Smooth Shear Research Articles

Shear resistance of assembled bentonite interface after confined water saturation and interfacial self-healing capacity

The requisite functions of a bentonite buffer in a deep geological repository depend on the sealing/healing of bentonite interfaces, with particular emphasis on the self-healing (automatic healing upon wetting) of assembled bentonite-bentonite interfaces. This study determined the shear resistance (including the peak shear strength and secant modulus) of densely compacted Gaomiaozi (GMZ) bentonite and its assembled interface after confined water saturation. The effect of bentonite dry density and saturation time on the shear resistance of saturated healed interfaces was elucidated, and the interfacial self-healing capacity was assessed. The results indicate that the shear resistance of the saturated healed interfaces increased with the bentonite dry density but had a non-monotonic correlation with the saturation time. For a given dry density of the bentonite, the saturated healed interface exhibits a lower peak shear strength than the saturated intact bentonite but a higher peak shear strength than the saturated separated interface. The saturated healed and separated interfaces have comparable shear moduli (secant moduli), which are lower than that of the saturated intact bentonite. The saturated healed interfaces display smooth shear failure planes, while the saturated assembled interfaces and intact bentonite exhibit comparable frictional angles. This indicates that interfacial self-healing plays a pivotal role in enhancing interfacial peak shear strength by facilitating microstructural bonding at the assembled interface. Finally, it can be stated that densely compacted GMZ bentonite has a robust interfacial self-healing capacity in terms of shear resistance. These findings contribute to the design of the bentonite buffer and facilitate the evaluation of its safe operation at specified disposal ages.

Mechanism of chip segmentation transition from shear slip to shear fracture of Zirconium-based bulk metallic glass in mechanical machining

An in-depth understanding of material cutting deformation forms the foundation for manufacturing Zirconium-based bulk metallic glass (Zr-based BMG) parts. This study explores the transition in chip segmentation mode from shear slip to shear fracture in Zr-based BMG during mechanical machining. The investigation covers chip micromorphology, cyclic cutting force, machined surface quality, and the themo-mechanical properties under various chip segmentation modes linked to cutting conditions. A size-dependent transition in chip segmentation has been observed in the cutting of Zr-based BMG. At a specific cutting speed, a smaller uncut chip thickness favors chip formation via shear slip, resulting in a smooth primary shear zone (PSZ). Conversely, a larger uncut chip thickness leads to chip shear fracture, with the PSZ displaying fishbone-like and dimple patterns. The integration of the modified cutting kinematic model with the free volume-dependent shear transformation zone (STZ) model suggests that the transition from shear slip to fracture is due to an increase in uncut chip thickness, which causes a greater STZ volume in the PSZ. When the STZ volume surpasses a threshold, the intrinsic structural recovery of material from shearing cannot fully compensate for the free volume softening, leading to excessive free volume that ultimately triggers shear fracture. During the chip shear fracture process, the evolution from fishbone-like patterns to dimples in the PSZ is attributed to faster crack propagation at greater uncut chip thickness or higher cutting speeds. In practical applications, it is advisable to avoid chip shear fracture during machining to maintain high-quality machined surface quality.

Comparison of hole expansion ratios in a dual phase steel for holes prepared by conventional punching and fine piercing

Dual phase steels generally exhibit poorer stretch flangeability compared to lower strength steels. Stretch flangeability is normally evaluated using hole expansion testing, wherein a specimen with a 10 mm central hole is expanded using a conical punch till onset of cracking at the edge of the hole. The percentage increase in hole diameter is referred to as the hole expansion ratio (HER). In this study, hole expansion ratios of a DP 600 steel were evaluated. Holes were prepared on a standard 90 mm x 90 mm specimen using two different hole preparation conditions: i) conventional punching where the hole edge exhibits both fracture and shear regions and ii) fine piercing where the hole edge exhibits almost completely smooth shear. Hole expansion testing was carried out on these specimens as per standard ISO 16630. The specimens with holes produced by fine piercing exhibited lower values of HER in spite of having a hole edge with a smooth surface. The specimens with holes prepared by conventional punching exhibited higher HER values. The influence of strain hardening of the cut edge during hole preparation and the surface roughness of the cut edge on the HER are discussed.

