In-situ Bonding Method For Soft Materials And Interface Mechanics For Composites

Over decades,composite structures have been widely used in many important fields,e.g.,functional devices,smart structures,etc.Interfaces of the composite structures,as bridges of the components,play critical roles in stress transfer,and thus have significant influences on the mechanical performance and stability of the structures.For soft materials,soft devices and soft machines have attracted tremendous amount of research interests in both academy and industry,and have been at the cutting-edge of research.However,the emergence of these devices has posed a fundamental challenge:hydrogels and elastomers without covalent bonds have low adhesion energy.While different methods are developed to bond hydrogels with hydrophobic elastomers,they still have severe limitations.In this thesis,we present an approach,i.e.,in-situ bonding method,to meet this challenge.It's a general approach to bond dissimilar polymer networks in various manufacturing processes.For composites,strengthening and toughening are still the key research areas.Interfacial debonding,frictional sliding,and voids are closely related to the failure of composites.We develop a new interface mechanical model and a computational model to comprehensively study the failure mechanisms of composites.It is expected that the present research will provide theory for designing,manufacturing,and developing of composite structures,e.g.,soft devices,soft machines,composites,etc.(1)In this thesis,we develop a general approach to bond dissimilar polymer networks in various manufacturing,such as printing,coating,etc.We mix silane coupling agents into the precursors of the networks,and tune the kinetics such that,when the networks form,the coupling agents incorporate into the polymer chains,but do not condensate.After a manufacturing step,the coupling agents condensate,add crosslinks inside the networks,and form bonds between the networks.This approach provides tough bonding,long-term stability,and extensive applicability,and enables independent bonding and manufacturing.Studies of both intra-network and inter-network condensation show that the bonding kinetics can be tuned by changing the temperature and pH,and by adding surfactants.(2)We demonstrate our approach in various manufacturing processes that form networks in different sequences,such as 3D printing,dip-coating,etc.We formulate oxygen-tolerant hydrogel resins by removing the conventional crosslinker MBAA and adding the silane coupling agents as the corsslinking and bonding elements.Oxygen-tolerant hydrogel resins can be used for spinning hydrogel fibers,3D printing,and dip-coating.We find that thin elastomer coatings enable hydrogels to sustain high temperatures,i.e.,120 ?,without boiling.(3)We develop a cohesive zone model(CZM)combining interfacial debonding,frictional sliding,and coupling between decohesion and friction.It has been implemented into a commercial software package ABAQUS as a user-defined element.Microbond test is carried out using an in-house developed tester.We verify this model by comparing the results of simulation and experiment.Dimensional analysis has been adopted to study the relationship between the mechanical behavior and the interfacial properties and the geometry of the structure.Dimensional considerations introduce a characteristic length,and the interfacial shear strength(IFSS)monotonically increases with the ratio of the characteristic length to the embedded length and is asymptotic to a horizontal line.(4)The interface debonding and damage progression from voids in syntactic foams are two detrimental processes that have significant negative impacts on the mechanical properties.The effects of these progressive damage processes are numerically investigated using a micromechanical approach.The tensile strength as a function of the interface properties,voids content,and hollow particle content and shell thickness has been analyzed in detail using axisymmetric computational models with explicit consideration of the matrix cracking and the interface debonding,which are modeled using the Extended Finite Element Method(XFEM)and the cohesive zone method(CZM).