Zishun LIU | XI'AN JIAO TONG UNIVERSITY (original) (raw)

Papers by Zishun LIU

Polymers, 2017

It is well known that elastic instabilities induce pattern transformations when a soft cellular s... more It is well known that elastic instabilities induce pattern transformations when a soft cellular structure is compressed beyond critical limits. The nonlinear phenomena of pattern transformations make them a prime candidate for controlling macroscopic or microscopic deformation and auxetic properties of the material. In this present work, the novel mechanical properties of soft cellular structures and related hydrogel-elastomer composites are examined through experimental investigation and numerical simulations. We provide two reliable approaches for fabricating hydrogel-elastomer composites with rationally designed properties and transformed patterns, and demonstrate that different geometries of the repeat unit voids of the periodic pattern can be used to influence the global characteristics of the soft composite material. The experimental and numerical results indicate that the transformation event is dependent on the boundary conditions and material properties of matrix material for soft cellular structures; meanwhile, the deformation-triggered pattern of matrix material affects the pattern switching and mechanical properties of the hydrogel-elastomer material, thus providing future perspectives for optimal design, or serving as a fabrication suggestion of the new hydrogel-elastomer composite material.

Structural Engineering and Mechanics, 2014

When the periodic cellular structure is loaded or swelling beyond the critical value,the structur... more When the periodic cellular structure is loaded or swelling beyond the critical value,the structure may undergo a pattern transformation owing to the local elasticinstabilities, thus leading to structure collapses and the change to anewconfiguration. Based on this deformation-triggered pattern, we have developed thenovel composite gel material. This designed material is a type of architecture materialwhich achieves special mechanical properties. In this paper, the mechanical behavior of the composite gel periodic structurewith pattern transformation has been studied through numerical simulations. When pattern transformation occurs, it can result in a different elastic relationshipcompared with the material at untransformed state. From the obtained nominal stress versusnominal strainbehavior, the Poisson's ratio and corresponding deformed structure patterns, we investigate the effects ofthe uniformly distributed gel inclusions oncomposite material, thereby having a better understanding ofthe characteristics of composite materials. We hope this study can provide future perspectives for thenew composite material.

International Journal of Solids and Structures, 2014

A polymer network can imbibe copious amounts of solvent and swell, the resulting state is known a... more A polymer network can imbibe copious amounts of solvent and swell, the resulting state is known as a gel. Depending on its constituents, a gel is able to deform under the influence of various external stimuli, such as temperature, pH-value and light. In this work, we investigate the photo-thermal mechanics of deformation of temperature sensitive hydrogels impregnated with light-absorbing nano-particles. The field theory of photo-thermal sensitive gels is developed by incorporating effects of photochemical heating into the thermodynamic theory of neutral and temperature sensitive hydrogels. This is achieved by considering the equilibrium thermodynamics of a swelling gel through a variational approach. The phase transition phenomenon of these gels, and the factors affecting their deformations, are studied. To facilitate the simulation of large inhomogeneous deformations subjected to geometrical constraints, a finite element model is developed using a user-defined subroutine in ABAQUS, and by modeling the gel as a hyperelastic material. This numerical approach is validated through case studies involving gels undergoing phase coexistence and buckling when exposed to irradiation of varying intensities, and as a microvalve in microfluidic application.

Theoretical and Applied Mechanics Letters, 2013

In periodic cellular structures, novel pattern transformations are triggered by a reversible elas... more In periodic cellular structures, novel pattern transformations are triggered by a reversible elastic instability under the axial compression. Based on the deformation-triggered new pattern, periodic cellular structures can achieve special mechanical properties. In this paper, the designed architecture materials which include elastomer matrixes containing empty holes or filled holes with hydrogel material are modeled and simulated to investigate the mechanical property of the periodic materials. By analyzing the relationship between nominal stress and nominal strain of periodic material, and the corresponding deformed patterns, the influence of geometry and shapes of the holes on the mechanical property of architecture material is studied in more details. We hope this study can provide future perspectives for the deformation-triggered periodic structures.

Carbon, 2012

Classical molecular dynamics with the AIREBO potential is used to investigate the thermal conduct... more Classical molecular dynamics with the AIREBO potential is used to investigate the thermal conductivity of both zigzag and armchair graphene nanoribbons possessing different densities of Stone-Thrower-Wales (STW) defects. Our results indicate that the presence of the defects can decrease thermal conductivity by more than 50%. The larger the defect density, the lower the conductivity, with the decrease significantly higher in zigzag than in armchair nanoribbons for all defect densities. The effect of STW defects in the temperature range 100-600 K was also determined. Our results showed the same trends in thermal conductivity decreases at all temperatures. However, for higher defect densities there was less variation in thermal conductivity at different temperatures.

