A Buckling-based Method for Measuring the Strain-Photonic Coupling Effect of GaAs Nanoribbons
Xue Feng
AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084
Center for Mechanics and Materials, Tsinghua University, Beijing 100084
KEYWORDS: Strain-photonic coupling, optoelectronic material, bandgap, buckling, nanoribbons
ABSTRACT: The ability to continuously and reversibly tuning the bandgap and strain-photonic coupling effect in optoelectronic materials is highly desirable for fundamentally understanding the mechanism of strain engineering and its applications in semiconductor. However, optoelectronic materials (i.e. GaAs) with their natural brittleness cannot be subject to directly mechanical loading such as tension or compression. Here, we report a strategy to induce continuous strain distribution in GaAs nanoribbons by applying structural buckling. Wavy GaAs nanoribbons are fabricated by transfer printing onto pre-strained soft substrate, and then the corresponding photoluminescence are measured to investigate the strain-photonic coupling effect. The theoretical analysis shows the evolution of bandgap due to strain and is consistent with the experiments. The results demonstrate the potential application of buckling configuration to delicately measure and tune the bandgap and optoelectronic performance.

This paper proposes a new model for the piezoelectric material with surface effects to describe gradient and surface effects. The constitutive equations and boundary conditions are derived by the variational method. Then this new model is used to solve the anti-plane problem of an infinity piezoelectricity with micro-hole. The impact of structure and surface properties of the material on the electromechanical fields is analyzed. The numerical results reveal that the surface/interface layer thickness has a great effect on the electromechanical couplings around the micro-hole. It is indicated that the thicker the surface layer is, the larger the effect of surface properties on the electromechanical responses is. In comparison, the internal length scales have a significant effect only on the electric field distributions, and their effect on stresses is very limited.

A novel numerical algorithm combining finite element sub-partition and substructure was developed to simulate crack extension. In this strategy, all elements group into three: elements split by a crack, elements encompassing a crack tip and normal elements. The first and second sorts of elements will be sub-partitioned. Grouping of elements is dynamic varying with crack extension. The sub-partitioned elements coving one crack are viewed as a sub-structure. This sub-structure is thereby dynamic also for its element member will increase when the crack extends. In this algorithm, a crack can extend along any path and free from the limit of the original mesh. The global re-mesh is not needed and the total degree of freedom keeps the same whether cracks extending or nucleating new cracks. This method was adopted to calculate the stress intensity factors of central crack in an infinite plane and to simulate the crack propagation in three-point bending beam to valid its accuracy. It was further used in micro-scale crack moving in the homogeneous material to demonstrate its adaptability for simulating of complex path cracking.

GaN-based 365 nm ultraviolet (UV) LEDs has various applications: curing, molding, purification, deodorization and disinfection etc. However, their usage is limited by very low output power, because of the absorption in the buffer GaN as well as thick n-type GaN layer. The GaN absorption layer (buffer GaN) removed flip chip (ARFC) along with n-type GaN thinning technique is one of the efficient ways to improve the output power of 365 nm UV LEDs. In this study, three different types of ARFC flip chip structures were fabricated to analyze the power enhancement in 365 nm UV LED. The first type comprises silicon as receptor wafer, which was bonded to epi-wafer using Au-Au bonding metal. Through-holes of 100 µm diameters were formed from Si substrate to n-GaN by using dry etching techniques, and filled with metals for the fabrication of n-electrode. Second type comprises copper plating method in which via-hole flip chip was electroplated with thick copper and third type comprises align wafer bonding method in which AlN is aligned and bonded to via-hole flip chip. All three types of structures undergo sequences of sapphire laser lift-off, buffer GaN removal and n-type GaN thinning. Here we present the detailed analysis on the physical, electrical and optical characteristics of three types of ARFCs.
* Correspondence to: sjy@sunmoon.ac.kr

The moisture absorption is the main drawback in natural fiber reinforced composites, causing remarkable degradations on such mechanical prosperities as elastic modulus and interface bonding. To quantify the relation between the water uptake and its effects on the mechanical properties, a non-linear constitutive model is proposed. We model the kinetic process under the framework of energy dissipation based on the nonequilibrium thermodynamics. The energy dissipation, corresponding to the water diffusion process is described by internal variables. And the mechanical degradations on the interface and the fiber are characterized by two degradation parameters. By comparison with the experimental data of sisal fiber reinforced composites, our model shows a good prediction for different fiber contents.

The mechanical behavior of film/substrate systems is continuously concerned due to the important application in a large number of fields in recent decades. In the present paper, the interface behavior of a thin-film adhering to a graded substrate is analyzed by a new model when the interface between the thin-film and the graded layer is subjected to a mismatched strain. The fundamental solution of a graded substrate under a pair of normal and tangential concentrated forces is obtained first, and then the mismatch problem of a thin-film bonded to the substrate is further investigated. Solving the obtained integral governing equation of Cauchy singularity leads to the interfacial shear stress, the normal stress in the film, as well as the stress intensity factor near the bonded edge. It is found that the interfacial mechanics of a thin-film bonded to a graded layer is significantly affected by varying gradient law, stiffness of the film, ratio of shear modulus and length scale of the graded transition layer. The results should be helpful for the design of systems with thin-films and functionally graded materials and could guide engineers to choose properly graded materials for particular applications.

The investigation is induced by the design of a new fluid-thermal acupuncture needle which is heated by fluid. Because the diameter of the flow tube for the acupuncture needle is less than 1 mm, the mechanical model of this needle can be regarded as a microtube. Then the microscale heat transfer model can be adopted to analyze the thermal-mechanical characteristics of the flow and heat transfer in the flow tube, which is implemented by the theoretical model and finite element method (FEM) software ANSYS. And the simulation results by two methods have good consistency. So the density of heat flow on the surface of the fluid-thermal acupuncture needle and the temperature of the acupunctured skin can be analyzed and designed by changing the temperature and flow speed of fluid¡¯s inlet from the theoretical and FEM method. The study tries our best to develop the basic theoretical method for the design of the fluid-thermal acupuncture needle and obtains some interested results.

