Dynamic Shear Stress Research Articles

Frictional heat from faulting of the 1999 Chi‐Chi, Taiwan earthquake

The frictional heat generated during an earthquake is thought to be a major portion of the total energy release. However, there have been no direct measurements of the heat generated by a large earthquake. We present an estimate of the frictional heat produced by the 1999 Chi‐Chi, Taiwan earthquake (Mw 7.6), from temperature anomaly that is measured in a borehole penetrating a large slip region (∼10 m) of the fault. A local increase in the temperature profile across the fault is interpreted to be the residual heat generated during the earthquake. Analyses of this temperature anomaly lead to a low estimate of the dynamic shear stress, 0.5 to 0.9 MPa, and the heat produced in the large slip region of the fault, 0.68 to 1.32 × 1017 J. The total frictional energy is estimated as 2.4 to 6.1 × 1017 J, indicating that seismic efficiency is 1 to 3%.

Effects of fluid dynamic induced shear stress on fungal growth and morphology

Industrial applications of filamentous microorganisms become more and more relevant for example for the production of pharmaceuticals. Therefore, pellet morphology is often requested. Knowledge about pellet growth behaviour at different fluid dynamic conditions is important, because many production processes are influenced by morphology. Morphology itself is influenced by the fluid dynamic conditions in a bioreactor. In the present study, pellet growth of Aspergillus niger AB1.13 was studied under different fluid dynamic conditions. A new experimental set-up was designed to guarantee comparability and reproducibility. Kinetic parameters of pellet growth were correlated with energy input and the maximum shear rate. The present work shows that kinetic parameters depend on the fluid dynamic conditions. Additionally, it could be demonstrated that for model development a correlation with the local maximum shear rate is of relevance especially in relation to transferability to other reaction systems with different set-up.

Glycocalyx-mimetic dextran-modified poly(vinyl amine) surfactant coating reduces platelet adhesion on medical-grade polycarbonate surface

A dextran-modified poly(vinyl amine) comb-like surfactant polymer, poly( N-vinyl dextran aldonamide -co-N-vinyl hexanamide) , that can surface-adsorb on hydrophobic polymeric substrates, was designed to improve the interfacial blood-compatibility of polymeric biomaterials. Medical-grade polycarbonate was selected as a model substrate because of its extensive use in blood-contacting biomedical devices like hemodialyzers, blood pumps and oxygenators. The surfactant polymer was physisorbed from aqueous solution onto the polycarbonate substrate. The surfactant coating was stable under dynamic shear conditions in whole blood, as confirmed by fluorescence microscopy and total internal reflection fluorescence (TIRF) experiments with fluorescein-labeled surfactant polymer. The coated disks and uncoated control disks were exposed to platelet-rich plasma (PRP) and whole human blood in a rotating disk system (RDS) to study platelet-adhesion under dynamic shear stress environments. Adhered platelets were stained with fluorescein isothiocyante (FITC)-tagged anti-CD41a monoclonal antibody and imaged by epifluorescence microscopy. Complimentary images were obtained by phase-contrast microscopy. Platelet adhesion on the surfactant-coated disks was reduced by ∼90%, compared with uncoated disks. The images also showed a concomitant reduction in platelet-derived microparticles on surfactant-coated disks, compared with uncoated disks. The results suggest potential application of carbohydrate-modified surfactant polymers as a glycocalyx-mimetic non-thrombogenic interfacial coating for blood-contacting biomaterials.

Dynamic shear stress at cell's membrane is governed by mechanical interaction between fluid and cell

Dynamic shear stress at cell's membrane is governed by mechanical interaction between fluid and cell

Effect of sock on biomechanical responses of foot during walking

Except the plantar pressure and gross joint motion, we know little about the mechanical state of a foot during walking. This study aimed at investigating the effect of wearing socks with different frictional properties on plantar shear, which is a possible mechanical risk factor of foot lesion development. A 3-D finite element model for simulating the foot-sock-insole contact was developed to investigate the biomechanical effects of wearing socks with different combinations of frictional properties on the plantar foot contact. The dynamic plantar pressure and shear stress during the stance phases of gait were studied through finite element computations. Three cases were simulated, a barefoot with a high frictional coefficient against the insole (0.54) and two socks, one with a high frictional coefficient against the skin (0.54) and a low frictional coefficient against the insole (0.04) and another with an opposite frictional properties assignment. Wearing sock of low friction against the insole to allow more relative sliding between the plantar foot and footwear was found to reduce the shear force significantly: at the rearfoot from 3.1 to 0.88 N, and at the forefoot from 10.61 to 1.61 N. The shear force can be further reduced to 0.43 N at the rearfoot, and 1.18 N at the forefoot, when wearing the sock with low friction against the foot skin and high friction set against the insole. Wearing sock with low friction against the foot skin was found to be more effective in reducing plantar shear force on the skin than the sock with low friction against the insole. The risk of barefoot walking in developing plantar shear related blisters and ulcers might be reduced by socks wearing especially those with low friction against the foot skin.

