Tunable band structures of polycrystalline graphene by external and mismatch strains

Lacking a band gap largely limits the application of graphene in electronic devices. Previous study shows that grain boundaries (GBs) in polycrystalline graphene can dramatically alter the electrical properties of graphene. Here, we investigate the band structure of polycrystalline graphene tuned by externally imposed strains and intrinsic mismatch strains at the GB by density functional theory (DFT) calculations. We found that graphene with symmetrical GBs typically has zero band gap even with large uniaxial and biaxial strain. However, some particular asymmetrical GBs can open a band gap in graphene and their band structures can be substantially tuned by external strains. A maximum band gap about 0.19 eV was observed in matched-armchair GB (5, 5) | (3, 7) with a misorientation of θ = 13° when the applied uniaxial strain increases to 9%. Although mismatch strain is inevitable in asymmetrical GBs, it has a small influence on the band gap of polycrystalline graphene.     

Modeling of ductile fracture: Significance of void coalescence

In this paper void coalescence is regarded as the result of localization of plastic flow between enlarged voids. We obtain the failure criterion for a representative material volume (RMV) in terms of the macroscopic equivalent strain (E_c) as a function of the stress triaxiality parameter (T) and the Lode angle (θ) by conducting systematic finite element analyses of the void-containing RMV subjected to different macroscopic stress states. A series of parameter studies are conducted to examine the effects of the initial shape and volume fraction of the primary void and nucleation, growth, and coalescence of secondary voids on the predicted failure surface E_c(T, θ). As an application, a numerical approach is proposed to predict ductile crack growth in thin panels of a 2024-T3 aluminum alloy, where a porous plasticity model is used to describe the void growth process and the expression for E_c is calibrated using experimental data. The calibrated computational model is applied to predict crack extension in fracture specimens having various initial crack configurations and the numerical predictions agree very well with experimental measurements.     

Influence of single rectangular groove on the flow past a circular cylinder

In the present study, flow control mechanism of single groove on a circular cylinder surface is presented experimentally using Particle image velocimetry (PIV). A square shaped groove is patterned longitudinally on the surface of the cylinder with a diameter of 50 mm. The flow characteristics are studied as a function of angular position of the groove from the forward stagnation point of the cylinder within 0° ≤ θ ≤ 150°. In the current work, instantaneous and time-averaged flow data such as vorticity, ω streamline, Ψ streamwise, u/U_o and transverse, v/U_o velocity components, turbulent kinetic energy, TKE and RMS of streamwise, u_rms and transverse, v_rms velocity components are utilized in order to present the results of quantitative analyses. Furthermore, Strouhal numbers are calculated using Karman vortex shedding frequency, f_k obtained from single point spectral analysis. It is concluded that a critical angular position of the groove, θ = 80° is observed. The flow separation is controlled within 0° ≤ θ < 80°. At θ = 80°, the flow separation starts to occur in the upstream direction. The instability within the shear layer is also induced on grooved side of the cylinder with frequencies different than Karman vortex shedding frequency, f_k.     

Flow structures behind a vibrissa-shaped cylinder at different angles of attack: Complication on vortex-induced vibration