The analytical shear band-type solutions are obtained at different periods for steady- state softening and hardening materials for the finite domain. For this purpose, strain gradient plasticity theory is discussed in detail and the constitutive equation for the gradient plasticity is solved using the analytic technique, i.e. homotopy analysis method (HAM), which is also constructed step by step, analyzed and implemented. The nonlinear governing partial differential equation is reduced to the non-dimensionalized nonlinear ordinary differential equation by using the appropriate similarity transformations. Convergent solutions are obtained with the help of optimal convergence-control parameter. Moreover, the error analysis has been performed to guarantee the convergence of our series solution. This present contribution addresses that HAM is a powerful tool for solving a complicated nonlinear problem and it deserves to be applied for more problems in deformation patterning phenomenon.

Many natural or man-made structures are thin or slender, and to model their deformations one can use reduced lower-dimension equations instead of the original three-dimensional ones. There is a long history in constructing proper 1-D beam equations or 2-D plate/shell equations, however until recently there still lacks a consistent method to do the dimension reduction for a general loading condition. In this talk, I shall present a systematic reduction method based on series expansions (coupled with asymptotic expansions for nonlinear problems). To illustrate the main ideas, starting from the 2-D field equations for the plane-stress problem, I shall give the derivation of a rigorous asymptotic beam theory, together with a pointwise error estimate. Then, the similar procedure is used to study stress-induced phase transitions in some slender/thin shape memory alloy (SMA) structures based on a constitutive model in literature. For a thin SMA layer, the reduced beam-like model consists of three differential equations for austenite, martensite and phase transition regions, based on which analytical solutions for the outer-loop, upper and lower plateau stresses are obtained and instability phenomena observed in experiments are captured. For a slender SMA cylinder (wire), the reduced equations lead to the generalization of the classical Ericksen’s 1-D results. Unlike the 1-D case, for which there infinitely-many equally stable states, the model with 3-D effect reveals there is only one optimal state (by the energy criterion). Also, the average axial stress is still the 1-D Maxwell stress, although there is a non-uniform distribution. Finally, based on the asymptotic method, we provide an analytical way to quantitatively determine the up-down-up material response and a systematic procedure to calibrate material constants through tension tests of a pre-twisted SMA tube.

Nearly 90% of service failures of metallic components and structures are caused by fatigue at a cyclic stress amplitude much lower than the tensile strength of the materials involved. A long-standing obstacle to developing better materials with higher fatigue limit and longer fatigue life has been that metals typically suffer from large, accumulative, irreversible damages in microstructure during fatigue, leading to history-dependent and unstable cyclic responses. Here, we report atomistic modeling and simulations of a history-independent and stable cyclic response in nanotwinned metals and validate the results with experimental observations from our collaborating groups. We demonstrate that this unusual behavior is governed by a type of highly correlated necklace dislocations formed in the nanotwinned metal under cyclic loading. Our findings reveal a new route to improve the fatigue life of engineering materials through tailor-designed microstructure.

Tumor growth is a complicated process involving genetic mutation, biochemical regulation, and mechanical deformation. In this paper, a thermodynamics-based nonlinear poroelastic model is established to interrogate the coupling among the mechanical, chemical, and biological mechanisms underpinning the growth of avascular tumors. A volumetric growth law accounting for mechano-chemo-biological coupling is proposed to describe the development of solid tumors. The regulating roles of stresses and nutrient transport in the growth of tumor spheroids are revealed under different surrounding environments. We show that the mechano-chemo-biological coupling triggers anisotropic and heterogeneous growth, responsible for the formation of layered structures in growing tumors. There exists a steady state, in which tumor growth is balanced by resorption. A phase diagram is constructed to illustrate how the elastic modulus and thickness of the confinements jointly dictate the volume of tumors at the steady state. The results are in consistency with relevant experimental results.

The crystalline-amorphous nanolaminates have been fabricated by incorporating the metallic glasses with the polycrystalline metals. Such kind of nanocomposite exhibits high strength and good ductility. However, the underlying deformation mechanisms in these nanocomposites are not well understood. Here we performed a series of large-scale molecular dynamic simulations to investigate the compressive deformation of crystalline-amorphous nanopillars consisting of alternating nanocrystalline Cu and amorphous Cu50Zr50 layers with the identical thickness. The simulation results show that the strength of material exhibits a maximum at the thickness of 3-5 nm. The atomistic simulations also reveal that as the layer thickness decreases, there exists a deformation mechanism transition from the local shear banding in individual amorphous layers to the co-deformation in both crystalline and amorphous layers. In the co-deformation regime, dislocation and shear transformation zone (STZ) are activated simultaneously in the crystalline and amorphous layers, respectively. There exists a complex interaction between dislocation and STZ through the crystalline-amorphous interface, to ensure plastic strain compatibility on the interface. A theoretical model involving dislocation slip and shear banding was proposed to explain the dependence of strength on the layer thickness.

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A finite elastic-viscoplastic constitutive model was proposed to study the influence of evolution of micro-structure on the macro-yield phenomena of amorphous glassy polymers. Within the rigorously irreversible thermodynamic framework, an internal-state variable, namely the sub-entanglement which is defined as the non-permanent entanglement, is employed to describe the macro-yield phenomena, i.e. the pre-yield nonlinearity, macro-yield transition and post-yielding softening. The initial density and the evolution of sub-entanglement during load application show an important effect on the macro-yield behaviors. The physical meanings of the parameters associated with the sub-entanglement evolution have been discussed. Thermal treatment histories will affect the initial density of sub-entanglement strongly. The capability of the proposed model was validated by predicting the deformation behavior of amorphous glassy polymers. The thermomechanical history dependent characteristics of the macro-yield behavior can be reproduced by the proposed model via the material parameters related to the sub-entanglement.

After briefly summarizing the main continuum mechanics milestones of the past founders up to 1960’s, an account of the transition to material mechanics in 1970’s is given. The most notable early contributions in China are reviewed and current activities in a variety of fields in emerging technologies are outlined. Some ground-breaking approaches initiated at the Academy’s Laboratory of Nonlinear Mechanics are discussed and the role of Wei Yang’s school in shaping the future of material mechanics in China and the world is pointed out.

The internal length is the governing parameter in strain gradient theories which among other things have been used successfully to interpret size effects at the microscale. It is usually introduced for dimensional consistency. However, exploring the physical mechanism of internal length scale and establishing its connection with material microstructure was at best tenuous. In the present presentation, we aimed to solve such critical problem through nanoindentation of Fe-Si bicrystals, the tension of tricrystalline and torsion of thin wires.

