Values Of Normal Stress Research Articles

Plastic deformation modeling of die-upset process for magnequench NdFeB magnets

The die upset of hot pressed NdFeB magnets modifies the equiaxed grains to platelets, develops the c-axis texture parallel to press direction, and improves magnetic properties. The mechanism of c-axis alignment has been suggested to be a combination of grain-boundary sliding and anisotropic grain growth in a direction normal to the applied stress. To clearly understand the role of deformation process in grain-boundary sliding or anisotropic grain growth simulations of the die-upset process were performed using ANATARES, a three-dimensional finite element method based deformation modeling software. The stresses and strains in the different regions of a cylindrical Nd–Fe–B magnet at different stages of the die-upset process were determined. The average value of the maximum principal stress (parallel to the upset direction) and total effective strain increased as the upset increased from 50% to 70%. The maximum principal stress and total effective strain show a maximum at the center and decrease in both the thickness and the radial direction due to friction at the die-wall/magnet interface. The stress and effective strain uniformity improves with increase in upset. The data agree well with the variations of texture in the magnet observed using pole figure measurements in this study and texture studies using a synchrotron source.

Effect of Manufacturing Circumstance on Bending Strength in Al-Al3Ni Functionally Graded Material.

The strength of Al-Al3Ni functionally graded material (FGM) has been studied through 3-point bending experiments for two different manufacturing conditions. Specimens with rectangular cross-section of 8 mm thickness×9 mm width are machined from thick-walled tube, which are manufactured in laboratory and vacuum atmosphere. They have graded compositions of Al3Ni phase in aluminum matrix in the radial direction. The nominal volume fractions of Al3Ni phase at the crack initiation site are Vf=41 and 15 vol.% under air, and Vf=35 and 0 vol.% under vacuum casting. Fracture of specimens occur more abruptly in the case of specimens manufactured in air than those made under vacuum condition. The fracture strengths are calculated from maximum bending load considering the graded Young's modulus and are plotted on the Weibull probability sheet. Each data set obeys a good linear relationship and the reliability of strength data is better in case of vacuum specimens. The average value of fracture stress improved with increase of volume fraction of Al3Ni for both the manufacturing conditions. However, in the case of vacuum manufactured specimens, coefficient of variation is small. The bending strength of Al-Al3Ni FGM is controlled by size, shape and distribution profile of Al3Ni particles, because the cleavage fracture of Al3Ni phase dominates the fracture process.

A VIDEO‐BASED METHOD FOR DETERMINATION OF AVERAGE STRESS‐STRAIN RELATIONS IN UNIAXIAL COMPRESSION OF SELECTED FOODS

ABSTRACTA video‐based method was applied to evaluate average stress‐strain relationships during uniaxial compression experiments of selected food materials. The determination of apparent Poisson ratios of Gouda cheese resulted in numbers close to the theoretical value for incompressible materials of 0.5. In case of such foodstuffs, mathematically corrected compressional stress based on the consideration of constant specimen volume showed almost similar results to stress values estimated on the basis of the true cross‐sectional area measured from lateral extension. For compressible materials such as apples or bread, which show a certain reduction of volume under stress or strain, the apparent Poisson ratios were strain‐dependent. Average stress values calculated from compressional force and actual specimen diameter were found to be different from either engineering stress and corrected stress.

Experimentally Verified Numerical Analysis of Internal Thermal Stresses in Alumina Ceramics with Grain Size Effect

The internal stress state after alumina ceramics cooling down was calculated using finite element method (FEM).The two numerical models of the orthotropic Al 2 O 3 material were taken for computations, using the so-called two-steps' method, for needs of pure orthotropic effect investigations. The calculations were made under such thermal conditions, that the diffusional stress relaxation may be omitted. The results of calculations prove grain size fluence on average stress value. FEM calculations have been verified by an experiment based on piezospectroscopy effect.

Numerical Analysis of Developing Turbulent Structure in S-Shaped Duct.

Numerical simulation has been performed for developing turbulent flow in S-shaped duct with a square section at a Reynolds number of 4×104. The S-duct was formed from two 22.5 degree bends with 40 mm hydraulic diameter and 280 mm mean radius of curvature. Straight ducts with hydraulic diameters of 7.5 and 50 are attached to the inlet and outlet planes of the S-duct, respectively. In calculation, an algebraic Reynolds stress model was adopted in order to predict anisotropic turbulent flow precisely, and a boundary-fitted coordinate system was introduced for coordinate transformation. The calculated results were compared with the experimental data using a laser Doppler velocimeter. In particular, the distributions of Reynolds stresses are compared in this report. As a result of comparison with the experimental data, it was found that the present method can accurately predict contours of streamwise normal stress which show the active generation of turbulence near the outer wall of the second bend. Moreover, it was clarified by estimating the production term that the high value of streamwise normal stress near the outer wall of the second bend is responsible for the velocity gradient of streamwise mean velocity with respect to the radial direction. The profiles of other normal stresses and shear stress are also predicted using the anisotropic turbulent model without great discrepancy.

