Negative Shear Research Articles

Static Characteristics of Improved Composite Box Girder with Corrugated Steel Webs

In order to accurately calculate the cross‐sectional stresses and deflections of the improved composite box girder with corrugated steel webs, the control differential equations and boundary conditions are established based on the energy variational principle, taking into account the effects of corrugated steel web properties, shear lag, and shear deformation. A cantilever composite box girder is used as a numerical example. The influence of width span ratio and corrugated steel web bending angle on the shear lag and fold effect is analyzed. The results show that the analytical solution is in good agreement with the finite element solution. A shear lag effect and a negative shear lag effect exist in the cantilever composite box girder under uniform load. The fold effect under concentrated load has a greater influence on the mechanical properties of the wing plate than under uniform load. With the increase of the width span ratio, the effects of both shear lag and fold effect increases. With the increase of the corrugated steel web bending angle, the fold effect is obviously enhanced, but the change of the bending angle has little effect on the shear lag effect.

Optimisation of intra-ply stitch removal for improved formability of biaxial non-crimp fabrics

Automated fabric forming solutions are required to meet the demand of liquid moulding processes, but wrinkling is a common problem for double-curvature parts due to a combination of the reinforcement type, manufacturing parameters and the part geometry. Local intra-ply stitch removal is introduced in the current work to improve the formability of a pillar-stitched biaxial NCF. An optimisation method is developed to remove stitches selectively, using a genetic algorithm coupled with a finite element model. Two criteria are defined to reduce the occurrence of forming defects whilst maintaining the integrity of the fabric. The first is to minimise the local shear angle across the surface of the ply and the second is to minimise the total stitch removal area. These criteria are combined into a single objective function and validated using a hemisphere forming case study. Experimental results confirm that macro-scale wrinkling can be successfully eliminated when intra-ply stitches are removed according to the optimised pattern. The stitch removal regions are distributed across both the positive and negative shear areas of the optimised NCF blank, indicating that local stitch removal can have a global effect on the formability. Perimeter shapes show that the optimum local stitch removal pattern enables a more balanced global material draw-in, demonstrating that the effect of stitch removal is not limited to the high shear regions. Removing stitches from just the over-sheared regions is therefore insufficient to fully mitigate wrinkles, justifying the need for the optimisation algorithm, as the optimised stitch removal pattern appears to be non-intuitive.

Research on shear lag effect of cantilever concrete box beam with double chambers

In this paper, the authors used finite element method to investigate the longitudinal and transverse shear lag effects and the effect of aspect ratio on the shear lag of double-chamber cantilever concrete box girder under uniform distributed load. The results show that the root section is positive shear lag, and the mid-span section is negative shear lag. In the longitudinal direction, positive and negative shear lag changes appear in box girder. The effect of shear lag at fixed end is significantly affected by aspect ratio, and the effect degree is different in different parts. The positive and negative values of shear lag must be determined by the stress curve.

Phase field crystal simulation of strain-induced square phase low-angle symmetric tilt grain boundary dislocation reaction

In this paper, the phase field crystal method is used to study the dislocation motion and reaction of the square phase symmetric tilt low-angle grain boundaries, and the dislocation configurations with different misorientation angles are analyzed under the action of applied strain. The geometric phase approach is used to characterize the strain field around the dislocations. The results show that after the solidification relaxation, the interfacial dislocations on both sides of the grain are distributed in parallel but opposite direction. With the increase of misorientation angle between grains, the number of dislocations increases, the spacing between them decreases, and the free energy of the system increases. Imposed by the applied strain, the grain boundary dislocations undergo climbing, launching, and reactive annihilation, with the free energy fluctuating. When the misorientation increases, the dislocation motion mode changes from climbing to climbing-sliping, resulting in more dislocation group configurations, and more reactions between dislocations and dislocation groups. For the dislocation reactions of different configurations, positive shear strain drives dislocations to approach, and negative shear strain drives dislocations to annihilate.

