Microscopic Modes Research Articles

The atomistic simulation study of Ag/MgO interface tension fracture

Metal/ceramic interfaces have wide applications and the interface fracture plays an important role in determining mechanical behaviors of related structures. The cohesive zone model is widely used to modelling the crack opening and extension, especially used to modelling the interface crack. Although this model is widely used, it is a macroscopic phenomenological model, and its atomistic scale mechanism attracts great attention. In this paper an interface atomistic model of Ag/MgO is used to study the microscopic interface opening mode and some interesting results are discovered. By considering all atomic interaction potentials related to the interfacial structure in the molecular mechanics calculation, the interface tension stress-displacement curves for several interface structures with different sizes are simulated, and fracture properties upon displacing Ag and MgO adjacent to the interface are revealed based on the simulation results and by developing a series model on interface fracture properties. The results indicate that the interface fracture strength is independent of the size of interface structures for ideal interfaces, the total tension displacement of interface structures increases and the fracture appears catastrophic characteristic with increasing unit thickness, which is explained well by the series model. Furthermore, the interface separation behaviors of interfaces with the atomic vacancy and dislocations are simulated, the study indicates that the interface strength decreases for the interfaces with defects, and the defects decrease the catastrophic tendency.

Mechanical property evaluation of woven jute–coir fiber based polymer composites

In this twenty-first century, the interest in using natural fibers as reinforcement in polymer composite material has increased significantly. These composite materials are replacing the traditional materials and reducing overall costs at great extent. Now this natural fiber based composite manufacturing has been a wide area of research and it is the most preferred choice for its superior properties like low density, stiffness, light weight and possesses better mechanical properties. In this work, an investigation has been carried out on jute and coir fiber reinforced (fabricating by hand lay-up technique) hybrid polymer composites. The effect of fiber orientation effect according to two jute fiber direction (0°/90° and 45°/45°) has also been investigated. The mechanical properties like tensile, flexural and impact properties of these composite materials has been studied and analyzed using experimental and numerical analysis (In Autodesk Simulation Mechanical). Fractured surfaces were comprehensively examined with scanning electron microscope to determine the microscopic fracture mode and to characterize the microscopic mechanism governing fracture. The maximum tensile strength (62 MPa), flexural strength (89.12 MPa) and impact strength (190.65 J/m2) is obtained from jute 45°/45°–coir 0°/90° composite. The investigation revealed that fiber orientation significantly affect the mechanical properties of these composites. The predicted results based on the FEM analysis were found to be in reasonable agreement with experimental observations.

A lab-on-phone instrument with varifocal microscope via a liquid-actuated aspheric lens (LAL).

In this paper, we introduce a novel concept of liquid-actuated aspheric lens (LAL) with a built-in aspheric polydimethylsiloxane lens (APL) to enable the design of compact optical systems with varifocal microscopic imaging. The varifocal lens module consists of a sandwiched structures such as 3d printed syringe pump functionally serves as liquid controller. Other key components include two acrylic cylinders, a rigid separator, a APL/membrane composite (APLMC) embedded PDMS membrane. In functional operation, the fluidic controller was driven to control the pressure difference and ALPMC deformation. The focal length can be changed through the pressure difference. This is achieved by the adjustment of volume change of injected liquid such that a widely tunable focal length. The proposed LAL can transform to 3 modes: microscopic mode (APLMC only), convex-concave mode and biconcave mode. It is noticeable that LAL in the operation of microscopic mode is tunable in focus via the actuation of APLMC (focal length is from 4.3 to 2.3 mm and magnification 50X) and can rival the images quality of commercial microscopes. A new lab-on-phone device is economically feasible and functionally versatile to offer a great potential in the point of care applications.