Complementary experimental studies have been conducted with a vibrissa-shaped cylinder at different angles of attack, through vortex-induced vibration (VIV) test in a wind tunnel, along with extensive measurements of wake dynamics in a water channel using time-resolved particle image velocimetry (TR-PIV). The VIV responses of an elastically mounted vibrissa-shaped cylinder are experimentally compared at various angles of attack in the range of θ = 0°–90°. At the reduced velocity of U₀/f₀D_h = 3–10 (f₀ being the system's natural frequency), the cross-flow displacement of the cylinders convincingly demonstrates that the vibrissa-shaped cylinder at a small angle of attack (θ ≤ 30°) is stable, and without appreciable displacement. Beyond θ = 30°, a prominent three-branched VIV response is readily identified, and increasing the angle of attack results in an upward shift of the synchronized region and a considerable intensification of the peak amplitude. Subsequently, TR-PIV measurements are made of the wake flow behind the vibrissa-shaped cylinder, to determine the spatio-temporally varying flow fields in two spanwise planes, i.e., the saddle and the nodal planes. Four systems with different angles of attack are chosen for comparison at Re_D = 1.8 × 10³, i.e., θ = 0°, 30°, 60° and 90°. In the two systems with θ = 0° and 30°, the wake regions feature weak velocity fluctuations in highly limited areas. However, increasing the angles of attack (to θ = 60° and 90°) gives rise to expanded recirculation zones, highly unstable flow reversals immediately behind the cylinder, and strengthened velocity fluctuations in the bulk wake regions. Cross-correlation of the fluctuating longitudinal velocities shows that at θ = 60° and 90° the energetic large-scale vortical structures form earlier, and they exert considerable influence on the near-wake fluid behind the cylinder. Finally, a sophisticated data-driven dynamic mode decomposition (DMD) process is used to extract the dominant unsteady structures in the four systems with different angles of attack. In the system with θ = 0°, two dominant DMD modes at frequencies St= 0.23 and 0.30 are identified in the saddle and the nodal planes, respectively, and those frequencies are St= 0.18 and 0.19 in the system with θ = 30°. The interaction between these dominant events at different frequencies tends to disrupt the formation of a strong vortex-shedding process. Therefore, the hydrodynamic force on the cylinder does not make a concerted contribution to suppressing the VIV behavior along the spanwise direction. In the systems with θ = 60° and 90°, the corresponding DMD modes exhibit much more synchronous, organized characteristics in the saddle and nodal planes, and unsteady events at the same frequencies are detected in both planes, reaching St = 0.14 (for θ = 60°) and 0.12 (for θ = 90°). These effects, along with the intensified vortex-shedding processes in the saddle and nodal planes, exert a concerted hydrodynamic force on the cylinder, causing it to start with an oscillatory state.     

Flow characteristics and flow-induced forces of a stationary and rotating triangular cylinder with different incidence angles at low Reynolds numbers

In this paper, the problem of two-dimensional fluid flow past a stationary and rotationally oscillating equilateral triangular cylinder with a variable incident angle, Reynolds number, oscillating amplitude, and oscillating frequency is numerically investigated. The computations are carried out by using a two-step Taylor-characteristic-based Galerkin (TCBG) algorithm. For the stationary cases, simulations are conducted at various incident angles of α=0.0–60.0° and Reynolds numbers of Re=50–160. For the oscillation cases, the investigations are done at various oscillating amplitudes of θ_max=7.5–30.0° and oscillating frequencies of F_s/F_o=0.5–3.0 considering two different incidence angles (α=0.0°, 60.0°) and three different Reynolds numbers (Re=50, 100, 150). The results show that the influences of key parameters (incidence angle, Reynolds number, oscillating amplitude, and oscillating frequency) are significant on the flow pattern and hydrodynamic forces. For the stationary cases, at smaller angle of incidence (α≤30.0°), Reynolds number has a large impact on the position of the separation points. When α is between 30.0° and 60.0°, it was found that the separation points are located at the rear corners. From a topological point of view, the diagram of flow pattern is summarized, including two distinct patterns, namely, main separation and vortex merging. A deep analysis of the influence of Reynolds number and incidence angles on the mean pressure coefficient along the triangular cylinder surface is presented. Additionally, for the oscillating cases, the lock-on phenomenon is captured. The dominant flow patterns are 2S mode and P+S mode in lock-on region at α=0.0°. It is found at α=60.0°, however, that the flow pattern is predominantly 2S mode. Furthermore, except for the case of F_s/F_o=2.0, the mean drag decreases as the oscillating amplitude increases for each Reynolds number at α=0.0°. At α=60.0°, the minimum mean drag for F_s/F_o=1.5 is lower than that for stationary case, and occurs at θ_max=15.0° (Re=100) and θ_max=22.5° (Re=150), respectively. Finally, the effect of Reynolds number on a rotational oscillation cylinder is elucidated.     