A grating rosette moire interferometry and incremental hole drilling combined system is developed to determine the magnitude of the residual stress along depth in an aluminum plate subjected to a uniform uniaxial tensile load. Performing in the plane 3-directional fringe analysis, the optical data contained in the moire interferograms are converted into values of strains in three directions corresponding to the grating rosette. The evaluation is carried out through the measurement of the in-plane displacement field in three directions generated by the introduction of the incremental small hole. The in plane 3-directional displacement fields are determined from the calculation of the optical in plane 3-directional phase distribution by means of a phase shifting method. The magnitude of the principal stresses is finally evaluated through a least-squares calculation and compared with the stress value applied to the specimen measured with strain gages.

Metallic glasses, due to the long-range disorder of atoms, expand locally or globally in response to shear, bearing a resemblance to Reynolds' dilatation of granular media. It has been widely accepted that shear flow is allowed in metallic glasses via a cascade of local dilatation events, which was mainly modeled by Spaepen's free volume theory or Argon's shear transformation theory. The shear-dilatation coupling leads to the pressure (or normal stress) sensitivity of macroscopic failure (flow and fracture) of metallic glasses, which essentially differs from that of their crystalline counterparts. Recently, it has been revealed that the dilatation itself can dominate brittle fracture of metallic glasses, where the crack tip propagates via cavitating events that involve a series of nanovoids nucleation and coalescence with very limited plastic growth. Therefore, the dilatation holds the key to understand the flow and fracture of metallic glasses. In this talk, we attempt to provide an up-to-date review on this aspect, but from a viewpoint of the dilatation. We first review the representative flow modes for metallic glasses, where the shear-dilatation correlation is highlighted. Theoretical as well as atomistic modeling and experimental work on the dilatation degree during shear banding at different stages are all discussed. Typical yielding criteria accounting for the dilatation effect are then described. We further present the atomic-level mechanism for local shear and dilatation events, respectively. Based on the inherent shear-dilatation competition, a united failure criterion is constructed, in which the governing factors to determine the ductile or brittle failure of metallic glasses are revealed.

Understanding the mechanism of martensitic transformation is of great importance in developing advanced high strength steels, especially Transformation-Induced Plasticity (TRIP) steels. The TRIP effect leads to enhanced work-hardening rate, postponed onset of necking and excellent formability. In-situ transmission electron microscopy has been performed to systematically investigate the dynamic interactions between dislocations and  martensite at micro-scale. Local stress concentrations, e.g. from notches or dislocation pile-ups, render free edges and grain boundaries favorable nucleation sites for  martensite. Its growth leads to partial dislocation emission on two independent slip planes from the hetero-interface when the austenite matrix is initially free of dislocations. The kinematic analysis reveals that activating slip systems on two independent {111} planes of austenite are necessary in accommodating the interfacial mismatch strain. Full dislocation emission is generally observed inside of austenite regions that contain high density of dislocations. In both situations, phase boundary propagation generates large amounts of dislocations entering into the matrix, which renders the total deformation compatible and provide substantial strain hardening of the host phase. These moving dislocation sources enable plastic relaxation and prevent local damage accumulation by intense slipping on the softer side of the interfacial region. Thus, finely dispersed martensite distribution renders plastic deformation more uniform throughout the austenitic matrix, which explains the exceptional combination of strength and ductility of TRIP steels.

Crystal plasticity at micron-nano scales involves many interesting issues. Some results are obtained for uniaxial compression experiments conducted on FCC single crystal micro-pillars, e.g. size effect and strain burst, etc. In the experiments, the mobile dislocations may escape from the free surface leading to the state of dislocation starved whereby an increase applied stress is necessary to nucleate or activate new dislocation sources. By performing in-situ TEM, the dislocation motion affected the material properties is observed. However, the atypical plastic behavior at submicron scales cannot be effectively investigated by either conventional crystal plastic theory or molecule dynamics simulation. The surfaces are transmissible and loading gradients are absent. Therefore, the strain gradient theory could not well explain these new mechanical behaviors dominated by single-arm discrete dislocation source since the Tayler low is no longer available at submicron scale. This, in turn, has led to develop new analytic and numerical models. Accordingly, a three dimensional discrete-continuous crystal plastic model is developed, which is coupling the discrete dislocation dynamics with finite element method. Three kinds of plastic deformation mechanisms for the single crystal pillar are investigated: (1) Single arm dislocation source controlled plastic flow; (2) Confined plasticity in coated pillars; (3) Dislocation starvation under low amplitude cyclic loadings. The predicted results agree well with the experimental data.

In this talk, we investigated bubble formation via acoustic levitation, during which a levitated droplet evolves from an oblate spheroidal shape to a flattened, thin bowl-shaped membrane, eventually forming a closed bubble. Through systematic experimentation and theoretical analysis, we demonstrated that the buckled geometry of the liquid membrane can drastically enhance the suction effect at the membrane rim, forming a significant pressure gradient inside the membrane which causes an abrupt area expansion and bubble formation. The insights presented here shed light on the acoustic manipulation and curving of other fluid/fluid interfaces, providing a reference for fabricating unique soft materials such as anti-bubbles and liquid onions.

Current strategies of computational crystal plasticity that focus on individual atoms or dislocations are impractical for real-scale, large-strain problems even with today’s computing power. Dislocation-density based approaches are a way forward but most schemes published to-date give a heavier weight on the consideration of geometrically necessary dislocations (GNDs), while statistically stored dislocations (SSDs) are either ignored or treated in ad hoc manners. In reality, however, the motions of GNDs and SSDs are intricately linked through their mutual (e.g. Taylor) interactions, and in fact, GNDs and SSDs are indistinguishable on a microstructural level, notwithstanding the fact that the GNDs are simply the portion of dislocations associated with the overall shape change of the crystal. A correct scheme for dislocation dynamics should therefore be the one commonly used in discrete dislocation dynamics (DDD) simulations, namely, an “all-dislocation” treatment that is equally applicable for all dislocations comprising both the GNDs and SSDs, with a rigorous description of the interactions between them.
In this paper, a new formulation for computational dynamics of dislocation-density functions, based on the above “all-dislocation” principle, is discussed. The dynamic evolution laws for the dislocation densities are derived by coarse-graining the individual density vector fields of all the discrete dislocation lines in the system, without distinguishing between GNDs and SSDs. The mutual elastic interactions between dislocations are treated in full by generalizing the elastic interactions between dislocation segments for dislocation densities, and reducing the Hirth-Lothe line-integral formulation into an algebraic form comprising only elementary functions which are straightforward enough for efficient numerical implementation. Other features in the model include forest (Taylor) hardening, generation due to the connectivity nature of dislocations, and dipole annihilation.

