High Stress Regions Research Articles

Intermediate-temperature creep behaviors of an equiatomic VNbMoTaW refractory high-entropy alloy

As one of refractory high-entropy alloys (RHEAs), VNbMoTaW is considered to be a promising candidate for elevated-temperature application. However, its creep behavior is rarely reported. In the present work, the compressive creep behaviors of an equiatomic VNbMoTaW RHEA with a grain size of 138 ± 36 μm were well studied over an intermediate temperature range (973–1173 K) and under high applied stress (130–520 MPa). The stress exponent of the alloy is found to remain stable (∼1) at relatively low temperatures (973–1073 K), whereas a stress-dependent transition behavior occurs at high temperatures (1123–1173 K), i.e., the stress exponent of the alloy changes from ∼1 in the low stress region (130–390 MPa) to ∼ 4 in the high stress region (390–520 MPa). Meanwhile, the creep activation energy increases from 139 to 156 kJ mol−1 at low temperatures to 307–373 kJ mol−1 at high temperatures. The low stress exponent and low activation energy at low temperatures suggest that the creep is controlled by dislocation pipe diffusion. The low stress exponent and relatively high activation energy in the high-temperature low-stress region suggest that the creep is controlled by lattice diffusion. In the high-temperature high-stress region, the prevalent dislocations detected by the post-mortem microstructural observation, the high stress exponent, and high activation energy suggest that the creep deformation is controlled by a lattice diffusion mediated dislocation climb process. These findings provide a fundamental understanding of the creep behavior and deformation mechanism of VNbMoTaW, which can be applied to design advanced creep-resistant RHEAs.

Transition between a nano-sized prismatic dislocation loop and vacancy cluster in α-iron: An atomic scale study

Plastic deformation and the presence of hydrogen enhance the concentration of vacancy-type defects in α-iron, wherein monovacancies can accumulate as a planar vacancy cluster (VC). The planar VC can then be nucleated into a corresponding prismatic dislocation loop (PDL), which can also be directly generated through cross-slip mechanisms of a screw dislocation. Inversely, a PDL can be converted to a corresponding VC. High densities of VCs and PDLs can significantly influence the mechanical properties of iron-based materials. Here, a quantitative energy barrier is established for the transition between nano-sized 1/2 〈111〉 PDLs and VCs using the nudged elastic band method for different applied strains. It was concluded that a cluster size of more than 19 vacancies can easily nucleate into a PDL if a VC with planar configuration is formed in α-iron. Further, the nano-sized VC to PDL transition is enhanced under compressive strain while the nano-sized PDL to VC transition is enhanced under tensile strain. This result indicates that nano-sized VCs generated by tensile deformation are converted to a PDL after unloading and diffusing away. Inversely, nano-sized PDLs that form by deformation can easily diffuse to high tensile stress regions near stress singularities, such as crack tips and triple-junctions of grain boundaries, and then accumulate as high-density VCs.

Finite element modeling of precast reinforced concrete wall with dual boundary elements under lateral load

This study aimed to stimulate the structural behaviour of the precast wall PC with dual boundary elements under constant axial load and incremental lateral load in terms of displacement control and assess its response based on the load–displacement response, stiffness degradation and modes of failure using Abaqus Software and validate the result of the tested specimen. Two approaches were used for defining the interaction between the steel reinforcement and the surrounding concrete where the high-stress regions are. Friction and cohesive behaviour using traction separation law were employed to define the interaction between the interfaces of the PC wall and post-cast concrete boundary elements. Furthermore, this study primarily focused on the nonlinear behaviour of concrete and steel materials, the bond-slip behaviour between concrete-to-steel interface, the interaction between precast and post-cast concrete interfaces, and load and boundary conditions. The concrete damage plasticity model was adopted to simulate the concrete behaviour and the cohesive and frictional behaviour to model the interaction between precast and post-cast concrete interfaces. The findings show that the initial response of both FE models agreed well with test rested specimen. However, Model I exhibited a stiffer response up to the yielding point. Furthermore, both FE models continued to exhibit ductile response, showing that the right-hand side post-cast cornet boundary element didn’t experience total crushing at the bottom right concrete.

