and orientations of particles,and the obtained rotation results agrees excellently with those from theory.These results also indicate that the rotation of hard particels can be tuned in a wide range by properly arranging their locations and orientations.Furthermore,detailed discussions are made on the extension of the established theory to cases including non-rigid particles,viscoelastic matrix and imperfect bonding.(4)A bioinspired soft-hard composite is designed to achieve negative Possion's ratio(NPR)deformation by ultilizing hard particle rotation.Based on the rotation angles of periodically arranged square particles and the competition between shrinking deformation of soft matrix and expansion induced by hard particle rotation,theoretical solution to the Poisson's ratio of soft-hard composites is deduced and indicates that the rotation per strain is a key parameter to achieve NPR behavior.The crucial role of particle rotation to NPR is further verified through comparison of theory,semi-theory and finite element analyses.Besides,optimization designs are carried out by taking maximum negative Poisson's ratio as objective,and the optimization searches under different particle distances show that identical or close orientation angles is helpful to the realization of NPR.With the optimized structure,composite test specimens are designed and printed by Polyjet 3D printing technique.The results obtained from both experimental tests and FEA further prove the deterministic effect of particle rotaion on the NPR behavior of the soft-hard composites.Besides,the experimental and FEA results for test models with different thicknesses also indicate a valley-shape variation of the Poisson's ratio with the increase of model thickness,and the Poisson's ratio breaks through the bounds of plane strain and plane stress cases by a large extent.This variation trend is found to be directly related to the increased particle rotation that is induced by out-of-plane shrinking deformation of the matrix.(5)The application of hard particle rotation to tunable phononic crystals(PCs)is investigated.A mechanically tunable PC is designed by embedding inclined elliptic metal particles(scatters)into soft rubber matrix.Numerical results show that the increase of strain can arouse narrowing,vanishing,borning and shifting changes to the band structure of the designed PC.More importantly,it is essential to take into account the significant influence of particle rotation on the variation of band structure with strain when designing mechanically tunable PCs,and under some situations,such rotation can facilitate precise control of the band structure of PCs via external loading.The present dissertation explored the design of several novel composite mateirals/structures based on the concept of soft-hard integration,developed relative modeling techniques and theoretical methods,and provided theoretical,numerical and experimental foundations for the novel designs.These studies can serve as references to the design of future hybrid materials/structures/devices with high performance and multifunctionality.