The relationship between grain size and n and r value of 3104 aluminum alloy sheet were investigated by tensile test along the different direction, on the basis of gain the 3104 aluminum alloy sheets that texture and configuration of the second-phase particle were almost the same but grain size was significantly different by means of appropriate heat treatment. The results showed that the size and dispersion of the remained constituent and dispersoid, the grain orientation in 3104 aluminum alloy sheet were approximately the same, but the grain size was from 12.65¦Im to 31.63¦Im. The strength of 3104 aluminum alloy sheet reduced slowly at first, and then rapidly as the grain size increased, which satisfied Hall-Petch formula. The n value of 3104 aluminum alloy sheet was increased slowly as the grain size increased from 13.80¦Im to 28.11¦Im, and the n value increased significantly as the grain size exceeded 29.19¦Im. The r value of 3104 aluminum sheet in 3 directions had no apparent variation with the increasing of grain size, which means that the r value was not sensitive to grain size in 3104 aluminum alloy sheet.

Ferroelectric polymers are flexible and easy to compliant with different forms of surfaces. Their low mechanical and acoustic impedance make them good candidates as sensors and transducers in the ultrasonic medical image, blood pressure, and pulse measurements, touch sensors in robotics, etc. [1-3]. For applications involving the external alternating electric field, ferroelectrics usually encounter the issue of electric fatigue after repetitive polarization switching. In this talk, we present a tailored electric cyclic process to enhance both the remnant polarization and fracture strength in Poly(vinylidene fluoride-trifluoroethylene) 65/35 mol % (P((VDF-TrFE 65/35)) copolymer films. The films suffer lower and higher field magnitude electric cycling successively.SEM and XRD results disclose the change of rod-like textures and crystallinity after electric cycling for the films. A microstructure evolution model, which addresses the effects of electric cycling on the ordering of the molecular chain structure, as well as on the fracture behavior, is proposed. The results indicate that electric cycling causes the molecular chains to be packed in a more ordered and closely packed structure. Consequently, the tensile strength is increased while the fracture strain is reduced for the copolymer films after electric cycling.

By now fracture mechanics has been considered as a mature branch of mechanics. However, there are still many outstanding questions in this branch that have not yet been answered. One such question for dynamic fracture propagation is the relation between fracture toughness and fracture velocity. Theoretically, there are two models proposed for solving dynamic fracture propagation problems: steady-state model and self-similar model. Using either model, the three independent variables in the mathematical formulation can be reduced into two. However, constant fracture velocity assumption has to be made in both models. To verify these theoretical models, researchers have been conducting controlled laboratory dynamic fracture experiments. Earlier laboratory experiments by Ravi-Chandar and Knauss showed that there is no clear relation between the fracture toughness and fracture velocity. Later Shukla et al. showed similar results. Another set of classical experiment by Rosakis showed the observation of limiting fracture velocity and suggested a relation between the toughness and fracture velocity, which has been adopted by the fracture textbook by Anderson. Spontaneous fracture tests by Xia et al. observed constant fracture velocity that is independent of fracture toughness, supporting the self-similar fracture propagation model. Recent dynamic fracture tests by Xia on brittle solids showed similar dependence of fracture toughness on the fracture velocity as that observed by Rosakis. Based on the available experimental data, two constitutive models governing the dynamic fracture propagation is proposed: 1. Self-similar model where fracture velocity is independent of fracture toughness and 2. Limiting fracture velocity model where the fracture toughness approaches infinite as the fracture velocity approaches the limiting velocity. The conditions where these models are applicable are discussed.

In this study, we take the view that frequency dependence of ferroelectric hysteresis is a result of direct competition between the speed of polarization evolution and the speed of external loading. We used the Ginzburg-Landau kinetic equation to evaluate the evolution of polarization vectors. We also devised a polycrystal model with a core-shell grain configuration to reflect the effect of the grain-boundary (GB) affected zone. The phase-field results showed that the coercive field tended to increases with frequency, but remnant polarization increased only slightly while the dielectric constant and piezoelectric constant d33, tended to decrease. We also found that, while both hysteresis and butterfly loops exhibited the familiar sharp tails at low frequency, the tails disappeared and the loops became elliptic- and kidney-shaped, respectively, at high frequency. The calculated low-frequency phenomena are widely supported by experiments, but the high-frequency ones are not commonly found in literature. We substantiated both types of findings with details of the underlying domain dynamics. They clearly showed a complete 180o polarization reversal at low frequency, but stopped mostly at 90o at high frequency. We also examined the influence of the kinetic coefficient and the loading amplitude and found that, as either increase, the elliptic and kidney shape of the loops would occur at a higher frequency. The calculated grain-size effects indicated that the remnant polarization, dielectric constant, and d33 all decreased with decreasing grain size. This is again widely supported by experiments. But we also found that the grain-size effect of coercive field is more complicated. It may increase or decrease, and it is the magnitude of spontaneous polarization of the GB affected zone that determines its outcome.

Implant-associated infection, a serious medical issue, is caused by the adhesion of bacteria to the surface of biomaterials; for this process, the surface roughness is an important property. Surface nanotopography of medical implant devices can control the extent of bacterial attachment by modifying the surface morphology; to this end, a model is introduced to facilitate the analysis of a nanoscale smooth surface subject to mechanical loading and in vivo corrosion. At nanometre scale rough surface promotes friction, hence reduces the mobility of the bacteria; this sessile environment expedites the biofilm growth. This manuscript derives the controlling equation for surface roughness evolution for metallic implant subject to in-plane stresses and predicts the in vivo roughness changes within 6 hours of continued mechanical loading at different stress level. This paper provides an analytic tool and theoretical information for surface nanotopography of medical implant devices.