Numerical Analysis of Reynolds Stress for Turbulent Flow Developing in a Rectangular-Sectioned 90 Degree Bend.

A numerical study has been performed on turbulent flow developing in a 90 deg curved duct of retangular cross section and an aspect ratio of 6. The ratio of bend mean radius of curvature to hydraulic diameter is 2.04 and straight ducts of 7.5 and 25.5 width are attached to the inlet and outlet of the bend, respectively. In addition to this straight duct. two-dimensional lateral contraction of the wind tunnel with an area ratio of 6 is considered as the numerical domain as well as the experimental apparatus. Calculated results were compared with the experimental data measured for Reynolds stress components in detail. In this calculation, an algebraic Reynolds stress model was adopted in order to predict anisotropic turbulence precisely, and a boundary-fitted coordinate system was introduced as the method of coordinate transformation. As a result of this examination, the present method can predict normal and shear stresses distributions of the inlet straight duct but cannot reproduce the characteristic destabilizing effect and shows an increase in the turbulence intensity close to the outer wall in the curved duct. The discrepancy may be caused by use of the wall function as the boundary condition. In the outlet straight duct. the calculated value of normal and shear stresses are smaller than those of the experiments.

A creep analysis of the deflection test for evaluating oxide growth stresses

During isothermal oxidation of one surface of a metal strip, growth-induced stresses will produce curvature of the metal/oxide composite sample. This observation has been used extensively elsewhere to evaluate the average stress developed within the oxide layer, but previous estimates have been based on an elastic stress analysis of the bending composite. At typical oxidation temperatures, creep deformation of both oxide and metal phases is much more likely. The present paper provides a stress analysis assuming such non-linear creep deformation. It considers the situation where the oxide experiences a net compressive stress and the metal a net tensile stress. The oxide neutral axis may be located either within the oxide layer or outside it, in the gas phase, depending on the thickness of the oxide and the relative creep strengths of the oxide layer and metal substrate. Evaluation of the stress distribution and average stress values in both the metal and oxide phases is provided for notional mechanical properties, within which the metal creep strength is varied widely. It is also shown how the assumption of elastic deformation can lead to large overestimates of stress values.

Global patterns of radiated seismic energy and apparent stress

Radiated energies from shallow earthquakes with magnitudes ≥5.8 that occurred between 1986 and 1991 are used to examine global patterns of energy release and apparent stress. In contrast to traditional methods which have relied upon empirical formulas, these energies are computed through direct spectral analysis of broadband seismic waveforms. Velocity‐squared spectra of body waves are integrated after they have been corrected for effects arising from depth phases, frequency‐dependent attenuation, and focal mechanism. The least squares regression fit of energy Es to surface wave magnitude Ms for a global set of 397 earthquakes yields log Es = 4.4 + 1.5Ms, which implies that the Gutenberg‐Richter relationship overestimates the energies of earthquakes. The least squares fit between Es and seismic moment M0 is given by the relationship Es = 1.6 × 10−5 M0, which yields 0.47 MPa as the average global value of apparent stress. However, the regression lines of both Es–Ms and Es–M0 yield poor empirical predictors for the actual energy radiated by any given earthquake; the scatter of data is more than an order of magnitude about each of the regression lines. On the other hand, global variations between Es and Mo, while large, are not random. When subsets of Es–M0 are plotted as a function of seismic region and faulting type, the scatter in data is substantially reduced. The Es–M0 fits for many seismic regions and tectonic environments are very distinctive, and a characteristic apparent stress τc can be derived. The lowest apparent stresses (<1.5 MPa) are associated with thrust earthquakes at subduction zones. The highest apparent stresses (>3.0 MPa) are associated with strike‐slip earthquakes that occur at oceanic ridge‐ridge transforms and in intraplate environments seaward of island arcs. Intermediate values of apparent stress (1.5 < τa < 3.0 MPa) are associated with strike‐slip earthquakes at incipient or transitional plate boundaries. In general, the dominant mode of failure for a tectonic environment is associated with the faulting type that has the lowest apparent stress. An energy magnitude ME can complement moment magnitude Mw in describing the size of an earthquake. ME, being derived from velocity power spectra, is a measure of seismic potential for damage. Mw, being derived from the low‐frequency asymptote of displacement spectra, is more physically related to the final static displacement of an earthquake. When earthquake size is ranked by moment, a list of the largest events is dominated by earthquakes with thrust mechanisms. When earthquake size is ranked by energy, the list of the largest events is dominated by strike‐slip earthquakes.