Secrets of the code: Do vascular endothelial cells use ion channels to decipher complex flow signals?

The ability of vascular endothelial cells (ECs) to respond to changes in blood flow is essential for both vasoregulation and arterial wall remodelling, while abnormalities in endothelial responsiveness to flow play an important role in the development of atherosclerosis. Endothelial flow responses also have important implications for the field of vascular tissue engineering. In response to changes in fluid dynamic shear stress, ECs exhibit humoral, metabolic, and structural responses. Significantly, ECs respond differently to different types of shear stress. For instance, steady shear stress elicits a profile of responses that differs drastically from oscillatory shear stress. Although our understanding of flow-induced signaling has advanced greatly over the past two decades, how ECs sense shear forces remains to be established. Furthermore, the mechanisms by which ECs discriminate among different flow waveforms are unknown. Activation of flow-sensitive ion channels is one of the most rapid known responses to flow in ECs. In this paper, we argue in favor of an important role for ion channels in shear stress sensing in ECs and propose that these channels may endow ECs with the ability to resolve components of a complex flow signal and hence distinguish among different types of flow.

Preliminary study on variation characteristics of ocean tide dynamic stress in crust and its relationship with earthquakes

The variation characteristics of dynamic stress in crustal blocks and its relationship with earthquakes are comprehensively studied by analyzing geophysical data and calculating the dynamic response of crustal blocks in North China to the tide level change of Bohai Sea and Yellow Sea using a 3D nonlinear dynamic finite element simulation. This study has noticed some new features of crustal dynamic stress: (a) High stress level appears at the place where the stress waves superpose and interference during the process of propagation, and the enhancement of dynamic shear stress is more significant as compared with that of compressive stress; (b) Variation of dynamic stress has influence on seismogenic environment and hence earthquake occurrence; (c) As viewed from the variation characteristics of crustal dynamic stress, the superimposing process of shear stress manifests the preparation process of earthquakes.

Can pseudotachylytes be used to infer earthquake source parameters? An example of limitations in the study of exhumed faults

Tectonic pseudotachylytes might be used to constrain earthquake source parameters, such as dynamic shear stress resistance, average dynamic friction and slip-weakening distance. Estimation of dynamic shear stress resistance and dynamic friction from field studies is based on the assumption that the volume of melt produced during coseismic slip is proportional to the frictional work converted to heat on the fault surface. Conditions conducive to a realistic estimate of dynamic shear resistance are: (i) the presence of large outcrop exposures that allow for estimation of the volume of pseudotachylyte; (ii) the presence of structural markers offset by faults in order to relate the displacement accommodated by the fault with the volume of melt produced; (iii) data that provide an estimate of the initial melt temperature; and (iv) determination of host-rock temperature and pressure conditions that may have existed during seismic faulting. An independent indication that steady-state friction in the presence of melts might be achieved during coseismic slip arises from the dependence of the fractal dimension of the fault profile (intersection of the fault surface with the outcrop surface) with displacement. This relation could also indicate the slip-weakening distance ( Hirose, T., Shimamoto, T., 2003. Fractal dimension of molten surfaces as a possible parameter to infer the slip-weakening distance of faults from natural pseudotachylytes. Journal of Structural Geology 25, 1569–1574). The above conditions are all satisfied in the case of the Gole Larghe Fault Zone, which consists of hundreds subparallel strike-slip faults that cut tonalites of the Adamello batholith (Italy). The thickness of pseudotachylyte-bearing faults increases with displacement. From displacement/thickness ratios and energy balance calculations, we determined the dynamic shear resistance for several pseudotachylyte-bearing faults. In the same faults, the fractal dimension of the fault profile increases from 1.0 to 1.16 with displacement. This was also observed in experiments where steady-state friction in the presence of melt was achieved ( Hirose, T., Shimamoto, T., 2003. Fractal dimension of molten surfaces as a possible parameter to infer the slip-weakening distance of faults from natural pseudotachylytes. Journal of Structural Geology 25, 1569–1574). However, we will show that the estimate of the dynamic shear stress resistance, average dynamic friction and slip-weakening distance in the studied faults is limited by the uncertainties to attribute the measured displacement to a single seismic rupture. Since many pseudotachylytes in the upper seismogenic crust overprint preexisting cataclasites, it is suggested that future field and experimental work should be addressed to determine microstructural indicators (i.e. evolution of cataclastic fabric with displacement) within cataclasites, which might constrain the contribution of the cataclastic, pre-pseudotachylyte displacement to the total displacement accommodated by the fault.