This talk will focus on in-situ mechanics for studying the mechanical behavior at atomic scale for nanometer-sized twins, single element metallic glass, and crystal BCC metals. In comparison to computational molecular dynamics, the experimental molecular dynamics with the in-situ high-resolution TEM is going to open a new approach to directly observe atomic-scaled deformation with in-situ mechanics. I will cover the high stress induced lattice disturbance, dislocation dipole nucleation and competition between slip and twinning in the deformation process of nano-sized body center cubic metals and deformation in single elemental metallic glasses.

Lithium ion batteries (LIBs) are broadly used in portable electronics. However, for more demanding applications such as powering electrical vehicles and serving as a power backup for the flexible energy source, the energy density, power density, and cycle lifetime of current battery technologies need to be improved significantly. To achieve these goals, one needs to understand the fundamental science of LIBs. In this context, we created the first working lithium ion cell inside the high vacuum of a transmission electron microscope (TEM), enabling real time atomic scale visualization of the charging and discharging processes of individual nanowire and nano particle electrodes. We have conducted in-situ TEM electrochemical cycling of numerous battery materials, particularly the high energy density anode materials such as SnO2, Si, Al, carbon nanotubes, and graphene. Several electrochemical mechanisms were observed and characterized in real-time, including lithiation induced stress, volume changes, phase transformations, pulverization, cracking, embrittlement, and mechanical failure in anode materials. I will present a comparison between our in-situ results and electrochemical studies on conventional battery electrodes and highlight how in-situ studies can have an important impact on the design of LIBs. In the future, we will need further advancements in in-situ characterization for understanding important processes in LIBs. For example, liquid cells are required in order to examine the electrochemical reactions between battery materials and the standard battery electrolytes, which are ethylene carbonate-based. Furthermore, the structure evolution needs to be correlated with the electrochemical measurements. Finally, I will discuss outstanding challenging issues and opportunities in the field of applications of in-situ electron microscopy in LIBs, and general nanoscience and nanotechnology research.

When the spatial scale goes down from macroscale to nanoscale, temporal scale will reduce to nano to femtosecond, and more importantly, the related energy scale of an externally applied field will drop for eighteen orders from joule to attojoule (1 nN times 1 nm = 6.42 eV), falling into the energy scale of the local fields of matter which consist of electronic structures, charge, molecular orbital and spin states. Therefore, at nanoscale, matters will show distinctly different performances from their bulk materials mainly due to the strong coupling between the local fields of matter and external applied fields, turning common materials such as carbon, even insulators, into functional nanomaterials with exceptional properties we expected for nanoelectronics, spintronics as well as energy conversion devices. In this talk, I will demonstrate by our recent findings (i) such exceptional functional properties in graphene, h-BN, transition metal dichalcogenides, black phosphorous and boron nanofilms; (ii) fabrication of two-dimensional materials and their one- and three- dimensional derivates by merging top-down and bottom-up methodology; (iii) their applications for sensors and generators.

What can we do if water is a tough solid? A hydrogel is an aggregate of a polymer network and water molecules--that is, a hydrogel is polymer-reinforced water. The polymer network makes the hydrogel a solid, but water molecules in the hydrogel retain its exceptional physical and chemical properties. Several recent findings show that hydrogels can achieve properties and applications well beyond previously imagined. Most existing hydrogels, like Jell-O and tofu, are fragile and dry out in open air. We make hydrogels as tough as rubber, and retain water in the low-humidity environment. Hydrogels are stretchable, transparent, ionic conductors. We demonstrate their use in artificial muscles, skins, and axons. We show that hydrogels outperform existing fire-retarding materials. We also demonstrate fiber-reinforced hydrogels that dissipate energy. This talk describes the mechanics and chemistry of these materials and applications.

Dielectric elastomer (DE) as one kind of electroactive polymers, has attracted extensive studies in the field of smart materials and structures, due to its superior properties, such as voltage-induced large deformation, fast response, quiet operation, high energy density, light weight and low cost. DE is widely used as a soft transducer with the unique electromechanical property. In this talk, first, I will present the recent progress in mechanics of soft materials, such as the wrinkling behavior of an inflated DE balloon, and puncture mechanics of DE membrane. Next, design and electromechanical properties investigation on soft DE transducers (such as soft DE robot, soft DE peristaltic pump, and so on) will be discussed.

Mechanics as a powerful tool has been widely exploited to unravel the failure mechanisms of materials and structures. With the increasing demand in storage/delivery of the renewable energy and the rapid advance in nanobiotechnology in the last decade, mechanics revives in electrochemistry, biology and nanomedicine. In this presentation, I will illustrate with examples on how mechanical stresses mediate electrochemical cycling in lithium ion batteries, how mechanical forces regulate biological functions and pathogenesis, and how the mechanistic understanding finds its ways to battery material design, mechanical energy harvesting, disease control, and nanomedicine innovation.

Assembly of three-dimensional (3D) micro/nanostructures in advanced functional materials has important implications across broad areas of technology. Existing approaches are compatible, however, only with narrow classes of materials and/or 3D geometries. This paper introduces ideas for a form of Kirigami that allows precise, mechanically driven assembly of 3D mesostructures of diverse materials from 2D micro/nanomembranes with strategically designed geometries and patterns of cuts. Theoretical and experimental studies demonstrate the applicability of the methods across length scales from macro to nano, in materials ranging from monocrystalline silicon to plastic, with levels of topographical complexity that significantly exceed those that can be achieved in any other way. A broad set of examples includes 3D silicon mesostructures and hybrid nanomembrane-nanoribbon systems, including heterogeneous combinations with polymers and metals, with critical dimensions that range from 100 nm to 30 mm. A 3D mechanically tunable optical transmission window provides an application example of this Kirigami process, enabled by theoretically guided design.

The eukaryotic genome is stored as a protein–DNA complex, the nucleosome, which is composed of H3, H4, H2A, and H4 histone proteins. The histone tails, variants of histones, the post-translational modifications on histone tails, and other DNA binding proteins have been shown to regulate nucleosome dynamics, which plays a critical role in gene expression. However, the regulation mechanism is still unclear on a molecular level. Here we present through computational studies on a single nucleosome particle and the particle with certain histone tail truncated. Our simulation unraveled the distinct function of H2A and H3 tails in stabilizing the nucleosome in the closed conformation: the H2A C-terminal tail switches linker DNA opening and closing simply by binding to DNA at different locations, whereas the H3 N-terminal tail regulates linker DNA by binding with a different pattern. Finally, we propose a model illustrating the mechanism by which H3 N-terminal and H2A C-terminal tails regulate nucleosome stability and dynamics.