Essential mechanical properties of materials can be assessed via the reverse analysis based on lo... more Essential mechanical properties of materials can be assessed via the reverse analysis based on loaddisplacement curves of dual indenters of different geometries. Two models namely the artificial neural networks (ANN) involving empirical risk minimization and the least squares support vector machines (LS-SVM) of the structural risk optimization group are constructed to determine the material properties via the load-indentation curves. The mapping of the load-indentation parameters to the material properties is formed and calibrated using function approximation procedure. Extensive large strain-large deformation finite element analyses were carried out to simulate the indentation of elasto-plastic materials obeying power law-strain hardening using both Berkovich and conical indenters. The study covers the material properties of a wide practical range with 680 datasets for each indenter. The results are displayed as surfaces describing the variations of load-indentation parameters and employed as inputs to the proposed neural network models. Both networks are robust and directly relate the the load-indentation parameters to the elasto-plastic material properties without involving iterative procedure. The method has wide potential applications on material characterization in semiconductor and thin film industries including MEMS and NEMS.

International Journal of Solids and Structures, 2013

In this paper, inhomogeneous deformation of a temperature-sensitive hydrogel has been studied and... more In this paper, inhomogeneous deformation of a temperature-sensitive hydrogel has been studied and analyzed under arbitrary geometric and boundary conditions. We present the governing equations and equilibrium conditions of an isothermal process based on the monophase gel field theory of hydrogel via a variational approach. We have adopted and implemented an explicit form of energy for temperature-sensitive hydrogel in a three-dimensional finite element method (FEM) using a user-supply subroutine in ABAQUS. For verification purpose, a few numerical results obtained by the proposed approach are compared with existing experimental data and analytical solutions. They are all in good agreement. We also provide several examples to show the possible applications of the proposed method to explain various complex phenomena, including the bifurcation, buckling of membrane, buckling of thin film on compliant substrate and the opening and closure of flowers.

Frontiers in Chemistry

Meso-scale models for hydrogels are crucial to bridge the conformation change of polymer chains i... more Meso-scale models for hydrogels are crucial to bridge the conformation change of polymer chains in micro-scale to the bulk deformation of hydrogel in macro-scale. In this study, we construct coarse-grain bead-spring models for polyacrylamide (PAAm) hydrogel and investigate the large deformation and fracture behavior by using Dissipative Particle Dynamics (DPD) to simulate the crosslinking process. The crosslinking simulations show that sufficiently large diffusion length of polymer beads is necessary for the formation of effective polymer. The constructed models show the reproducible realistic structure of PAAm hydrogel network, predict the reasonable crosslinking limit of water content and prove to be sufficiently large for statistical averaging. Incompressible uniaxial tension tests are performed in three different loading rates. From the nominal stress-stretch curves, it demonstrated that both the hyperelasticity and the viscoelasticity in our PAAm hydrogel models are reflected. The scattered large deformation behaviors of three PAAm hydrogel models with the same water content indicate that the mesoscale conformation of polymer network dominates the mechanical behavior in large stretch. This is because the effective chains with different initial length ratio stretch to straight at different time. We further propose a stretch criterion to measure the fracture stretch of PAAm hydrogel using the fracture stretch of CC bonds. Using the stretch criterion, specific upper and lower limits of the fracture stretch are given for each PAAm hydrogel model. These ranges of fracture stretch agree quite well with experimental results. The study shows that our coarse-grain PAAm hydrogel models can be applied to numerous single network hydrogel systems.

$Research paper thumbnail of Effects of temperature on the fracture and fatigue damage of temperature sensitive hydrogels$

RSC Advances

The fracture energy increases rapidly, when the temperature is higher than the VPTT, mainly becau... more The fracture energy increases rapidly, when the temperature is higher than the VPTT, mainly because of the hydrophobic interactions becomes dominant.

Polymers

Due to its excellent temperature sensitivity, the Poly(N-isopropylacrylamide) (NIPA) hydrogel has... more Due to its excellent temperature sensitivity, the Poly(N-isopropylacrylamide) (NIPA) hydrogel has attracted great interest for a wide variety of applications in tissue engineering and regenerative medicine. NIPA hydrogel undergoes an abrupt volume phase transition at a lower critical solution temperature (LCST) of 30–35 °C. However, the mechanical behaviors of NIPA hydrogel induced by phase transition are still not well understood. In this study, phase transition effects on mechanical properties of NIPA hydrogel are quantitatively studied from experimental studies. The mechanical properties of NIPA hydrogel with the LSCT around 35 °C are systemically studied with varying temperatures (31–39 °C) under a tensile test. We find that the mechanical properties of NIPA hydrogel are greatly influenced by phase transition during the tension process. The maximum nominal stress and maximum stretch above the LCST are larger than those of below the LCST. The Young’s modulus of NIPA hydrogel is a...

