Showing papers on "Shear stress published in 2018"

Tailoring force sensitivity and selectivity by microstructure engineering of multidirectional electronic skins

Abstract: Electronic skins (e-skins) with high sensitivity to multidirectional mechanical stimuli are crucial for healthcare monitoring devices, robotics, and wearable sensors. In this study, we present piezoresistive e-skins with tunable force sensitivity and selectivity to multidirectional forces through the engineered microstructure geometries (i.e., dome, pyramid, and pillar). Depending on the microstructure geometry, distinct variations in contact area and localized stress distribution are observed under different mechanical forces (i.e., normal, shear, stretching, and bending), which critically affect the force sensitivity, selectivity, response/relaxation time, and mechanical stability of e-skins. Microdome structures present the best force sensitivities for normal, tensile, and bending stresses. In particular, microdome structures exhibit extremely high pressure sensitivities over broad pressure ranges (47,062 kPa−1 in the range of <1 kPa, 90,657 kPa−1 in the range of 1–10 kPa, and 30,214 kPa−1 in the range of 10–26 kPa). On the other hand, for shear stress, micropillar structures exhibit the highest sensitivity. As proof-of-concept applications in healthcare monitoring devices, we show that our e-skins can precisely monitor acoustic waves, breathing, and human artery/carotid pulse pressures. Unveiling the relationship between the microstructure geometry of e-skins and their sensing capability would provide a platform for future development of high-performance microstructured e-skins. Customizing piezoresistive sensors with different microscale geometries makes it easier for ‘electronic skin’ devices to sense forces in different directions. Hyunhyub Ko and colleagues from South Korea’s Ulsan National Institute of Science and Technology report that carbon nanotube/silicone elastomer composites fabricated into three shapes—micro-domes, pyramids, and pillars—have unique responses to mechanical stress and deformations. Experiments with interlocked pairs of micropatterned films revealed hemispherical shapes were best at sensing tensile and bending stresses, as well as minute changes to pressure. Micropillars, on the other hand, exhibited strong sensitivity to shear stress. With help from computer simulations, the researchers identified changes in contact area and localized stress as the critical factors needed to guide design of multidirectional force sensitivity. Prototype e-skin devices containing the interlocked microshapes successfully monitored bio-signals including breath patterns, spoken words, and arterial blood pressure. We present piezoresistive electronic skins with tunable force sensitivity and selectivity in response to multidirectional forces (normal, shear, tensile, bending) by engineering microstructure geometries (dome, pyramid, pillar). Microdome structures present the best force sensitivities for normal, tensile, and bending stresses. On the other hand, for shear stress, micropillar structures exhibit the highest sensitivity. As proof-of-concept demonstrations, the e-skins are used for wearable healthcare devices to precisely monitor various bio-signals including sound, human breath, and artery/carotid pulse pressures.

Large correction in opening wedge high tibial osteotomy with resultant joint-line obliquity induces excessive shear stress on the articular cartilage

Abstract: The purpose of this study was to analyse the resultant stress induced by joint-line obliquity after HTO for varus knee deformity using a three-dimensional (3D) finite element model analysis. The geometrical bone data used in this study were derived from commercially available human bone digital anatomy media. The 3D knee models were developed using 3D computer-aided design software. The articular surface was overlaid with a 2-mm-thick cartilage layer for both femoral and tibial condyles. Ligament structures were simulated based on properties reported in previous anatomical studies. Regarding the loading condition, isolated axial loads of 1200 N with lateral joint-line inclinations of 2.5°, 5°, 7.5°, and 10° in reference to the horizontal axis were applied to the femur to simulate the mechanical environment in a knee with joint-line obliquity. A steep rise of shear stress in the medial compartment was noted in the model with obliquity of 5° or more. This laterally directed shear stress exhibited an incremental increase in accordance with the obliquity angle. The maximum shear stress value in the medial cartilage increased from 1.6 MPa for the normal knee to 3.3, 5.2, and 7.2 MPa in the joint-line obliquity models with 5°, 7.5°, and 10° of obliquity, respectively. The effects of HTO for varus knee deformity on the amount/distribution of stresses in the articular cartilage were analysed using a 3D finite element model. It was shown that joint-line obliquity of more than 5° induced excessive shear stress in the tibial articular cartilage. A large amount of correction in OWHTO with a resultant joint-line obliquity of 5° or more may induce detrimental stress to the articular cartilage. Double-level osteotomy should be considered as a surgical option in this situation.