The contact mechanics of a UK railway ballast

A newly developed apparatus was used to investigate the contact mechanics between particles of a common UK railway ballast. The data were then compared with the models currently and commonly used in geotechnical discrete-element method (DEM) analyses, but the discrepancy between the predictions and measurements is large even at small loads. The first contact behaviour was much softer than the Hertz–Mindlin model for smooth spheres, both in normal and shear. Even if the normal loading could be fitted better with a model that accounted for roughness, the elastic nature of these models could not capture the plasticity that is evident on unloading. Large displacement shear cycles caused not only an increase of inter-particle friction, as discussed previously, but also a significant degree of particle interlock arising from the wear of the contacts. While there were no rate effects for sliding shearing, contact ageing displacements could be observed at pre-failure loads, although these stabilised after a few days. Pre-failure cyclic loading resulted in stiffnesses that increased in lateral loading but decreased in normal loading; in both cases, however, this stabilised after a few tens of cycles, while the stiffnesses at the reversals of the large displacement cycles did not evolve with continued cycling. The presence of water did not affect the lateral stiffness and the influence of the normal load was consistent with Hertz–Mindlin, even if the absolute values were much softer. It can be concluded that the current commonly used models for DEM analyses are not applicable for ballast/crushed rock and alternative models should also focus on plasticity at the asperity scale.

Dimensionality reduction for regularization of sparse data-driven RANS simulations

Data assimilation can reduce the model-form errors of RANS simulations. A spatially distributed corrective parameter field can be introduced into the closure model, whose optimal values can be efficiently found by an adjoint method and gradient-based optimization. When assimilating experimental data, which in most cases are sparsely distributed or based on a low-resolution grid, the inverse problem is highly underdetermined and thus ambiguous. Therefore, to reduce the ambiguity, regularization is required. Established regularization approaches such as total variation and Sobolev gradient methods can produce smooth and physically meaningful velocity fields in many cases. However, if the measurements are located close to walls, spiky and unphysical wall shear stress profiles can occur. A new regularization strategy based on a piecewise linear approximation of the corrective field is proposed. This method is shown to lead to a very accurate free stream velocity field and smooth wall shear stress profiles. The resulting skin friction drag error for the case of flow over periodic hills was around 1.38% which is seven times lower than the error obtained with the Sobolev gradient and two orders of magnitude lower than that obtained with the other two methods.

Effects of shearing processing on the quality and magnetic properties of electric steel sheet

Mechanical manufacturing process of electric motor induces a residual stress, which increases the iron loss, and affects the efficiency and performance of machine. In this paper, a shear test was carried out on a non-oriented electrical steel sheet with a thickness of 0.50 mm and a relative clearance range of 2–12% of the thickness, and the shear force was detected during shearing process. The sheared surface morphology, microhardness distribution and magnetic properties after the shearing processing were also tested. The obtained experimental results indicate that: as the shear clearance increases, the shear force increases, the plastic deformation zone enlarges, and the flow of microstructure becomes intense. In addition, as the shear clearance increases, the depth and work-hardening degree of the work-hardened layer increase, resulting in degraded magnetic properties of the electrical steel. Moreover, when the lateral clearance is 5%, a complete and smooth shear section can be obtained.

Shear behaviour of seawater sea-sand coral aggregate concrete beams reinforced with FRP strip stirrups

