Values Of Normal Stress Research Articles

A class of vortex flows in granular medium

The authors study numerically a problem on vortex flows in bounded domain in granular material using discrete element method. The granular material is composed of spherical particles with the assigned distribution of radii. Dry friction and rolling resistance at particle interfaces are taken into account. The kinematic patterns of granular material specimen deformation are described, and the trajectories of some particles are shown. The article gives values of normal and shear stresses acting at the study domain boundaries from the side of the material.

Study of Covalent/Ionic Cross-Linked Modification on Physical and Mechanical Properties of Chitosan Film as Potential Material in Medical Application

The aim of this study was to investigate the effects of covalent and ionic cross-linked reactions which were respectively done by using genipin and tripolyphosphate (tpp), on the structure and mechanical properties of chitosan film. Both cross-linked and uncross-linked films were prepared by solution casting method and characterized. FTIR spectra showed no characteristic of –OCH3 peak from genipin at 1444 cm-1 which resulted by a new covalent bonding in chitosan film. Reduction in absorption intensity at 1560 cm-1 wave number in chitosan cross-linked tpp films were due to the presence of ionic interaction between the positive charged of amino group in chitosan and negatively charged of phosphate group by tpp. The pattern area from the XRD results showed that the covalent cross-linked had significantly changed on the chitosan`s degree of crystallinity. The water contact angle on the surface of covalent/ionic cross-linked chitosan film reached the highest θ at 82.72° which indicated more hydrophobic properties was formed. Covalent/ionic cross-linked chitosan also showed the higher mechanical strength with average tensile stress value at 71.25 MPa. All finding results demonstrated that cross-linked modification on the chitosan film had successfully reduced the film’s hydrophilicity and increased the mechanical properties of the film.

Influence of operation factors on brittle fracture initiation and critical local normal stress in SE(B) type specimens of VVER reactor pressure vessel steels

A complex of mechanical tests and fractographic studies of VVER-1000 RPV SE(B) type surveillance specimens was carried out: the brittle fracture origins were revealed (non-metallic inclusions and structural boundaries) and the correlation between fracture toughness parameters (CTOD) and fracture surface parameters (CID) was established. A computational and experimental method of the critical local normal stress determination for different origin types was developed. The values of the critical local normal stress for the structural boundary origin type both for base and weld metal after thermal exposure and neutron irradiation are lower than that for initial state due to the lower cohesive strength of grain boundaries as a result of phosphorus segregation.

Considerations regarding the stress relieving in a special product

The special product has a steel bearing structure located inside the shoulders and it is fitted with a steel cylinder loaded by an internal pressure pi. The value of the maximum internal pressure is high, at limit being equal to the allowable stress (σa = pi), where pi = 3000 MPa. The uniform stresses decrease if there is an external pressure applied on the cylinder. Therefore, it must be produced external pressure in order to reduce the stress ctmax., in other words to achieve a relieving operation. The external diameter of the internal tube is larger than the internal diameter of the external tube. In the normal temperature conditions these two cylinders cannot be assembled. After it is heated up, expanded cylinder is mounted on the internal cylinder and after the quenching, on the contact surface it appears a relieving pressure p. We are interested in computing the parameters of this technological procedure, for the classic way of relieving, namely by heating up the external cylinder. After processing by cutting, radius R2 of the internal tube is greater than δ = 0.05 mm radius R2 of the external tube. After cooling, the two tubes have a common radius R2, which will have an average value between the values they had before the assembling process. This is why the external tube has to suffer a thermal expansion, the internal tube suffering a compression, so that the two displacements summed, measured perpendicular to the contact surface, have a value of δ = 0.05 mm. After the relieving operation, on the contact surface between the two tubes will be produces a contact pressure, p. In the paper we study the effect of the relieving pressure p, onto the stresses if there would be not relieving and the variation of stresses considering relieving of both the internal tube and the external tube. The highest values of the stresses are those on the internal face of the bearing. If calculations are in accordance with the third failure theory it results that the maximum tangential stresses are the highest values. The values of normal and shear stress influence the phenomenon of relieving in the bearing because high internal pressures inside the bearing may break the internal tube leading to collapse of the structure.

