Intense Shear Research Articles

Elucidating how correlated operation of shear transformation zones leads to shear localization and fracture in metallic glasses: Tensile tests on Cu[sbnd]Zr based metallic-glass microwires, molecular dynamics simulations, and modelling

The plastic deformation of metallic glasses (MG) is well-known to occur via shear transformation zones (STZs) on the scale of atomic clusters, yet fracture of MG takes place via shear bands of the micron scale. So far, understanding on how the operation of STZs leads to shear localization and fracture remains limited. In this work, tensile tests on Cu/Zr-based MG micro-wires show that both the first-yield and fracture stress exhibit the Weibull distribution, and fractography reveals that shear localization in the form of intense shear bands leads to shear fracture. Molecular dynamics (MD) simulations show that shear bands form via the correlated emergence and operation of discrete STZs close to one another. To describe how the stochastic yet correlated occurrence of the discrete STZs leads to shear localization, a model is constructed to relate the probability of the successive operation of discrete STZs, to their nucleation density. The model predicts that, if nucleation density of the STZs grows along the strain path, as prior shear events triggers the emergence of new STZs, then successive occurrence of discrete shear events speeds up rapidly to an asymptotic state which is exactly the condition of shear localization and shear banding. Furthermore, the MD results suggest an exponential growth law for the occurrence of the STZs along the strain path, which also gives predictions in good agreement with the experimental Weibull distributions of the first-yield stress of Cu/Zr-based MGs.

Two-phase flow measurements of an unsteady breaking bore

A key feature of breaking bores, jumps, and spilling breakers is the roller region, characterised by intense shear and recirculation, associated with air bubble entrainment and splashing. Detailed unsteady air–water flow measurements were conducted in a breaking bore propagating in a large-size channel, using an array of dual-tip phase-detection probes and an ultra-high-speed video camera. The results showed a steep roller front, with a very-dynamic air–water bubbly region, albeit with a relatively limited air–water roller region. In this study, a major challenge was the inconsistency in light intensity linked to the travelling nature of the bore. A simple flow visualisation technique was applied to retrieve the two-dimensional side-looking profile of the roller edge and average void fraction. The results were validated independently with a phase-detection probe. While the probe data lacked spatial variability, the study reinforces the needs of high-quality validation data set, including in unsteady transient flows.

In situ investigation of strain heterogeneity and microstructural shear bands in rolled Magnesium AZ31

A comprehensive characterization of microstructural strain accommodation is performed on rolled Magnesium AZ31 tensioned in the rolling direction (RD). Scanning microscopic digital image correlation (SMDIC) is applied over macroscopic areas, strain-mapping ∼106 grains in situ at their natural scale. Further, for rigorous volumetric interpretation, two samples have been tested in RD-tension to monitor both lateral material planes (perpendicular to transverse and normal directions of the plate) with SMDIC. Very different strain localization patterns are observed on the two faces. Each pattern family is isolated by logical filtering for a quantitative analysis of their shearing orientations, allowing volumetrically-compatible and surface-mediated structures to be distinguished. It is found that the most intense strain localization takes place on microstructural bands in a rhombic pattern that are inherited from the rolling process. These are found to activate at the onset of the elastic-plastic transition and rapidly incur the heaviest strains, primarily through basal slip due to the weaker texture of the band grains. The deformation of the bulk matrix surrounding these bands exhibits localization patterns consistent with the predominant activity of basal and prism slip. The two-face measurement provides a unique handle on the degree of surface effects and shows them to be significant in this case of intense plastic anisotropy and directional shear banding. It is directly shown that the activity of the microstructural bands suppresses the r-ratio, the primary macroscopic measure of anisotropy in these plates, as suspected in the literature. The importance of accounting for the spatial-coordination of orientations in these highly-anisotropic polycrystals is clearly presented.

Influence of Diameter on the Templated Crystallization of Polyethylene/Carbon Material Fiber Composites under Intense Shear Flow.