Numerical study on the oscillation of a transient bubble near a confined free surface for droplet generation

In the present work, the oscillation of a spark-created bubble near a confined water–air interface and the ensuing droplet generation and ejection are studied numerically using the boundary element method. The interface is accorded by the top opening of either one of the following symmetrical configurations, which are distinguished by the value of angle between their vertical symmetry axis and lateral wall (i.e., θ): (i) a centrally perforated horizontal flat plate (θ = 90°) and (ii) vertically placed cylinder (θ = 0°), nozzle (θ > 0°) and diffuser (θ < 0°). Furthermore, the influences of the effective parameters such as the strength parameter (i.e., the intensity of local energy input), the bubble-free surface distance (standoff distance) and the nozzle size on the bubble dynamics and droplet formation and ejection processes are investigated. It was found that the moment at which the bubble attained its maximum volume was advanced as θ increased. In addition, by decreasing θ the attraction of the bubble toward the free surface during its expansion phase and its migration from the free surface during its contraction phase became stronger. Furthermore, for the nozzle case, by increasing θ, the volume of the droplet was increased. It was also found that by increasing the strength parameter, the volume of the droplet increased and its pinch-off happened earlier. Finally, as the standoff distance was increased, the volume of the droplet increased and its pinch-off was delayed.     

Growths of mechanical elasticity and electrical conductance of graphene nanoplatelet/poly(lactic acid) composites under strong electric field: correlation with time evolution of higher order structure of graphene nanoplatelets

For a composite of poly(lactic acid) containing graphene nanoplatelets (GNPs) at a low concentration (0.34 vol%), this study examined growths of mechanical and electrical properties under an alternating current (AC) electric field, focusing on field-induced GNP structures governing those properties. The composite was subjected to the AC field (60 Hz) of various intensities E for various times t _E at 190 °C. A fraction of randomly oriented GNPs was aligned by the field and then connected into columns, as suggested from optical microscopy. This structural evolution led to qualitatively similar growths of low-frequency storage modulus and static electrical conductivity. The key quantity for understanding this growth was a time t _E* for occurrence of short circuit that detected formation of GNP columns conductively bridging the electrodes. The growths of both modulus and conductivity for various E were summarized as functions of a reduced variable, t _E/t _E*, confirming the growths commonly reflected the evolution of the GNP columns. However, the modulus grew fast and leveled off by t _E/t _E* ~ 1, whereas the conductivity kept growing gradually even at t _E/t _E* > 1. This difference was discussed in relation to the matrix chains and leftover GNPs out the column.     

Stress and finite-strain analysis of a circular ring under diametral compression

Using a moiré, large-strain analysis method, a complete solution is shown in this paper of the fields of strain and stress for a circular ring subjected to diametral compression between two flat platens. The isotheticsu andv, obtained using 1000-lines-per-inch gratings, were differentiated photographically by the shifting technique (moiré-of-moiré) to determine ?u/?x, ?v/?y, ?u/?y and ?v/?x. Using the exact finite strain-displacement relationship, the Eulerian strains ? _x ^E , ? _y ^E and γ _xy ^E were computed. From these, the principal Eulerian strains were obtained. These results were verified with the isochromatics obtained from a large-deformation photoelasticity analysis. The ring was made of a polyurethane rubber which exhibits a linear relationship between natural strain and a newly introduced concept of “natural stress”. The Eulerian strains were converted to natural strains, and from these natural stresses were computed using the newly developed concept. Results are presented graphically for the whole field of the ring.     

Surface characteristics of spruce veneers and shear strength of plywood as a function of log temperature in peeling process

The objective of this study was to determine the effect of temperature of spruce (Picea orientalis L.) logs during peeling process on surface roughness, adhesive wettability, colour variation of veneer, and shear strength of plywood made from these veneer sheets. Veneer samples were manufactured from the logs after they were kept for 3 h and 24 h to reach to average temperatures of 52 °C and 32 °C, respectively. A fine stylus method was used for surface roughness evaluation of the veneer produced from two types of the logs and it was found that the samples peeled from the logs with a temperature of 52 °C had significantly better roughness values than those of manufactured from the logs with 32 °C at a 95% confidence level. Wettability of veneer samples was determined with contact angle measurements according to the sessile drop method. Urea formaldehyde (UF) and phenol formaldehyde (PF) resin drops were used in contact angle measurements. Contact angles of PF resin drops on veneers were similar for each peeling temperature while the contact angles of UF glue resin on veneers produced from the logs with 32 °C were lower than those of produced from the logs with 52 °C. Small colour difference was measured (indicated by a low ΔE value) on veneer samples depending on the log temperature. The highest shear strength value was determined for the plywood manufactured from veneers obtained from the logs with 52 °C by using UF glue.     