Direct Shear Testing on Unsaturated Silty Soils to Investigate the Effects of Drying and Wetting on Shear Strength Parameters at Low Suction

A modified conventional direct shear device was used to measure unsaturated shear strength of two silty soils at low suction values (0 ~ 50 kPa) that were achieved by following drying and wetting paths of soil water characteristic curves (SWCCs). The results revealed that the internal friction angle of the soils was not significantly affected by either the suction or the drying wetting SWCCs. The apparent cohesion of soil increased with a decreasing rate as suction increased. Shear stress-shear displacement curves obtained from soil specimens subjected to the same net normal stress and different suction values showed a higher initial stiffness and a greater peak stress as suction increased. A soil in wetting exhibited slightly higher peak shear stress and more contractive volume change behavior than that of soil in drying at the same net normal stress and suction.

Temperature dependence of stress in CVD diamond films studied by Raman spectroscopy

Abstract Evolution of residual stress and its components with increasing temperature in chemical vapor deposited (CVD) diamond films has a crucial impact on their high temperature applications. In this work we investigated temperature dependence of stress in CVD diamond film deposited on Si(100) substrate in the temperature range of 30 °C to 480 °C by Raman mapping measurement. Raman shift of the characteristic diamond band peaked at 1332 cm-1 was studied to evaluate the residual stress distribution at the diamond surface. A new approach was applied to calculate thermal stress evolution with increasing tempera­ture by using two commonly known equations. Comparison of the residts obtained from the two methods was presented. The intrinsic stress component was calculated from the difference between average values of residual and thermal stress and then its temperature dependence was discussed.

On the modeling of tilted fixed-guided flexible beams under tension

We derive explicit solutions for a fixed-guided slender suspension beam that is initially straight and tilted with respect to the moving direction of its sliding end. The beam experiences substantial axial forces during the tension, resulting in a nonlinear boundary value problem. We consider sliding end displacements in the direction that cause longitudinal tension along the beam. We first propose an exact approach, leading to analytical solutions for various physical variables such as the transverse force and deflection profile, in terms of the axial force and the positive real solution of a third-order algebraic equation. We also propose an alternative approximate solution based on a second-order equation, which provides closed-form analytical solutions for the physical variables. We also introduce analytical validation techniques for the underlying assumptions. Consistency with nonlinear finite-element analysis is also addressed. Moreover, the results of the approximate method are represented by dimensionless formulas, generating charts to predict solutions for arbitrarily assigned beam parameters. Magnitudes of the normal and shear stress values are also included to consider the effects of yield and shear strengths as the limiting factors at large deflection conditions.

Influence of Phase Shift and Amplitude Ratio on the Principal Stresses and Directions in Multiaxial Fatigue Testing

The maximum values of normal and shear stresses are the basic parameters which influence directly the initiation and propagation of multiaxial fatigue cracks.Based on the above, the first part of the paper presents an analysis of principal stresses (normal and shear) in case of symmetrical tension-compression loadings with superimposed phase-shifted symmetrical torsion cycles. The influence of stress amplitude ratio and phase shift on the maximum (normal and shear) stresses and on the directions of the planes along which these act is analyzed and graphically represented using stress hodographs.The second part of the paper highlights the possibility of using the maximum value of the normal or shear stress as base parameter for durability studies under multiaxial fatigue, based on existing experimental data. The mentioned data is correlated with the results of an original experimental program carried out by the authors on 41Cr4 steel and conclusions are formulated with regard to the role of maximum shear stress in life-time calculation.

