Zishun Liu

Uniqueness of reverse analysis from conical indentation tests

The curvature of the loading curve, the initial slope of the unloading curve, and the ratio of the residual depth to maximum indentation depth are three main quantitiesthat can be established from an indentation load-displacement curve. A relationship among these three quantities was analytically derived. This relationship is valid for elasto-plastic material with power law strain hardening and indented by conical indenters of any geometry. The validity of this relationship is numerically verified through large strain, large deformation finite element analyses. The existence of an intrinsic relationship among the three quantities implies that only two independent quantities can be obtained from the load-displacement curve of a single conical indenter. The reverse analysis of a single load-displacement curve will yield non-unique combinations of elasto-plastic material properties due to the availability of only two independent quantities to solve for the three unknown material properties.

Advances in Mechanics of Soft Materials: A Review of Large Deformation Behavior of Hydrogels

Hydrogels possess magnificent properties which may be harnessed for novel applications. However, this is not achievable if the mechanical behaviors of hydrogels are not well understood. This paper aims to provide the reader with a birds eye view of the mechanics of hydrogels, in particular the theories associated with deformation of hydrogels, the phenomena that are commonly observed, and recent developments in applications of hydrogels. Besides theoretical analyses and experimental observations, another feature of this paper is to provide an overview of how mechanics can be applied.

ANALYTICAL SOLUTIONS OF POLYMERIC GEL STRUCTURES UNDER BUCKLING AND WRINKLE

One of the unique properties of polymeric gel is that the volume and shape of gel can dramatically change even at mild variation of external stimuli. Though a variety of instability patterns of slender and thin film gel structures due to swelling have been observed in various experimental studies, many are not well understood. This paper presents the analytical solutions of swelling-induced instability of various slender and thin film gel structures. We have adopted the well developed constitutive relation of inhomogeneous field theory of a polymeric network in equilibrium with a solvent and mechanical load or constraint with the incremental modulus concept for slender beam and thin film gel structures. The formulas of buckling and wrinkle conditions and critical stress values are derived for slender beam and thin film gel structures under swelling-induced instability using nonlinear buckling theories of beam and thin film structures. For slender beam structure, we construct the stability diagram with the...

Optical and mechanical models for a variable optical attenuator using a micromirror drawbridge

In this paper, we develop optical and mechanical models of a surface micromachined variable optical attenuator (VOA) having an initial tilt angle. The proposed models are employed to compare various characteristics of the VOA such as snap-down voltage, attenuation versus driving voltage, and the rise time and the fall time based on the experimental data. In addition, the analytical models of the static and dynamic behavior of the drawbridge are verified using the results from the finite element method. We obtain reasonable accuracy between the results from the experiment and the proposed models.

A transition from localized shear banding to homogeneous superplastic flow in nanoglass

A promising remedy to the failure of metallic glasses (MGs) by shear banding is the use of a dense network of glass-glass interfaces, i.e., a nanoglass (NG). Here we investigate the effect of grain size (d) on the failure of NG by performing molecular dynamics simulations of tensile-loading on Cu50Zr50 NG with d = 5 to 15 nm. Our results reveal a drastic change in deformation mode from a single shear band (d ∼ 15 to 10 nm), to cooperative shear failure (d ∼ 10 to 5 nm), to homogeneous superplastic flow (d ≤ 5 nm). Our results suggest that grain size can be an effective design parameter to tune the mechanical properties of MGs.

Nano-optomechanical Actuator and Pull-Back Instability

This paper studies the nonlinear behavior of a nano-optomechanical actuator, consisting of a free-standing arc in a ring resonator that is coupled to a bus waveguide through evanescent waves. The arc deflects when a control light of a fixed wavelength and optical power is pumped into the bus waveguide, while the amount of deflection is monitored by measuring the transmission spectrum of a broadband probe light. This nanoactuator achieves a maximal deflection of 43.1 nm, with a resolution of 0.28 nm. The optical force is a nonlinear function of the deflection of the arc, leading to pull-back instability when the control light is red-tuned. This instability is studied by a combination of experiment and modeling. Potential applications of the nanoactuator include bio-nanomotor, optical switches, and optomechanical memories.