In this study, carbon fibers (CFs) and carbon nanofibers (CNFs) were introduced into polyethylene (PE) and intensive shear flow was imposed on the melt during the melt second flow caused by gas penetrating the melt during the gas-assisted injection molding (GAIM). The effect of fiber diameter on crystalline morphologies of obtained composites was deeply studied. The results revealed the fact that no matter whether CFs or CNFs were introduced into the PE matrix or not, the orientation degree of PE lamellae would increase during the melt second flow. Hybrid shish-kebab structures were difficult to be formed in GAIM CF/PE composites and only a few oriented PE lamellae overgrew on the local surface of CFs. However for GAIM CNF/PE composites, the formation of hybrid shish-kebab structures was clearly observed in the entire thickness. The structural diversity was mainly ascribed to the difference of CF diameter, showing obvious size-dependent effect.

Responses of Yellow Sea Cold Water Mass to Typhoon Bolaven

A two-month seabed-mounted observation (YSG1 area) was carried out in the western Yellow Sea Cold Water Mass (YSCWM) using an RDI-300K acoustic Doppler current profiler (ADCP) placed at a water depth of 38 m in late summer, 2012. On August 2012, Typhoon Bolaven passed east of YSG1 with a maximum wind speed of 20 m s−1. The water depth, bottom temperature, and profile current velocities (including u, v and w components) were measured, and the results showed that the typhoon could induce horizontal current with speed greater than 70 cm s−1 in the water column, which is especially rare at below 20 meters above bottom (mab). The deepening velocity shear layer had an intense shear velocity of around 10 cm s−1 m−1, which indicated the deepening of the upper mixed layer. In the upper water column (above 20 mab), westward de-tide current with velocity greater than 30 cm s−1 was generated with the typhoon’s onshore surge, and the direction of current movement shifted to become southward. In the lower water column, a possible pattern of eastward compensation current and delayed typhoon-driven current was demonstrated. During the typhoon, bottom temperature variation was changed into diurnal pattern because of the combined influence of typhoon and tidal current. The passage of Bolaven greatly intensified local sediment resuspension in the bottom layer. In addition, low-density particles constituted the suspended particulate matter (SPM) around 10 mab, which may be transported from the central South Yellow Sea by the typhoon. Overall, the intensive external force of the Typhoon Bolaven did not completely destroy the local thermocline, and most re-suspended sediments during the typhoon were restricted within the YSCWM.

Design strategy for high plasticity and strength in metallic glasses: A molecular dynamics simulation study

The poor plasticity of metallic glasses (MGs) limits their application as structural materials. How to enhance dramatically the plasticity of the MGs without compromising the strength is a highly attractive topic. Here, the effect of layer thicknesses and analogous flow defects concentrations (AFDCs) on the mechanical behavior of amorphous/amorphous (A1/A2) nano-multilayers is investigated by molecular dynamics method. The results indicate, when layer thickness is small (from 1.8 to 8.5 nm), for any given layer thickness, the plastic deformation of A1/A2 nano-multilayer evolves from a “shear localization” fashion to a “uniform deformation” mode, and eventually to a “shear localization” deformation again with the increase of AFDC. Comparing with the monolithic MGs, the A1/A2 nano-multilayers with optimum AFDC manifesting uniform deformation achieve a perfect combination of high strength and superior plasticity. What is more, the optimum AFDC could always be found for the sample with arbitrary layer thickness, and shows an upward tendency with increasing layer thickness. When layer thickness is large (13.0 and 25.0 nm), no matter how the AFDC changes, the optimum AFDC disappears for determined layer thickness. In this case, the plastic deformation behavior is arrested in A1 soft layer, resulting in intense shear localization and the reduced plasticity.

Rheological properties of fresh and reconstituted milk protein concentrates under standard and processing conditions

As the processability of fresh and reconstituted milk protein concentrates crucially depends on their rheological properties, a considerable amount of studies focuses on this topic. By means of a direct comparison, we are the first to clearly show that distinct rheological differences can exist between fresh and reconstituted milk protein concentrates under standard and processing conditions. We show that reconstituted milk protein concentrates made from commercial milk protein powders exhibit higher viscosities than fresh ones. Furthermore, we found that during intense shearing, the reconstituted milk protein concentrates undergo a loss of structure, which manifests itself in a significant viscosity decrease. The inverse effect can be observed for fresh milk protein concentrates. Besides these differences, the reconstituted milk protein concentrates exhibit gel–like properties above a certain protein content. We attribute these observations to protein–protein interactions in the milk protein powder, which are induced by manufacturing and/or storing conditions. Our results demonstrate that rheological properties of fresh and reconstituted milk protein concentrates are quantitatively not invariably interchangeable. Thus, the purpose of this article is to emphasize the necessity for researchers and engineers to take into account the rheological particularities of different milk protein concentrates prior to usage.