Characterization of three formulations of a synthetic foam as models for a range of human cancellous bone types.

Porous polyurethane foams were prepared from Daro foam components with a range of mechanical properties to simulate human trabecular bone. Ratios of 10.0:5.0, 10.0:7.9, and 10.0:10.0 isocyanate to resin were mixed, cured, and cut into cubes. Properties were determined from uniaxial compression to 50% of the original cube height at a strain rate of 1.2 mm/s. Electron microscopy was used to characterize the foam structure. Average compressive yield stress values, ultimate compressive stresses, and elastic moduli ranged from 4.44 to 2.79, 5.61 to 3.28, and 134.0 to 110.1 MPa, respectively, for the three formulations. The foam materials showed a similar morphology of spherical bubbles, and the average bubble size tended to decrease as the ratio of isocyanate to resin increased even though the bubble size differences were not statistically significant. The results indicate that large blocks of foam can be prepared with consistent mechanical properties simulating a range of trabecular bone properties so that implants can be tested for various patient populations.

Prediction of Soil Stresses Beneath a Rigid Wheel

A finite element modelling technique is being developed as a management tool that can be used to predict and avoid excessive soil compaction. Values of normal stress between a rigid wheel and the soil were obtained using an instrumented bar across the width of the wheel. These values were used to apply loads to the finite element model. A non-linear stress-strain relationship was used that shows that soil compaction is a function of both normal and shearing stress. The linear-elastic parameters, Young's Modulus and Poisson's ratio, are updated at small increments of load to follow the non-linear stress-strain relationship closely. Values of octahedraI normal (mean normal) and major principal stress are predicted accurately in some situations but not at the high load condition in an initially uniformly loose soil profile.

The effects of thermal nitridation and reoxidation on the interfacial stress and structure of silicon dioxide gate dielectrics

Electron spin resonance spectroscopy is used to study the effects of thermal NH3 nitridation and subsequent reoxidation on the structure of the dielectric/silicon interface. This is accomplished by study of the Pb center, an interfacial point defect. The values of g⊥ in Pb spectra observed in the nitrided oxide and reoxidized nitrided oxide systems differ from Si/SiO2 systems, suggesting that the average value of the tensile stress in the silicon substrate increases upon nitridation while reoxidation acts to return the interfacial stress to prenitridation levels. The implications of these structural changes upon device performance are discussed.

3D analysis of failure modes in the Charpy impact test

A three-dimensional analysis of the Charpy V-notch specimen subject to impact loading is carried out, using a data parallel implementation in a transient three-dimensional finite element programme. An elastic-viscoplastic constitutive relation for a porous plastic solid is used to model ductile fracture by the nucleation and subsequent growth of voids to coalescence. Cleavage is modelled in terms of attaining a critical value of the maximum principal normal stress over a specified material volume. A convected coordinate Lagrangian formulation is employed and the discretization is based on twenty-node brick elements with 2*2*2 Gauss points. The equations of motion are integrated numerically by an explicit integration procedure using a lumped mass matrix. The predictions of the full three-dimensional calculations are compared with those of corresponding plane-strain analyses.

Turbulent stresses downstream of three mechanical aortic valve prostheses in human beings

High levels of turbulent stresses resulting from disturbed blood flow may cause damage to red blood cells and platelets. The purpose of this study was to evaluate the spatial distribution and temporal development of turbulent stresses downstream of three mechanical aortic valve prostheses in human subjects: the St. Jude Medical, the CarboMedics, and the Starr-Edwards silicone rubber ball. Blood velocity measurements were taken at 17 measuring points in the cross-sectional area of the ascending aorta 5 to 6 cm downstream of the aortic anulus with the use of a perivascular pulsed Doppler ultrasound system. Turbulence analysis was done for each of the 17 measuring points by calculating the radial Reynolds normal stresses within 50 msec overlapping time windows during systole. By coordinating the calculated Reynolds normal stress values for each time window and for all measuring points, computerized two-dimensional color-coded mapping of the turbulent stress distribution during systole was done. For the St. Jude Medical valves the highest Reynolds normal stress (27 to 63 N/m2) were found along the central slit near the vessel walls. The temporal development and spatial distribution of Reynolds normal stresses for the CarboMedics valves were quite similar to those of the St. Jude Medical valves with maximum Reynolds normal stress values ranging from 19 to 72 N/m2. The typical Reynolds normal stress distribution for the Starr-Edwards silicone rubber ball valves was asymmetric, revealing the highest Reynolds normal stresses (11 to 56 N/m2) at various locations in the annular region between the ball and the vessel wall. The spatial distribution and temporal development of turbulent stresses downstream of the three investigated mechanical aortic valve prostheses correlated well with the superstructure of the valves. The maximum Reynolds normal stresses for the three valve types were in the same order of magnitude with exposure times sufficient to cause sublethal damage to red blood cells and platelets.

