Showing papers on "Hydrostatic stress published in 2008"

Influence of epitaxial strain on the ferromagnetic semiconductor EuO : First-principles calculations

Abstract: From first-principles calculations we investigate the electronic structure and the magnetic properties of $\mathrm{EuO}$ under hydrostatic stress and the appropriate biaxial stress for epitaxial films. There is a complex interdependence of the $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ and $\mathrm{Eu}\phantom{\rule{0.3em}{0ex}}4f$ and $5d$ bands on the magnetism in $\mathrm{EuO}$, and decreasing lattice parameters is an ideal method to increase the Curie temperature ${T}_{c}$. Compared to hydrostatic pressure, the out-of-plane compensation that is available to epitaxial films diminishes this increase in ${T}_{c}$, although the ${T}_{c}$ increase is nonetheless significant due to the small value of Poisson's ratio for $\mathrm{EuO}$. We find the semiconducting gap closes at a 6% in-plane lattice compression for epitaxy, at which point a significant conceptual change must occur in the active exchange mechanisms.

Neutron diffraction residual stress analysis of zirconia toughened alumina (ZTA) composites

Abstract: Measurements of the residual stresses in zirconia toughened alumina (ZTA) composites, containing 1.7, 14 and 22 vol% yttria-stabilized zirconia (3Y–TZP) were obtained by neutron diffraction. Over the range of volume fraction investigated, the hydrostatic stress in alumina and zirconia phases varies roughly linearly with the zirconia content. It is shown that these features can be qualitatively understood by taking into consideration the thermal expansion mismatch between the ZrO 2 and Al 2 O 3 grains. In addition, a decrease of the Al 2 O 3 line broadening is observed, which implies a decreasing micro-residual strain due to the ZrO 2 particles in the alumina. It is inferred that the residual strain field is highly hydrostatic, and that a decrease of the grain boundary shear stress occurs as a function of the reinforcement volume fraction. This phenomenon is more important in the case of 1.7 vol% zirconia composite because the zirconia particles are in the nanometre size range, with a narrow distribution.

Effects of Deformation-Induced Constraint on High-Cycle Fatigue in Ti Alloys with a Duplex Microstructure

Abstract: The effects of the deformation behaviors of individual phases in Ti alloys with a duplex microstructure containing a mixture of primary α grains and lamellar α + β colonies have been analyzed by applying the finite-element method (FEM) to the relevant microstructure. The softer primary α grains and the harder lamellar α + β colonies are treated as distinct microstructural units with different constitutive properties. The FEM results indicate that the softer (primary α) phase, which reaches yielding first, tends to concentrate both the plastic strain and the hydrostatic stress, whose magnitudes depend on the volume fraction of the primary α grains and the load levels. The insight from the FEM analyses has been used to modify the linear relation between alternating stress and mean stress in a Haigh or modified Goodman diagram by incorporating a microstructure-induced hydrostatic stress term, leading to a set of bilinear relations for high-cycle fatigue (HCF) failures of Ti alloys with a duplex microstructure. The applicability of the micromechanical approach to treating the mean stress effects in HCF failure of Ti-6Al-4V with various duplex microstructures is elucidated by comparison against experimental data in the literature.

Analysis of thermo-mechanical process occurred in magnesium alloy AZ31 sheet during differential speed rolling

Abstract: Rolling is an effective method used to impose a large amount of plastic deformation to metallic materials in a form of sheet. As the plastic deformation increases at appropriate temperatures, the grains become smaller in size and the mechanical properties of the sheet are improved. A recent study found that the grains of magnesium alloy AZ31 sheets are recrystallized and reduced to 3 μm in diameter by a differential speed rolling at an elevated temperature. The present study analyzed the thermo-mechanical processes occurring in the sheet during differential speed rolling by the rigid-thermoviscoplastic finite element method. Findings include a significant increase of plastic deformation by severe shear deformation, a notable increase of temperature by plastic deformation as well as by friction, and an incredible decrease of rolling force by low hydrostatic stress in magnitude at the deformation zone.

Boundary Perturbation Solution for Nearly Circular Holes and Rigid Inclusions in an Infinite Elastic Medium

Abstract: The boundary perturbation method is used to solve the problem of a nearly circular rigid inclusion in a two-dimensional elastic medium subjected to hydrostatic stress at infinity. The solution is taken to the fourth order in the small parameter epsilon that quantifies the magnitude of the variation of the radius of the inclusion. This result is then used to find the effective bulk modulus of a body that contains a dilute concentration of such inclusions. The corresponding results for a cavity are obtained by setting the Muskhelishvili coefficient K equal to -1, as specified by the Dundurs correspondence principle. The results for nearly circular pores can be expressed in terms of the pore compressibility. The pore compressibilities given by the perturbation solution are tested against numerical values obtained using the boundary element method, and are shown to have good accuracy over a substantial range of roughness values.