The preparation of concrete for remote islands or reefs using locally available seawater, sea sand, and coral aggregate can greatly shorten the construction period and mitigate the shortage of river sand and natural gravel resources. This study is the first to experimentally investigate the shear behaviour of seawater sea-sand coral aggregate concrete (SSCAC) beams reinforced with non-corrosive carbon fibre-reinforced polymer (CFRP) strip stirrups. The test parameters included concrete strength, concrete type, and stirrup configuration. The experimental results suggested that CFRP-reinforced SSCAC beams exhibited a brittle shear failure mode because of the sudden CFRP rupture at the bend portions. The ratio of the measured maximum strain to the ultimate tensile strain of the CFRP strip stirrups varied from 36.6% to 47.8%. The critical diagonal shear cracks of SSCAC beams directly penetrated through the coral aggregates and split the aggregates, whereas those in the natural aggregate concrete (NAC) beams bypassed the natural gravel. In this regard, the SSCAC beams exhibited approximately 10% lower shear strength than their NAC counterparts owing to the loss of the aggregate interlocking effect after the occurrence of smooth diagonal shear cracks. The higher the strength grade of SSCAC, the greater the shear strength and service load.

Cerebral multi-autoregulation model based enhanced external counterpulsation treatment planning for cerebral ischemic stroke.

Enhanced external counterpulsation (EECP) treatment for cerebral ischemic stroke patients with differing severity of stenosis, is subject to uncertainties due to the varying effects of the cerebral autoregulation mechanism on haemodynamics. The current study reports the development of a cerebral multi-autoregulation (MR) mathematical model, based on cerebral arteriole regulation of neurogenic, vascular smooth muscle reflex and shear stress mechanisms which takes into account the severity of stenosis. The model was evaluated by comparison to authentic clinical measurements of cerebral autoregulatory efficiency. Then it was applied to a 0D/3D geometric multi-scale haemodynamic model of a cerebral artery. Haemodynamic indicators were calculated under different pressurization durations of EECP to evaluate the efficacy for different stenosis lesions. Moderate stenosis of 50% to 60% produced excessive time-averaged wall shear stress in the distal area of the stenosis (>7 Pa) during prolonged pressurization and may result in damage to vascular endothelial cells. However, prolonged pressurization did not result in haemodynamic risk for severe stenosis of 70% to 80%, indicating that the duration of pressurization may be extended with increasing severity of stenosis. The current MR model accurately simulated cerebral blood flow and has relevance to the simulation of cerebral haemodynamics in a clinical setting.

Additive manufacturing of B4C‐W‐based composites via fused deposition molding using highly filled granular feedstocks

AbstractThe miniaturization and mobility of nuclear reactors have become an important trend in the development of nuclear energy. In order to simplify the design of shielding materials with improved complexity and reduced weight, 3D B4C‐W‐based composites were fabricated via fused deposition molding using highly‐filled granular feedstocks containing 62 vol% B4C‐W powders (boron carbide accounted for 30 wt% and tungsten for 70 wt%) and 38 vol% polymer binders (60 wt% Carnauba wax, 22 wt% polypropylene, 13 wt% polystyrene, and 5 wt% stearic acid). The rheological properties and microstructure of the feedstock and extruded filaments were clarified. Roles of the printing parameters including extrusion temperature, platform temperature and deposited‐layer height in the morphology of 3D composites were investigated in detail. Extruded filaments with good shape retention, dense fracture surface, and uniformly dispersed B4C‐W grains were achieved, benefiting from the smooth printing and shear thinning behaviour of the feedstock. Defects including warping, small pores or stacking pores could be formed under improper printing parameters, owing to the poor bonding strength between deposited layers induced by thermal internal stress or decomposition of wax. 3D composites with large size of 130 × 130 × 5 mm were fabricated, which showed a satisfactory compressive strength of 34.8 MPa. This work showed a facile route to fabricate 3D radiation shielding materials based on highly‐filled polymers.

Optimisation and numerical simulation of shearing blade used for citrus seedling grafting