Measurement of the saturation magnetostriction constant of Co-rich amorphous films

The small-angle magnetization rotation (SAMR) is extended to measure the magnetostriction constant of magnetic thin films with an accuracy about +or-1*10/sup -8/. It is found that the average value of internal stress existing in the sample can be determined during the measurement. The magnetostriction constant of Co-rich amorphous film is measured, and is about 7*10/sup -8/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fault-striae analysis: A turbo pascal program package for graphical presentation and reduced stress tensor calculation

We describe integrated computer programs for graphical presentation, on-screen selection, and reduced stress tensor calculation of fault-striae data. The input consists of dip azimuth and dip inclination of faults, bearing and plunge of associated striae, relative slip-sense of the hanging wall, and degree of confidence in the slip-sense determination. The data are checked for inaccurate measurements, if necessary corrected (program CHECK), and represented on the equal area, lower hemisphere stereonet either as fault poles (program F-SH) or as great circles (program F-S). In both situations, arrows point in the direction of relative hanging-wall slip; their heads express the confidence levels in slip-sense determination. Program ORIENT plots dip azimuth and dip inclination of faults, striae bearing and plunge, and rake as rose diagrams, enabling a quick recognition of the dominant fault types. Program TURNER determines the best-fitting contraction and extension axes ( P- B- T axes method) for each fault-striae data set. Program INVERS calculates the principal stress directions and the stress ratio according to the direct inversion method. The angle between the calculated direction of maximum shear stress along the fault plane and the measured striae is determined for each data set, and the distribution of these angles is plotted in a histogram. The relative values of normal and shear stresses are plotted in a normalized Mohr circle diagram. Graphical output is in standard HPGL files, suitable for publication-quality plotting by commercial word-processing and graphics programs. Data handling by the program package is demonstrated with a test file. Finally, the output of our programs is compared with that provided by published routines implementing different techniques.

Monitoring of the concrete dam of the Boguchany hydroelectric station in the construction period

1. Since the start of construction of the Boguchany hydrostation, 1184 remote transducers, which are used for monitoring the state of sections of the concrete dam and foundation during the construction period, have been installed in the concrete dam as of October 1992. An automated system for processing and analyzing the data of observations has been created and is being introduced, which will make possible prompt monitoring of the state of sections of the concrete dam and foundations in the construction period. 2. The stress state of the columns during construction is determined mainly by the temperature regime of the concrete of the columns. The values of normal stresses in the columns of the monitored section on vertical areas are within ±1 MPa. An increase of compressive stresses is observed on horizontal areas, which is related to an increase of the load as the columns are built up. According to the monitoring data, the column joints have a width within 0.5–2.0 mm, which is sufficient for grouting. Under all columns of the dam section the deformations of the rock foundation are due to the load from the mass of concrete, and in the spillway section, in addition, from the mass of water. An increase of compressive strains diminishing with time is observed under the effect of a constant load on the foundation (interruption in concreting).

Subepicardial fiber strain and stress as related to left ventricular pressure and volume.

In a mathematical model of the mechanics of the left ventricle (LV) by Arts et al. (1), assuming uniformity of fiber stress (sigma f) and fiber strain (delta epsilon f) in the wall during the ejection phase, fiber stress and fiber strain were related to LV cavity pressure (Plv), LV cavity volume (Vlv) and wall volume (Vw) by the following pair of equations: sigma f = Plv (1 + 3 Vlv/Vw) and delta epsilon f = 1/3 delta ln (1 + 3 Vlv/Vw). The ratio of Vlv to Vw appeared to be the most important geometric parameter, whereas the actual LV shape was of minor importance. The relationships on fiber strain and stress were evaluated experimentally in six anesthetized open-chest dogs during normal and elevated (volume loading) end-diastolic LV pressure. Subepicardial fiber strain was measured simultaneously in 16 adjacent regions of the LV anterior wall, using optical markers that were attached to the epicardial surface and recorded on video. Changes in Vlv were measured by use of four inductive coils sutured to the LV in a tetrahedric configuration. Vw was measured postmortem. During control as well as hypervolemia the following results were found. At the anterior free wall of the LV, the slope of the estimated linear relationship between measured and calculated fiber strain was 1.017 +/- 0.168 (means +/- SD), which is not significantly different from unity. Calculated fiber stress corresponded qualitatively and quantitatively with experimental results reported on isolated cardiac muscle. Calculated subepicardial contractile work per unit of tissue volume was not significantly different from global pump work as normalized to Vw. These findings support the assumption of homogeneity of muscle fiber strain and stress in the left ventricular wall during the ejection phase. Furthermore, average values of fiber stress and strain can be estimated on the basis of measured left ventricular pressure and volume.