Numerical Simulation of a Striated Piezoresistive MEMS Pressure Sensor on Circular Silicon Diaphragm: A Finite Element Method-Based Study

Micro-Electro-Mechanical Systems (MEMS) technology has been heavily incorporated into fabrication of sensors due to its undoubtful advantages such as efficient form-factor, low-power consumption and less bulk manufacturing cost. One of the first sensors to undergo transformation in the micro-scale has been the pressure sensor. Pressure sensors are used excessively in various fields such as automobile, aerospace, aviation, biomedical, consumer electronics, etc. Various researches are being conducted to optimize the design of MEMS-based pressure sensors to meet the specific requirements of different fields. This work presents a piezoresistive pressure sensor design with striated-shaped piezoresistors instead of the conventional bar-shaped piezoresistors. Such a configuration allows positioning the short arms of the striated design effectively on the narrow high stress regions. The piezoresistors are embedded on a circular diaphragm. The performance advantage of such a design is validated through analytical models and numerical simulations. Finite Element Analysis (FEA) is deployed using COMSOL Multiphysics software to achieve precise simulation results. The results are then compared to the existing literature. The proposed design achieves a sensitivity of 40.365 mV/V/MPa under a pressure range of 0–1 MPa. The proposed design performs better than existing literature and can be used in applications that require high sensitivity over a long pressure range, for example, in barometric altimeters.

Effect of the in-situ formed CuO additive on the fracture behaviour and failure mechanism of Ag-SnO2 composites

Oxide additives can significantly improve the mechanical properties of ceramic particle-reinforced metal matrix composites. In this study, the fracture behaviour and failure mechanism of the Ag-SnO2 composites without and with in-situ formed CuO additive are investigated using in-situ scanning electron microscope (SEM) and numerical simulation. The results show that the CuO additive significantly enhances the fracture toughness of the Ag-SnO2 composite. The initiation and propagation of secondary cracks near the primary crack tip are effectively inhibited, and large dimples are formed around SnO2 particles. The improvement in the mechanical properties is attributed to the stress concentration relief in the Ag matrix and enhanced interfacial strength. Interface debonding at the sharp corners of the SnO2 particles initiates the formation of cracks in the Ag-SnO2 composite without CuO. The initiated cracks propagate through the high-stress concentration region at an angle of 45° to the loading direction, resulting in a matrix fracture. For the Ag-SnO2 composite with CuO nanoparticles, the propagation rate of the primary cracks reduces significantly, and microcracks are primarily induced near clustered SnO2 particles. Moreover, the damage initiation shifts from the SnO2/Ag interface to the Ag metal matrix with reduced SnO2 particle aggregation in the Ag-SnO2(CuO) composite.

Cross-linker Mobility Governs Fracture Behavior of Catch-Bonded Networks.

While most chemical bonds weaken under the action of mechanical force (called slip bond behavior), nature has developed bonds that do the opposite: their lifetime increases as force is applied. While such catch bonds have been studied quite extensively at the single molecule level and in adhesive contacts, recent work has shown that they are also abundantly present as crosslinkers in the actin cytoskeleton. However, their role and the mechanism by which they operate in these networks have remained unclear. Here, we present computer simulations that show how polymer networks crosslinked with either slip or catch bonds respond to mechanical stress. Our results reveal that catch bonding may be required to protect dynamic networks against fracture, in particular for mobile linkers that can diffuse freely after unbinding. While mobile slip bonds lead to networks that are very weak at high stresses, mobile catch bonds accumulate in high stress regions and thereby stabilize cracks, leading to a more ductile fracture behavior. This allows cells to combine structural adaptivity at low stresses with mechanical stability at high stresses.

Notch Fatigue Life Prediction Model Considering Stress Gradient Influence Depth and Weight Function

Notch characteristics significantly affect the fatigue performance of engineered components, for which the stress gradient effect is worth careful consideration. The traditional stress gradient analysis method based on the Coffin–Manson equation does not take into account the stress gradient influence range regarding the definition of the stress gradient correction factor, nor the high-stress gradient region, which has a greater influence on fatigue life. To address the aforementioned problems, a new notch fatigue life model is proposed in this paper. First, the stress–strain field at the root of the notch is analyzed to define the depth of stress gradient influence, following which the influence of the low-stress gradient region is reduced by a weighting function in the calculation of the stress gradient correction factor. Finally, to validate the method, three sets of experimental data, including TC4, GH4169, and EN8B, were used and compared with three other models. The results demonstrate that the predicted lifetimes of the new model are all within a 2-fold dispersion band, and the prediction ability is better than that of the other three models.