Stress states and moment rates of a two-asperity fault in the presence of viscoelastic relaxation

Abstract. A fault containing two asperities with different strengths is considered. The fault is embedded in a shear zone subject to a constant strain rate by the motions of adjacent tectonic plates. The fault is modelled as a discrete dynamical system where the average values of stress, friction and slip on each asperity are considered. The state of the fault is described by three variables: the slip deficits of the asperities and the viscoelastic deformation. The system has four dynamic modes, for which analytical solutions are calculated. The relationship between the state of the fault before a seismic event and the sequence of slipping modes in the event is enlightened. Since the moment rate depends on the number and sequence of slipping modes, the knowledge of the source function of an earthquake constrains the orbit of the system in the phase space. If the source functions of a larger number of consecutive earthquakes were known, the orbit could be constrained more and more and its evolution could be predicted with a smaller uncertainty. The model is applied to the 1964 Alaska earthquake, which was the effect of the failure of two asperities and for which a remarkable post-seismic relaxation has been observed in the subsequent decades. The evolution of the system after the 1964 event depends on the state from which the event was originated, that is constrained by the observed moment rate. The possible durations of the interseismic interval and the possible moment rates of the next earthquake are calculated as functions of the initial state.

Strategies for stiffness analysis of laminates with microdamage: combining average stress and crack face displacement based methods

Many approximate analytical models have been developed to calculate stress state between intralaminar cracks with the aim to predict the degradation of certain elastic property (most often axial modulus or shear modulus) of cross‐ply laminate. Often they are plane stress solutions and laminate constants like Poisson's ratios cannot be considered. On the other hand the so called GLOB‐LOC approach, presented in WWFE III, allows calculation of any thermo‐elastic property of a general symmetric laminate with an arbitrary number of cracks in each layer provided that two local parameters – average and normalized crack opening displacement (COD) and crack face sliding displacement (CSD) are known. In this paper relationships are derived expressing these two parameters (COD and CSD) with average value of transverse stress and in‐plane shear stress perturbation between cracks. Expressions are exact and independent on the approximations in the stress model. As examples, average perturbation functions for two shear lag models and Hashin's variational model are used to calculate damaged laminate properties that would not be available in original formulation: Poisson's ratio and thermal expansion coefficients. Predictions are compared with test data for GF/EP laminates and with more accurate predictions based on FEM calculations.

Study of 3-D stress development in parent and twin pairs of a hexagonal close-packed polycrystal: Part II – crystal plasticity finite element modeling

Stress heterogeneity within each individual grain of polycrystalline Zircaloy-2 is studied using a crystal plasticity finite element (CPFE) model. For this purpose, the weighted Voronoi tessellation method is used to construct 3D geometries of more than 2600 grains based on their center-of-mass positions and volumes as measured by three-dimensional X-ray diffraction (3DXRD) microscopy. The constructed microstructure is meshed with different element densities and for different numbers of grains. Then a selected group of twin and parent pairs are studied. It is shown that the measured average stress for each grain from the 3DXRD experiment is within the stress variation zone of the grain modeled in the CPFE simulation. Also, the CPFE average stress calculation for each grain is in good agreement with the measured average stress values. It is shown that upon considering the stress variations within each grain, stresses in the parent and twin are quite different if they are plotted in the global coordinate system. However, if the stress tensor is rotated into the local coordinate system of the twin habit plane, all the stress components averaged over the presented population are close, except for the shear acting on the twin plane and the transverse stress. This result is significant as it provides information needed to model such parent-twin interactions in crystal plasticity codes.

Computational fluid dynamic study of hemodynamic effects on aortic root blood flow of systematically varied left ventricular assist device graft anastomosis design

ObjectivesTo quantify the range of blood flow parameters in ascending aorta that can result from various angulations of outflow graft anastomosis of a left ventricular assist device (LVAD) to the aortic wall, as a means to understand the mechanism of aortic valve insufficiency. MethodsA realistic aorta model with LVAD anastomosis was generated from computed tomographic images of a patient. Based on this model, the LVAD anastomosis geometry parameters, such as anastomosis locations, inclination angle, and azimuthal angle (cross-sectional plane) of the graft, were varied, to create 21 models. With the assumption of no flow passing the aortic valve, and a constant flow rate from the LVAD cannula, computational fluid dynamics simulations were used to study the blood flow patterns in the ascending aorta. In addition, pulsatile flows were assumed in the LVAD cannula, with the aortic valve opened during peak systole, for 2 specific anastomosis configurations, to evaluate the influence of the pulsatile flow profile and the transvalvular flow on the aortic flow patterns. ResultsChanges in the inclination angle, from 60° to 120°, or the azimuthal angle, from 90° to 120°, or moving from a lower to a higher anastomosis position, causes significant changes for all flow parameters. A lower anastomosis location, an inclination angle ≥90°, and an azimuthal angle of 60° or 120° are all capable of reducing blood flow stagnation in the aortic root and producing normal wall shear stress and moderate pressure values in the region. ConclusionsCarefully chosen anastomosis geometry is likely to be able to generate a close-to-normal hemodynamic environment in the aortic root. Greater knowledge of aortic valve remodeling may make possible the creation of favorable flow patterns in the aortic root, through optimization of surgical design to reduce or delay the occurrence of aortic valve insufficiency.