Negative thermal expansion of quartz glass at low temperatures: An ab initio simulation study

Using a mixed classical Molecular dynamics (MD)/ab initio simulation scheme combined with a quasi-harmonic approximation, we calculate the linear thermal expansion coefficient αL(T) in vitreous silica glasses. The systems are first cooled down by classical MD simulations. Then they are structurally relaxed by ab initio DFT calculations. The vibrational properties are calculated employing the frozen phonon method, and these results are finally used to calculate the Helmholtz free energy as a function of volume. In agreement with experiments, our simulations predict that αL(T) is negative at low temperatures up to T ≈ 150 K. In this low-temperature regime, the simulation results are in quantitative agreement with experiments. To elucidate the origin of the negative thermal expansion, we analyze in detail the microscopic mode Grüneisen parameters in the system and show that the anomalous behavior of αL(T) can be related to the fact that the Grüneisen parameters for the lowest modes become negative at low temperatures – i.e., the lowest eigenfrequencies become stiffer with increasing volume.

Microscopic Hydrodynamic Modes in a Binary Hard Sphere Mixture

We derive analytic microscopic expressions for the shear viscosity, the speed of sound, and the decay rates of the hydrodynamic modes in a hard sphere binary gas mixture directly from the spectral properties of coupled Boltzmann equations. We show that the analytic expressions give good agreement with experimental viscosity data and to the results of light scattering experiments on noble gas binary mixtures.

Assessing nitrous oxide effect using electroencephalographically-based depth of anesthesia measures cortical state and cortical input.

Existing electroencephalography (EEG) based depth of anesthesia monitors cannot reliably track sedative or anesthetic states during n-methyl-D-aspartate (NMDA) receptor antagonist based anesthesia with ketamine or nitrous oxide (N2O). Here, a physiologically-motivated depth of anesthesia monitoring algorithm based on autoregressive-moving-average (ARMA) modeling and derivative measures of interest, Cortical State (CS) and Cortical Input (CI), is retrospectively applied in an exploratory manner to the NMDA receptor antagonist N2O, an adjuvant anesthetic gas used in clinical practice. Composite Cortical State (CCS) and Composite Cortical State distance (CCSd), two new modifications of CS, along with CS and CI were evaluated on electroencephalographic (EEG) data of healthy control individuals undergoing N2O inhalation up to equilibrated peak gas concentrations of 20, 40 or 60% N2O/O2. In particular, CCSd has been devised to vary consistently for increasing levels of anesthetic concentration independent of the anesthetic's microscopic mode of action for both N2O and propofol. The strongest effects were observed for the 60% peak gas concentration group. For the 50-60% peak gas levels, individuals showed statistically significant reductions in responsiveness compared to rest, and across the group CS and CCS increased by 39 and 42%, respectively, while CCSd was found to decrease by 398%. On the other hand a clear conclusion regarding the changes in CI could not be reached. These results indicate that, contrary to previous depth of anesthesia monitoring measures, the CS, CCS, and especially CCSd measures derived from frontal EEG are potentially useful for differentiating gas concentration and responsiveness levels in people under N2O. On the other hand, determining the utility of CI in this regard will require larger sample sizes and potentially higher gas concentrations. Future work will assess the sensitivity of CS-based and CI measures to other anesthetics and their utility in a clinical environment.

Effects of Heating Rate on the Dynamic Tensile Mechanical Properties of Coal Sandstone during Thermal Treatment

The effects of coal layered combustion and the heat injection rate on adjacent rock were examined in the process of underground coal gasification and coal-bed methane mining. Dynamic Brazilian disk tests were conducted on coal sandstone at 800°C and slow cooling from different heating rates by means of a Split Hopkinson Pressure Bar (SHPB) test system. It was discovered that thermal conditions had significant effects on the physical and mechanical properties of the sandstone including longitudinal wave velocity, density, and dynamic linear tensile strength; as the heating rates increased, the thermal expansion of the sandstone was enhanced and the damage degree increased. Compared with sandstone at ambient temperature, the fracture process of heat-treated sandstone was more complicated. After thermal treatment, the specimen had a large crack in the center and cracks on both sides caused by loading; the original cracks grew and mineral particle cracks, internal pore geometry, and other defects gradually appeared. With increasing heating rates, the microscopic fracture mode transformed from ductile fracture to subbrittle fracture. It was concluded that changes in the macroscopic mechanical properties of the sandstone were result from changes in the composition and microstructure.