Study on fatigue fracture of SW400 fine-grained high-strength steel T-lap joint

The SW400 fine-grained high-strength steel T-lap joints were welded by submerged arc welding. The influence of welding technology on the geometrical characteristics of welds and fatigue performance of the joints were studied by radial fatigue test with different welding parameters. The numerical analysis was used to study the characteristics of the stress distribution. And the fatigue fracture morphology was analyzed by scanning electron microscope. The results show that with the increase of welding line energy, the thickness of the weld increased, and the propagation direction of the fatigue crack changed from the weld to the base metal. When the thickness was less than 4 mm, the high-stress region was in the area of the weld around the weld root, and the fatigue crack propagated to the weld. When the thickness was greater than 4.5 mm, the high-stress region of AE and AS were in the weld around the root, where the crack propagated to the weld, while in the middle of the weld high-stress region was in the area of base metal around the weld root, crack propagated to the base metal. With the increase in the thickness of the weld, the maximum mises stress decreased, and the crack propagation rate decreased. The time of crack initiation was very short, which occurred in the high-stress region of the weld root in the middle of the weld.

Superior resistance to high–temperature creep in an additively manufactured precipitation–hardened CrMnFeCoNi high–entropy alloy nanocomposite

A carbon–containing CrMnFeCoNi high–entropy alloy (C–HEA) nanocomposite was additively manufactured via a laser powder bed fusion (LPBF) process combined with subsequent heat treatment. Exposure of the LPBF C–HEA to this additional heat–treatment step not only resulted in heterogeneous–structured grains of which the substructure is decorated with a dislocation network, but also uniformly distributed nanosized carbide precipitates at the grain and substructure boundaries. A multistep creep test conducted on the LPBF C–HEA at 873 K enabled the stress exponent, n, to be obtained with values of 3.03 in the low stress region (175–250 MPa) and 6.99 in the high stress region (250–325 MPa). The LPBF C–HEA alloy exhibited superior high–temperature creep resistance (i.e., the creep rate and threshold stress were minimized) compared to other CrMnFeCoNi HEAs. The mechanism underlying the creep resistance and unique creep deformation behavior of the LPBF C–HEA alloy is discussed on the basis of electron channeling contrast imaging and electron backscatter diffraction analyses.

Impact of Modern Irrigation Methods on Groundwater Storage and Land Subsidence in High-water Stress Regions

Water resources management in high-stress regions is a large-scale problem affecting food and water security. Therefore, modern irrigation systems such as sprinklers, subsurface, and drip irrigations are applied to manage the water shortage in regions of the Nile delta. Because of the increase in population rate and climate change, freshwater supplies are significantly limited and must be efficiently managed. Consequently, Egypt plans to implement MIS in the old land of the Nile delta. The current study aims to investigate and model the recharge from the excess irrigation system that affects groundwater storage and land subsidence unexpectedly. The visual modflow was applied to investigate the groundwater heads and drawdown. At the same time, the analytical solution based on Terzaghi theory, and the surfer software was used to estimate and map the land subsidence.For this reason, we investigated the groundwater drawdown in the high-water shortage of Egypt’s Nile delta. Three scenarios were considered by changing irrigation methods and their Impact on land subsidence in the Nile Delta Aquifer (NDA). Therefore, the results of this study demonstrated that the groundwater drawdown reached 2.60 m, 4.20 m, and 6.50 m, respectively. In contrast, the land subsidence outstretched 26 m, 44 m, and 65 cm for the sprinkle sub-surface. Future design infrastructures such as pipelines, roads, irrigation networks, canals, and water structures should consider land subsidence due to modern irrigation systems (MIS).

Adaptive load feedback robustly signals force dynamics in robotic model of Carausius morosus stepping.