Analysis of Stress Concentration between Fillet and Hole in a Stepped Plate under Tensile Load by Photoelasticity

Abstract Stress concentrations around discontinuities, such as a hole or a sudden change in cross section of a structural member, have great important cause in the most materials failure because the stress near the points of application of concentrated loads can reach values much larger than the average value of the stress in the member. This paper presents the stress concentrations between fillet and hole at different locations in a stepped plate under tensile loading. The analysis for interaction effect of stress concentration

Experimental investigation of the flow past passive vortex generators on an airfoil experiencing three-dimensional separation

The use of passive vortex generators (VGs) as a simple and effective way to delay or suppress separation on an airfoil optimized for wind turbine blades is examined experimentally. The profile experiences three-dimensional separation of the Stall Cell type. Pressure, Flow Visualization and Stereo Particle Image Velocimetry experiments are discussed for the case of triangular counter rotating VGs with common flow up. For a Reynolds number of 0.87×106, Stall Cell formation is delayed for 5°, and lift increases up to α=15°. In total, maximum lift increases by 44%, while drag increases by 0.002 at pre-stall angles of attack. At α=16° the flow bifurcates between separated and attached flow conditions. At α=10° the flow is examined in detail and an investigation on the turbulence characteristics is carried out by correlating Reynolds stresses production to time averaged flow gradients. Strong turbulent interaction is observed between the two vortices and the underlying flow up to 37.2 VG heights downstream of the VGs, while further downstream (up to 47.2 VG heights) diffusion governs the flow. A wandering motion of the VG vortices leads to increased v`v`¯ normal stress values between the two vortices.

Estimation of the Level of Residual Stress in Wires with a Magnetic Method

AbstractResidual stress present in wires after drawing process affects their magnetic properties. The paper presents a concept to estimate the level of residual stress on the basis of measurements of hysteresis loops. In order to describe the effect qualitatively the Jiles-Atherton-Sablik description is adapted. On the basis of variations in hysteresis loop shapes the average values of residual stress in wires for different single draft values are determined. It was found that the estimated average values by magnetic stresses are comparable with the results of numerical modeling and experimental studies.

Evaluation of Interfacial Fracture Strength in Micro-Scale Components Combined with High-Voltage Environmental Electron Microscopy

A novel experimental method that enables evaluation of interfacial strength of micro-components subjected to well-controlled various environments is proposed. The method combines a high-voltage transmission electron microscope (HVEM) equipped with an open type environmental cell (EC) and a quantitative nano-mechanical holder. The experiments were conducted by using focused-ion-beam-processed micro-cantilever specimens containing interfaces of dissimilar materials (silicon (Si) and copper (Cu)). The in situ observation of fracture phenomenon along a target interface (Si/Cu) and the simultaneous measurement of fracture load were successfully implemented under vacuum and gaseous environments (N2 gas and N2/H2 mixed gas) whose maximum pressure was controlled at 5 × 103 Pa. The stress distribution at the onset of fracture was calculated in detail by finite element analysis. The strength against interfacial debonding was then evaluated by the local normal stress value at the interface free-edge. The proposed technique, which can also be combined with simultaneous electron probe analyses, is highly versatile when compared to the conventional nano-mechanical strength evaluations under limited environments.

