Numerical Study On Nonlinear Mechanics Of Large Deformation,Contact And Adhesion Of Soft Materials

Soft materials cover a wide of spectrum ranging fromrubbery,smart elastomers and hydrogel to biomimetics or biotissues like such as cell,skin,etc.It exhibits nonlinear mechanical behaviors in many scenarios,such as large deformation under complex coupled fields,in contact and adhesion problems.Understanding such complex nonlinear mechanical behaviors is critical for the prediction and design of material property,cellular biomechanical characteristics.However,it is difficult to study these behaviors by an analytical method.Hence Finite Element(FE)numerical method is proposed to solve these complex problems,while many challenges still exist in building up the models efficiently and the complicated constitutive relation and boundary conditions.The dissertation aims to study the nonlinear mechanical behaviors of soft materials(large deformation,contact and adhesion)through theoretical analysis and FE simulation.In the first part of dissertation,covalent adaptable network polymer(so called Vitrimer)has been studied.Vitrimer,as a thermoset,whose crosslinked networks can be rearranged due to the initiations of chemical reactions(e.g.,bond exchange reaction)when a specific external stimulus(light,heat or change of chemical environment)is given.The resulting networks with microscopic dynamic rearrangement lead to macroscopic rheological mechanical properties,such as stress relaxation,creep,etc.In addition,it contributes to surface welding and recycling based on powder state.This recent developed polymers,which combine the structural stability of thermosets and the reprocessing ability of thermoplastics,have potential applications in many fields.Therefore,the study on mechanics modeling of vitrimer is useful to the prediction of mechanical behaviors and design in practice.One typical application scenario,i.e.reshaping of polymer,has been presented in the work.Based on a previously developed constitutive model and the the secondary development of FE commercial software ABAQUS,we develop a user material subroutine(UMAT)to describe the complex constitutive of the material,and investigate thermoforming process of this material under a non-uniform and evolving temperature field through numerical simulations.Our focus is on the complex coupling between mechanical deformation,heat conduction and bond exchange reactions.These results can be utilized to design the conditions for reprocessing and may help in the future study of recycling process based on the powder fusion under pressure.In the second part of dissertation,we discuss the large deformation and contact problem with complex boundary conditions of soft materials.Specifically,the mechanical behavior of a cell's entry into the confined channel has been studied.The process widely exists in cell mechanical experiments,like micropipette,micro fluidic,tumor cell migration.The previous simulation work did not implement the exact prescribes of external loads,leading to the neglection of friction and the incapable of accurate calculation for cell mechanical response.Our work develops a computational algorithm based on the secondary development of ABAQUS to update real-time loading boundary,and take the friction into account.The model ignores the internal cytoskeleton of cell and simplifies it into a viscoelastic sphere,while the Neo-Hookean model is used to represent the elastic to describe the large deformation of the cell in the confined channel and the Prony series is adapted for viscos.The work compares the cell's mechanical responses to the different loading methods,and also studies the effects of friction.The developed method in this work can be extended to other complex cases with different shapes,materials,loads and friction conditions.In the third part of dissertation,we study the delamination of rigid objects from an elastic substrate with finite thickness,which is a fundamental adhesion problem.Our work is motivated by anti-icing coatings and marine fouling release coatings,where the adhesion strength of the coatings is a core parameter defined as the normal pull-force divided by the contacting area.It is of great importance to study critical pull-force and the adhesion mechanism between the rigid punch and the substrate and.In 2D plane strain case where a rigid punch is adhered to an elastic half-space,we derive the analytical solution revealing the relation of substrate's modulus,the pull-off force,loading position and angle,etc.,and obtains the fundamental mechanical mechanism for the lower angled pull-off force(i.e.the incorporation of uneven normal traction causes delamination to initiate locally near an edge of the contact).In 3D case where the thickness of elastic substrate is finite,we develop a finite element model with cohesive zone approach to simulate the delamination of a rigid punch from an elastic substrate with different loading positions,angles and geometry parameters.The results theoretically explain the various delamination modes under the normal separation.For shear separation,we indentify three delamination modes and systematically discussed the relation between critical pull-ofif force and the delamination modes.In particular,3D simulations reveals another mechanism,i.e.interface cavitation,for inducing local delamination and thus reducing the pull-off force.The results will facilitate the development of low adhesion coatings like soft material coatings with mechanical heterogeneities.In summary,the dissertation studies three typical nonlinear mechanical problems of soft materials using Finite Element method,which are constitutive model for large deformation,contact and adhesion.Specifically,the reprocessing of vitrimer,a cell's entry into confined channel and delamination of a rigid punch from an elastic substrate have been investigated thoroughly.Our work provides necessary guidance on using Finite Element method for solving the nonlinear problems of soft material.