Polaritonic modes in a dense cloud of cold atoms

We analyze resonant light scattering by an atomic cloud in a regime where near-field interactions between scatterers cannot be neglected. We first use a microscopic approach and calculate numerically the eigenmodes of the cloud for many different realizations. It is found that there always exists a small number of polaritonic modes that are spatially coherent and superradiant. We show that scattering is always dominated by these modes. We then use a macroscopic approach by introducing an effective permittivity so that the atomic cloud is equivalent to a dielectric particle. We show that there is a one-to-one correspondence between the microscopic polaritonic modes and the modes of a homogeneous particle with an effective permittivity.

Mechanical property evaluation of glass–jute fiber reinforced polymer composites

The natural fibers such as jute, coir, hemp, sisal etc. are randomly used as reinforcements for composite materials because of its various advantages such as low cost, low densities, low energy consumption over conventional fibers. In addition, they are renewable as well as biodegradable, and indeed wide varieties of fibers are locally available. In this study, glass–jute fiber reinforced polymer composite is fabricated, and the mechanical properties such as tensile, flexural and impact behavior are investigated. The materials selected for the studies were jute fiber and glass fiber as the reinforcement and epoxy resin as the matrix. The hand lay‐out technique was used to fabricate these composites. Fractured surface were comprehensively examined in scanning electron microscope (SEM) to determine the microscopic fracture mode. A numerical procedure based on the finite element method was then applied to evaluate the overall behavior of this composite using the experimentally applied load. Results showed that by incorporating the optimum amount of jute fibers, the overall strength of glass fiber reinforced composite can be increased and cost saving of more than 30% can be achieved. It can thus be inferred that jute fiber can be a very potential candidate in making of composites, especially for partial replacement of high‐cost glass fibers for low load bearing applications. Copyright © 2016 John Wiley & Sons, Ltd.

Modeling of slip, twinning and transformation induced plastic deformation for TWIP steel based on crystal plasticity

One of the most critical issues in development of micromechanics models for TWIP steel is to establish the continuum constitutive model which can accurately represent and model the characteristic plastic deformation at macro level. However, the uncertainty in describing the evolution of state variables based on crystal plasticity theory poses a great challenge in handling the complex plastic deformation with different deformation mechanisms and their complicated interactions and interplays at microscopic scale and thus becomes a non-trivial issue. Many attempts to address this issue by coupling slip and twinning or slip and transformation have been proven to be efficient via comparing and corroborating the predicted texture evolution using crystal plasticity theory with experiment. An accurate constitutive model, however, needs to be established to articulate and model the interactions of slip, twinning and transformation, which have been observed in experiment. In this paper, a micromechanics model for modeling of slip, twinning and transformation induced plasticity of twinning-induced plasticity (TWIP) steel is proposed by using the crystal plasticity approach. The model serves as a feasible approach to reflecting the micro deformation mechanisms during the plastic deformation process of TWIP crystals. The phase transformation is introduced and represented by the rate-dependent constitutive model. The algorithms for realization of the developed model are implemented in ABAQUS/Standard platform using UMAT. Furthermore, different deformation mechanisms of the microscopic plastic deformation modes of TWIP steel are analyzed based on the proposed models. The simulation results by using the developed model reveals that both twinning and transformation have an obvious effect on hardening and transformation, which cause the decrease of stress of single crystal, and the sequence of transformation and twinning rotation can be determined according to the proposed model.

Novel dual-function lens with microscopic and vari-focus capability incorporated with an aberration-suppression aspheric lens.

Substantial aberrations are ubiquitous in many conventional adaptive lenses due to the existence of deformable interface and thus inevitably compromise the optical performance. In this paper, we introduce a novel concept of dual-function fluidic lenses (DFFL) with a built-in aspheric polydimethylsiloxane lens (APL) to enable the design of a compact optical system with tunable imaging and aberration suppression properties. This is achieved by varying both hydrostatic pressures (i.e. adjusting the injected liquid volume change) such that a widely tunable focal length and the simultaneously integrated APL for aberrations correction. DFFL can transform to 4 modes: microscopic mode (APL only), APL/concave mode, APL/plano mode, and APL/convex mode. Focal tunability of DFFL from 12/8 mm to about 90/65 mm (DI water/ethanol) is demonstrated without any mechanical moving components. Aberration characterization is carried out systematically and the low cost, high performance microscopic mode can be easily achieved by actuating the contact between APL and PDMS membrane. In addition, DFFL turning to microscopic mode (focal length 7.32 mm and magnification 50X) can rival the images quality of commercial microscopes.