Examining the Wind Shear Theory of Sporadic E With ICON/MIGHTI Winds and COSMIC‐2 Radio Occultation Data

AbstractThe wind shear theory is widely accepted as an explanation for the formation of a sporadic E (Es) layer, but the direct comparison of Es with the local wind shear has been limited due to the lack of neutral wind measurements. This study examines the role of the vertical wind shear for Es, using signal‐to‐noise ratio profiles from COSMIC‐2 radio occultation measurements and concurrent measurements of neutral wind profiles from the Ionospheric Connection Explorer. It is observed that the Es occurrence rate and average S4 index are correlated with the negative vertical shear of the eastward wind, providing observational support for the wind shear theory. Es can be observed even when the vertical wind shear is positive, which is interpreted as metallic ion layers generated at an earlier time.

Shear resistance of a novel wet connection for prefabricated composite beams under shear-bending coupling loading

This paper proposed a new wet connection joint applied to the prefabricated composite beam, which connecting the precast concrete slabs with full depth and the precast steel beam with the shear studs by placing the short transverse reinforcements into the reserved slots and post-pouring the concrete. A series of push-out experiments including 25 specimens was conducted to evaluate the longitudinal shear performances of this new wet connecting form under the coupling of shear loading and negative moment. Results showed that this new wet connecting method also presented good composite action compared to the cast-in-situ steel-concrete composite beam with the same design details. Three parameters including the stud diameter, the post-cast concrete strength and the short transverse reinforcement ratio were primary factors affecting the shear resistance and failure mode of prefabricated composite beams. Due to the plastic development of short transverse reinforcements placed in the reserved slots, the loading-slip curve corresponding to the concrete splitting failure presented a slow failure trend. Considering the combination of the negative moment and shear loading, the use of upper layer transverse reinforcement also could limit the cracking of the post-cast concrete. In addition, post-pouring the high-performance concrete could further strengthen the shear capacities of the wet-connecting region and stud connectors but restrict the ductility development. Finally, the formulas describing the loading-slip curve and calculating the shear strengths were optimized respectively.

Effects of scour-hole dimensions and bulb positions on the lateral response of under-reamed pile in soft clay

The failures of pile foundations in water-borne structures are often attributed to the scouring phenomenon. Even though many studies are available on the countermeasures against scour around the pile, no attempt is made to increase the load-carrying capacity of pile foundation by changing the pile geometry. Therefore, in the present study, the effect of scouring on the lateral response of a single under-reamed pile in soft clay is investigated using three-dimensional numerical analysis. The variables considered in the present study are embedment ratio and bulb position of the under-reamed pile, depth, slope angle, and bottom width of the scour-hole. For a given scour-hole dimension, the lateral load-carrying capacity of the under-reamed pile increases; whereas, deflection, maximum negative shear force, and bending moment in the shaft portion reduce compared to the pile without bulb. A notable improvement in the pile response is found when the bulb is placed just below the scour-hole. The lateral load-carrying capacity of the under-reamed pile reduces with an increase in depth and bottom width of scour-hole from the pile surface and a decrease in slope angle of scour-hole. Pile responses reduce with the consideration of stress history due to excavation.

Alfvén Modes in the Plasma of T-15MD and TRT Tokamaks with a Negative Magnetic Shear

Alfvén Modes in the Plasma of T-15MD and TRT Tokamaks with a Negative Magnetic Shear

The evolution of local stress during deformation twinning in a Mg-Gd-Y-Zn alloy

In this study, we perform in-situ tensile testing and high-resolution electron backscatter diffraction (HR-EBSD) to monitor the twin local stresses at different stages: (i) before twinning, (ii) twins that have propagated into the parent crystal, and (iii) thickening of the twin lamella. The resolved shear stress (RSS) at the twin interface is calculated during deformation twinning, which indicates that the RSS evolution is a dynamic process. Negative shear stress is observed during the propagation stage, which provides evidence for the twinning shear transformation-induced stress reversal. Furthermore, a local Schmid factor (LSF) is used to evaluate the extent to which the local stress is dominated by the RSS on the identified twin plane. The experimental results indicate that the LSFs evolve with strain and are at their maximum level before twinning. The initiated twins are correlated with the highest LSFs compared with the unactivated variants. In addition, the product of the Luster Morris m′ value times the LSF of the twin variant may reveal a threshold for the slip-induced twinning process.