Yield stress measurements and microstructure of colloidal kaolin powders clusterized and dispersed in different liquids

Rheologicat measurements of four colloidal kaolin powders dispersed in water, paraffinic oil and liquid rubber have been done at solid concentration （9 （VS/VL） in the range 8-33%. In quasi-static conditions the yield stress τ° values were derived. An S-shaped relationship between τ° and （9 exists, that allows to evaluate the percolation threshold θc values. In water θc ranges between 29% and 33%, while it is between 17% and 21% in oil and in liquid rubber. In water the microstructure units, i.e. the primary clusters among kaolin crystallites, are denser and bigger than the ones in the two other liquids. Furthermore, in quasistatic condition, the aqueous microstructure units have less tendency to cling together to form a network that can span all over the liquid phase. The different kaolin dispersions at θ equal to 22% are characterized by yield stress ranging between 7.S Pa and 59 Pa in water, 66-250 Pa in oil and 230-770 Pa in liquid rubber. It has been found that for any kaolin types the log τ°/ θ evaluated near to the percolation threshold is a conservative parameter of the three different liquids. These results are useful to design colloidal dispersions with selected microstructure.     

Behavior of graphite/epoxy plates with holes under biaxial loading

An experimental investigation was conducted to study the behavior under biaxial tensile loading of quasiisotropic graphite/epoxy plates with circular holes and to determine the influence of hole diameter on failure. The specimens were 40 cm×40 cm (16 in.×16 in.) laminates of [0/±45/90] _s layup. Four hole diameters, 2.54 cm (1.00 in.), 1.91 cm (0.75 in.), 1.27 cm (0.50 in.) and 0.64 cm (0.25 in.), were investigated. Deformations and strains were measured using strain gages and birefringent coatings. Equal biaxial loading was introduced by means fo four whiffle-tree grip linkages and controlled with a servohyraulic system. Initially, the circumferential strain is uniform around the boundary of the hole. Subsequently, with increasing load, regions of high strain concentration with nonlinear response develop at eight characteristic locations 22.5 deg off the fiber axes. Failure in the form of cracking and delamination initiates at these points. Maximum strains at failure on the hole boundary reach values up to twice the ultimate strain of the unnotched laminate. The effect of hole diameter on strength was described satisfactorily using an average biaxial-stress criterion. Good correlation was also obtained with theoretical predictions based on a tensor-polynomial failure criterion for the lamina and a progressive degradation model.     

Biaxial testing of [O2/±45] s graphite/epoxy plates with holes

An experimental investigation was conducted to study the behavior under biaxial-tensile loading of [O₂/±45] _s graphite/epoxy plates with circular holes and to determine the influence of hole diameter on failure. The specimens were 40-cm×40-cm (16-in.×16-in.) graphite/epoxy plates of [O₂/±45] _s layup. Four hole diameters, 2.54 cm (1.00 in.), 1.91 cm (0.75 in.), 1.27 cm (0.50 in.) and 0.64 cm (0.25 in.), were investigated. Deformations and strains were measured using strain gages and birefringent coatings. Biaxial tension in a 2∶1 ratio was applied by means of four whiffle-tree grip linkages and controlled with a servohydraulic system. Stress and strain redistributions occur around the hole at a stress level corresponding to localized failure around the 67.5-deg location and nonlinear strain response at the 0-deg location. Maximum measured strains at failure on the hole boundary are higher (approximately 0.016) than the highest ultimate strain of the unnotched laminate (0.010). Two basic patterns of failure were observed: (a) horizontal cracking initiating at points off the horizontal axis and accompanied by extensive delamination of the subsurface ±45 deg plies, and (b) vertical cracking along vertical tangents to the hole and accompanied by delamination of the outer 0-deg plies. The strength reduction ratios are lower than corresponding values for uniaxial loading by approximately 16 percent, although the stress-concentration factor under biaxial loading is lower.     