On the recovery of fibres by tape lifts, tape scanning, and manual isolation

The recovery of fibre traces via tape lifting, tape scanning and manual isolation is investigated. The recovery efficiency of taping was determined using different tapes, donor textiles, and receptor textiles. It was determined that tape lifts generally recover over 90% of extraneous fibres that had been transferred by direct contact with a donor textile. The recovery via tape scanning was evaluated by the preparation of a set of 15 tapes that contained a number of target fibres on a background of other fibres. The tapes presented varying difficulty and were investigated by trained fibre examiners. The examiners were asked to locate the target fibres and to provide their opinion on the difficulty of the search. As expected, the efficiency decreases for more difficult searches. It was determined that the efficiency of the search was influenced by the microscopic illumination modes used by the examiner. A final experiment investigated the recovery by manual isolation of fibres from non-textile items. It was determined that all fibres from tie cables and knives were recovered, except in those cases where the recipient item contained many fibres. In addition, the examiners correctly sampled each of three fibre populations present on a sample of duct tape.

Simulation study of hysteresis in the gradient-flux relation in toroidal plasma turbulence

Global nonlinear simulations with heat modulation are carried out to understand the turbulent transport mechanism in toroidal plasmas. Rapid propagation of the heat modulation and a hysteresis in the gradient-flux relation are found in the turbulent simulation of drift-interchange modes. A global mode is excited nonlinearly, and the nonlinear couplings with Reynolds stress take a finite temporal duration for self-consistent redistribution of the energy. The mode also has a seesaw effect: increase of the amplitude of the global mode at one position affects the turbulence at the other radial position not by inducing the radial flux by itself, but by absorbing the energy from microscopic modes. Successive excitations of microscopic modes cause the accelerated propagation of the flux change like turbulence spreading after the onset of modulation. Owing to these non-diffusive processes, the hysteresis appears in the gradient-flux relation, which is compared with experiments.

Behavioural observation of laminated polymer composite under uniaxial quasi-static and cyclic loads

In this study, we investigate the mechanical performance of woven glass fiber reinforced polymer composite laminates subjected to quasi-static and cyclic loading at room temperatures using the unnotched and open hole specimens. The mechanical behaviour and damage in composite laminates are greatly affected by the existence of notches with different parameters such as the size of the holes, number of layers and stacking sequences. The objective of this study is to investigate and evaluate the mechanical behavior of composite lamination structures using unnotched and notched specimens by experimentally and numerically. The materials selected for the studies were chopped strand mat (CSM)/woven roving fabric (WR) as the reinforcement and epoxy resin as the matrix. The hand lay-out technique was used to fabricate these composites. Fractured surfaces were comprehensively examined in a optical microscope and scanning electron microscope (SEM) to determine the microscopic fracture mode and to characterise the microscopic mechanism governing fracture. A numerical procedure based on the finite element method was then applied to evaluate the overall fatigue behaviour of the unnotched and open hole polymer laminate composites using the experimentally applied load. The predicted results based on the FEM analysis were found to be in reasonable agreement with experimental observations.

Ferrell–Berreman Modes in Plasmonic Epsilon-near-Zero Media

We observe unique absorption resonances in silver/silica multilayer-based epsilon-near-zero (ENZ) metamaterials that are related to radiative bulk plasmon-polariton states of thin-films originally studied by Ferrell (1958) and Berreman (1963). In the local effective medium, metamaterial description, the unique effect of the excitation of these microscopic modes is counterintuitive and captured within the complex propagation constant, not the effective dielectric permittivities. Theoretical analysis of the band structure for our metamaterials shows the existence of multiple Ferrell–Berreman branches with slow light characteristics. The demonstration that the propagation constant reveals subtle microscopic resonances can lead to the design of devices where Ferrell–Berreman modes can be exploited for practical applications ranging from plasmonic sensing to imaging and absorption enhancement.