Animals utilize a number of neuronal systems to produce locomotion. One type of sensory organ that contributes in insects is the campaniform sensillum (CS) that measures the load on their legs. Groups of the receptors are found on high stress regions of the leg exoskeleton and they have significant effects in adapting walking behavior. Recording from these sensors in freely moving animals is limited by technical constraints. To better understand the load feedback signaled by CS to the nervous system, we have constructed a dynamically scaled robotic model of the Carausius morosus stick insect middle leg. The leg steps on a treadmill and supports weight during stance to simulate body weight. Strain gauges were mounted in the same positions and orientations as four key CS groups (Groups 3, 4, 6B, and 6A). Continuous data from the strain gauges were processed through a previously published dynamic computational model of CS discharge. Our experiments suggest that under different stepping conditions (e.g., changing "body" weight, phasic load stimuli, slipping foot), the CS sensory discharge robustly signals increases in force, such as at the beginning of stance, and decreases in force, such as at the end of stance or when the foot slips. Such signals would be crucial for an insect or robot to maintain intra- and inter-leg coordination while walking over extreme terrain.

A deep dive into the static force transmission of the human masticatory system and its biomechanical effects on the temporomandibular joint

ObjectiveThis study aims to investigate the biomechanical behavior and reveal the force transmission patterns of the human masticatory system through advanced three-dimensional finite element (FE) models. MethodsThe FE model was constructed according to the medical images of a healthy male adult. It contains full skull structures, detailed temporomandibular joints (TMJs) with discs, complete dentitions, masticatory muscles, and related ligaments. Several static bite scenarios were simulated to demonstrate the effects of bite positions and muscle force recruitments on the force transmission patterns. ResultsMolar occlusal surfaces are the primary force transmission region for clenching. Sensitivity analysis demonstrated that the stiffness of the bite substance would not alter the force transmission patterns but could affect the maximum contact stresses on the discs and the occlusal surfaces. During the unilateral clenching tasks, the high-stress region on the discal surfaces shifted ipsilaterally. The presence or absence of the molar cushions would significantly affect the biomechanical response of the masticatory system. SignificanceFE analysis is an effective way of investigating biomechanical responses involving complicated interactions. Enriching the static analysis of the masticatory system with a detailed model can help understand better how the forces were transmitted and the significance of TMJs during the clenching process.

Stress and stretching regulate dispersion in viscoelastic porous media flows.

In this work, we study the role of viscoelastic instability in the mechanical dispersion of fluid flow through porous media at high Péclet numbers. Using microfluidic experiments and numerical simulations, we show that viscoelastic instability in flow through a hexagonally ordered (staggered) medium strongly enhances dispersion transverse to the mean flow direction with increasing Weissenberg number (Wi). In contrast, preferential flow paths can quench the elastic instability in disordered media, which has two important consequences for transport: first, the lack of chaotic velocity fluctuations reduces transverse dispersion relative to unstable flows. Second, the amplification of flow along preferential paths with increasing Wi causes strongly-correlated stream-wise flow that enhances longitudinal dispersion. Finally, we illustrate how the observed dispersion phenomena can be understood through the lens of Lagrangian stretching manifolds, which act as advective transport barriers and coincide with high stress regions in these viscoelastic porous media flows.

Microstructural, mechanical and thermodynamic properties ınvestigation of the novel rare earth-free multicomponent Mg-15Al-8Ca-3Zn-2Ba alloy

There has been a significant increase in research and development efforts to meet the growing demand for environmentally friendly magnesium (Mg) alloys. Studies are currently exploring various alloying element combinations to meet demanding specifications. In this study, it was aimed to examine the usability of aluminum (Al), calcium (Ca), and zinc (Zn) elements together with barium (Ba), and to investigate the mechanical and thermodynamic properties of the obtained multicomponent alloy system. SEM and hardness tests were used to examine the microstructural and mechanical features of Mg alloys. In the SEM analysis, the alloy was determined to consist of an ?-Mg matrix, a blocky compact structure containing Ba (Mg17Ba2), a regional eutectic structure (Ca2Mg6Zn3), and independently growing lamellae (Al2Ca). The general hardness analysis results of the alloy, measured with Brinell and Vickers tests, were determined to be ~77 and ~82, respectively. The indentation test also revealed that stress transfer to the Al2Ca laves phase is possible, depending on the orientation of the slip plane between the matrix and the Al2Ca phase. It was also observed that cracks developed in the indentation test on the Mg17Ba2 intermetallic phase were only formed in the high-stress regions of the structure, and their propagation was limited. According to thermodynamic analysis, the ?Hmix value is -2.73 kJ/mol, the ?Smix value is 5.95 J/molK, the ? value is 34%, the ?? value is 0.14, and the ? value is 2.03. The obtained thermodynamic data were found to be compatible with the microstructural development of the alloy.

