Puncture Mechanics,Toughening Mechanism,and Applications Of Soft Materials

Different from rigid materials,soft materials,such as elastomers and hydrogels have low moduli and can be stretched to large deformation.Meanwhile,soft smart materials are able to respond to various stimuli,like electric field,temperature,p H,chemicals,etc.,by deformation,change of transparency,swelling or deswelling,etc..Soft materials play irreplaceable roles in various applications,typically in stretchable devices,soft robotics,medical devices,and wearable devices.In applications,the reliability of soft materials is one of the most important concerns.Generally speaking,a stretchable film might suffer puncture failure by continuous indention by rigid objects.The toughness of a soft material quantitatively describe the resistance to a crack propagation.Recently,new applications are emerging by the combining of soft materials with other devices or materials.In this thesis,we study the soft materials in terms of puncture mechanics,toughening mechanism,and applications in the following four parts:(1)We study the puncture failure of a elastomeric membrane by cylindrical indenters experimentally and analytically.We focus on the effect of friction between the membrane and the indenter on the puncture failure of the membrane.We introduce the critical 1I(the first invariant of the right Cauchy-Green deformation tensor)to successfully predict the puncture failure of the membrane.We find out that a membrane will be punctured through more easily under a low friction condition.Also,a crack is left on the membrane in a low friction condition,while a circular hole is left in a high friction condition.This study can help to uncover the puncture mechanism of elastomeric membrane and to design soft membranes and structures with enhanced puncture resistance.(2)We put forward the concept of elastic dissipater to reveal the contribution of elastic energy to the toughness of soft materials.We first perform a series of exepriments to tear polyacrylamide(PAAm)hydrogels with various thicknesses.Based on the experimental results,we then develop the generalized Lake-Thomas model.In the classic Lake-Thomas model the single layer of polymer chain lying across the crack plane is taken as the elastic dissipater.In the generalized Lake-Thomas model,we regard the materials which occupy the fracture process zone as the elastic dissipater.The propagation of the crack will separate this zone into two and dissipate the elastic energy in this zone.Therefore,the release of the elastic energy by the elastic dissipater contributes to the toughness of the material.The elastic dissipater can help the design of materials with high toughness,high fatigue resistance and low hysteresis.(3)To more easily,environmental-friendly,safely,and efficiently combine the hydrogels with other materials or devices in the form of coatings,we invent a new strategy,called hydrogel paint.Hydrogel paint is compatible to various hydrogels(like slippery hydrogel,temperature/p H sensitive hydrogel,etc.),as well as different substrates(like metals,plastics,elastomers,etc.).Hydrogel paint can enable strong bonding between coatings and substrates,and play key roles in various applications,like medical devices,stretchable devices,and soft robotics.(4)We combine the dielectric elastomer actuator(DEA)with two-dimensional photonic crystals(2D PCs)to achieve structural coloration.The structural colors generated by the 2D PCs can be tuned through both the in-plane and out-of-plane actuation modes of the DEA.The in-plane expansion of DEA subjected to voltage will increase the distance between two adjacent lattice spaces of the 2D PCs,while the outof-plane wrinkle pattern will change the angle the light goes through the 2D PCs.We establish an analytical model to investigate the relationship between the actuation voltage and the structural color for the in-plane actuation mode of DEA.This model is also used to optimize the pre-stretch and the size of the actuation area of the DEA by considering the actuation efficiency.We find that a higher pre-stretch and smaller actuation area are preferable for a higher actuation efficiency.The tunable 2D PCs realized by DEA can be applied in color changeable soft skin for camouflage and smart optical devices,such as adjustable optical grating.