A constitutive model for simple shear of dense frictional suspensions

Abstract: Discrete particle simulations are used to study the shear rheology of dense, stabilized, frictional particulate suspensions in a viscous liquid, toward development of a constitutive model for steady shear flows at arbitrary stress. These suspensions undergo increasingly strong continuous shear thickening (CST) as the solid volume fraction ϕ increases above a critical volume fraction, and discontinuous shear thickening (DST) is observed in a range of ϕ. When studied at controlled stress, the DST behavior is associated with nonmonotonic flow curves of the steady-state shear rate as a function of stress. Recent studies have related shear thickening to a transition from mostly lubricated to predominantly frictional contacts with the increase in stress. In this study, the behavior is simulated over wide ranges of concentration, dimensionless shear stress, and coefficient of interparticle friction. The simulation data have been used to populate the lubricated-to-frictional rheology model of Wyart and Cates [Phy...

Failure Characteristics of Granite Influenced by Sample Height-to-Width Ratios and Intermediate Principal Stress Under True-Triaxial Unloading Conditions

Abstract: The failure modes and peak unloading strength of a typical hard rock, Miluo granite, with particular attention to the sample height-to-width ratio (between 2 and 0.5), and the intermediate principal stress was investigated using a true-triaxial test system. The experimental results indicate that both sample height-to-width ratios and intermediate principal stress have an impact on the failure modes, peak strength and severity of rockburst in hard rock under true-triaxial unloading conditions. For longer rectangular specimens, the transition of failure mode from shear to slabbing requires higher intermediate principal stress. With the decrease in sample height-to-width ratios, slabbing failure is more likely to occur under the condition of lower intermediate principal stress. For same intermediate principal stress, the peak unloading strength monotonically increases with the decrease in sample height-to-width. However, the peak unloading strength as functions of intermediate principal stress for different types of rock samples (with sample height-to-width ratio of 2, 1 and 0.5) all present the pattern of initial increase, followed by a subsequent decrease. The curves fitted to octahedral shear stress as a function of mean effective stress also validate the applicability of the Mogi–Coulomb failure criterion for all considered rock sizes under true-triaxial unloading conditions, and the corresponding cohesion C and internal friction angle φ are calculated. The severity of strainburst of granite depends on the sample height-to-width ratios and intermediate principal stress. Therefore, different supporting strategies are recommended in deep tunneling projects and mining activities. Moreover, the comparison of test results of different σ2/σ3 also reveals the little influence of minimum principal stress on failure characteristics of granite during the true-triaxial unloading process.

Cracking Processes and Coalescence Modes in Rock-Like Specimens with Two Parallel Pre-existing Cracks

Abstract: The cracks in a rock tend to initiate, propagate, and coalesce under loading. Based on the digital image correlation (DIC) method, uniaxial compression tests are carried out on rock-like specimens with various arrangements of two parallel cracks. The full-field strain and failure features of the rock-like materials are observed and analysis by a self-developed code. Two process zones are defined according to the differences between the shear strain field and the tensile strain field: a shear process zone and a tensile process zone. The following results are obtained in this study. (1) Three coalescence modes can be observed using the DIC method: a shear coalescence mode, a tensile coalescence mode, and a mixed coalescence mode. (2) At the microscopic level, the bridge angle and crack arrangement affect the formation of the process zone; at the macroscopic level, they determine the crack propagation path and the failure mode. (3) The peak strength of the rock-like specimen is related to the crack inclination angle and the bridge angle. (4) Numerical modeling by the expanded distinct element method and the strain strength criterion simulates the different coalescence modes of the experimental study efficiently.

Impact of Cavitation, High Shear Stress and Air/Liquid Interfaces on Protein Aggregation.

Abstract: The reported impact of shear stress on protein aggregation has been contradictory. At high shear rates, the occurrence of cavitation or entrapment of air is reasonable and their effects possibly misattributed to shear stress. Nine different proteins (α-lactalbumin, two antibodies, fibroblast growth factor 2, granulocyte colony stimulating factor [GCSF], green fluorescence protein [GFP], hemoglobin, human serum albumin, and lysozyme) are tested for their aggregation behavior on vapor/liquid interfaces generated by cavitation and compared it to the isolated effects of high shear stress and air/liquid interfaces generated by foaming. Cavitation induced the aggregation of GCSF by +68.9%, hemoglobin +4%, and human serum albumin +2.9%, compared to a control, whereas the other proteins do not aggregate. The protein aggregation behaviors of the different proteins at air/liquid interfaces are similar to cavitation, but the effect is more pronounced. Air-liquid interface induced the aggregation of GCSF by +94.5%, hemoglobin +35.5%, and human serum albumin (HSA) +31.1%. The results indicate that the sensitivity of a certain protein toward cavitation is very similar to air/liquid-induced aggregation. Hence, hydroxyl radicals cannot be seen as the driving force for protein aggregation when cavitation occurs. Further, high shear rates of up to 108 s-1 do not affect any of the tested proteins. Therefore, also within this study generated extremely high isolated shear rates cannot be considered to harm structural integrity when processing proteins.

