Values Of Normal Stress Research Articles

An apparatus for studying the response of cultured

In order to simulate the hemodynamic environment of human arteries in vivo, we designed a laminar flow apparatus which can precisely simulate the normal stress and shear stress to which cultured endothelial cells are exposed. Under both normal and abnormal physiological conditions, this apparatus can accurately control and adjust the values of normal stress and shear stress and the frequency of pulse waves, as well as the amplitude of pulsatile flow. This in vitro apparatus provides an experimental platform for studying the response of the biological characteristics of cultured endothelial cells in a hemodynamic environment.

Baffled cup and end-effects of a vane-in-a-large cup rheometer for Newtonian fluids

A four-blade vane in a large cup was tested for Newtonian fluids using a controlled stress rheometer. The end-effects of a hypothetical cylinder were evaluated as a function of vane distance from the base of the container. A hypothetical cylinder radius and an effective height of the vane that includes the contribution of the “end-effect” were estimated. Also, the effect of a non-baffled and a baffled cup was investigated. Average values of shear stress and shear rate were calculated assuming that the shear conditions are similar to those existing in an analogous concentric cylinder. Furthermore, experimental angular velocity as a function of torque was correlated when using the power and Reynolds dimensionless numbers. Flow characterization with a vane-in-a-large cup is possible if laminar flow is verified. End-effects are minimized when vane is placed at the surface of fluid and the distance between the bottom of the vane and the base of the container is higher than a half of the vane height (for vane heights equal or higher than the vane diameter). Baffled did not show an effect on the laminar flow contrary to the large effect observed in the turbulent flow. For shear factors calculations of a vane-in-a-large cup, Couette analogy must only be applied using the finite cup radius equations, and corrected for non-Newtonian flow when necessary.

Turbulent stress measurements downstream of three bileaflet heart valve designs in pigs☆

Mechanical heart valves can cause thromboembolic complications, possibly due to abnormal flow patterns that produce turbulence downstream of the valve. The objective of this study was to investigate whether three different bileaflet valve designs would exhibit clinically relevant differences in downstream turbulent stresses. Three bileaflet mechanical heart valves (Medtronic Advantage), CarboMedics Orbis Universal and St. Jude Medical Standard) were implanted into 19 female 90 kg pigs. Blood velocity was measured during open chest conditions in the cross sectional area downstream of the valves with 10 MHz ultrasonic probes connected to a modified Alfred Pulsed Doppler equipment. As a measure of turbulence, Reynolds normal stress (RNS) was calculated at three different cardiac output ranges (3-4, 4.5-5.5, 6-7 L/min). Data from 12 animals were obtained. RNS correlated with increasing cardiac outputs. The highest instantaneous RNS observed in these experiments was 47 N/m2, and the mean RNS taken spatially over the cross sectional area of the aorta during systole was between 3 N/m2 and 15 N/m2. In none of the cardiac output ranges RNS values exceeded the lower critical limit for erythrocyte or thrombocyte damage for any of the valve designs. Reynolds normal stress values were below 100 N/m2 for all three valve designs and the difference in design was not reflected in generation of turbulence. Hence, it is unlikely that any of the valve designs causes flow induced damage to platelets or erythrocytes.