Influence of hydrological communication between basement-rooted faults and hydraulic fractures on induced seismicity: A case study

The mechanical conditions that allow seismogenic faults to be activated in distinctive positions and moments remain uncertain. An integrated approach is proposed to quantify the hydrological connection between stimulated wells and associated seismogenic faults during fracturing stimulation. A case study in Fox Creek, Alberta, was conducted to demonstrate the applicability of this integrated approach. Ant tracking was used to identify the subvertical basement-rooted faults from a three-dimensional seismic reflection survey. A coupled flow–geomechanics simulation was conducted to quantify the well–fault hydraulic connection in terms of pore pressure diffusion and poroelastic stress perturbation during fracturing stimulation. Three identified basement-rooted faults were corroborated by the focal mechanisms and spatial distribution of induced siemsicity events. The negative shear stress gradient indicates the downward shear growth during hydraulic connections. The induced earthquakes were triggered by fluid diffusion through hydraulic fractures along high-permeability fault damage zones downwards into the basement. This basal fault slip was attributed primarily to the elevated pore pressure along the fault plane in response to fracturing fluid injection. The moderate distance (879 m for NS-oriented wells and 749 m for NW-SE-oriented wells) between the future horizontal wellbores and critically-stressed faults could mitigate the effects of well-fault hydraulic connection and reduce the seismicity risks.

Influence of circulating energetic ions on MHD instabilities in negative magnetic shear plasma

The influence of circulating energetic ions (CEIs) on double tearing mode (DTM) is investigated by a global kinetic-MHD hybrid simulation. It is shown that the co-CEIs with a small orbit width have a destabilizing effect on the DTM. As the orbit width increases, this destabilizing effect is reduced. If the orbit width is large enough, the DTM is transformed to single tearing modes due to the co-CEIs, so the mode becomes more stable than the DTM without CEIs. The effect of the counter-CEIs with small orbit width is similar to that of the co-CEIs. For a large orbit width, the counter-CEIs also have a stabilizing effect. If the energetic ion beta is larger than a threshold, an energetic particle driven mode (EPM) is excited due to the resonance between counter-CEIs and the bulk plasma. The effects of energetic ion beta, orbit width and resistivity on this EPM are studied systematically. The onset threshold of EPM depends on the orbit width and resistivity. It decreases with orbit width increasing, while it increases with resistivity increasing. The mode transition from DTM to EPM with energetic ion beta increasing is also found for large orbit width.

Effects of clay content on the shear behaviors of sliding zone soil originating from muddy interlayers in the Three Gorges Reservoir, China

The evolution of sliding zone soil originating from a muddy interlayer in the Three Gorges Reservoir (TGR) in China is accompanied by an increase in clay content, and the subsequent changes in its mechanical properties control the initiation or reactivation of large-scale bedrock landslides. In this paper, using sliding zone soil samples from the Xingfusi landslide in the Wanzhou area, two groups of remolded saturated specimens were prepared with clay contents of 25.0% and 12.5%. Direct shear creep tests and a series of ring shear tests were carried out to investigate the shear behaviors of the specimens with different clay contents. On this basis, the effect of clay content on the evolution of landslides was discussed. The results show that (1) during the long-term creep process before landslide initiation, the shear creep modulus of sliding zone soil in steady creep state significantly decreases with an increase in clay content, which results in the increase in cumulative creep displacement. (2) The increase in clay content in sliding zone soil will lead to a reduction in peak shear strength and residual frictional resistance. In the drained shearing on saturated sliding zone soil with a high clay content, the excess pore-water pressure on the sliding surface cannot dissipate as rapidly due to the small permeability of sliding zone soil with a high clay content, which may favor postfailure movement. (3) With an increase in shear displacement rate, the residual strength first shows a weak “negative shear rate effect” and then shows a significant “positive shear rate effect”. However, with an increase in clay content, the initial shear rate for the residual strength showing a “positive rate effect” will decrease. (4) Sliding zone soil with a high clay content generally does not exhibit strength recovery, while sliding zone soil with a low clay content clearly recovers its peak residual strength due to volume change of the shear zone caused by secondary compression after a long period of reconsolidation. In this case, a greater external load or reduction in effective normal stress needs to be applied to reactivate the landslide.