Promoting osteoblast proliferation on polymer bone substitutes with bone-like structure by combining hydroxyapatite and bioactive glass

Mimicking the structural features of natural bone has been demonstrated to bring pronounced advantages for mechanical reinforcement of polymeric orthopedic substitutes that are composed of bioinert polymer matrix and bioactive fillers. However, to trigger effective bone formation and implant integration, the bioactivity of bone substitutes plays a vital role. We hypothesized that the use of hydroxyapatite (HA) and bioactive glass (BG), compared to the use of HA alone, could improve the biological properties of polymer-based bone substitutes. Herein, high-density polyethylene (PE) composites loaded with HA and BG were fabricated using a modified injection molding machine that can provide intense shear flow to regulate the hierarchical structure of the composites. Morphological observation revealed that bone-like structures were formed in both HA/PE and BG/HA/PE composites, showing highly oriented interlocked shish kebabs. In addition, the bioactive fillers were distributed uniformly. Osteoblast proliferation was promoted by the combination of HA and BG. The mechanism was the upregulation of Runx2 expression (1.51 ± 0.17) with BG and the activation of the TAZ/YAP (1.41/0.64) signaling pathway, which accelerated the generation of ossification-related proteins. BG can regulate microRNA to promote the mRNA expression of Runx2. The silencing of Runx2 expression can inhibit BG-induced osteoblast proliferation. These results suggest that the BG/HA/PE composites having a bone-like structure have high potential as bone substitutes to repair large bone defects.

Grain Refinement by Extension Twin in Mg Alloy during Asymmetrical Rolling

The work looked into the grain refinement process of Mg alloy during asymmetrical rolling with a focus on the role of twin. The present sample was deformed at ambient temperature by single operation with the height reduction of 50% at the roll speed ratio of 1:4 for the upper and lower rolls having the same dimension. From the electron backscatter diffraction analysis in the surface region where intense shear strain was imparted, a number of { 10 1 ¯ 2 } extension twins with a width of ~1 µm were detected clearly in most of the deformed grains. Moreover, the average misorientation angle of the deformed grains in the top region was found to be ~32°, which was two times higher than that in the center area where the extension twin was detected rarely. As a result, the microstructure in the top region was refined significantly down to be ~1.1 µm with an aid of twin activities that would be discussed in this study.

On the Dynamic Fragmentation and Lubrication of Coseismic Landslides

AbstractThe three‐dimensional discrete element method has been employed to analyze the dynamic fragmentation and lubrication mechanisms of coseismic Tangjiashan landslide induced by the 2008 Ms 8.0 Wenchuan earthquake. The numerical results show that the internal rock damage occurs and propagates gradually along the basal failure plane due to seismic shaking. At the peak seismic shaking, a sudden increase of tensile and shear stresses can lead to the complete breakage of basal bonds. This is associated with a sudden relief of overburden stress and rapid decrease of basal stress ratio. Thus, the slope fails as a whole and moves quickly downslope. In this process, several large transversal cracks develop at the middle and upper rear regions, disintegrating the slope mass into several large blocks. During landslide propagation, the thickness of basal fragmented layer increases progressively due to intense shearing, and the basal stress ratio reduces accordingly from 0.68 to 0.28. The reduction of landslide basal stress ratio occurs when the strong basal resistance is overcome by seismic‐ and gravity‐induced shear forces together with intense particle rearrangements. It can be quantified by vibrational and rotational granular temperatures of the basal shear layer, with the peak values of 35.2 and 11.6 m2/s2, respectively. The widespread internal slope fragmentation and subsequent lubrication have been identified as the key mechanisms governing landslide motion, which appear to be the intrinsic features of landslide irrespective of its triggering mechanism. The seismic shaking is more relevant to the detachment than to the spreading of landslide mass.