Manual grafting for citrus seedlings is still the most common grafting technique, which is a labour-intensive and time-consuming procedure. This study investigated the effect of blade geometry and operating parameters, including bevel angle, slide cutting angle, blade thickness, and loading rate on the shear quality, shear force and shear energy of citrus rootstock seedlings. Box-Behnken design was employed to conduct experiments and explore the interaction effect of selected factors on indicators. The optimal structure and operating parameters were determined to be 285 mm min −1 for loading rate, 27° for bevel angle, 50° for slide cutting angle, and 2.83 mm for blade thickness. Under these conditions, the predicted values of the evaluation indices were determined to be 66.70% for percentage of smooth shear without splitting failure, 747.98 N for average value of the maximum shear force and 55.48 mJ mm −2 for specific shear energy. Based on the optimal parameters, with the help of reverse modelling method, a numerical simulation model was established to simulate the shearing process and analyse the stress distribution in the stalk and blade. The maximum effective stresses of the rootstock specimen and blade in the shearing process were about 28.11 and 229.82 MPa, respectively. This study indicates that optimisation of operational parameters of shearing blade will result in improving shear quality and saving significant energy. The results are beneficial to practical applications in design and development of shearing device for future citrus grafting robots. • Shearing blade of citrus rootstock was optimised by using Box-Behnken design. • Quantifying damage level of citrus rootstocks to evaluate shear quality of blade. • Establishing rootstock shearing model by using reverse modelling method. • Stress distribution in rootstock and blade were predicted through co-simulation.

Comparative Study of Marine and Lacustrine Shale Reservoirs from the Viewpoint of Rock Mechanics

Marine and lacustrine shales are two major important types of organic-rich shales that are usually deposited in the stable ocean basin and deep lake/swamp settings, respectively. In the past decade, commercial development of marine shale gas resources has been achieved. The underdeveloped lacustrine shale also showed great oil and gas potential. Is it feasible to simply copy the development mode of marine shale gas to lacustrine shale? This is what this paper would attempt to answer from the viewpoint of rock mechanics. Using borehole cores from two typical marine and lacustrine shale gas wells, a series of experiments, including X-ray diffraction (XRD), triaxial compression, fracture surface three-dimensional (3D) scanning, and scanning electron microscopy (SEM), were implemented to comprehensively reveal the mechanical difference between marine and lacustrine shales. Postfailure fracture complexity and surface roughness were quantified by fractal dimension and morphology reconstruction, respectively. A new index BInew, considering the influences of the complete stress–strain curve and fracture characteristics, was proposed to evaluate the brittleness of these two types of shales. Results showed that the mechanical properties of marine and lacustrine shales were quite different and should be exploited by following their own characteristics. Mechanical parameters of lacustrine shale, such as compressive strength, Young’s modulus, cohesive strength, and internal friction angle, were much lower than those of marine shale. Different from the typical shear plus bedding splitting failure mode of marine shale, lacustrine shale usually formed only one smooth shear fracture. Based on the newly proposed index, the brittleness of lacustrine shale was significantly lower than that of marine shale. Huge discrepancy in quartz and clay mineral contents and the consequent petrophysical structure divergence were the reasons for the marked mechanical difference between marine and lacustrine shales. Differentiation strategies and effects of hydraulic fracturing for these two kinds of formations were also discussed.

Scour modeling based on immersed boundary method: A pathway to practical use of three-dimensional scour models

Local scour around structures is an important hydraulic phenomenon that involves multiple coupled physical processes, including turbulent flow, sediment transport, and the evolution of erodible bed. This paper presents a new three-dimensional (3D) scour model, ibScourFoam, which is based on an immersed boundary method. This model is capable of dealing with scour around complex structures and thus it paves the way for practical use of 3D scour models. A special wall function has been developed to overcome the non-smoothness problem of wall shear stress in previous immersed boundary methods. Smooth and accurate wall shear is of critical importance for successful simulation of scour. The model was validated against cases reported in the literature and one flume experiment conducted in this work. The validation case of flow and scour around a vertical pile was reproduced well by our model. The flume experiment is for scour around a partially-buried, horizontal cylinder placed on a foundation. The horizontal cylinder is of finite length and the scour is initiated by flow contraction. The model revealed the distinct stages of scour in the experiment. The complex process of undermining and exposure of initially buried cylinder and foundation was captured well by the model.