Inverse Pseudo Hall-Petch Relation in Polycrystalline Graphene

Understanding the grain size-dependent failure behavior of polycrystalline graphene is important for its applications both structurally and functionally. Here we perform molecular dynamics simulations to study the failure behavior of polycrystalline graphene by varying both grain size and distribution. We show that polycrystalline graphene fails in a brittle mode and grain boundary junctions serve as the crack nucleation sites. We also show that its breaking strength and average grain size follow an inverse pseudo Hall-Petch relation, in agreement with experimental measurements. Further, we find that this inverse pseudo Hall-Petch relation can be naturally rationalized by the weakest-link model, which describes the failure behavior of brittle materials. Our present work reveals insights into controlling the mechanical properties of polycrystalline graphene and provides guidelines for the applications of polycrystalline graphene in flexible electronics and nano-electronic-mechanical devices.

INHOMOGENEOUS LARGE DEFORMATION KINETICS OF POLYMERIC GELS

This work examines the dynamics of nonlinear large deformation of polymeric gels, and the kinetics of gel deformation is carried out through the coupling of existing hyperelastic theory for gels with kinetic laws for diffusion of small molecules. As finite element (FE) models for the transient swelling process is not available in commercial FE software, we develop a customized FE model/methodology which can be used to simulate the transient swelling process of hydrogels. The method is based on the similarity between diffusion and heat transfer laws by determining the equivalent thermal properties for gel kinetics. Several numerical examples are investigated to explore the capabilities of the present FE model, namely: a cube to study free swelling; one-dimensional constrained swelling; a rectangular block fixed to a rigid substrate to study swelling under external constraints; and a thin annulus fixed at the inner core to study buckling phenomena. The simulation results for the constrained block and one-dimensional constrained swelling are compared with available experimental data, and these comparisons show a good degree of similarity. In addition to this work providing a valuable tool to researchers for the study of gel kinetic deformation in the various applications of soft matter, we also hope to inspire works to adopt this simplified approach, in particular to kinetic studies of diffusion-driven mechanisms.

Comparing the effects of dispersed Stone–Thrower–Wales defects and double vacancies on the thermal conductivity of graphene nanoribbons

Classical molecular dynamics with the AIREBO potential is used to investigate and compare the thermal conductivity of both zigzag and armchair graphene nanoribbons possessing various densities of Stone-Thrower-Wales (STW) and double vacancy defects, within a temperature range of 100-600 K. Our results indicate that the presence of both kinds of defects can decrease the thermal conductivity by more than 80% as defect densities are increased to 10% coverage, with the decrease at high defect densities being significantly higher in zigzag compared with armchair nanoribbons. Variations of thermal conductivity in armchair nanoribbons were similar for both kinds of defects, whereas double vacancies in the zigzag nanoribbons led to more significant decreases in thermal conductivity than STW defects. The same trends are observed across the entire temperature range tested.

Pattern formation in plants via instability theory of hydrogels

In this paper, we demonstrate how deformation patterns of leaves and fruits in growing and drying processes can be described via the inhomogeneous field theory. The distorted deformation of ribbed leaves and the ridge formation on fruit surfaces can be understood as the energy-minimizing mechanical buckling patterns. The swelling and de-swelling induced instabilities of various membrane structures or elastic sheets on elastic or gel-like substrates are simulated using the inhomogeneous field theory of a polymeric network in equilibrium with solvent and mechanical constraints. The article describes briefly the inhomogeneous field theory of hydrogel deformation and the buckling patterns of thin hydrogel films on thick substrates. The theory is then adopted to simulate the growth and drying processes of leaves and fruits through the buckling phenomena observed in the film gel of various shapes, geometric proportions, chemical potentials and mechanical constraints. The key idea is to show that the hydrogel deformation theory can capture the deformation process and various states of plant growth or drying. The study has been made in an attempt to mimic the shapes of fruits and leaves from the swelling/deswelling patterns of hydrogel films. The study provides the possibility of exploring the origin of the intriguing natural phenomena of leaves and fruits.