Analysis of stress and strain concentration around a centralized elliptical hole in a monodomain liquid crystal elastomer sheet

Liquid crystal elastomers (LCEs) are a kind of soft materials which couple the high elastic deformability with the unique properties of liquid crystals. Such combination endows notched LCEs unusual stress and strain concentration behaviors, which is preliminary discussed by us in the case of a large sheet with a centralized circular hole. Here we extend our study from circular holes to more generalized, elliptical holes. We investigate the stress and strain fields around a centralized elliptical hole in a LCE sheet subjected to uniaxial stretches by finite element methods. The effects of the shape factor, defined as the ratio of the minor-to-major axis of the ellipse, are mainly concerned. Compared with common elastomers like neoHookean, LCEs get higher stress and strain concentration factors (SCFs), and the location of maximal strain deviates slightly from that of maximal stress. With the decrease of the shape factor (a sharper ellipse) the SCFs of LCEs rise much severer than those of neoHookean, and the deviation is diminishing. In addition, as the stretch increases, the region near the root is getting close to an approximately uniaxial stress state, and the high strain region will extend non-monotonously along the hole edge, which differs from a monotonous extension of the high stress region. All the unusual behaviors of LCEs above are attributed to the spontaneous strain induced by director rotation. Attempts to correlate stress and strain concentration behaviors to the spontaneous strain are made in this article.

Advanced heat-resistant martensitic steels: Long-term creep deformation and fracture mechanisms

Creep behavior at 650 °C of the 9% Cr martensitic steels with different W, N and B contents was investigated using the threshold stress concept. Creep strength breakdown and the change in the “apparent” stress exponent were revealed at the applied stresses ranging from 140 to 120 MPa in both steels. Moreover, the threshold stresses, which were estimated by extrapolating the two parts of curves “Minimum creep rate vs. Applied stress” corresponding to the high and low stresses at a creep rate of 10−11 s−1, decreased from 94 … 98 MPa for both steels to 30 and 65 MPa in the 9Cr3W and 9Cr1.5WBN steels, respectively, at the transition from the high stress region to the low stress region. The sources of these threshold stresses are considered to be dislocations, the M23C6 carbides and Laves phase particles located at the low-angle lath boundaries as well as MX carbonitrides. Analysis of true stress exponent revealed no evidence for the change in creep mechanism at the transition from the high stresses to low stresses. This was creep controlled by high temperature climb of dislocations due to lattice diffusion. On the other hand, the creep fracture mechanism in the 9Cr3W steel was changed from the transgranular fracture at the high stresses to the intergranular fracture at the low stresses. This was accompanied with the formation of the high fraction of the coarse particles of Laves phase.

Satellite based detection of pre-earthquake thermal anomaly, co-seismic deformation and source parameter modelling of past earthquakes

The presented work brings satellite-based detection of pre-earthquake transient thermal anomaly and coseismic deformation mechanism using InSAR (SAR Interferometry) observations of the two major earthquakes that occurred recently (the Iran-Iraq border earthquake of Nov. 12, 2017 (Mw 7.3) and the New Mirpur, Pakistan earthquake of 24th Sept. 2019 (Mw 5.4)). Near Iran-Iraq border earthquake, a rise of 4–6 ​°C from the normal temperature in both day and night Land Surface Temperature (LST) maps along the SW part of the fault is observed. In case of the New Mirpur, Pakistan earthquake, a rise of 3–5 ​°C from normal temperature in both day and night LST maps around the SSW and NW side of the epicenter is observed. Further, Sentinel-1A microwave satellite data employing InSAR technique have been used to estimate co-seismic ground deformations and fault geometry and slip distribution of both earthquakes. The results show that both events occurred due to blind reverse fault mechanism dipping NE with shallow dip. The weightage of strike slip component and dip slip component is more or less equal to each other in total slip distribution for Iran-Iraq border earthquake. Similarly, New Mirpur, Pakistan earthquake have more strike slip components than dip slip components in the weightage in total slip distribution. Considerable number of aftershocks have also occurred in the regions as estimated by coulomb static stress along the SW side of the fault geometry of Iran-Iraq border earthquake and for New Mirpur, Pakistan earthquake, along the SSW, NW, and east side of the fault geometry. The correlation between high stress regions before and after the occurrence of earthquakes using thermal anomaly map and Coulomb static stress map has been also observed.