Magnetic resonance elastography

The goal of our research is to develop MRI-based methods for assessing the mechanical properties of tissues in vivo. We have focused on a novel MRI technique for visualizing propagating acoustic shear waves [Science 269, 1854–1857 (1995)]. Suitable dynamic shear stress for Magnetic Resonance Elastography (MRE) can be generated by surface drivers, inertial effects, acoustic radiation pressure, or endogenous physiologic mechanisms. The MRE acquisition sequence is capable of visualizing cyclic tissue motion of less than 1 micron in displacement amplitude, with imaging times ranging from 100 ms to several minutes. Inversion algorithms based on continuum mechanics are used to process the acquired data to generate maps of mechanical properties such as depict stiffness, viscosity, attenuation, and anisotropic behavior. We have applied MRE to assess specimens of a variety of tissues, ranging in stiffness from lung to cartilage. Human studies have demonstrated that it is feasible to apply MRE to quantitatively image the mechanical properties of skeletal muscles, gray and white matter in the brain, thyroid, kidney, liver, and skin. Our preliminary clinical studies have to date applied MRE to observe changes in tissue mechanical properties in patients with breast, brain, and thyroid tumors, liver fibrosis, and diffuse diseases of skeletal muscle.

Interaction of Wall Shear Stress Magnitude and Gradient in the Prediction of Arterial Macromolecular Permeability

Large spatial shear stress gradients have anecdotally been associated with early atherosclerotic lesion susceptibility in vivo and have been proposed as promoters of endothelial cell dysfunction in vitro. Here, experiments are presented in which several measures of the fluid dynamic shear stress, including its gradient, at the walls of in vivo porcine iliac arteries, are correlated against the transendothelial macromolecular permeability of the vessels. The fluid dynamic measurements are based on postmortem vascular casts, and permeability is measured from Evans blue dye (EBD) uptake. Time-averaged wall shear stress (WSS), as well as a new parameter termed maximum gradient stress (MGS) that describes the spatial shear stress gradient due to flow acceleration at a given point, are mapped for each artery and compared on a point-by-point basis to the corresponding EBD patterns. While there was no apparent relation between MGS and EBD uptake, a composite parameter, WSS(-0.11) MGS(0.044), was highly correlated with permeability. Notwithstanding the small exponents, the parameter varied widely within the region of interest. The results suggest that sites exposed to low wall shear stresses are more likely to exhibit elevated permeability, and that this increase is exacerbated in the presence of large spatial shear stress gradients.

Slow dynamics of supercooled m-toluidine investigated by mechanical spectroscopy

Dynamics of supercooled m-toluidine close to the glass transition have been investigated by dynamic shear modulus measurements and stress relaxation experiments. The viscoelastic response of this material follows time-temperature-superposition in the temperature range investigated. Comparison with results at ultrasonic frequencies suggests the existence of a secondary relaxation. A change of the temperature dependent viscosity from a Vogel-Fulcher-Tammann behavior to another regime at low temperatures is also found. Compared to most inorganic glass formers, the viscosity of m-toluidine at the glass transition is approximately two orders of magnitude lower. The shear relaxation times are characterized by the same temperature dependence as the viscosity. They are in reasonable agreement with the results of previous ultrasonic measurements. The conclusions of the present work agree with recent results obtained by high resolution dielectric spectroscopy.

Vascular endothelial wound closure under shear stress: role of membrane fluidity and flow-sensitive ion channels