An analysis of the brittle-ductile transition in dynamic crack growth

The brittle-ductile transition in dynamic crack growth is investigated through the numerical analysis of a plane strain edge cracked specimen, subject to impulsive loading at one end. The material is characterized as an elastic-viscoplastic solid, with a temperature dependent flow strength. Thermal softening due to adiabatic heating and a model for ductile failure by void nucleation, growth and coalescence are incorporated into the constitutive relation. The ductile void growth mechanism involves two populations of void nucleating particles; discretely modelled low strength inclusions that give large voids near the crack tip at an early stage and homogeneously distributed small second phase particles that require large strains for void nucleation. Cleavage is modelled in terms of a spatially non-uniform, but temperature and strain rate independent, critical value of the maximum principal normal stress. The numerical results show a clear transition from cleavage dominated crack growth at low temperatures to purely ductile crack growth at higher temperatures. There is an accompanying large increase in the material's resistance to dynamic crack initiation and growth. The computed crack growth behaviour is a direct outcome of the material description; no ad hoc dynamic fracture criterion is employed. Effects of variations in the material model on the brittle-ductile transition are explored.

A Study on the Longitudinal Bending Stress Distribution in a Double Hull Tanker Taking Account of the Effects of Warping in Bending

In International Maritime Organization, they decided that an oil tanker newly constructed after 1992 A. D. should have the devices for prevention of oil pollution, and one choice of these devices was to have the double hull.In such an oil tanker, we often adopt more space between longitudinal bulkheads. And the more the heigh tensile steel is used, the heigher the stress level in hull structure will become generally. It is, therefore, expected that the effects of warping in bending upon the stress distribution due to longitudinal bending can not be neglected under the heavy ballast condition in which the rate of change of shearing force becomes large.In this paper, at first, we have carried out some numerical calculations on longitudinal bending stress distributions in the double hull tanker of 140, 300 DWT., applying the method of analysis which has been proposed by the two of us previously. Sea state has been chosen as the regular wave whose wave height equal to 10 metres and wave length equal to the ship's length.After careful consideration, we have following conclusions : (1) The values of normal stresses due to warping in bending come to about from 20% to 30% of those due to bending under the heavy ballast condition. Especially at the gunwale part and/or at the bilge part near in the midship section, it reaches its maximum.(2) Under the hogging condition, the normal stress due to warping in bending is in the same plus/ minus sign of those due to bending at a gunwale/bilge part, within the region where the shearing force is increasing. Under the sagging condition, the situations are reversible.(3) We can regard the ship's hull girder as thin walled constant cross section beam, so far as the longitudinal stress components in the parallel part of ship's hull girder are concerned.

Miscible displacement in a Hele-Shaw cell

We formulated a theory of simple mixtures of incompressible miscible liquids in terms of the mass averaged velocityu and the solenoidal volume averaged velocityW, We derived simplified equations for miscible displacement in a Hele-Shaw cell. We obtained a steady solution of these equations corresponding to displacement under gravity with prescribed values of concentration and mean normal stress at the inlet and exit of the cell. We studied the stability of this steady flow. This differs from previous works which treat the stability of unsteady miscible displacement using a quasi-static assumption and classical equations based on divu=0. In our problem, replacingu withW gives rise to a difference in the mean normal stress, which alters the pressure drop across the cell and changes the velocity of free fall. We found that the stability equations are the same in the two formulations, but the boundary conditions are slightly different; however the difference will be small if diffusion is slow or the thickness of the cell is small. The results show that steady miscible displacement in a Hele-Shaw cell is stable to long and short waves. Within certain ranges of parameters, the displacement of glycerin into water can be unstable. This instability is basically of a Rayleigh-Taylor type, regularized by diffusion. As the diffusion parameterS becomes smaller, the waves of disturbances become finer and are confined to an increasingly thin diffusion layer. Water displacing glycerin is always stable. This is due to the fact that the steady equilibrium profile is not steep enough to create a fingering instability.