EFFECT OF SURFACE CRACKING ON RESPONSES OF FLEXIBLE PAVEMENTS STRUCTURE

Pavement cracking is a major factor of failure in sof pavements structure, In spit of this, seldom intention has be en considered how is the pavement structure has been affected when surface cracking a re presented and/or progressed. On the other hand, higher costs are presented for m aintenance and repair. Generaly, pavement surface conditions in terms of pavement su rface cracks, rutting, roughness and pit-holes are the main factors used to establis h criteria for pavement maintenance and repair. Structural adequacy factor of the pavem ent is usualy neglected. The accurate decision making processes for maintenance and repair works must be done not only due to functional conditions, but also due to structural conditions. Finite element analysis is used in this research p aper using the ANSYS-10 program. SOLID45, and SOLID185 elements represent u nbounded layers, and asphalt cement layers respectively to evaluate the stresses in a flexible pavement system under different conditions. These conditions were: (1) Th e vertical load of a single wheel was modeled as uniform pressure placed directly at the edge of the transverse crack. (2) The effects of seasonal differentiation of the pave ment material properties during winter and summer. and (3) Various crack depthes wh ich represent 25% , 50%, 75% and 100% of the asphalt layer thicknesses are also, considered in the analysis. The horizontal normal stresses along the top and bottom of the AC layer, and vertical stresses at selected points in the base has been in vestigated. Comparisons of stresses finally were made for all scases. It is found that the presence of crack in the AC layer significantly increases both the vertical stresses and the horizontal stresses in the AC layer which i ndicate a greater potential for tensile stresses outside the tire treads than in the middle of the treads. With the wheel load located at the edge of transverse crack there is considerable change in the values of horizontal and vertical normal stresses in the a sphalt pavement layers. Seasonal variation has significant effect on the normal stre sses values, where the maximum horizontal tension stresses (σyy ) is higher in winter than that in summer with abo ut 66% . Finally, it is recommended that at crack depth rati o (CDR) 50% pavement must be rehabilitated to prevent initiation of new crack s or rutting of asphalt pavement surface. In other side the winter season is consid ered the critical for initiation of cracking, as summer season is considered critical f or rutting occurrence.

Molecular dynamics simulations on atomic friction between self-assembled monolayers: Commensurate and incommensurate sliding

Atomic friction between self-assembled monolayers (SAMs) on Au(1 1 1) has been studied through molecular dynamics simulations, with emphasis on the comparison of the performances of commensurate and incommensurate SAMs in relative sliding. Results show that the shear stress on commensurate SAMs exhibits a clean periodic pattern, manifesting the atomic stick–slip friction, while random fluctuations and a much lower average value of the shear stress are observed for incommensurate sliding. The different frictional behavior can be traced to the difference in molecule movements, especially in the collective nature of the motion. In commensurate sliding, all molecules move synchronously in each period and phase, but they swing randomly and independently for incommensurate monolayers. Simulations provide clear evidence that under the same working conditions friction forces on commensurate SAMs are always higher than those in incommensurate cases. The results also show a linear dependence of shear stress on normal pressure and logarithmic dependence on sliding velocity.

FINITE ELEMENT SIMULATIONS OF THE PHYSIOLOGICAL AORTIC ROOT AND VALVE SPARING CORRECTIONS

A 3-D model of the aortic root was developed consisting of aortic valve, sinus of Valsalva and aortic duct. Through finite element modeling, the mechanical behavior of the valve was investigated simulating four different configurations: physiological (baseline), aneurysmatic and corrected with two different sparing techniques, named after David and Yacoub, respectively. These surgical procedures imply the replacement of the sinus of Valsalva with a synthetic graft, inside which the cusps are resuspended. The final goal of this study consisted in the assessment of the mechanical effects of sinuses aneurysm on the whole root, as well as the effectiveness of the two aforementioned techniques in restoring valve functionality and normal stress values on the root substructures. Based on the assumption of three-leaflet symmetry, only one third of the aortic root was modeled. Two cardiac cycles were simulated and the mechanical behavior of the aortic root was evaluated in terms of stress pattern, contact pressure and contact areas observed during leaflets coaptation and when contact between the leaflet and the graft occurs. Both the sparing techniques are able to restore coaptation of the aortic valve, but for the David procedure, contact between the leaflet and the graft wall was detected.