Sufficiently rapid healing of vascular endothelium following injury is essential for preventing further pathological complications. Recent work suggests that fluid dynamic shear stress regulates endothelial cell (EC) wound closure. Changes in membrane fluidity and activation of flow-sensitive ion channels are among the most rapid endothelial responses to flow and are thought to play an important role in EC responsiveness to shear stress. The goal of the present study was to probe the role of these responses in bovine aortic EC (BAEC) wound closure under shear stress. BAEC monolayers were mechanically wounded and subsequently subjected to either "high" (19 dyn/cm(2)) or "low" (3 dyn/cm(2)) levels of steady shear stress. Image analysis was used to quantify cell migration and spreading under both flow and static control conditions. Our results demonstrate that, under static conditions, BAECs along both wound edges migrate at similar velocities to cover the wounded area. Low shear stress leads to significantly lower BAEC migration velocities, whereas high shear stress results in cells along the upstream edge of the wound migrating significantly more rapidly than those downstream. The data also show that reducing BAEC membrane fluidity by enriching the cell membrane with exogenous cholesterol significantly slows down both cell spreading and migration under flow and hence retards wound closure. Blocking flow-sensitive K and Cl channels reduces cell spreading under flow but has no impact on cell migration. These findings provide evidence that membrane fluidity and flow-sensitive ion channels play distinct roles in regulating EC wound closure under flow.

Exact transient full–field analysis of a finite crack subjected to dynamic anti–plane concentrated loadings in anisotropic materials

In this study, the elastodynamic full–field response of a finite crack in an anisotropic material subjected to a dynamic anti–plane concentrated loading with Heaviside–function time dependence is investigated. A linear coordinate transformation is introduced to simplify the problem. The linear coordinate transformation reduces the anisotropic finite–crack problem to an equivalent isotropic problem. An alternative methodology, different from the conventional superposition method, is developed to construct the reflected and diffracted wave fields. The transient solutions are determined by superposition of two proposed fundamental solutions in the Laplace transform domain. The fundamental solutions to be used are the problems for applying exponentially distributed traction and displacement on the crack faces and along the crack–tip line in the Laplace transform domain, respectively. Exact analytical transient solutions for dynamic shear stresses, displacement and stress–intensity factor are obtained by using the Cagniard–de Hoop method of Laplace inversion and are expressed in explicitly compact formulations. The solutions have accounted for the contributions of all diffracted waves generated from two crack tips. Numerical results for the time history of shear stresses and stress–intensity factors during the transient process are calculated based on analytical solutions and are discussed in detail. The transient solutions of stresses have been shown to approach the corresponding static values after the first eight waves have passed the field point. The dynamic stress–intensity factor will reach a maximum value when the incident wave arrives at the crack tip, and remain constant before the first diffracted wave generated from the other crack tip arrives, and then will oscillate near the static value. A simple explicit expression of the dynamic overshoot for stress–intensity factors is derived as a function of the location for applied loadings, the crack length and material constants.

The steady state and dynamic rheological properties of telechelic associative polymer solutions

In this study, steady state, dynamic shear and normal stress differences of the polymer solutions containing telechelic hydrophobically modified ethoxylated urethane (HEUR) associative polymers have been investigated. The intermediate steady state and dynamic shear thickening were observed for both ascending and descending shear rate and frequency, respectively. The dynamic amplitude stress sweep measurements showed that maxima in G′ and G″ occurred at the frequencies of 1, 5 and 10 rad/s. At lower shear rates and frequencies, the ratio of complex viscosity to steady shear viscosity was observed to be greater than unity. This phenomenon is believed to be associated with the formation of a micro-gel like structure. The modified Cox–Merz rule (Delaware–Rutgers rule) explained the dynamic and steady state viscosity behaviors well in the shear thinning zone but not at the plateau or shear thickening regions.

Characterization of rheology of fresh fiber reinforced cementitious composites through ram extrusion

Rheology behavior of fresh short fiber reinforced cementitious composites was investigated through ram extrusion. The experimental results were interpreted by a six-parameter Benbow-Bridgwater model. The effects of water contents, matrix composition, fiber volume and fiber types on the die entry and die land pressure were investigated and related to the compositions of the fresh short fiber reinforced cementitious composites. The six rheological parameters, together with the dynamic bulk stress and dynamic surface shear stress, were derived and compared for various composites. It was found that fresh short fiber reinforced cementitious composites had an obvious pseudo-plasticity and shear-thinning behavior. The water content, fiber concentration, fiber type and cement matrix influenced the rheology of the fresh composites in different mechanisms. The quantitative rheology parameters, such as bulk yield stress, dynamic bulk stress, wall shear yield stress and dynamic surface shear stress, provided promising interpretations for experimental results.