Functional materials and energy materials display a range of impressive multi-field coupling effect, such as the coupling between strain, strain gradient, electric, magnetic and thermal field etc. The extreme sensitivity of this coupling effect to material composition, structure and external stimuli offers possibilities for designing new devices. By combining first-principles theory and neutron/synchrotron techniques, we investigated a novel coupling effect between strain gradient and electric polarization (flexoelectricity) and revealed atomic vibrations play a critical role in the multi-field coupling effect in the functional materials and energy materials.

Multiscale and multiphysics materials modeling addresses the challenging materials problems that involve multiple physical phenomena at multiple spatial and temporal scales. In this talk, I will present the multiscale and multiphysics models developed in my research group with a recent focus on energy storage materials and advanced structure materials. Our study of rechargeable lithium ion batteries for energy storage applications reveals a rich spectrum of electrochemically-induced mechanical degradation phenomena. The work involves a tight coupling between multiscale chemomechanical modeling and in situ nanobattery testing. Our study of nanostructured metals and alloys elucidates the effects of nanostructures on the size-dependent ultrahigh strengths and surface/interface-mediated deformation mechanisms. Finally, I will present my perspectives on the multiscale and multiphysics modeling that require a synergistic integration of engineering physics and applied mathematics, in order to design the advanced structural and functional materials to realize their potential to the full.

Numerical Simulations of propagation and coalescence of cracks in rocks under thermo-seepage-mechanical coupling condition in in the Framework of General Particle Dynamics
Numerical Simulations of propagation and coalescence of cracks in rocks under thermo-seepage-mechanical coupling condition in in the Framework of General Particle Dynamics
Numerical Simulations of propagation and coalescence of cracks in rocks under thermo-seepage-mechanical coupling condition in in the Framework of General Particle Dynamics

In the framework of linear elasticity, singularities occur in domains with non-smooth boundaries. Particularly in fracture mechanics, the local stress field near stress concentrations is of interest. In this work, the internal force singularities at the tip of an arbitrarily inclined semi-infinite crack terminating at the interface of two dissimilar materials are investigated on the basis of Reissner¡¯s plate theory. Using the eigenfunction expansion method, the eigenequation of the corresponding problem is derived explicitly by directly solving the governing equations of Reissner¡¯s plate theory in terms of three generalized displacement components. The focus is on the calculation of the singularity order as a fundamental quantity in fracture mechanics. The singularity orders of the moments and shear force at the crack tip are determined by the dominant eigenvalues whose real parts lie between 0 and 1. The influences of the bi-material parameters and the crack inclination angle on the moment and shear force singularity orders are discussed in detail. Specially, the variations of the shear force singularity order with the bi-material parameter and the crack inclination angle are analyzed in mathematics. It is proved that the shear force singularity order is a completely monotonic function of the bi-material parameter and the inclination angle. Some numerical results are given in order to prove the validity of the present study.

The peridynamic theory replaces the conventional differential equation of motion of continuum mechanics with an integral formula, leading to a strongly non-local theory that accounts for long range interactions among material points. It also facilitates treatment of discontinuities, initiation and evolution of damage in continua. We will present the solutions of the Green¡¯s functions for point forces in one-, two- and three-dimensional domains, as well as the Green¡¯s functions for general diffusion problems within the formalism of peridynamics. We show that these peridynamic Green¡¯s functions can be uniformly expressed as classical solutions plus Dirac functions, and convergent non-local integrals. They have different values close to the source of loading, but approach the classical theory when the non-local length tends to zero or the considered material point is far away from the loading point. The Green¡¯s functions can be used to develop methods to solve elastic and diffusion problems in infinite, semi-infinite and finite domains.
Keywords: Peridynamic theory, Green¡¯s functions, Elasticity, Diffusion

A real-space phase field model is employed to investigate the domain switching-induced shielding or anti-shielding of a magnetically impermeable crack in ferromagnetic materials with single and multi-domain states. Phase field simulations demonstrate that magnetization switching takes place from the crack tip due to the highly concentrated stress in a ferromagnetic thin plate with a stationary crack when the external mechanical load exceeds a critical value . Based on the stress and magnetic field obtained from phase field simulations, an I-integral method is established to calculate the crack-tip stress intensity factors (SIFs) in the ferromagnetic thin plate subjected to different values of mechanical load. The I-integral is not affected by the size of integral area and domain walls, which is applicable to the large-scale domain switching. The calculation results indicate that domain switching decreases the crack-tip stress intensity factors, resulting in domain-switching toughening in ferromagnetic materials. Furthermore, the domain switching in the multi-domain ferromagnetic plate induces a much larger decrease of stress intensity factor than in the single-domain one, which implies the multi-domain state is tougher than the single domain state for ferromagnetic materials.

Damage and failure rule of nano-particle ceramic coatings bonded on the alloy substrates are studied by observing crack evolution in the coating systems under in situ three- and four-point bending tests with corresponding load-displacement curves. A general damage and catastrophic failure model for the nano-particle ceramic coatings is proposed based on the elastic-brittle material model. Simultaneously, a mathematical model of damage and catastrophic is also used to describe the failure of the nano-particle ceramic coatings. The results indicate that the damage increases with increasing stress and obeys the power-law characteristics with the power exponent of 1/2, the damage tends to be complete as the stress reaches the failure point. The damage rate increases rapidly when the stress is near to the failure point and shows the power law singularity of -1/2. The experimental results of thin and thick coatings are all in agreement with the predictions based on the models.

The electrocaloric effect of ferroelectric materials, which occurs significantly near the first-order paraelectric/ferroelectric transition Curie temperature, has a tremendous prospect in solid-state cooling devices. In the present work, thermodynamics analysis and phase field simulations were conducted to demonstrate the mechanical compression-induced two types of pseudo-first-order phase transition, which could occur at a temperature below the Curie temperature. Thus, in one material there may coexist ultrahigh positive and negative electrocaloric effects, which are associated with the two pseudo-first-order phase transitions and tunable by the magnitude of the compression. The mechanical compression-induced pseudo-first-order phase transition and the coexistence of positive and negative electrocaloric effects establish a novel technology to design and manufacture next generation of solid-state cooling devices.