Turbulent flow over a D-section bluff body: a numerical benchmark

A numerical benchmark of different turbulence closures was performed to investigate the turbulent wake flow behind a submerged D-shaped bluff body. The numerical models included steady two-equation Reynolds-Averaged Navier–Stokes models and Large Eddy Simulations (LES), which were compared to planar and stereoscopic particle image velocimetry measurements. The k- $$\omega$$ SST low-Reynolds number model was found to be better adapted to capture the intense shear layer at the top of the D-section compared to the other two-equation models. Yet, the k- $$\epsilon$$ and k- $$\omega$$ SST high-Reynolds number models demonstrate higher performance in the recirculation region. LES was also performed over the D-section to determine the influence of the sub-grid scale models on the prediction of the vortical structures. The Wale model together with central difference schemes showed a better overall agreement over the standard Smagorinsky model, which appears too dissipative. A spectral analysis was performed in the wake region, yet no distinct shedding frequencies could be found. A proper orthogonal decomposition was applied to the LES results to extract the mean flow dynamics and the coherent structures.

The Rich Dynamics of Jupiter’s Great Red Spot from JunoCam: Juno Images

Abstract We have used high-resolution images obtained with JunoCam onboard the Juno spacecraft during its close flyby of Jupiter on 2017 July 11, to study the dynamics of the Great Red Spot (GRS) at the upper cloud level. We have measured the horizontal velocity and vorticity fields using the clouds as tracers of the flow. We have analyzed a variety of cloud morphologies that serve to characterize different underlying dynamic processes. Long undulating dark gray filaments (2000–10000 km) circulate around the outer part of the vortex moving at high speed (∼120–140 m s−1) where mesoscale waves (wavelength 75 km) indicate stable conditions in this region. At mid distance from the center, a large eddy (radius ∼500 km) is observed in a region of intense horizontal wind shear whereas on the opposite side, compact cloud clusters with cell sizes of ∼50 km, indicative of shallow convection, are observed. The core of the GRS (∼5000 × 3000 km2) is turbulent where the circulation has weakly cyclonic and anticyclonic regions. This variety of phenomena occurs in the upper ammonia cloud layer and haze (thickness ∼20–50 km) that represents the top of a dynamical system with a much deeper circulation.

Calculation of Parameters of Granular Polymer Material Plasticization Process Under Intense Shear Impact

The characteristics of the granular polymer material plasticization process in a plasticizer shear flow are studied. A mathematical model of the process, which fully conforms to the modern concepts of the plasticization mechanism and allows determination of the key process parameters, particularly the plasticization time, under intense shear impact on the polymer granules, is proposed. This model provides the possibility of performing technological calculations even at the stage of designing mixing equipment for preparation of highly viscous polymer solutions.

Simulated Kelvin–Helmholtz Waves over Terrain and Their Microphysical Implications

Abstract Two Kelvin–Helmholtz (KH) wave events over western Washington State were simulated and evaluated using observations from the Olympic Mountains Experiment (OLYMPEX) field campaign. The events, 12 and 17 December 2015, were simulated realistically by the WRF-ARW Model, duplicating the mesoscale environment, location, and structure of embedded KH waves, which had observed wavelengths of approximately 5 km. In simulations of both cases, waves developed from instability within an intense shear layer, caused by low-level easterly flow surmounted by westerly winds aloft. The low-level easterlies resulted from blocking by the Olympic Mountains in the 12 December case, while in the 17 December event, the easterly flow was produced by the synoptic environment. Simulated microphysics were evaluated for both cases using OLYMPEX observations. When the KH waves were within the melting level, simulated microphysical fields, such as hydrometeor mixing ratios, evinced considerable oscillatory behavior. In contrast, when waves were located below the melting level, the microphysical response was attenuated. Turning off the model’s microphysics scheme and latent heating resulted in weakened KH wave activity, while removing the Olympic Mountains eliminated KH waves in the 12 December event but not the 17 December case. Finally, the impact of several microphysics parameterizations on KH activity was evaluated for both events.