Impact of Normal and Shear Stresses Due to Wheel Slip on Hydrological Properties of an Agricultural Clay Loam: Experimental and New Computerized Approach

The main purpose of this study was to evaluate the effect that mechanical stresses acting under the slipping driving wheels of agricultural equipment have on the soil’s pore system and water flow process (surface runoff generation during extreme event). The field experiment simulated low slip (1%) and high slip (27%) on a clay loam. The stress on the soil surface and changes in the amounts of water flowing from macropores were simulated using the Tires/tracks And Soil Compaction (TASC) tool and the MACRO model, respectively. Taking a 65 kW tractor on a clay loam as a reference, results showed that an increase in slip of the rear wheels from 1% to 27% caused normal stress to increase from 90.6 kPa to 104.4 kPa at the topsoil level, and the maximum shear contact stress to rise drastically from 6.0 kPa to 61.6 kPa. At 27% slip, topsoil was sheared and displaced over a distance of 0.35 m. Excessive normal and shear stress values with high slip caused severe reductions of the soil’s macroporosity, saturated hydraulic conductivity, and water quantities flowing from topsoil macropores. Assuming that, under conditions of intense rainfall on sloping land, a loss in vertical water flow would mean an increase in surface runoff, we calculated that a rainfall intensity of 100 mm h-1 and a rainfall duration of 1 h would increase the runoff coefficient to 0.79 at low slip and to 1.00 at high slip, indicating that 100% of rainwater would be transformed into surface runoff at high slip. We expect that these effects have a significant impact on soil erosion and floods in steeper terrain (slope > 15°) and across larger surface areas (> 16 m2) than those included in our study.

Plotting of Rational Sections of Hydrotechnical Tunnels

Results of solution of problems related to the plotting of rational cross-sectional shapes of hydrotechnical tunnels with respect to a given normal tangential-stress distribution on its perimeter are presented as a function of the depth of embedment of the tunnel and values of the coefficient of thrust of the enclosing rock. It is demonstrated that the condition whereby there are no points on the perimeter of the tunnel at which values of the tangential normal stresses exceed the ultimate tensile and compressive strengths of the enclosing rock may serve as a criterion for this purpose. Examples of the problem’s solution are presented.

Effect of Stress Perturbations on the Dynamics of a Complex Fault

A plane fault containing two asperities subject to a constant strain rate by the motion of tectonic plates is considered. The fault is modelled as a discrete dynamical system where the average values of stress, friction and slip on each asperity are considered. The state of the fault is described by the slip deficits of the asperities. We study the behaviour of the system in the presence of stress perturbations that are supposed to be due to dislocations of neighbouring faults. The fault complexity entails consequences that are not present in the case of a homogeneous fault. A stress perturbation not only changes the occurrence time of the following earthquake but may also sensitively change the slip amplitude and area, hence the seismic moment, of the earthquake, as well as the position of its hypocentre. The greatest changes take place when simultaneous slip of asperities is involved. A Coulomb stress value can be assigned to each asperity. The change in the difference between the Coulomb stresses of the two asperities is a measure of how much the system gets closer to or farther from the condition for simultaneous slip. As an example, we consider the effect of the 1960 Great Chilean Earthquake on the two-asperity fault that produced the 2010 Maule earthquake and calculate the changes in the moment rate and in the total seismic moment. It results that, in the absence of the 1960 earthquake, the Maule earthquake would have occurred several decades later and would have involved a different sequence of modes, so that the moment rate function would have been very different, with a longer duration and a greater seismic moment.

Analysis of non-linear strain-softening behaviour around tunnels

This paper studies the influence of non-linear stress–strain of post-failure on the ground response curve of a circular tunnel excavated in rock mass. Thus, the influence of variations of both the quality of rock mass and the confining stress around the tunnel due to the advancement of the tunnel face are addressed on the value of drop modulus. A new procedure is presented to obtain the softening parameter and its critical value, and the accuracy and practical application of the proposed procedure are shown through examples. The effects of variable drop modulus and the value of the critical softening parameter on the ground response curve are investigated. The results show that, to obtain a ground response curve, the variable drop modulus (non-linear behaviour) in the post-failure region can be approximated by a constant drop modulus (linear behaviour post-failure) if it is calculated by using both quality of rock mass before failure and the average value of confinement stress within the plastic region.