Experimental study on mechanical characteristics of granite under high temperatures and triaxial stresses

Abstract: Extracting heat from hot dry rock(HDR) and nuclear waste disposal both need researches on granite mechanical characteristics at high temperature and high pressure in depth.Utilizing the 20 MN servo-controlled triaxial rock testing machine with high temperature and pressure developed by the authors,the research of thermal deformation and failure characteristics of large size granite samples of φ 200 mm×400 mm at high temperature and triaxial stresses through recent half a year is carried out;and the characteristics of thermo-mechanical parameters such as linear thermal expansion coefficient and elastic modulus changing with temperature,are also achieved.The research results indicate that:(1) Under 3D hydrostatic stress,granite′s thermal deformation course can be divided into slow deformation stage at low temperature,fast deformation stage at medium-high temperature and slow deformation stage at high temperature.Thermal expansion coefficient measured without stress state may excessively magnify the rock′s thermal expansion or thermal effect and will result in wrong conclusion,so the thermal expansion coefficient measured with stress state can reflect the actual state of rock mass.(2) Granite shows feature of expansion in volume at low stress while shrinkage in volume after exceeding a certain stress under high temperature and triaxial stresses.It is a different feature contrasted with that of normal temperature.(3) The failure shape of granite at high temperature is of shear failure,while granite will transform to ductility under high confining pressure and high temperature.(4) The elastic modulus of granite decreases with the increase of temperature under confining pressure.The variety course can be divided into three stages,i.e.slow decreasing stage,fast decreasing stage and constant stage.

Modeling the effect of microstructural features on the nucleation of creep cavities

Abstract: A crystal plasticity based finite element model has been applied to study the deformation of metals at the microstructural length scale, in order to determine the effect of various microstructural features on the nucleation of creep cavities. The deformation model captures the non-uniform distributions of the equivalent plastic strain and the hydrostatic stress within the different grains of the microstructure when subjected to cyclic loading conditions. The influence of various microstructural features such as grain boundaries, triple junctions, and second-phase particles, on the strain and stress fields is examined through the simulations. The results indicate that the various microstructural parameters, such as grain orientation, presence of the precipitates and their shape, and alignment of the boundaries with respect to the loading direction influence the strain and stress distributions, and therefore, the conditions that favor the nucleation and growth of creep cavities.

Deviatoric stress measurements at high pressure and temperature

Abstract: X‐ray diffraction of samples at high pressure and temperature provide information not only on the unit cell dimensions, but also on the deviatoric stress in the sample. Macroscopic stress is defined by the relative strains inferred from the different diffraction lines in an elastically anisotropic sample. Cubic materials are particularly useful for determining the macroscopic stress since, under hydrostatic stress, all diffraction lines will display the same strain. Measurements on samples with a superconducting wiggler synchrotron source in a large volume high pressure apparatus (SAM85), capable of generating 15 GPa pressure and 1500 °C temperature have been inverted for deviatoric stress as a function of pressure and temperature for NaCl and gold. Deviatoric stress determinations provide information on the yield strength of the sample. In addition, the presence of deviatoric stress will significantly affect the pressure calibration based on a diffraction standard. Measurements in a diamond anvil cell at room temperature with a neon pressure medium demonstrate that a systematic pressure error of up to 2 GPa occurs at about 30 GPa using gold as the pressure standard. This error is significantly reduced by heating to only 100 °C. Microscopic deviatoric stress is inferred from peak broadening. Strength measurements have been made on diamond at temperatures up to 1500 °C at 10 GPa in SAM85 from this type of data.

Fabrication of Cu Nanowires at Predetermined Positions by Utilising Stress Migration

Abstract: Stress migration (SM) is a phenomenon whereby atoms diffuse from a region of lower stress towards a region of higher stress because of the hydrostatic stress gradient. In this study, the fabrication of Cu nanowires at predetermined positions by controlling the direction of the atomic flux caused by SM is analysed. Cu nanowires are fabricated by rearranging accumulated atoms into wire-like crystals. First, the sites at which Cu atoms accumulate are found from finite element (FE) analysis and from the results obtained from experiments. Next, a technique is proposed for accumulating atoms by SM at predetermined sites. Finally, the successful fabrication of Cu nanowires at these sites is demonstrated.