Investigating Time Dependent Stress Changes Globally Following Large Earthquakes (M≥7)

Triggered earthquakes can cause disproportionate damages depend on their magnitudes. In fact, there is a causal link between the spatial distribution of those events and the stress changes induced by the mainshock. Co-seismic stress loading is one of the key factors in determination of triggering mechanism. However, the time lags ranging hours to years and the stress diffusion over wider areas cannot be evaluated with the co-seismic process alone. In some cases, the stress interactions for long periods and larger areas can be attributed to post-seismic viscoelastic relaxations. In this study, M≥7 earthquakes from the Global Centroid Moment Tensor (GMCT) catalogue are modelled as dislocations to calculate shear stress changes on following earthquake nodal planes near enough to be triggered. The catalogue scanned for all other events (4.5<M<7) that occurred within ±2° from the centroid rupture planes. According to Omori law, which is one of the most reliable time predictable diagram of aftershock distributions, 10-year periods were used for the stress calculations. The events that had computed within ±0.01 to ±1 MPa stress change limits, considered as potential triggered events. The global co-seismic stress calculations show that 60.6% of the triggered events occurred in regions where the shear stress increased. The global stress change by incorporation viscous flow into co-seismic stress change were also tested. In this case, an increase in the rate of triggered events in both positive (15%) and negative (9%) shear stress areas were obtained. Despite the rate of triggered events has climbed significantly in both areas, only 2% of the changes have been computed globally. These rates are highly depending on fault mechanism across the plate boundaries. Thrust faults in the catalogue, for example, influence the stress distribution over broader regions and the dimension of fault ruptures. Therefore, thrust faults in the catalogue prevails the global statistics for both co-seismic and viscoelastic stress calculations. The results also demonstrate the significant effect of viscous flow, following large earthquakes, which cannot be neglected in stress interaction analysis.

Negative interlayer shear effect on a double-layered van der Waals material resonator

In sharp contrast to the previous view that interlayer shear enhances bending stiffness of van der Waals material, here we report a decreasing bending stiffness and natural frequency in double-layered ${\mathrm{MoS}}_{2}$ (${\mathrm{DLMoS}}_{2}$) nanoresonators with special stacking orders. Further theoretical analysis shows that the decreasing frequency is mainly due to the negative interlayer shear modulus in ${\mathrm{DLMoS}}_{2}$ nanoresonators with some stacking orders and twisting angles. Detailed analysis of the molecular structure reveals the essential roles of the interfacial potential in the interlayer shear modulus. The present study provides valuable physical insights for understanding the mechanical behaviors of double-layered van der Waals material resonators.

Investigation on the Exit Burr Formation in Micro Milling.

The burr on micro part has harmful effect on the dimensional accuracy and service performance. The original control of exit burr formation during micro milling is desirable and advisable. In this paper, the formation mechanism of exit burr was studied based on the varying cutting direction during micro milling. Three exit burr control strategies were concluded, the material properties embrittlement, the support stiffness increasing and machining parameter optimizing operations. Then, micro milling experiments were carried out to investigate the exit burr morphology and size. It was found that the exit burr formation was attributed to the change of material flowing path at the exit surface, which was caused by the negative shear deformation zone that was induced by the discontinuous shape features. Different exit burr morphologies were classified; the triangle exit burr type was caused by the varying exit burr growing direction along the exit surface. The optimal machining parameters in micro milling to obtain a small exit burr were suggested.

Mechanical behavior and constitutive relation of the interface between warm frozen silt and cemented soil

To study the shear mechanical properties and deformation mechanism of the interface between cemented soil and warm frozen soil, the key parameters are provided for the design and evaluation of cemented soil structure in warm frozen soil ground. In this study, a series of negative temperature direct shear tests of the cast-in-place cemented soil-frozen soil interface were conducted under the influence of different factors. The binary medium model was used toinvestigate the mechanical stress and deformational mechanisms of the interface, and the damage rate and stress sharing rate functions were introduced. Assuming that the strength of each element of the interface obeyed the Weibull probability distribution, the constitutive equation of the warm frozen soil-cemented soil interface was established. The results showed that the stress-displacement curve of the interface was a strain-softening type, and the entire deformation process of the interface was divided into four stages: the linear elastic, elastic-plastic, softening, and residual stability stages. The constitutive model based on the binary medium theory revealed the micro stress–strain mechanism of the interface and better described the entire stress–displacement process of the interface and the strain softening phenomenon, which can provide a basis for the numerical simulation and theoretical calculation of the structure in the frozen soil ground.