Paleomagnetism of IODP Site U1380: Implications for the Forearc Deformation in the Costa Rican Erosive Convergent Margin

The destructive nature of subduction erosion poses challenges to fully understanding the evolution of erosive convergent margins that are critical to understanding crustal recycling and seismogenesis. Forearc deformation holds important clues to the evolution of erosive convergent margins. Here we present detailed paleomagnetic and structural analyses of IODP Site U1380 cores from the middle slope of the forearc of the Costa Rican erosive convergent margin. The analyses reveal a strong deformation zone from ~490 to ~550 mbsf that is characterized by abundant fissility/foliations shallower than the bedding. Similar relatively strong deformation zones are recognized from the frontal prism and upper slope sites, and are broadly correlative, forming a zone of strong deformation across the forearc. This zone spans ~2.0 to 1.83 Ma and the deformation likely occurred briefly at ~1.80 Ma. The widespread, short-lived, and strong deformation is interpreted as a result of intense subhorizontal shear following the rapid forearc subsidence driven by the dramatic subduction erosion associated with the abrupt onset of the Cocos Ridge subduction. Given the typical occurrence of forearc subsidence by subduction erosion, similar styles of deformation are probably common in other erosive convergent margins as well.

On the Structure and Mechanical Properties of Multilayered Composite, Obtained by Explosive Welding of High-Strength Titanium Alloys

One of the ways to simultaneously increase the strength and the fracture and impact toughness of structural materials is by producing multilayered materials. In this paper we discuss the structure and properties of a seven-layer composite obtained by explosive welding of high-strength titanium alloys. The structure of the composite was characterized using light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). At the interfaces between plates, formation of waves and vortices was observed. The wave formation is discussed with respect to the kinetic energy loss. The vortices consisted of a mixture of two alloys and possessed a martensitic structure comprising α′ and β phases of titanium. Localized plastic deformation occurred along the interfaces during explosive welding by formation of shear bands. The most intensive shear banding occurred in the vicinity of the upper interfaces. The local hardness at the interfaces increased due to the formation of the quenched structures. The interfaces between titanium alloys positively influenced the impact toughness of the composite, which increased in comparison with that of bulk titanium alloys by a factor of 3.5. The strength characteristics of the composite remained at the same level as that of the bulk material (1100–1200 MPa).

Near‐Inertial Waves and Their Underlying Mechanisms Based on the South China Sea Internal Wave Experiment (2010–2011)

AbstractBased on mooring array data from the South China Sea (SCS) Internal Wave Experiment, in this paper, five burst events of near‐inertial waves (NIWs) in the northern SCS that occurred during August 2010 to April 2011 are reported. Among these NIWs, three were induced by typhoons Lionrock, Meranti, and Megi in 2010. Due to the differences in typhoon characteristics, mooring measuring ranges, distances between typhoon centers and moorings, and local conditions, the NIWs induced by the three typhoons show various intensities and structures at all three moorings. An interesting phenomenon is that the weaker typhoon Meranti generates stronger NIWs than the stronger typhoon Megi, which is largely due to the larger translation speed of Meranti. The e‐folding time of near‐inertial variance suggests that the damping of typhoon‐induced NIWs is related to local conditions. In addition to the typhoon‐induced NIWs, two NIW burst events were found in December 2010 and March 2011 at one mooring; however, these events were absent from the two adjacent moorings. Through analysis, we ruled out the local winds, lateral propagation, and parametric subharmonic instability as the causes of these NIWs. Although the real mechanisms remain unknown, the NIWs that occurred in December 2010 and March 2011 exhibit different features from the typhoon‐induced NIWs. First, these NIWs have larger vertical wave numbers and smaller wavelengths than the typhoon‐induced NIWs. Second, these NIWs can cause intense shear in deep water, whereas intense shear of typhoon‐induced NIWs mainly exist in the upper ocean (approximately 150–200 m).

The Effect of Fuel Injection Equipment of Water-In-Diesel Emulsions on Micro-Explosion Behaviour

The number and size distributions of the water dispersed phase have a significant effect on both the long-term stability of an emulsion, and the probability of micro-explosions inside an engine. The emulsions are subjected to intense pressure and shear flow in the fuel injection equipment resulting in changes in the number and size distributions of the dispersed phase. These changes, in turn, have significant effects on the micro-explosion behavior of the droplets. To our knowledge, these effects are not known and have not been reported previously. To uncover some of these effects we carried out a comprehensive experimental investigation on an emulsion spray of 10% water (by volume) in diesel at different injection pressures of 500, 1000 and 1500 bar. A measurement system consisting of a high-speed camera was used to visualize the droplets’ micro-explosions and a thermocouple measured the temperature. Our measurements indicated that the emulsion shear in the injector nozzle shifted the emulsion droplet size distribution towards the smaller end resulting in a delay in the onset of micro-explosion. This delay in the onset of the micro-explosion is thought to be due to the decrease in the dispersed water coalescence rate which, in turn, increases the stability of the emulsion. The results also show that this delay in the onset of micro-explosion, and the temperature required for its onset, increased with injection pressure.