Short-term variations of surface magnetism and prominences of the young Sun-like star V530 Per

Aims. We investigate magnetic tracers in the photosphere and the chromosphere of the ultra-rapid rotator (P ~ 0.32 d) V530 Per, a cool member of the open cluster α Persei, to characterize the short-term variability of the magnetic activity and large-scale magnetic field of this prototypical young, rapidly rotating solar-like star. Methods. With time-resolved spectropolarimetric observations spread over four close-by nights, we reconstructed the brightness distribution and large-scale magnetic field geometry of V530 Per through Zeeman-Doppler imaging. Simultaneously, we estimated the short-term variability of the surface through latitudinal differential rotation. Using the same data set, we also mapped the spatial distribution of prominences through tomography of Hα emission. Results. As in our previous study, a large dark spot occupies the polar region of V530 Per with smaller, dark, and bright spots at lower latitudes. The large-scale magnetic field is dominated by a toroidal, mostly axisymmetric component. The maximal radial field strength is equal to ~1 kG. The surface differential rotation is consistent with a smooth Sun-like shear dΩ = 0.053 ± 0.004 rad d−1, close to the solar shear level. The prominence pattern displays a stable component that is confined close to the corotation radius. We also observe rapidly evolving Hα emitting structures, over timescales ranging from minutes to days. The fast Hα evolution was not linked to any detected photospheric changes in the spot or magnetic coverage.

Parametric study of aluminum bar shearing using Johnson-Cook material modeling

Previous studies of the shearing process demonstrated that clearance and shear rate are the most influential parameters on the geometry of sheared billets. This paper illustrates a parametric numerical study of the impact of these parameters on the quality of the shear surface using the finite element simulation of shearing. In order to account for interactions between stress state evolution and the associated heating during shearing, a fully coupled thermo-mechanical simulation method was adopted. The influence of stress state, strain rate, and temperature on the material behavior were taken into account by using Johnson-Cook plasticity and ductile failure models. Many simulations were carried out involving diverse shear rates and shear clearances. The relationship between the parameters of shear surface geometry and the temperature was illustrated and proven. Contrary to the expectation of high-speed shearing performance, a burr free smooth shear surface was found using a low shear rate. This study illustrates a numerical strategy to determine the best shear clearance-rate set for aluminum alloy Al7075-T6 bars that minimizes the shear surface defects.

An immersed boundary method with y+‐adaptive wall function for smooth wall shear

AbstractImmersed boundary (IB) method with a wall function for high Reynolds number flows is attractive when the boundary is complex and evolving. However, when combined with IB method, existing wall functions cannot produce smooth wall shear stress, which is important for many processes such as erosion. The root cause is the discontinuity between the log‐law and laminar layers in most wall functions. A wall function in IB methods is typically enforced through IB cells. However, for complex and evolving boundaries, IB cells can be located in either the log‐law layer or the laminar layer, which follow different laws. To remedy this, a new method is introduced with a y+‐adaptive strategy. The idea is that when an IB cell is too close to an IB, it is replaced by a neighboring fluid cell further away from the boundary. Consequently, all IB cells are in the log‐law layer. The resulting wall shear stress is much smoother. This adaptive strategy is a compromise between how accurate the location of an IB wall is represented and the smoothness of simulated wall shear. Example cases in 1D, 2D, and 3D show the y+‐adaptive wall function produces results compared well with theory and experiments.

On the smallness condition in linear inviscid damping: monotonicity and resonance chains

We consider the effects of mixing by smooth bilipschitz shear flows in the linearized Euler equations on . Here, we construct a model which is closely related to a small high frequency perturbation around Couette flow, which exhibits linear inviscid damping for L sufficiently small, but for which damping fails if L is large. In particular, similar to the instability results for convex profiles for a shear flow being bilipschitz is not sufficient for linear inviscid damping to hold. Instead of an eigenvalue-based argument the underlying mechanism here is shown to be based on a new cascade of resonances moving to higher and higher frequencies in y, which is distinct from the echo chain mechanism in the nonlinear problem.