Because of their superior ballistic and mechanical properties, lightweight materials like titaniu... more Because of their superior ballistic and mechanical properties, lightweight materials like titanium alloy Ti-6Al-4V and aluminum alloy AA5083-H116 have been frequently used for aerospace and military applications. Tests of ballistic limit velocity are limited by expensive nature of the experimental setup, and indeed, that can be resolved by adopting numerical simulations. Numerical study involving the finite element method (FEM) suffers from severe element distortion problem when used for high velocity impact analysis. Therefore, the coupled smooth particle hydrodynamics-finite element method (SFM) has been adopted to study the perforation of Ti-6Al-4V and AA5083-H116 target plates with thicknesses of 26.72 mm and 25 mm, respectively. In the SFM, smooth particle hydrodynamics (SPH) method is incorporated in the severely distorted regions and the FEM otherwise. Effects of strain rate and adiabatic heating are significant for high velocity impact problems, and hence, a constitutive model incorporating effects of high strain rate and adiabatic temperature is proposed. The constitutive model is verified and finally adopted for perforation studies. Close correlation between the numerical and experimental ballistic limit velocities are accomplished. The study shows that the proposed method is able to emulate the failure mechanisms of the target plates without any numerical problem.

International Journal of Applied Mechanics

Hydrogels and shape memory polymers (SMPs) possess excellent and interesting properties that may ... more Hydrogels and shape memory polymers (SMPs) possess excellent and interesting properties that may be harnessed for future applications. However, this is not achievable if their mechanical behaviors are not well understood. This paper aims to discuss recent advances of the constitutive models of hydrogels and SMPs, in particular the theories associated with their deformations. On the one hand, constitutive models of six main types of hydrogels are introduced, the categorization of which is defined by the type of stimulus. On the other hand, constitutive models of thermal-induced SMPs are discussed and classified into three main categories, namely, rheological models; phase transition models; and models combining viscoelasticity and phase transition, respectively. Another feature in this paper is a summary of the common hyperelastic models, which can be potentially developed into the constitutive models of hydrogels and SMPs. In addition, the main advantages and disadvantages of these ...

The morphological characteristics of a wrinkled film are largely determined by the state of stres... more The morphological characteristics of a wrinkled film are largely determined by the state of stress at the onset of the instability. For surfaces compressed equibiaxially, it is well established that ridge-based structures, including herringbone or labyrinth patterns, provide the lowest energy state for stresses far exceeding critical buckling. For near-critical stresses, the equilibrium morphology is less understood.

Surface instabilities play key roles in a variety of contexts including flexible electronics, sur... more Surface instabilities play key roles in a variety of contexts including flexible electronics, surfaces with switchable properties, tissue engineering and biosensors. Wrinkles for elastic bilayers are well known to result from a balance between the bending energy of the stiffer skin layer and the stretching/compression of the softer substrate.

In the final stage of ceramic sintering, pores can either move with, or separate from, grain boun... more In the final stage of ceramic sintering, pores can either move with, or separate from, grain boundaries. The outcome is critical to the resulting ceramics. This paper studies an axisymmetric model of a single pore on a moving grain boundary. Two rate processes, grain boundary migration and surface diffusion, are concomitant. Surfaces move to reduce the total surface and grain boundary energy. A finite element method is formulated to simulate the transient separation process.

abstract The emergence of wearable electronics is leading away form glass substrates for the disp... more abstract The emergence of wearable electronics is leading away form glass substrates for the display backplane, to plastic and metal. At the same time the substrate thickness is reduced to make displays lighter. These two trends cooperate toward the development of compliant substrates, which are designed to off load mechanical stress from the active circuit onto the substrate. Compliant substrates made the circuit particularly rugged against rolling and bending.

Mechanical energy and electrical energy can be converted to each other by using a dielectric elas... more Mechanical energy and electrical energy can be converted to each other by using a dielectric elastomer transducer. Large voltage-induced deformation has been a major challenge in the practical applications. The voltage-induced deformation of dielectric elastomer is restricted by electromechanical instability (EMI) and electric breakdown.

Droplets in air or liquids under electrical voltages appear in diverse processes from thunderstor... more Droplets in air or liquids under electrical voltages appear in diverse processes from thunderstorm cloud formation, ink-jet printing, electrospinning nanofibers to electrospray ionization. In these processes, the electrostatic energy competes with surface energy of the drops and causes sharp tips to form on the ends of the drops. Here, we report a physically distinct scenario for droplets in solid matrices under voltages.

When a gas is injected into a bubble in an elastomer, the bubble may first expand gradually, and ... more When a gas is injected into a bubble in an elastomer, the bubble may first expand gradually, and then snap suddenly to a large size. This snap-through instability is analyzed here using a model that accounts for both the surface tension and the limiting stretch of the elastomer.