Stress-Dependent Elastic Behaviour of a Titanium Alloy at Elevated Temperatures

In this study the influence of stress and temperature on the elastic modulus during fully-reversed low cycle fatigue of the titanium alloy Ti6242 is examined. The change of the elastic properties with stress manifests itself in a crescent-like shaped hysteresis loop of stress vs. plastic strain at very low amplitudes, i.e. below the technical yield stress. A quadratic extension of Hooke??s law with a second constant ??k?? is applied. The parameters are determined all along the unloading curve in tension and compression. The approach results in the alignment of the hysteresis loop so that they become vertical, i.e. the elastic strain is accurately described. The value and sign of ??k?? depend on whether the deformation occurs in tension or compression. Like the Young??s modulus E ₀, ??k?? also depends on temperature. At temperatures up to 550°C the values of ??k?? in tension and compression do not change during fatigue life. However, at 650°C thermally activated slip processes lead to changes of both, E ₀ and ??k??.     

Size-dependent effect on biaxial and shear nonlinear buckling analysis of nonlocal isotropic and orthotropic micro-plate based on surface stress and modified couple stress theories using differential quadrature method

The size-dependent effect on the biaxial and shear nonlinear buckling analysis of an isotropic and orthotropic micro-plate based on the surface stress,the modified couple stress theory(MCST),and the nonlocal elasticity theories using the differential quadrature method(DQM)is presented.Main advantages of the MCST over the classical theory(CT)are the inclusion of the asymmetric couple stress tensor and the consideration of only one material length scale parameter.Based on the nonlinear von K′arm′an assumption,the governing equations of equilibrium for the micro-classical plate considering midplane displacements are derived based on the minimum principle of potential energy.Using the DQM,the biaxial and shear critical buckling loads of the micro-plate for various boundary conditions are obtained.Accuracy of the obtained results is validated by comparing the solutions with those reported in the literature.A parametric study is conducted to show the effects of the aspect ratio,the side-to-thickness ratio,Eringen’s nonlocal parameter,the material length scale parameter,Young’s modulus of the surface layer,the surface residual stress,the polymer matrix coefficients,and various boundary conditions on the dimensionless uniaxial,biaxial,and shear critical buckling loads.The results indicate that the critical buckling loads are strongly sensitive to Eringen’s nonlocal parameter,the material length scale parameter,and the surface residual stress effects,while the effect of Young’s modulus of the surface layer on the critical buckling load is negligible.Also,considering the size dependent effect causes the increase in the stiffness of the orthotropic micro-plate.The results show that the critical biaxial buckling load increases with an increase in G12/E2and vice versa for E1/E2.It is shown that the nonlinear biaxial buckling ratio decreases as the aspect ratio increases and vice versa for the buckling amplitude.Because of the most lightweight micro-composite materials with high strength/weight and stiffness/weight ratios,it is anticipated that the results of the present work are useful in experimental characterization of the mechanical properties of micro-composite plates in the aircraft industry and other engineering applications.     

Large bending behavior of creased paperboard. I. Experimental investigations

The large bending behavior of a creased paperboard is studied in the range of rotation θ ? [0°, 180°] – new results, apparently not reported previously in literature – with the aim to point out some crucial aspect involved in an adaptive robotic manipulation of the industrial cartons.The loading tests show a great variability of the mechanical behavior, depending dramatically on the crease indentation depth (also for the specimens obtained from the same carton): (a) when the damage induced during the crease formation is relatively small, the bending response is unusually complex: the moment constitutive function, m_L(θ), presents (up to) two peaks followed by unstable branches; (b) for greater indentation, the m_L(θ) is monotone.In the unloading case the response m_U(θ) is always monotone and is practically independent of the formation conditions of the crease. These behaviors can be easily described analytically using (piecewise) third degree splines.In a companion paper, the erection of a typical carton corner with unstable constitutive behavior is fully analyzed to detect the possible criticalities.     