Effect of foreign object damage on high-cycle fatigue strength of titanium alloy for aero-engine blade

To address the fatigue behavior of damaged aero-engine blade, in the current paper, the research on high cycle fatigue (HCF) strength of TC11 titanium alloy after subjected to foreign object damage (FOD) were presented. First of all, FOD simulation tests were carried out on simulated flat blade samples using gas cannon test system. The macro and micro impact damage of samples were examined in detail. The FOD test results shows that the microscopic characteristics of damage area include smooth area, micro cracks, micro notches, shear bands and loss of material (LOM). Then, HCF tests were carried out on the damaged flat blade samples using HCF fatigue testing machine through stepping method. The undamaged standard test specimens were also tested for HCF limit as baseline. The fatigue results show that the damage depth L2 related to impact velocity has a great influence on fatigue limit. The greater the impact velocity, the lower the fatigue limit. Microscopic observation on fatigue fracture was conducted by metallographic microscope and found that FOD would change the microstructure of material in damage area, made the phase to be slender and in a certain arrangement direction.

An Improved Immersed Boundary Method for Simulating Flow Hydrodynamics in Streams with Complex Terrains

Three-dimensional (3D) computational fluid dynamic (CFD) simulations have gained substantial popularity in recent years for stream flow modelling. The complex terrain in streams is usually represented by a 3D mesh conforming to the terrain geometry. Such terrain-conforming meshes are time-consuming to generate. In this work, an immersed boundary method is developed in an existing terrain-conforming CFD model named U2RANS as an alternative, in which terrains are represented implicitly in the Cartesian background mesh. An improved two-layer wall function is proposed in the framework of the k-ε turbulence model, with the aim of producing accurate and smooth wall shear stress distribution and paving the way for future model development on sediment transport and scour modeling. The improvement overcomes the inherent discontinuity and nonlinearity of the two-layer velocity profile, which causes error in the estimation of shear velocity. The new algorithm utilizes a distance control on the image point in immersed boundary method and a modification of velocity prediction in the laminar layer. The improved immersed boundary method is tested with 1D, 2D, and 3D cases, and comparisons with flume experiments show promising results.

Wave interactions in neutrally stable shear layers: Regular and singular modes, and non-modal growth

A recent letter [J. R. Carpenter and A. Guha, “Instability of a smooth shear layer through wave interactions,” Phys. Fluids 31, 081701 (2019)] compared the neutral modes of a smooth two-dimensional shear profile without an inflection point to the modes of its corresponding piecewise-linear profile. The regular mode in the smooth profile was identified as the one least sensitive to the numerical resolution, while the singular modes displayed high sensitivity. Here, we provide a physical interpretation using a wave interaction approach for understanding the structure and behavior of both the regular and singular modes. The regular modes are the interfacial Rossby waves located at the concentrated mean vorticity gradient of the shear profile. In contrast, the singular modes result from a one way phase-locking interaction between singular vorticity disturbances, passively advected by the mean flow at different levels of the profile, and the interfacial Rossby waves. We show that this one way interaction can also lead to a sustained non-modal growth of the interfacial Rossby waves that cannot be captured by standard eigenvalue analysis.

The viscous Holmboe instability for smooth shear and density profiles

The Holmboe wave instability is one of the classic examples of a stratified shear instability, usually explained as the result of a resonance between a gravity wave and a vorticity wave. Historically, it has been studied by linear stability analyses at infinite Reynolds number, $Re$, and by direct numerical simulations at relatively low $Re$ in the regions known to be unstable from the inviscid linear stability results. In this paper, we perform linear stability analyses of the classical `Hazel model' of a stratified shear layer (where the background velocity and density distributions are assumed to take the functional form of hyperbolic tangents with different characteristic vertical scales) over a range of different parameters at finite $Re$, finding new unstable regions of parameter space. In particular, we find instability when the Richardson number is everywhere greater than $1/4$, where the flow would be stable at infinite $Re$ by the Miles-Howard theorem. We find unstable modes with no critical layer, and show that despite the necessity of viscosity for the new instability, the growth rate relative to diffusion of the background profile is maximised at large $Re$. We use these results to shed new light on the wave-resonance and over-reflection interpretations of stratified shear instability. We argue for a definition of Holmboe instability as being characterised by propagating vortices above or below the shear layer, as opposed to any reference to sharp density interfaces.