Study of Bearing Model on the Internal Forces of the Wide Bearing Continuous Beam

The standingpile has been applied quite extensive in our country, which is one of the three major structure forms of the quay. Take the internal force calculation of wharf track beam in Jiangsu Taizhou for example, this paper studied the bar system finite element method processing method of rigid support and elastic support. Using different bearing support mode to calculate the track beam internal force, the results show that the corresponding internal force of different models has big differences. The negative bending moment and shear force of point support model are large. The effect of support width and elastic support was considered in the continuous beam model of elastic distribution supporting and should be considered prior to other supporting. In the process of similar design, the bearing width can be increased. This is conducive to the uniform distribution of internal force, and make the structure more reasonable.

Time-domain investigation of an external cloak for antiplane elastic waves

Space folding techniques based on non-monotonic transforms lead to a new class of 2D isotropic cloaks with a constant negative shear modulus and a spatially varying negative density for antiplane elastic waves. We consider an external cloak consisting of a core with positive shear modulus and density, and a shell with simultaneously negative shear modulus and density. Such a core–shell resonant system creates a virtual folded region outside the shell. To handle such negative physical parameters in the time-domain, a two-step strategy is used: (i) assuming resonant (Drude and Lorentz-types) effective parameters in the frequency-domain; (ii) returning to the time-domain by applying the formalism of the auxiliary fields. We numerically show that, at the designed central frequency, scattering of a cylindrical antiplane elastic wave incident upon a finite set of small clamped obstacles is drastically reduced after a lapse of time, when they are placed in the close neighborhood of the external cloak. However, cloaking efficiency decreases when the source is placed in the intense near field of the external cloak and moreover at short times, cloaking breaks down.

Nonlinear Simulations of Gravity Wave Tunneling and Breaking over Auckland Island

AbstractHorizontally dispersing gravity waves with horizontal wavelengths of 30–40 km were observed at mesospheric altitudes over Auckland Island by the airborne advanced mesospheric temperature mapper during a Deep Propagating Gravity Wave Experiment (DEEPWAVE) research flight on 14 July 2014. A 3D nonlinear compressible model is used to determine which propagation conditions enabled gravity wave penetration into the mesosphere and how the resulting instability characteristics led to widespread momentum deposition. Results indicate that linear tunneling through the polar night jet enabled quick gravity wave propagation from the surface up to the mesopause, while subsequent instability processes reveal large rolls that formed in the negative shear above the jet maximum and led to significant momentum deposition as they descended. This study suggests that gravity wave tunneling is a viable source for this case and other deep propagation events reaching the mesosphere and lower thermosphere.

Impurity effects on trapped electron modes in tokamak plasmas with inverted electron density profile

Impurity effects on trapped electron modes (TEMs) in tokamak plasmas with inverted electron density profile (IEDP) are numerically investigated with a gyrokinetic integral eigenmode equation. It is found that different from the negative gradient of normal electron density profile, the positive gradient of the IEDP has a stabilizing effect on TEM in the presence of impurity ions. The electron temperature gradient threshold for TEM excitation increases not only with the increasing absolute value of IEDP but also with increasing impurity content. Furthermore, the effects of different impurity species and different impurity peaking profiles on TEMs with the IEDP are analyzed in detail. It is shown that there is a transition point of impurity density profile, on both sides of which the impurity has opposite effects on TEM. The dependence of such a transition point on electron temperature and density gradients is obtained numerically. Besides, the synergistic effects of ion temperature gradient and impurity density gradient are studied, in which a similar transition point of the ion temperature gradient is also identified in the case of outwardly peaked impurity density profile. In addition, impurity effects on the characteristics of mode structure and on the radial transport coefficients in positive and negative magnetic shear regions are discussed as well based on quasi-linear mixing length estimation.