Vibrational Raman scattering from surfaces of III-V semiconductors: Microscopic and macroscopic surface modes

With ongoing instrumental improvements Raman spectroscopy (RS) is advancing into the study of surface vibrational modes of semiconductors. On compound semiconductors, like the III–V's, two distinct types of surface modes occur, microscopic modes being vibrations confined to the surface and macroscopic modes penetrating much deeper into the bulk, dependent on the electromagnetic boundary conditions. Both mode types depend on surface properties, the microscopic ones on the atomic scale, and the macroscopic ones on the scale of the surface polariton wavelength. While the former one delivers surface atomic structure information, the latter one may be useful for instance to study the morphology of surfaces, such as the shape of semiconductor nanowires. We discuss Raman spectra obtained on the atomically well-defined III–V(110) model surfaces and recent results obtained on isolated III–V nanowires. The comparison of both gives insight in the capabilities of Raman scattering from surface phonons: The contributions of surface phonons, surface resonances, and macroscopic modes (Fuchs–Kliewer modes, surface phonon polaritons) to the Raman spectra become evident.

Multicomponent seismic forward modeling of gas hydrates beneath the seafloor

We investigated the effect of microscopic distribution modes of hydrates in porous sediments, and the saturation of hydrates and free gas on the elastic properties of saturated sediments. We simulated the propagation of seismic waves in gas hydrate-bearing sediments beneath the seafloor, and obtained the common receiver gathers of compressional waves (P-waves) and shear waves (S-waves). The numerical results suggest that the interface between sediments containing gas hydrates and free gas produces a large-amplitude bottom-simulating reflector. The analysis of multicomponent common receiver data suggests that ocean-bottom seismometers receive the converted waves of upgoing P- and S-waves, which increases the complexity of the wavefield record.

On violation of local closure of transport relation in high-temperature magnetized plasmas

Rapid propagation of heat modulation and a hysteresis in the gradient-flux relation are found in a global nonlinear simulation of drift-interchange mode turbulence in toroidal helical plasmas. A global mode is excited nonlinearly and induces the turbulence flux in a limited radial region. The nonlinear couplings take a finite temporal duration for redistributing the energy. The mode also has a seesaw effect: increase of the amplitude of the global mode, at the other radii, works to absorb the energy form microscopic modes to suppress the turbulence. Successive excitations of microscopic modes cause the accelerated propagation of change of the heat flux like turbulence spreading after the onset of modulation. Owing to these processes, the hysteresis appears in the gradient-flux relation, which is compared with experiments.

Mechanics and mechanisms of fatigue in a WC–Ni hardmetal and a comparative study with respect to WC–Co hardmetals

There is a major interest in replacing cobalt binder in hardmetals (cemented carbides) aiming for materials with similar or even improved properties at a lower price. Nickel is one of the materials most commonly used as a binder alternative to cobalt in these metal-ceramic composites. However, knowledge on mechanical properties and particularly on fatigue behavior of Ni-base cemented carbides is relatively scarce. In this study, the fatigue mechanics and mechanisms of a fine grained WC–Ni grade is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to analyze the fatigue susceptibility of the studied material. It is found that the fatigue sensitivity of the WC–Ni hardmetal investigated is close to that previously reported for Co-base cemented carbides with alike binder mean free path. Additionally, fracture modes under stable and unstable crack growth conditions are inspected. It is evidenced that stable crack growth under cyclic loading within the nickel binder exhibit faceted, crystallographic features. This microscopic failure mode is rationalized on the basis of the comparable sizes of the cyclic plastic zone ahead of the crack tip and the characteristic microstructure length scale where fatigue degradation phenomena take place in hardmetals, i.e. the binder mean free path.

Transient stress evolution in repulsion and attraction dominated glasses

We present results from microscopic mode coupling theory generalized to colloidal dispersions under shear in an integration-through-transients formalism. Stress-strain curves in start-up shear, flow curves, and normal stresses are calculated with the equilibrium static structure factor as only input. Hard spheres close to their glass transition are considered, as are hard spheres with a short-ranged square-well attraction at their attraction dominated glass transition. The consequences of steric packing and physical bond formation on the linear elastic response, the stress release during yielding, and the steady plastic flow are discussed and compared to experimental data from concentrated model dispersions.