Microstructure evolution of Ti-6Al-2Zr-1Mo-1V alloy and its mechanism in multi-pass flow forming

Multi-pass flow forming processes are one of the major approaches to producing thin-walled titanium cylindrical parts with large cross-sections in which microstructure characteristics have a significant effect on the tailoring of desirable properties. In this study, the microstructure evolution of the Ti-6Al-2Zr-1Mo-1V alloy and its mechanism in a multi-pass flow forming process are systematically studied based on the deformation characteristics during the process. The results show that waved grains formed in the process are due to the compressive stress in the rolling direction (RD) and the non-uniform stress field in the deformation area. With the compression and shear deformation induced in multi-pass flow forming, non-standard {0002} basal plane texture is formed whose {0002} poles are tilted from the normal direction (ND) to the RD and the circumferential direction (CD) with an angle of 20° and 30°, respectively. It is found that α grains are refined by two mechanisms, viz., continuous dynamic recrystallization (CDRX) and deformation-induced intense and localized shearing (DILS). CDRX mainly promotes secondary α grain refinement, while DILS has significant effects on primary α grain refinement under the larger deformation and on secondary α grain refinement during the entire process. Meanwhile, the grains refined by DILS with different orientations facilitate further deformation. This effect of grain refinement leads to improvement of the yield strength in the RD and the CD by 12.42% and 19.05%, respectively, compared annealed samples of the cylindrical part with the original one. These findings provide a basis for microstructure control and property tailoring of Ti-6Al-2Zr-1Mo-1V cylindrical parts in flow forming processes.

The effects of microfluidization on the physical, microbial, chemical, and coagulation properties of milk

This work examines the use of mild heat treatments in conjunction with 2-pass microfluidization to generate cheese milk for potential use in soft cheeses, such as Queso Fresco. Raw, thermized, and high temperature, short time pasteurized milk samples, standardized to the 3% (wt/wt) fat content used in cheesemaking, were processed at 4 inlet temperature and pressure conditions: 42°C/75 MPa, 42°C/125 MPa, 54°C/125 MPa, and 54°C/170 MPa. Processing-induced changes in the physical, chemical, and microbial properties resulting from the intense pressure, shear, and cavitation that milk experiences as it is microfluidized were compared with nonmicrofluidized controls. A pressure-dependent increase in exit temperature was observed for all microfluidized samples, with inactivation of alkaline phosphatase in raw and thermized samples at 125 and 170 MPa. Microfluidization of all samples under the 4 inlet temperature and processing pressure conditions resulted in a stable emulsion of fat droplets ranging from 0.390 to 0.501 μm, compared with 7.921 (control) and 4.127 (homogenized control) μm. Confocal imaging showed coalescence of scattered fat agglomerates 1 to 3 μm in size during the first 24 h. We found no changes in fat, lactose, ash content or pH, indicating the major components of milk remained unaffected by microfluidization. However, the apparent protein content was reduced from 3.1 to 2.2%, likely a result of near infrared spectroscopy improperly identifying the micellar fragments embedded into the fat droplets. Microbiology results indicated a decrease in mesophilic aerobic and psychrophilic milk microflora with increasing temperature and pressure, suggesting that microfluidization may eliminate bacteria. The viscosities of milk samples were similar but tended to be higher after treatment at 54°C and 125 or 170 MPa. These samples exhibited the longest coagulation times and the weakest gel firmness, indicating that formation of the casein matrix, a critical step in the production of cheese, was affected. Low temperature and pressure (42°C/75 MPa) exhibited similar coagulation properties to controls. The results suggest that microfluidization at lower pressures may be used to manufacture high-moisture cheese with altered texture whereas higher pressures may result in novel dairy ingredients.