Negative Shear Research Articles

An Experimentalapproach to Investigate the Effect of Process Conditions on Exit Burr formation in Shapingan Aluminiumalloy

At the exit edge of the wokpiece, burrs are usually found during shaping operation. Gradual rotation of the positive shear plane to the negative shear plane may be the reason behind it. This is due to absence of any support material at the exit end of cut. In the present work, a number of orthogonal shaping experiments is performed on aluminium alloy (4600-M) in dry environment under different cutting velocities, orthogonal rake angles and exit edge bevel angles to observe the occurrence of burr formation. Experimental results are analysed through ANOVA to find out the significance of the effect of cutting parameters and exit edge bevel angle to control the exit burr formation. It is noted that exit edge bevel angle has the most significant effect. An exit edge bevel angle of 15° is recommended in shaping operation when burr becomes negligible.

Ideal MHD Stability Prediction and Required Power for EAST Advanced Scenario

The Experimental Advanced Superconducting Tokamak (EAST) is the first fully superconducting tokamak with a D-shaped cross-sectional plasma presently in operation. The ideal magnetohydrodynamic (MHD) stability and required power for the EAST advanced tokamak (AT) scenario with negative central shear and double transport barrier (DTB) are investigated. With the equilibrium code TOQ and stability code GATO, the ideal MHD stability is analyzed. It is shown that a moderate ratio of edge transport barriers' (ETB) height to internal transport barriers' (ITBs) height is beneficial to ideal MHD stability. The normalized beta βN limit is about 2.20 (without wall) and 3.70 (with ideal wall). With the scaling law of energy confinement time, the required heating power for EAST AT scenario is calculated. The total heating power Pt increases as the toroidal magnetic field BT or the normalized beta βN is increased.

Mechanical effect of rotating non-spherical particles on failure in compression

When non-spherical particles (grains, blocks) in a rock or geomaterial under compression are sufficiently detached to enable rolling under shear stress the moment equilibrium dictates that further increase in displacement requires reduced shear stress. Thus the rolling non-spherical particle produces an effect of apparent negative stiffness whose value is proportional to the magnitude of the compressive stress. We model geomaterials with rolling particles at a macroscale as a matrix containing inclusions with negative shear modulus. We consider two extreme types of inclusions: spherical inclusions and shear cracks with negative stiffness shear springs inside. We show that there exists a critical concentration of inclusions at which the effective shear modulus abruptly becomes negative and the geomaterial loses stability. Furthermore, there exists a special value of negative shear modulus/stiffness of inclusions (and hence the magnitude of the compressive stress) at which the critical concentration is zero, such that the first rolling particle induces the global instability. This stress is of the order of the shear modulus the geomaterial exhibits at the advanced stage of fracturing when the particles are sufficiently detached to enable the rolling.

Structural, elastic, phonon and electronic properties of a MnPd alloy

The structural, elastic, phonon and electronic properties of a MnPd alloy have been investigated using the first-principles calculation. The calculated lattice constants and electronic structure agree well with the experimental results. The microscopic mechanism of the diffusionless martensitic transition from the paramagnetic B2 (PM-B2) phase to the antiferromagnetic L10 (AFM-L10) phase through the intermediate paramagnetic L10 (PM-L10) phase has been explored theoretically. The obtained negative shear modulus C′ = (C11 — C12)/2 of the PM-B2 phase is closely related to the instability of the cubic B2 phase with respect to the tetragonal distortions. The calculated phonon dispersions for the PM-L10 and AFM-L10 phases indicate that they are dynamically stable. However, the AFM-L10 phase is energetically most favorable according to the calculated total energy order, so the PM-L10 → AFM-L10 transition is caused by the magnetism rather than the electron—phonon interaction. Additionally, the AFM-L10 state is stabilized through the formation of a pseudo gap located at the Fermi level. The calculated results show that the CuAu-I type structure in the collinear antiferromagnetic state is dynamically and mechanically stable, thus is the low temperature phase.

Experimental Study on Exit Edge Defects in Two Dimension Cutting of SiCp/Al Composites

In this paper, the influence of the tool rake angle, cutting depth and cutting speed on exit edge defect sizes are discussed by the two dimension cutting experiment of SiCp/Al composites. The sizes are measured by using the Keyence VHX-1000C type super preview 3D microscopy. The results indicate that exit edge defect sizes are influenced greatly by tool rake angle and cutting depth, and are influenced slightly by the cutting speed. The edge defect sizes have an overall downward trend when the tool rake angle is smaller than 5°, and have an apparent upward trend when the tool rake angle (γ0) is 10°.The length (L), height (H) of exit edge defects and the negative shear angle increase with increasing of the cutting depth.