Evidence of instantaneous dynamic triggering during the seismic sequence of year 2000 in south Iceland

We analyze the coseismic stress perturbation during the 17 June 2000 south Iceland seismic sequence; the main shock (Ms 6.6) was followed by three large events within a few tens of seconds (8, 26, and 30 s) located within 80 km. The aim of this paper is to investigate short‐term fault interaction and instantaneous triggering. This happens when a fault perturbed by a stress change fails before the end of the transient stress perturbation. We compute the shear, normal, and Coulomb stress changes as functions of time in a stratified elastic half‐space by using discrete wave number and reflectivity methods. We calculate dynamic stresses caused by the main shock at the hypocenters of these three subsequent events. Our numerical results show that the onset of the last two events is slightly delayed with respect to the arrival time of the second positive peak of Coulomb stress variation, while the first event occurred after the first positive stress peak. We have also analyzed the response of a spring slider system representing a fault governed by a rate‐ and state‐dependent friction law, perturbed by shear and normal stress variations caused by the main shock. The fault response to the computed stress perturbations is always clock advanced. We have found suitable constitutive parameters of the modeled fault that allow the instantaneous dynamic triggering of these three earthquakes. If the initial sliding velocity is comparable with the tectonic loading velocity, we obtained failure times close to the observed origin times for low values of the initial effective normal stress.

Particle image velocimetry applied to suspensions of millimetric-size particles using a vane-in-a-large-baffled-cup rheometer

Millimetric particles suspended in four different fluids were sheared and video-recorded using a controlled-stress rheometer with a four-blade vane-in-a-large-cup. Experimental situations where the suspension next to the vane moves as a solid cylinder, i.e., where the Couette analogy can be applied, were determined. Velocity fields were measured by particle image velocimetry (PIV) in various suspensions: resin particles in oil (creamed particles), resin particles in water (settled particles), acetate particles in glycerin, and uniformly suspended gelatin particles in a commercial beverage. Data were expressed in terms of the Reynolds number calculated from experimental angular velocity measurements. The suspensions revealed flow patterns which include a considerable amount of vortices initially originating close to the millimetric-size particle bed and thereafter distributing and spreading within the large-baffled-cup. Flow characterization with a vane-in-a-large-cup was only possible for acetate particles in glycerin undergoing laminar flow, as verified by PIV and magnitude of the Reynolds number. Average shear stress and shear rate values were calculated and results for the suspension viscosity were in agreement to those predicted by the model of Happel (Viscosity of suspensions of uniform spheres. Journal of Applied Physics, 1957, 28(11), 1288–1292).

Residual stress development in Pb(Zr,Ti)O 3/ZrO 2/SiO 2 stacks for piezoelectric microactuators

The residual stress of multilayers in piezoelectric microelectromechanical systems structures influences their electromechanical properties and performance. This paper describes the development of residual stress in 1.6 μm Pb(Zr 0.52,Ti 0.48)O 3 (PZT)/0.3 μm ZrO 2/0.5 μm SiO 2 stacks for microactuator applications. The residual stresses were characterized by wafer curvature or load-deflection measurements. PZT and zirconia films were deposited on 4-in. (100) silicon wafers with 0.5 μm thick thermally grown SiO 2 by sol–gel processes. After the final film deposition, the obtained residual stress of PZT, ZrO 2, and SiO 2 were 100–150, 230–270, and − 147 MPa, respectively. The average stress in the stack was ∼ 80 MPa. These residual stresses are explained in terms of the thermal expansion mismatch between the layers and the substrate. Load-deflection measurements were conducted to evaluate localized residual stresses using released circular diaphragms. The load-deflection results were consistent with the average stress value from the wafer curvature measurements. It was found that more reasonable estimates of the stack stresses could be obtained when mid-point vertical deflection data below 6 μm were used, for diaphragms 0.8–1.375 mm in diameter.

Modeling evaluation of the fluid‐dynamic microenvironment in tissue‐engineered constructs: A micro‐CT based model