Numerical simulations on the settling dynamics of an ellipsoidal particle in a viscoelastic fluid

The settling dynamics of a rigid ellipsoid in an unbounded viscoelastic fluid under inertialess conditions is studied through direct numerical simulations. The governing equations are solved by the finite element method with an Arbitrary Lagrangian–Eulerian formulation to handle the particle motion. The viscoelastic fluid is modeled through the Giesekus constitutive equation. Simulations are carried out up to a value of Deborah number of 5. The settling of prolate and oblate spheroidal particles is first addressed. The sedimentation, lift, and angular velocities are computed as a function of the orientation angle and for aspect ratios from 1/8 to 8. Regardless of the particle shape, initial orientation, and Deborah number, the particle rotates to align its longest axis along the force direction. A high extensional stress region behind the particle is observed at high aspect ratios due to the large curvature of the tip, leading to a fast decay of the axial fluid velocity downstream and to the appearance of a negative wake. Similarly, a triaxial ellipsoid reaches a final orientation with major axis parallel to the falling direction. The particle shape affects the orientational dynamics, both in terms of the orbits followed by the orientation vectors and the time needed to reach the equilibrium orientation, and the steady-state settling velocity. The fastest sedimentation rate is observed for a prolate spheroid with aspect ratio of about 2 whereas the slowest one for a high aspect ratio oblate spheroid. Triaxial ellipsoids settle with rates in between these two limiting behaviors.

Numerical assessment of riverbank filtration using gravel back filter to improve water quality in arid regions

The main challenge of water resource management in high-stress regions, especially in developing countries, is in adopting non-traditional methods to obtain safe drinking water in desired quantities. In Egypt, the riverbank filtration (RBF) system is one of the most common solutions to solve rivers’ water quality issues. Several sites have been investigated, and the system has demonstrated tremendous potential. The drinking water plant in Embaba, Giza, Egypt, is considered in this study to improve the quality of the abstracted water through the vertical well system. The numerical code of MODFLOW and MT3D is used to simulate the impact of using the emplacement of the gravel-pack filter (GPF). Three different scenarios were investigated: the first consists of vertical GPF; the second is horizontal GPF for different geometries, depths, widths or thicknesses, lengths, and permeability of the filter material; and, the third is drilling a pipe filter through the riverbed for different pipe numbers, depths, and the material’s hydraulic conductivity. The results revealed that the riverbank filtration sharing (RBFS) rate was increased by increasing the filter width or thickness at the riverside, the filter pipe numbers, the length of the horizontal filter, and the permeability of filter material. At the same time, the thickness of the river bed decreased by increasing the filter width at the groundwater side. Also, the RBFS was increased by increasing the filter width or the thickness in the two directions and the pipe length. However, it returned to decrease again due to groundwater sharing. Thus, the RBF design should carefully consider the gravel-pack, pipe filter geometry, and permeability impact rate of RBFS.

Nanograin formation and cracking mechanism in Ti alloys under very high cycle fatigue loading

This paper shows that nanograins appear in a locally high-stress region for very high cycle fatigue of TC17 alloy, and twinning is a main contributor to nanograin formation. The locally high stress results in twinning or slip in preferentially oriented α grains. Then, the interaction between twin systems or dislocations induces the formation of dislocation cells or walls, nucleation of microbands, and finally the nanograins. As a result, the nanograin regions and the boundaries between the nanograin and coarse grain regions become preferential sites for crack initiation and early growth. The finite element analysis demonstrates the nanograin-related fatigue cracking behavior.