Ab initio study of shear strain effects on ferroelectricity at PbTiO3 thin films

Ferroelectric thin film with the perovskite ABO3 structure have been widely used in technology applications, e.g., actuators in MEMS/NEMS and nonvolatile random access memories (FeRAM). In order to clarify the effect of the shear strain on the ferroelectricity, the PbTiO3 thin film as a typical one is chosen. The focus of this study is to put on the PbO-terminated (1​×​1) and c(2​×​2) surfaces and the TiO2-terminated (1​×​1) surface. Based on ab initio density functional theory calculations with the local density approximation, we have found out that in both the PbO and TiO2-terminated (1​×​1) models, the ferroelectricity in the PbO layers was enhanced under the positive shear strain while it was suppressed under the negative one. For the TiO2 layers, the ferroelectricity was slightly enhanced and sharply suppressed under the positive and negative shear strains, respectively. In the PbO-terminated (2​×​2) model, the AFE phase was suppressed by the FE phase under the positive shear strain while the opposite trend was found under the negative shear strain. For the PbO layers, the ferroelectricity was enhanced under the positive and negative shear strains. For the TiO2 layers, the influence of the negative shear strain on the ferroelectricity was larger than that of the positive one. In addition, the ideal strength of the PbTiO3 thin film with the different terminations was investigated as well.

Discussion of “Critical assessment of pile modulus determination methods” 1Appears in the Canadian Geotechnical Journal, 48(10): 1433–1448 [doi: 10.1139/t11-050

Lam and Jefferis (2011) have produced a timely, well written, and worthy paper. I would like to add a few comments to the authors’ presentation of the strain measurements. Moreover, the authors’ recommendations regarding the E-modulus to be used for determining the load distribution in a pile subjected to a static loading test from the strain measurements need both to be emphasized and moderated, which is what the following brief compilation of case records aims to do. The authors’ paper presents the changes of strain of up to 100+ μ3 in the pile occurring as a result of the concreting and during the set-up time between pile construction and the static loading test. I think it is worthwhile to emphasize that the changes are caused by three different processes: temperature change (heating and cooling), swelling of the concrete, and reconsolidation by the soil (Fellenius et al. 2004). First, changes of strain are imposed during the hydration process as a consequence of the differences in thermal response between the concrete and the steel (in the sister bar gages). The hydration process causes an increase of temperature, which takes place over several hours, about 24 h rather than the 3 h the authors report. At first, the concrete is fluid, and there is very little strain or stress transferred between the concrete and the sister bars. Second, when the concrete starts to harden, bonding develops between the rebars and the concrete, and a further temperature increase will result in an apparent elongation — tension — of the rebars. During the subsequent cooling, which can take weeks or months, the concrete (usually) reduces more than the rebars, manifested by a shortening of the rebars — apparent compression — co-occurring with tension in the concrete. During the following period, the concrete absorbs water from the soil, which results in a volume increase — swelling — recorded as a rebar elongation — apparent tension — and, conversely, compression in the concrete. Third, at depth, when the soil recovers from the disturbance imposed by the construction, it usually tends to settle, which causes negative direction shear forces to develop, resulting in an increase of load in the pile — noticeable as residual load, which the authors showed to have taken place for the subject pile. Consequently, when the static loading test commences, a state of stress and strain exists in the pile that to some extent will affect the response to the applied load increments, as registered by the strain gages. The evaluation of the strain-gage records, in particular when the secant stiffness method is applied, needs to consider this, as suggested for a few of the following case histories.

Elastic composite with negative stiffness inclusions in antiplane strain

An elastic composite consisting of an isotropic matrix with isotropic cylindrical inclusions of negative shear modulus is considered in the antiplane strain approximation. We show that the effective moduli of a composite with negative stiffness inclusions are defined uniquely and can be determined by using the conventional elastic solution where the positive moduli are replaced with the negative ones. We compute the effective shear modulus of the composite with interacting negative stiffness inclusions using the differential self-consistent method. We found that for concentrations of inclusions below a certain critical value the composite is stable and has positive effective shear modulus that is higher than that of the matrix. Thus negative stiffness inclusions stiffen the matrix, as has been suggested by R. Lakes. However, as soon as the concentration of inclusion exceeds the critical value the effective modulus of the composite abruptly drops to a negative value and the composite becomes unstable. Correspondingly, the value of the negative effective stiffness depends upon the stabilising device. The critical concentration depends upon the value of the negative modulus of inclusions; there exists a value of negative shear modulus at which the critical concentration is zero, i.e. the composite is always unstable.