Natural cartilage remodels both in vivo and in vitro in response to mechanical stresses, hence mechanical stimulation is believed to be a potential tool to modulate extra-cellular matrix synthesis in tissue-engineered cartilage. Fluid-induced shear is known to enhance chondrogenesis in engineered cartilage constructs. The quantification of the hydrodynamic environment is a condition required to study the biochemical response to shear of 3D engineered cell systems. We developed a computational model of culture medium flow through the microstructure of a porous scaffold, during direct- perfused culture. The 3D solid model of the scaffold micro-geometry was reconstructed from 250 micro-computed tomography (micro-CT) images. The results of the fluid dynamic simulations were analyzed at the central portions of the fluid domain, to avoid boundary effects. The average, median and mode shear stress values calculated at the scaffold walls were 3.48, 2.90, and 2.45 mPa respectively, at a flow rate of 0.5 cm(3)/min, perfused through a 15 mm diameter scaffold, at an inlet fluid velocity of 53 microm/s. These results were compared to results estimated using a simplified micro-scale model and to results estimated using an analytical macro-scale porous model. The predictions given by the CT-based model are being used in conjunction with an experimental bioreactor model, in order to quantify the effects of fluid-dynamic shear on the growth modulation of tissue-engineered cartilage constructs, to potentially enhance tissue growth in vitro.

Characteristics of subglacial drainage systems deduced from load-cell measurements

AbstractRock tunnels beneath Engabreen, northern Norway, permit access to the ice-bedrock interface beneath a 210 m thick glacier. Eight load cells have been installed in the bedrock of the glacier sole along a 22 m transect. With some interruptions, the load cells have been logged at 15 min intervals since December 1992; here we analyse the records until 2003. Load-cell signals measure stresses acting normal to the bedrock, and usually log the pressure of the thin water film between the basal ice and the bed. Occasionally there are distinct pressure events, characterized by short-lived (hours) local minima, often followed by a maximum before decaying to background load-cell pressure. The amplitudes of these pressure events are of the order of 0.01–1 MPa and depend on the placement of the sensor and the state of the subglacial drainage system. We identify winter and summer pressure regimes. The winter regime is characterized by few pressure events of large pressure amplitude influencing all load cells. A lag of 0–6 days is observed between surface forcing (rain) and pressure events during winter. The summer regime typically has periods of daily pressure events of low amplitude. No delay is seen between surface forcing and pressure events during the summer regime. In summer, the onset of a pressure event is correlated with a local maximum of the derivative of the subglacial discharge record, whereas no such relation is found during the winter regime. The transition from winter to summer (May/June) is easily detectable and is strongly correlated with a rapid increase in subglacial discharge and the transition to a dominating R-channel system. The autumn transition is less clearly defined, but has usually occurred by the beginning of November. Stress bridging, an increase in bed pressure at the edge of low-pressure channels, is recorded during the summer regime. Water pressures at the bed are connected or unconnected to the drainage system. Pressure increases in the connected system, beyond local normal stress values, lead to an uplift of the connected system and a pressure drop in the unconnected system. The occurrence of pressure events is determined by the capacity of the drainage system. Uplift is controlled by local normal stress values and not mean ice-overburden pressure.

Social support, life events and physical symptoms: A prospective study of chronic and recent life stress in men and women

The objective of this study was to examine whether functional social support buffers the effects of chronic and recent life stress on physical symptom reporting in men and women and assess whether perceptions of support remain stable over time. A longitudinal design was used. Self-ratings of stressful experiences were completed every 3 months over a period of 1 year. Physical symptoms were assessed at 6 and 12 months, and perceptions of social support were measured at baseline and at 12 months. Three groups were formed on the basis of average stress values for the first and second 6 months of the year: chronic low stress, chronic high stress and a recent high-stressed group consisting of individuals who reported low stress for the first 6 months, but high stress for the second 6 months of the year. As expected, high life stress was associated with greater symptom reporting. Perceptions of support remained stable and did not decline over the 12 months. After controlling for negative affectivity and physical symptoms at 6 months, no main effects of social support were found on reported physical health. However, perceived social support did moderate physical symptom reporting in those reporting recent high stress. For chronically high-stressed individuals there was no significant difference in physical symptoms between those with high or low social support. It was concluded that social support moderates the impact of recent but not chronic life stress on physical symptom reporting.