There exist surging societal needs for products made from renewable and sustainable resources that are biodegradable, carbon neutral and non-petroleum based. Wood cellulose fibers, the major components of paper, are obtained from plants and represent one of the most abundant and renewable materials on earth. Wood cellulose fibers have an intrinsically hierarchical structure, which holds promises to enable an array of highly desirable properties and thus could enable unconventional applications beyond their traditional use. In this talk, I will showcase an unconventional application of wood cellulose fibers: An anomalous scaling law of strength and toughness of cellulose nanopaper. The quest for both strength and toughness is perpetual in advanced material design; unfortunately, these two mechanical properties are generally mutually exclusive. A general mechanism to address the conflict between strength and toughness still remains elusive. We report a first-of-its-kind study of the dependence of strength and toughness of cellulose nanopaper on the size of the constituent cellulose fibers. Surprisingly, we find that both the strength and toughness of cellulose nanopaper increase simultaneously (40 and 130 times, respectively) as the size of the constituent cellulose fibers decreases (from a mean diameter of 27 μm to 11 nm), revealing an anomalous but highly desirable scaling law of the mechanical properties of cellulose nanopaper: the smaller, the stronger and the tougher. The findings of this research could lead to a new class of high-performance engineering materials that are both strong and tough, a Holy Grail in materials design. To this end, we have demonstrated high-performance fibers by hybridizing wood cellulose and graphene oxide.

Self-assembly processes, often driven by mechanical interactions between different parts in a material, can lead to formation of tubes, 3D structures, or devices with unique properties. The stimuli for these self-assembly processes can be thermal, chemical, or even by living cells or through intrinsic material properties such as lattice parameter differences. Here I present a few case studies of how chemical stimuli can be used to control shape formation of soft materials. The first case demonstrates mismatch strain-driven curvilinear shape formation by folding of polymer films induced by differential swelling upon chemical stimulation. Experiments with combined top-down and bottom-up approach demonstrate capabilities to form various curvilinear shapes. Finite element modeling of these systems is used to guide the shape formation processes, leading potentially to origami folding. Another case involves controlling the polymerization of a gel in a confined configuration leading to buckled 3D configurations. Such polymerization (growth) induced 3D shape may be used to study complex shape formation processes of biological systems such as organs of complex shapes. Based on the mechanics understanding of these mechanical mechanisms, one can design different 3D shapes including origami.

Size effect of nanomaterials is essentially induced by the large ratio of surface to volume. Many experiments have been carried out to explore the size effect in nanomaterials, including characterizing surface atomic structures by using electron diffraction and scanning-probe microscopy. The experimental findings provide a convincing demonstration that size effect plays an important role in the mechanical property of nanomaterials. A typically physical parameter to describe theoretically the surface of nanomaterials is the surface energy density. Within the framework of continuum mechanics, a new theory for nanomaterials based on surface energy density is proposed to predict size effect (surface effect) of nanomaterials. In contrast to previous theories, the linearly elastic constitutive relationship that is usually adopted to describe the surface layer of nanomaterials is not invoked. The surface elastic constants, which is difficult to determine experimentally, are no longer needed in the new theory. Instead, a surface-induced traction to characterize the surface effect in nanomaterials is derived. Only the surface-energy density of bulk materials and the surface-relaxation parameter are needed in the new theory. Both Parameters have clearly physical meanings and are very easy to determine through experiment and simple MD simulation. The new theory is further used to characterize the elastic property of several kinds of nanomaterials or nanostructures, whose predictions agree very well with existing numerical or experimental results. It is demonstrated that the present theory is much convenient for predicting the mechanical behavior of nanomaterials in contrast to the existing ones.

To overcome the shortcomings in conventional high-rise steel structure, such as material waste, long construction period, massive construction waste and environmental pollution,we are promoting the development and application of industrialized prefabricated high-rise steel structure system in China. A new type of high-rise steel structure, which can be assembled by parts under integrated industrial process was developed.
In this paper, the history of research on buckling capacity for steel members with slender section in home and aboard is reviewed. To achieve the innovations on industrial assembly for high-rise steel structure system, basic research has been carried out systematically about the connections, columns, floorslabs and lateral force resisting systems. To study the seismic behavior of prefabricated H-beam to square tubular column connection with stiffener, six prefabricated H-beam to square tubular column connections were tested under the lateral cyclic loads. The influence factors included dimensions of stiffener, connect type of stiffener and inner diaphragm. The hysteresis loop, skeleton curve, bearing capacity, strength and stiffness degradation performance, ductility performance and energy dissipation capacity of each specimen were analyzed comparatively.Related performances are assessed through theoretical analysis, model experiment and case studies, such as stability, load-carrying capacity, deformations under earthquake action and failure modes. Combined with the deformation of diagonal braces, stress development, and load - displacement curve, the stability bearing capacity of the structure were determined. The flexible layout can significantly change the stress performance of joints, increase the lateral stiffness of columns, increase the bearing capacity of whole structure, and reduce the calculative length coefficient of diagonal braced column.
Some suggestions about research topics are proposed as follows: the interaction between local buckling and global buckling, distortion buckling of cold formed steel members, how to control width-thick-ratio of components and plastic hinges of members efficiently in seismic design, general principles in revising the Code for Design of Steel Structures, etc

The relationship between structural hierarchy and viscosity of geomedium is examined. It is showed that viscosity and characteristic strain rate of rock mass vary at different structural levels. There exists a one-to-one correspondence between characteristic scale level and strain rate, which have inversely proportional relationship. High viscosity with low characteristic strain rate is for macro-level, while meso- and micro-levels are characterized by low viscosity and high characteristic strain rate. Generally, with the increase of strain rate, deformation and fracture take place at decreasing scale levels, and viscosity gradually decreases. With high characteristic strain rate at meso- and micro-levels, the viscosity is inversely proportional to strain rate at these levels. Based on the analysis of viscosity at different structural levels, unified description of viscosity at different structural levels is suggested and applied to the description of the strength-strain rate sensitivity of rock mass.