Impact of Shear Stress on Early Vein Graft Remodeling: A Biomechanical Analysis

In this study, an analytical technique was developed to estimate the dynamic, three-dimensional wall shear and tensile forces within the arterial circulation and applied to our established model of flow-mediated graft remodeling within the rabbit carotid system. Through selective unilateral distal ligation, vein grafts were exposed to distinct flow environments, characterized by a sixfold difference in mean flow rate. Implanted vein grafts were harvested at 1, 3, 7, 14, and 28 days to evaluate graft morphology. Hemodynamic and real-time imaging data, obtained at graft implantation and harvest, served as input for estimation of dynamic shear stress and wall tension. Marked differences in the remodeling process were observed in high vs. low flow grafts, with low flow grafts demonstrating accelerated intimal hyperplasia and reduced outward remodeling. The impact was a peak in the maximum and minimum shear stress at Day 7, with a delayed increase in lumen diameter leading to partial normalization of the wall shear by Day 28. Intramural wall tension reached a maximum at Day 3, with an increase in wall thickness leading to a significant reduction in these forces by Day 14. Despite the significant morphologic changes, no differences in the incremental moduli of elasticity were observed.

Dynamic punch shear characterization of plain weave graphite/epoxy composites at room and elevated temperatures

In many situations, composite structures are subjected to dynamic loading. To design such structures, there is a need of dynamic material properties. In the current investigation, a compression Split Hopkinson Pressure Bar was modified to load the samples in punch shear mode. Experimental studies were conducted to characterize 24 layer plain weave graphite/epoxy laminate samples. Laminates were manufactured using vacuum assisted resin infusion molding process. Samples of nominal size 26 mm × 13 mm × 4 mm were subjected to dynamic punch shear test at three different pressure settings that yielded strain rates in the range of 1415–3601/s at room and elevated temperatures of 51.7 °C, 79.4 °C and 107.2 °C. Dynamic shear stress–strain properties were evaluated. Failure modes were characterized through optical and scanning electron microscopy. Results of the tests indicate that the dynamic shear strength increases with increase in the strain rate and decreases with temperature for a given strain rate. Higher temperature causes the softening of matrix.

Reply to comment by N. Koizumi et al. on “Coseismic hydrological changes associated with dislocation of the September 21, 1999 Chichi earthquake, Taiwan”

Reply to comment by N. Koizumi et al. on “Coseismic hydrological changes associated with dislocation of the September 21, 1999 Chichi earthquake, Taiwan”

Dynamic shear stress in parallel-plate flow chambers

An in vitro model using a parallel-plate fluid flow chamber is supposed to simulate in vivo fluid shear stresses on various cell types exposed to dynamic fluid flow in their physiological environment. The metabolic response of cells in vitro is associated with the wall shear stress. However, parallel-plate flow chambers have not been characterized for dynamic fluid flow experiments. We use a dimensionless ratio h/ λ v, in determining the exact magnitude of the dynamic wall shear stress, with its oscillating components scaled by a shear factor T. It is shown that, in order to expose cells to predictable levels of dynamic fluid shear stress, two conditions have to be met: (1) h/ λ v<2, where h is the distance between the plates and λ v is the viscous penetration depth; and (2) f 0< f c/ m, where the critical frequency f c is the upper threshold for this flow regime, m is the highest harmonic mode of the flow, and f 0 is the fundamental frequency of fluid flow.

Development and characterization of hydrocarbon polyol polyurethane and silicone magnetorheological polymeric gels

AbstractMagnetorheological polymeric gels (MRPG) have been developed for use in semi‐active magnetorheological fluid (MRF) dampers and other magnetorheological (MR) devices. The novel MRPGs are prepared by suspending iron particles in polymeric gels. Off‐state (i.e, no applied magnetic field) viscosity and settling behavior can be controlled through the selection of polymeric gels. In this study, tunable rheological properties were investigated with a piston‐driven flow type rheometer with a shear rate varying from 20 s−1 to 6,000 s−1. Silicone MRPG (with 84.5 wt % iron particles) has controllable viscosity and a high shear yield stress over a wide range of shear rates. Silicone MRPG (79.5 wt % iron particles) has the lowest viscosity of those studied. Polyurethane MRPG has the lowest settling rate. The order of addition of magnetic particles and polymer during the polymerization process affects the MRPG final off‐state apparent viscosity (80% increase in apparent viscosity for silicone MRPG polymerized after adding iron particles). This indicates that polymer gels modify the surface properties of the magnetic particles, causing interaction among particles. The dynamic shear yield stress is higher for fluids with better dispersion stability. Polyurethane MRPG, which has the lowest settling rate, has a high dynamic yield stress (23 kPa at 350 mT). Both dynamic and static shear stress values of the MRPGs were found to be similar in magnitude (5–8 kPa at 120 mT for silicone MRPG with 84.5 wt % iron particles and polyurethane MRPG), indicating that MRPGs can provide consistent performance in devices. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1176–1182, 2004