A slab method study of strain hardening and friction effects in cold foil rolling process

In the present work, strain hardening and friction models are incorporated in the foil-rolling model developed by earlier researchers. The strain hardening effect is incorporated by means of power law and a more realistic friction model, viz. Wanheim and Bay's model is used. A parametric study has been carried out to show the effect of Wanheim and Bay's friction model and strain hardening on the roll force and roll torque. It is observed that friction model has great influence on qualitative and quantitative predictions of foil-rolling process. Strain hardening also influences the results and needs to be incorporating in a foil-rolling model. Taking average value of flow stress to account for this effect does not work in the case of severe strain-hardening.

Elastic–plastic response of unpressurised pipes subjected to axially-long radial indentation

Elastic–plastic force-deflection analysis of unpressurised pipes with long axial indentations are carried out. Solutions from an analytical method are compared to the corresponding finite element solutions and experimental test results. The analytical method is based on a simple energy-based approach developed to predict the initial gradients of the force-deflection curves and the limit loads of the indented rings using linear beam bending theory and upper bound theories. A comprehensive finite element study of indented unpressurised pipes is performed to study their sensitivities to the elastic–plastic responses. The finite element analysis covers six different materials, four different geometries and two different boundary conditions, and is compared with the corresponding analytical solutions. The results presented in this paper indicate that by using the analytical solutions for limit loads incorporating a representative flow stress as the average value of the yield stress and ultimate tensile stress, it is possible to obtain reasonably accurate predictions for the peak loads.

Influence of added concrete compressive strength on adhesion to an existing concrete substrate

An experimental study was performed to evaluate the bond strength between two concrete layers of different ages, considering different mixtures of added concrete, with different strengths. The specimens first had the roughness of the substrate surface increased by sand blasting. Later, the new concrete was added. Afterwards, slant shear tests were performed to quantify the bond strength in shear. These tests indicated that increasing the compressive strength of the added concrete relative to the compressive strength of the substrate concrete improves the bond strength and changes the rupture mode from adhesive to monolithic. A finite element analysis showed that, increasing the difference between the compressive strengths of the added concrete and the substrate concrete, higher values of normal stress are present in the interface, for the same level of shear stress. The study presented in this paper reveals that the added concrete, normally with higher compressive strength than the substrate concrete, may possibly have an influence on concrete-to-concrete bond strength. Therefore, design of shear at the interface between concrete cast at different ages, according to Eurocode 2, could be improved, since here only the lowest strength concrete is considered.

Comparison of Residual Shear Strengths from Back Analysis and Ring Shear Tests on Undisturbed and Remolded Specimens

Slope stabilization procedures for six landslides in the Niigata prefecture in Japan afforded an excellent opportunity to examine the residual drained shear strength of weathered mudstones. Installation of 3 m diameter drainage galleries allowed undisturbed block samples to be collected which contained the slickenside surface of the failure plane. Ring shear tests were conducted on undisturbed samples and remolded samples of the same material. In addition to the laboratory testing, back analysis was performed to calculate the residual shear strength parameters of the slopes. Good agreement was found between the residual friction angles determined from back analysis, tests on undisturbed samples, and tests on remolded specimens. Many of the landslide materials exhibited a small cohesion at low values of effective normal stress, but the secant friction angle achieved a constant value at higher normal stresses.

Bending and Shearing Characteristics of Sunflower Stalk Residue

Bending stress, modulus of elasticity, shearing stress and specific shearing energy were determined for sunflower (Heliantus annus L.) stalk. The bending forces were measured by a cantilever test in the field at different moisture contents and the bending stress and the modulus of elasticity were calculated from these data. For measuring the shearing forces, the stalk specimens were severed by using a computer aided cutting apparatus. The specific shearing energy was calculated by using the area under the shearing force versus displacement curve. The bending tests were conducted at four different moisture contents. The bending stress decreased as the moisture content increased. The average bending stress value varied between 37.77 and 62.09 MPa. The modulus of elasticity in bending also decreased as the moisture content and diameter of stalks increased. The average modulus of elasticity varied between 1251.28 and 2210.89 MPa. The shearing stress and the specific shearing energy were also determined at five moisture contents according to the stalk regions. The results showed that the shearing stress and the specific shearing energy increased as the moisture content increased. The maximum shearing stress and specific shearing energy were 1.07 MPa and 10.08 mJ mm(-2), respectively. Both the shearing stress and the specific shearing energy were found to be higher in the lower region of the stalk due to structural heterogeneity.