Finite Element and Experimental Analysis of Edge Defects Formation during Orthogonal Cutting of SiCp/Al Composites

In this paper, a finite element method was used to dynamically simulate the process of the edge defects formation during orthogonal cutting SiCp/Al composites. The influence of the cutting speed, cutting depth and rake angle of the PCD insert on the size of the edge defects have been investigated by using scanning electron. According to the simulated results, it can be provided that the cutting layer material has an effect on transfer stress and hinder the chip formation in the critical transition stage, and the critical transition point and distance are defined in this stage. The negative shear phenomenon is found when the chip transit to the edge defects in the flexure deformation stage, so the process of the chip formation is the basis of the edge defects formation. In addition, the relationship between the nucleation and propagation direction of the crack and the variation of the edge defect shape on the workpiece was investigated by theory, and it found that the negative shear angle formation is the primary cause of the edge defect formation. A mixed mode crack is found in the crack propagation stage. The sizes of edge defects were measured by the experiment and simulation, and the edge defect size decrease with the increasing of tool rake angle, while increase with increasing cutting depth and cutting speed.

Elastic metamaterials with negative bands

The unusual properties of a metamaterial come from special resonances supported by its resonating structure units. Guided by a previously developed effective medium theory, which links the resonances of the microstructures and the unusual properties, two types of elastic metamaterials in two dimensions were designed with different resonant structures in their building blocks that exhibit multiple negative dispersion bands with special characteristics. The first type possesses negative mass density and negative shear modulus simultaneously over a large frequency regime, which leads to a negative band for shear waves only. Mode conversion takes place at the interface of the metamaterial and the common solids. The second type is able to produce negative effective moduli in different frequency regimes within a large frequency regime of negative effective mass density. This results in a super anisotropic negative band and a negative band that only compressional is allowed. All of these unusual properties are demonstrated by simulations. This work was supported by Hong Kong RGC Grant No. 605008, HKUST604207 and KAUST start-up package.

Fence probe measurement of flow reversal in separating turbulent boundary layers

A small fence probe was evaluated for measurements in the time-dependent flow reversal region of the transition from boundary layer to separated flow. For moderate and high Reynolds numbers, the fence probe is demonstrated to be a usable tool for the measurement of the reverse flow associated with separation. Although the present probe pressure transducer system was limited to approximately 200 Hz, pulses of positive and negative shear stress were readily detected. At or near the location of zero surface shear stress, the measurements were limited by the signal-to-noise ratio. For the separated flow investigated, a marked reduction in the pressure gradient occurred when the fence probe indicated approximately 20 % reversal for the higher Reynolds numbers. The reversal increased to 24 % for the lower Reynolds numbers. The measurements indicate that flow reversal alone may not be adequate to identify the degree of separation. Upstream of turbulent boundary layer (intermittent) separation, the duration of the reversed shear stress was found to be very short (0.002–0.007 s), suggesting a local, small-scale, impulse-type separation. At and beyond the location of intermittent separation, the shear stress reversal duration was an order of magnitude longer. Estimates of the maximum and minimum surface shear stress in the separation region were also obtained with the fence probe.

Surface/interface effects on elastic behavior of a screw dislocation in an eccentric core–shell nanowire

The elastic behavior of a screw dislocation which is positioned inside the shell domain of an eccentric core–shell nanowire is addressed with taking into account the surface/interface stress effect. The complex potential function method in combination with the conformal mapping function is applied to solve the governing non-classical equations. The dislocation stress field and the image force acting on the dislocation are studied in detail and compared with those obtained within the classical theory of elasticity. It is shown that near the free outer surface and the inner core–shell interface, the non-classical solution for the stress field considerably differs from the classical one, while this difference practically vanishes in the bulk regions of the nanowire. It is also demonstrated that the surface with positive (negative) shear modulus applies an extra non-classical repelling (attracting) image force to the dislocation, which can change the nature of the equilibrium positions depending on the system parameters. At the same time, the non-classical solution fails when the dislocation approaches very close to the surface/interface with negative shear modulus. The effects of the core–shell eccentricity and nanowire diameter on dislocation behavior are discussed. It is shown that the non-classical surface/interface effect has a short-range character and becomes more pronounced when the nanowire diameter is smaller than 20nm.