Wall shear stress from jetting cavitation bubbles

Abstract: The collapse of a cavitation bubble near a rigid boundary induces a high-speed transient jet accelerating liquid onto the boundary. The shear flow produced by this event has many applications, examples of which are surface cleaning, cell membrane poration and enhanced cooling. Yet the magnitude and spatio-temporal distribution of the wall shear stress are not well understood, neither experimentally nor by simulations. Here we solve the flow in the boundary layer using an axisymmetric compressible volume-of-fluid solver from the OpenFOAM framework and discuss the resulting wall shear stress generated for a non-dimensional distance,γ = 1.0 (γ = h/Rmax, where h is the distance of the initial bubble centre to the boundary, and Rmax is the maximum spherical equivalent radius of the bubble). The calculation of the wall shear stress is found to be reliable once the flow region with constant shear rate in the boundary layer is determined. Very high wall shear stresses of 100 kPa are found during the early spreading of the jet, followed by complex flows composed of annular stagnation rings and secondary vortices. Although the simulated bubble dynamics agrees very well with experiments, we obtain only qualitative agreement with experiments due to inherent experimental challenges.

Influence of Porosity on the Flexural and Free Vibration Responses of Functionally Graded Plates in Thermal Environment

Abstract: This paper examines the influence of porosities on the flexural and free vibration response of functionally graded material (FGM) plates based on the authors’ recently developed non-polynomial higher-order shear and normal deformation theory. The theory accommodates the nonlinear variation in the in-plane and transverse displacements in the thickness coordinates. It also contains the hyperbolic shear strain shape function in the displacement field with only four unknowns. A new mathematical model has also been proposed to incorporate the effects of porosity in the FGM plate. Various numerical examples have been solved to ascertain the accuracy, efficiency, and applicability of the present formulation. The effects of porosity, volume fraction index, plate thickness, aspect ratio, boundary conditions and temperature have been discussed in details. The obtained results can be treated as a benchmark for future studies.

Blood Pump Design Variations and Their Influence on Hydraulic Performance and Indicators of Hemocompatibility.

Abstract: Patients with ventricular assist devices still suffer from high rates of adverse events. Since many of these complications are linked to the flow field within the pump, optimization of the device geometry is essential. To investigate design aspects that influence the flow field, we developed a centrifugal blood pump using industrial guidelines. We then systematically varied selected design parameters and investigated their effects on hemodynamics and hydraulic performance using computational fluid dynamics. We analysed the flow fields based on Eulerian and Lagrangian features, shear stress histograms and six indicators of hemocompatibility. Within the investigated range of clearance gaps (50-500 µm), number of impeller blades (4-7), and semi-open versus closed shroud design, we found association of potentially damaging shear stress conditions with larger gap size and more blades. The extent of stagnation and recirculation zones was reduced with lower numbers of blades and a semi-open impeller, but it was increased with smaller clearance. The Lagrangian hemolysis index, a metric commonly applied to estimate blood damage, showed a negative correlation with hydraulic efficiency and no correlation with the Eulerian threshold-based metric.

Analytical solution of a Maxwell fluid with slip effects in view of the Caputo-Fabrizio derivative ⋆

Abstract: Couette flows of an incompressible Maxwell fluid with non-integer order derivative without singular kernel due to the motion of a bottom flat plate are analyzed under the slip boundary condition. An analytical transform approach is used to obtain the exact expressions for both velocity field and shear stress. Three particular cases from the general results with slip at the wall are obtained. These solutions, which are organized in simple forms in terms of exponential and trigonometric functions, can be conveniently engaged to obtain known solutions from the literature. The control of the new non-integer order derivative on the velocity and shear stress of the fluid is analyzed for some flows with practical applications. The non-integer order derivative with non-singular kernel is more appropriate for handling mathematical calculations of the obtained solutions.

Characterization of Cell Damage and Proliferative Ability during and after Bioprinting.

Abstract: When a biomaterial solution containing living cells is subject to bioprinting, the cells experience process-induced stresses, including shear and extensional stresses. These process-induced stresses breach cell membranes and can lead to cell damage, thus reducing cell viability and functioning within the printed constructs. Studies have been conducted to determine the influence of shear stress on cell damage; however, the effect of extensional stress has been typically ignored in the literature until the recently collected evidence of its importance. This paper presents a novel method to characterize and quantify the cell damage caused by both shear and extensional stresses in bioprinting. In this method, cell damage law is first established to relate cell damage to shear stress based on the experiments with a rheometer; the process-induced shear stress experienced by cells in bioprinting is represented, and the established cell damage model is applied to calculate the degree of cell damage caused by shear stress in bioprinting; then cell damage caused by extensional stress is inferred from the difference between the total cell damage and the amount of cell damage attributed to shear stress. With the obtained magnitude of extensional stress from fluidic simulation, the model that relates extensional stress to cell damage is established; the bioprinting process-induced cell damage attributed to both shear and extensional stresses is therefore presented. Schwann cells and myoblasts were used as examples to validate the models. Comparison between experimental and simulation results shows the effectiveness of the models presented in this paper. Moreover, the viability and proliferative ability of cells in the first 72 h after bioprinting is investigated, with the results illustrating that the process-induced forces affect not only cell viability but also their proliferative ability after bioprinting.