Compressive response and failure of braided textile composites: Part 1—experiments

Experimental results obtained by examining the planar biaxial compression/tension response of carbon 2D triaxial braided composites (2DTBC) are reported in this paper. These experiments were motivated by a need to examine the failure of 2DTBC in a state of stress that would be similar to what is experienced by the walls of a tubular member under compressive crush loads. Results obtained from a series of biaxial tests that were conducted with different proportional displacement loading ratio combinations of compression and tension are reported. In all cases, the dominant failure mechanism under such a stress state is the buckling of the bias and axial tows within the composite. Full field surface displacement data is acquired concurrently during all biaxial and some uniaxial tests using the technique of digital speckle photography. Digital images of the specimen surface that is illuminated with a He-Ne laser are acquired at discrete time intervals during the loading history using a high-resolution digital camera. These images are stored and analyzed to obtain the incremental inplane surface displacement field, Δu(x,y) and Δv(x,y). From these, the incremental inplane surface strains Δε_x, Δε_y and Δγ_xy are obtained by numerical differentiation. The present paper, which is the first in a two part series, is devoted to the biaxial experimental results pertaining to 2DTBC failure.     

Computational study of microparticle effect on self-propelled jumping of droplets from superhydrophobic substrates

We present three-dimensional numerical simulations, employing a lattice Boltzmann method for three-phase system of liquid, gas, and solid, and investigate the influence of a solid particle on the dynamic and departure of a droplet after coalescence on superhydrophobic substrates. A particle can be removed autonomously by the jumping motion of the droplet, which partially or fully covers the particle. This spontaneous removal from superhydrophobic substrates is achieved by converting surface energy to kinetic energy, independent of gravity. We discussed the effect of size, wettability and initial placement of particle on the evolution of lateral and vertical motion of the droplet. The results indicate that the droplet with a fully immersed particle, as in the floating mechanism, reaches to the same equilibrium height as a particle-free droplet. However, the droplet with a partially immersed particle, as in the lifting mechanism, can have a substantial jumping velocity compared to a particle-free droplet. As the size of the partially immersed particle approaches its critical limit, which is equal to the size of the droplet, the droplet jumping and transport from the substrate is enhanced. Besides the particle size, the particle wettability can result in a considerable droplet jumping velocity. A particle with a neutrally wetting contact angle (i.e. 90°) is found to elevate the transport of the droplet to a higher distance from the substrate relative to a partially wetting case (i.e. 60°). In the lifting removal mechanism, unlike the floating removal mechanism, the particle initial placement is highly critical for the detachment of the merged droplet from the substrate, as well as the elevation of the detached droplet to a longer distance from the substrate. For a partially immersed particle, the critical particle initial position from the substrate above which the droplet-particle system does not jump away from the substrate is independent of particle size and wettability and is about 1.5r_d where r_d is the initial size of the droplet.     

A critical review of methods for determining stress-intensity factors from isochromatic fringes

Two-parameter methods of fracture analysis for determining the stress-intensity factor from photoelastic isochromatic-fringe data were critically reviewed. The methods of Irwin, Bradley and Kobayashi, and Smith were developed in detail and differences in the three approaches were noted. Theoretical fringe loops were generated for a crack of length 2a in a semi-infinite plate with biaxial loading. These fringe loops were used to compare the three analysis methods and to determine the accuracy of each method. All three methods give a close estimate of the stress-intensity factor, with the Bradley-Kobayashi differencing procedure providing the most precise estimate ofK. However, if measurement errors become excessive (larger than 2 percent) the differencing procedure magnifies these errors and the original method proposed by Irwin is the recommended approach. The two-parameter methods can be employed to determineK to within ±5 percent, provided the angle of tilt of the isochromatic-fringe loop is 73 ≤θ _m < 139 deg. Ifθ _m is outside this range, the two-parameter methods should not be employed.