Aortic Valve Replacement for Aortic Insufficiency: Valve Type as a Determinant of Systolic Strain Recovery

Left ventricular (LV) 3D systolic strain decreases in absolute value postoperatively and does not recover in patients who undergo aortic valve replacement (AVR) for chronic aortic insufficiency (AI). We investigated whether choice of valve prosthesis (mechanical [St. Jude], bioprosthetic [bovine pericardial], Ross procedure) had a significant impact on strain recovery in this surgical population. MRI with tissue-tagging was performed on 14 patients with chronic AI both before and 28 +/- 13 months after AVR. Average values of LV systolic strain and end-systolic stress (ESS) were computed from MRI data for the LV. Three types of prosthetic valve were examined (Ross procedure n = 4, bovine pericardial n = 5, and St. Jude n = 5). Overall, systolic strain, ESS, LV volumes, ejection fraction, and LV mass all changed significantly following AVR. Comparisons between individual valve types revealed no differences in any of these measurements. Patients who received a mechanical valve had a greater decrease in the absolute value of systolic strain following surgery compared to patients from the nonmechanical group (Ross procedure and bioprosthetic valve). Comparisons between the Ross group and the prosthetic group (St. Jude and bioprosthetic) produced no significant differences in strain, ESS, LV volume, and mass. These early results suggest that valve prosthetic type may be a factor in efforts to improve strain recovery after AVR for AI, although further investigation is warranted. MRI with tissue-tagging may be a useful tool for comparing the impact of prosthetic valve choice on incompletely recovered systolic strain following AVR for chronic AI.

Wave Momentum and Radiative Stresses in Elastic Solids

The physical meaning of the notions of wave momentum and radiative stress in elastic bodies is considered. Of great importance for defining and calculating these quantities are the variables in which the wave processes in the medium are described. In Euler variables, the radiative stress is defined as the time average of the momentum flux of the medium across the boundary of a volume that is stationary in space. It differs from the radiative stress calculated in Lagrangian variables by the value of the convective component of momentum flux involved with the motion of particles in the medium. It is shown that, physically, the wave momentum can be defined only in Euler variables. In the linear approximation of the small perturbation theory the wave momentum is a quadratic component of the total mechanical momentum of the medium. The integrals of motion of linear acoustic equations correspond to it, as well as to energy. In the second approximation of the perturbation theory, the wave momentum no longer has a rigorous definition. However, the concept of wave momentum borrowed from the linear approximation can be used for the calculation of average values of stresses and deformations, provided the body is not moving as a whole.

Role of overconsolidation on sand–geomembrane interface response and material damage evolution

The behavior of high-density polyethylene (HDPE) reinforcement elements is critical to the overall performance of many geotechnical structures including landfills, retaining walls, embankments, and shallow foundations. The reinforcement elements, which may consist of manufactured sheets or strips, must be properly installed in order to transfer stresses from areas of concentrated loading to reinforcing zones. While attention is given during construction to avoid contact damage of reinforcement elements, the construction process exerts stresses on the reinforcement element that may substantially exceed the stresses acting throughout the structure's service life. This paper examines the influence of overconsolidation of HDPE reinforcement elements on interface performance through examination of its effect on sand–smooth geomembrane interfaces. A range of overconsolidation and operational normal stress values are selected based on conditions typically present in practice. Results show the peak friction coefficient to increase substantially with overconsolidation magnitude while the effect on the residual friction coefficient is minimal. The magnitude of the increase in peak friction coefficient is shown to be dependent on both the damage induced by the smooth geomembrane and the degree to which sand particles adjacent to the interface undergo damage in the form of breakage during shear. The damage induced by the smooth geomembrane and the sand particles are presented in terms of the level of overconsonsolidation, confinement stress during shear, and particle characteristics such as angularity and hardness.