Similarity and structure of wall turbulence with lateral wall shear stress variations

Abstract: Wall-bounded turbulence, where it occurs in engineering or nature, is commonly subjected to spatial variations in wall shear stress. A prime example is spatially varying roughness. Here, we investigate the configuration where the wall shear stress varies only in the lateral direction. The investigation is idealised in order to focus on one aspect, namely, the similarity and structure of turbulent inertial motion over an imposed scale of stress variation. To this end, we analyse data from direct numerical simulation (DNS) of pressure-driven turbulent flow through a channel bounded by walls of laterally alternating patches of high and low wall shear stress. The wall shear stress is imposed as a Neumann boundary condition such that the wall shear stress ratio is fixed at 3 while the lateral spacing $s$ of the uniform-stress patches is varied from 0.39 to 6.28 of the half-channel height $\unicode[STIX]{x1D6FF}$ . We find that global outer-layer similarity is maintained when $s$ is less than approximately $0.39\unicode[STIX]{x1D6FF}$ while local outer-layer similarity is recovered when $s$ is greater than approximately $6.28\unicode[STIX]{x1D6FF}$ . However, the transition between the two regimes through $s\approx \unicode[STIX]{x1D6FF}$ is not monotonic owing to the presence of secondary roll motions that extend across the whole cross-section of the flow. Importantly, these secondary roll motions are associated with an amplified skin-friction coefficient relative to both the small- and large- $s/\unicode[STIX]{x1D6FF}$ limits. It is found that the relationship between the secondary roll motions and the mean isovels is reversed through this transition from low longitudinal velocity over low stress at small $s/\unicode[STIX]{x1D6FF}$ to high longitudinal velocity over low stress at large $s/\unicode[STIX]{x1D6FF}$ .

Microscopic Origins of Shear Jamming for 2D Frictional Grains.

Abstract: Shear jamming (SJ) occurs for frictional granular materials with packing fractions ϕ in ϕ_{S}<ϕ<ϕ_{J}^{0}, when the material is subject to shear strain γ starting from a force-free state. Here, ϕ_{J}^{μ} is the isotropic jamming point for particles with a friction coefficient μ. SJ states have mechanically stable anisotropic force networks, e.g., force chains. Here, we investigate the origins of SJ by considering small-scale structures-trimers and branches-whose response to shear leads to SJ. Trimers are any three grains where the two outer grains contact a center one. Branches occur where three or more quasilinear force chain segments intersect. Certain trimers respond to shear by compressing and bending; bending is a nonlinear symmetry-breaking process that can push particles in the dilation direction faster than the affine dilation. We identify these structures in physical experiments on systems of two-dimensional frictional discs, and verify their role in SJ. Trimer bending and branch creation both increase Z above Z_{iso}≃3 needed for jamming 2D frictional grains, and grow the strong force network, leading to SJ.

Shear resistance of stiffened steel corrugated shear walls

Abstract: In recent years, the steel corrugated shear walls (SCSWs) are widely used in building structures to serve as lateral force resistant members. For some practical engineering applications that the width of the infilled SCSWs in frame structure is much greater than its height, it is common to add vertical stiffening systems to the SCSWs, thus forming the stiffened SCSWs (SSCSWs), and the stiffening system is composed of a pair of vertical stiffeners installed on both sides of the corrugated plate and the connecting high-strength bolts. In this paper, the shear resistant behavior of the SSCSWs is investigated via FE analyses considering both the geometrical and material nonlinearities, and over 300 models are analyzed through elastoplastic numerical process. The comparison of the shear resistant behavior of SSCSWs with different stiffening rigidities is performed, which indicates that the stiffening system can effectively restrain the out-of-plane displacements of the corrugated wall, and can improve both shear resistance and ductility of the SSCSWs. Then a transition rigidity ratio of the stiffening system is proposed to reflect the critical value of the stiffening rigidity that the out-of-plane displacements of the corrugated plate are fully restrained at the bolted locations. Correspondingly, curve fitted formula of the transition rigidity ratio is provided to enable a conservative prediction. Finally, shear buckling formulas are fitted to reveal the relationship between the reduction factor and the normalized aspect ratio, and they are validated to be able to conservatively predict the ultimate shear stress of SSCSWs. Accordingly, some design recommendations are presented, which could provide valuable references for practical design of SSCSWs.