Continuous carbon nanotube (CNT) fibers are made of numerous well aligned meso-scale bundles, where micro-scale CNTs are closely packed together. Hence, the mechanical properties of the CNT fibers, including tensile strength, Young¡¯s modulus and flexibility, greatly benefit from the hierarchical structure of CNTs. To better understand how the CNT fibers gain their mechanical performance, a comprehensive investigation of the mechanical behavior and microstructural evolution of the CNT fibers is conducted using coarse-grained molecular dynamics simulations. Then, simulations of monotonic and cyclic tensile tests are also conducted to study plastic deformation behavior of the CNT fiber. Besides, it is found that the CNT fibers can be further enhanced through polymer infiltration during the spinning process. Accordingly, to reveal the enhancing mechanism from the microscopic aspect, the self-densified microstructure and enhanced properties of the CNT/polymer composite fibers are also analyzed by performing coarse-grained molecular dynamics simulations. Moreover, the dependence of the mechanical behavior on the microstructure of the composite fiber under different strain rates is revealed. Further, the roles of chain size and density of the polymers on the reinforcement effect are explored as well.

In order to analyze thaw consolidation of permafrost beneath roadway embankment, the concept of the effective consolidation time is established. A 3-D large strain thaw consolidation model based on Eulerian description is presented and applied to analyze consolidation behaviors of thawed permafrost layer under embankments of the Qinghai-Tibet highway. It is found that thaw consolidation is controlled by several factors, including load, the effective consolidation time as well as the characteristic drainage length. Combination of these factors makes the effect that in the initial operation years of the highway, degree of thaw consolidation increases. After a certain number of years, it decreases mainly due to the increase in the characteristic drainage length and decrease in the effective consolidation time. Pore water then accumulates in the post-thawed domain, which would take some residual consolidation time to dissipate. This explains the phenomenon that in some permafrost areas on the Qinghai-Tibet plateau permafrost has already completely thawed, while settlement of roadway embankment continuously develops.

Thin polystyrene (PS) film has shown anomalous properties such as reduced glass transition temperature when the thickness is below 100 nm. However, few attentions have been paid to its mechanical properties, which are important in understanding the stability and reliability of polymer nano-structures. In this work, we aim at elucidating the size dependent mechanical properties of ultrathin polystyrene films using molecular dynamics (MD) simulations. Coarse grained MD samples of free-standing PS films with different thicknesses were generated using the augmented phantom chain growth method. Active deformations were applied by moving two repulsive walls to determine the size dependent mechanical properties of the films. The distribution of local atomic mobility was investigated through dividing the films into equidistant bins along thickness and calculating the mean square displacement (MSD) for each bin. The local mobility of atoms at different positions of the chain as well as the mobility of the entire chain are also investigated. The results indicate the existence of a softened surface layer with reduced modulus and enhanced local atom mobility compared to the bulk state. It shows the deformation has an enhancing effect on the local atom mobility, especially along the thickness direction. This work can provide insights into the size dependent mechanical properties of ultrathin PS films.

Dislocation mobility as a function of stress is the fundamental law for the deformation in crystalline materials. However, even cutting-on-edge microscopes are incapable of recording the position of a stressed dislocation within the fine time window. Hence direct observation for one to deduce the speed-stress relationship of dislocations is still missing. Using large-scale molecular dynamics simulations, we obtain the angstrom scale spatial and picosecond level temporal information of an obstacle-free twinning partial dislocation in face centred cubic crystals. The dislocation exhibits two limiting speeds: The first is subsonic and occurs when the resolved shear stress is on the order of hundreds of megapascal. While the stress is raised to gigapascal level, an abrupt jump of dislocation velocity, from subsonic to transonic, occurs. The two-speed limits are governed respectively by the local transverse and longitudinal phonons associated with the stressed dislocation, as the two types of phonons influence dislocation gliding at different stress level.

We examine the critical strain–to–onset of void collapse and void coalescence via homogenization–based micromechanics analyses. The critical strains are established by examining the energetics of a voided unit cell throughout its loading history. Cells of varying initial void volume fraction f0 are subjected to a full range of the Lode parameter L and stress triaxiality T ranging from low (and negative) to high, and loci of strain–to–onset of void collapse and void coalescence are obtained. Numerical results show that the loci are discontinuous functions of T, with the existences of transition zone separating loci of strain–to–onset of void collapse and that of void coalescence; within the zones neither of the two void behaviors is predicted at low triaxiality. Stress state dependence of void collapse and void coalescence are investigated and presented in the form of phase maps, analogous to phase diagrams; they illustrate and distinguish clearly in the T−L space, regions in which void collapse, void coalescence and neither of the two are possible.

Currently, magnetic field or spin torques generated by power-dissipating currents are often used for the magnetization switching in spintronic devices. In order to revolutionize the spintronic devices for low-power consumption, fast response, low loss, and device minimization, the magnetization switching purely with a voltage via the magnetoelectric (ME) coupling has been proposed. Many efforts have been made on the switching via ME coupling induced by the piezoelectric strain. Intrinsically, this is a strain-assisted switching process. However, in thin film multiferroic heterostructures, the strain-mediated ME coupling is often limited due to the substrate clamping effect. In this talk, we demonstrate the charge-mediated ME coupling for voltage-driven magnetization switching in nanomagnets via a multiscale study combining first-principles calculations and magnetization dynamics. We focus on the ME coupling in metal-magnet-insulator nanoheterostructures. Taking the Pt-FePt-MgO nanoheterostructure as a model system where FePt is an attractive magnet for memory and logic devices, we firstly apply first-principles calculations to determine the relationship between magnetic anisotropy energy (MAE) and the electric field (E). MAE change of the system is shown to be attributed to the electric field induced charge in the FePt-MgO interface. The dependence of the MAE-E relationship on the layer number of FePt and the lattice variation of the system is also studied. Then the MAE-E relationship is input to the magnetization dynamics analysis of a single-domain FePt nanomagnet in the shape of an elliptical cylinder. The switching dynamics are studied in terms of different ramp rates, pulse widths, amplitudes of the voltage, in order to find the conditions for a fast and deterministic 180 degree switching. Finally, the voltage-driven magnetization switching at finite temperature is considered.

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