Reconstruction of magnetohydrodynamic equilibrium configurations with hollow current density profiles in the HL-2A tokamak

Magnetohydrodynamic equilibrium configurations with hollow current density profiles in the HL-2A tokamak are reconstructed for pellet injection discharges. The experimental data of pressure profiles, resulting from electron temperature measurements and a model density profile, analogous to that measured in the JET experiment, and Mirnov pickup coil signals are employed in the reconstructions. It is explicitly shown for the first time that configurations with hollow current density profiles may be realized by multiple pellet injection in HL-2A tokamak experiments. The reconstructed results show there exists a negative magnetic shear region in the plasma core which would benefit confinement improvement.

Relaminarization of turbulent channel flow using traveling wave-like wall deformation

Effect of traveling wave-like wall deformation (i.e. peristalsis) in a fully developed turbulent channel flow is investigated by means of direct numerical simulation. We not only demonstrate that the friction drag is reduced by wave-like wall deformation traveling toward the downstream direction, but also show that the turbulence is completely suppressed (viz. the flow is relaminarized) under some sets of parameters. It is also found that at higher amplitude of actuation the relaminarized flow is unstable and exhibits a periodic cycle between high and low drag. The drag reduction is caused by suppression of random Reynolds shear stress component due to stabilization. The quadrant analysis reveals that traveling wave-like wall deformation makes strongly negative random Reynolds shear stress in the converging region.

Strength and deformation characteristics of cement-mixed gravelly soil in multiple-step triaxial compression

When backfill is mixed with cement and compacted in the field, it is very difficult to obtain the multiple similar samples needed to determine a Mohr–Coulomb (M–C) failure envelope by a set of triaxial compression (TC) tests at different levels of effective confining pressure (σ′hs), due to the inevitably large heterogeneity. The effects of the intermediate loading histories, with and without large stress–amplitude unload/reload cycles, on the stress–strain properties of compacted cement-mixed well-graded gravelly soil, particularly the peak strength, were evaluated in multiple-step loading (ML) drained TC tests using a single specimen. A set of single-step loading (SL) tests at different σ′hs were also performed. Similar specimens, prepared in the laboratory, were used for the tests in this study. Although the M–C failure envelope can be quite accurately determined by relevant ML tests, the peak strength may be underestimated if (a) TC loading is ceased far before the peak stress state or (b) TC reloading is started after the peak stress state has been passed at previous loading steps. Cases (a) and (b) may take place in both ML tests, with increasing and decreasing σ′h, while Case (b) may take place in ML tests with decreasing σ′h. The reloading stress–strain relation at the intermediate stage in ML tests may become very soft due to the additional damage caused by a negative irreversible shear strain increment occurring in the immediately preceding unloading process. This effect gradually decreases during reloading, while it totally disappears once large-scale yielding has started, with essentially no effect on the peak strength. It is of the first priority to perform ML tests by increasing σ′h. It is recommended that ML tests be performed by decreasing σ′h if possible, because the peak stress may be closely reached at some intermediate steps. An upper envelope of results from a pair of ML tests, with increasing and decreasing σ′h, could be more representative of the true M–C failure envelope.

The Madden–Julian Oscillation in CCSM4

Abstract This study assesses the ability of the Community Climate System Model, version 4 (CCSM4) to represent the Madden–Julian oscillation (MJO), the dominant mode of intraseasonal variability in the tropical atmosphere. The U.S. Climate Variability and Predictability (CLIVAR) MJO Working Group’s prescribed diagnostic tests are used to evaluate the model’s mean state, variance, and wavenumber–frequency characteristics in a 20-yr simulation of the intraseasonal variability in zonal winds at 850 hPa (U850) and 200 hPa (U200), and outgoing longwave radiation (OLR). Unlike its predecessor, CCSM4 reproduces a number of aspects of MJO behavior more realistically. The CCSM4 produces coherent, broadbanded, and energetic patterns in eastward-propagating intraseasonal zonal winds and OLR in the tropical Indian and Pacific Oceans that are generally consistent with MJO characteristics. Strong peaks occur in power spectra and coherence spectra with periods between 20 and 100 days and zonal wavenumbers between 1 and 3. Model MJOs, however, tend to be more broadbanded in frequency than in observations. Broad-scale patterns, as revealed in combined EOFs of U850, U200, and OLR, are remarkably consistent with observations and indicate that large-scale convergence–convection coupling occurs in the simulated MJO. Relations between MJO in the model and its concurrence with other climate states are also explored. MJO activity (defined as the percentage of time the MJO index exceeds 1.5) is enhanced during El Niño events compared to La Niña events, both in the model and observations. MJO activity is increased during periods of anomalously strong negative meridional wind shear in the Asian monsoon region and also during strong negative Indian Ocean zonal mode states, in both the model and observations.

Formation of internal transport barriers in Globus-M tokamak in regime with early neutral heating beam switch-on

A series of experiments have been performed on the Globus-M spherical tokamak under conditions with additional plasma heating by a beam of neutral atoms at the stage of plasma current buildup. This regime allowed a region with negative magnetic shear to be formed in the inner plasma region adjacent to the magnetic axis. Under these conditions, the stability margin coefficient at the axis was greater than unity. This scenario of plasma heating leads to the transition to a regime of improved confinement with the formation of a broad temperature profile and peaked electron density profile due to the formation on internal barriers. Results of experiments and numerical simulations of transport processes using the ASTRA code are presented.

Constrained least-squares solution and regularization in inverse boundary element analysis of photoelastic models involving contact

Existing work by the present team has involved the development of an inverse boundary element approach for fitting unknown boundary conditions to full-field stress data obtained using photoelasticity, treating the displacements on the unknown region as the solution variables. However, the recovered boundary conditions sometimes show unrealistic features such as negative contact pressures or shear tractions which exceed realistic frictional values; noisy or incomplete input data make these problems worse and can make the results oscillatory. The work described here exploits available constrained least-squares techniques in order to express the contact conditions as linear inequality constraints upon the displacement solution. A variant of Tikhonov regularization is also used to penalize against oscillatory traction distributions on the unknown boundary. The chosen implementation, solved using the LSSOL algorithm, can handle the issue of rigid-body motion without the need for manual imposition of restraints. The technique is illustrated using simulated and real examples, demonstrating that the technique correctly reconstructs the individual principal stresses.

Solution structure and stability of the DNA undecamer duplexes containing oxanine mismatch

Solution structures of DNA duplexes containing oxanine (Oxa, O) opposite a cytosine (O:C duplex) and opposite a thymine (O:T duplex) have been solved by the combined use of 1H NMR and restrained molecular dynamics calculation. One mismatch pair was introduced into the center of the 11-mer duplex of [d(GTGACO6CACTG)/d(CAGTGX17GTCAC), X = C or T]. 1H NMR chemical shifts and nuclear Overhauser enhancement (NOE) intensities indicate that both the duplexes adopt an overall right-handed B-type conformation. Exchangeable resonances of C17 4-amino proton of the O:C duplex and of T17 imino proton of O:T duplex showed unusual chemical shifts, and disappeared with temperature increasing up to 30°C, although the melting temperatures were >50°C. The O:C mismatch takes a wobble geometry with positive shear parameter where the Oxa ring shifted toward the major groove and the paired C17 toward the minor groove, while, in the O:T mismatch pair with the negative shear, the Oxa ring slightly shifted toward the minor groove and the paired T17 toward the major groove. The Oxa mismatch pairs can be wobbled largely because of no hydrogen bond to the O1 position of the Oxa base, and may occupy positions in the strands that optimize the stacking with adjacent bases.

DNS study on mechanism of small length scale generation in late boundary layer transition

It has been widely accepted that small length vortices, a key mechanism for late flow transition and turbulence generation, are generated by “large vortex breakdown”. However, according to our recent DNS results, the hairpin vortex structure remains very stable and never breaks down. We also recognize strong multiple level negative and positive spikes and consequent multiple level high shear (HS) layers near the bottom of the boundary layer. From our DNS results, we found a second sweep of the first level rings gathering energy from the inviscid region downdraft and generating positive spikes and consequent HS layers. With the instability of the HS layer, new vortex rings and consequent new sweeps are generated. Then, the second level negative and positive spikes and high shear layers are formed near the laminar sub-layer. According to our observation, all small length scales (turbulence) are found around high shear layers generated by multilevel positive spikes and the wall surface, especially near the bottom of the boundary layer, but no small length vortices are found generated by “large vortex breakdown”. This paper provides detailed description and analysis for the energy transfer paths and small length scale (turbulence) generation and sustenance observed by our DNS.