New motorcycle helmets with metal foam shell

Abstract: New motorcycle helmets are designed with metal foam shell and their impact behaviour has been studied. Impact experiments have been performed on a first set of prototype helmets with metal foam shell at standard impact locations. Numerical simulations are performed and the predicted headform acceleration is validated with experimental data. The biomechanical characteristics of head impact were studied with both metal foam and ABS helmets. The helmet with metal foam shell performed reasonably well compared to ABS helmet. Motorcycle crash helmets absorb the shock and cushion the human head so that the time of actual impact is extended. They spread the impact over large area of the human head thereby decreasing the pressure at any one point. The outer shell in regular helmets is made of thermoplastic (either ABS or Polycarbonate) which is heavy and stiff. The outer shell spreads the impact over large area of the helmet and prevents the penetration of sharp objects. To make more comfortable for a rider it is desirable to reduce its weight. One way of doing this is using an outer shell of lighter material without compromising on its dynamic performance and safety. One group of materials, which reduce weight and absorb energy, is the porous materials. Metal foams are a class of cellular materials and have many interesting properties such as high stiffness in conjunction with low specific weight combined with good energy absorption characteristics. These unique characteristics make them useful for applications range from automobile bumpers to aircraft crash recorders. Outer shell in motorcycle helmets can be one such application for metal foams, which needs to be studied. In a helmet besides the energy absorbed by the polymer foam, metal foams can also absorb energy because of their porous nature and can prevent the penetration of sharp objects. Metal foams based on aluminium or nickel are the most commonly used at present in various applications. METAL FOAMS A general overview over the mechanical properties of cellular materials can be found in Gibson and Ashby (1). The stress-strain curve of polystyrene foams (like EPS) and metal foams have similar shapes although their yield points are very different. In compression after yielding, strains will increase at almost constant stress and once the foam is compressed (or densified) then the stresses will start rising again. For solid metals with isotropic mechanical behaviour, the von Mises yield criterion is widely used and the yield surface is independent of hydrostatic stress. It is assumed that the elastic volumetric energy does not affect the plastic flow of metals. Metal foams exhibit plastic flow for a pure hydrostatic stress condition. It follows that the elastic volumetric energy affects the plastic flow of foams and it is necessary to extend the yield criterion to take this effect into account. Several studies were made to investigate the constitutive response of aluminum foams in the recent past. Deshpande and Fleck (2) studied the isotropic and continuum based metal foams and included a hydrostatic stress term in the yield function to take into account the volume changes in the foam. Figure 1 shows the stress-strain behaviour of aluminium foam for 500 kg.m -3 density. Hanssen et al (4) established an extensive experimental database for the structural behaviour of aluminium foam- filled extrusions. They discussed and compared various material models for aluminium foam. Hanssen et al (5) carried out experimental bird strike tests on aluminium foam based double sandwich panels. They predicted the failure of structural components with aluminium foam in bird-strike events through a numerical model.

On the Balance Equation for Stresses Concentrated on Curves

Abstract: Conditions of equilibrium of forces are formulated for a force system in a continuous body which consists of a bulk stress and a stress concentrated on a curve. The system of forces is interpreted as a tensor valued measure and the equilibrium of forces is interpreted in a weak sense. The divergence theorem is proved for bulk stress fields with the corresponding singularity. Examples of such stress fields are given.

Constraint Evaluation And Effects On J-R Resistance Curves For Pipes Under Combined Load Conditions

Abstract: Single edge cracked under tension (SENT) specimens appear as an alternative to conventional fracture specimens to characterize fracture toughness of circumferentially cracked pipes. The similarities of stress and strains fields between SENT specimens and cracked pipes are now well known. However, these similarities are not so well established for the case of circumferentially cracked pipes under combined loading conditions (i.e. internal pressure plus bending). This work presents a numerical analysis of crack-tip constraint of circumferentially surface cracked pipes and SENT specimens using full 3D nonlinear computations. The objective is to examine combined loading effects on the correlation of fracture behavior for the analyzed crack configurations. The constraint study using the J-Q methodology and the h parameter gives information about the fracture specimen that best represents the crack-tip conditions on circumferentially flawed pipes under combined loads. Additionally, simulations of ductile tearing in a surface cracked plate under biaxial loading using the computational cell methodology demonstrate the negligible effect of biaxial loadings on resistance curves. Introduction Structural defects in pressurized piping systems are very often surface cracks that form during fabrication or during in-service operation. The fracture conditions of these crack configurations contrast sharply to fracture conditions present in conventional deeply cracked specimens (i.e. CT or SENB). Single edge cracked under tension specimens (SENT) appear as an alternative to conventional fracture specimens to characterize fracture toughness of circumferentially cracked pipes. The similarities of stress and strains fields between SENT specimens and cracked pipes are now well determined. However, these similarities are not so well established for the case of circumferentially cracked pipes under combined load conditions (i.e. internal pressure plus tension, internal pressure plus bending, etc.). Heating and cooling cycles, ground movement and frost heave are examples of situations that produce yielding in the pipe metal when it is submitted to biaxial loading. To further understand the effects of combined loading conditions on fracture behavior of flawed pipes, this work presents a numerical investigation of crack-tip constraint of circumferentially surface cracked pipes and SENT specimens using full 3D nonlinear computations. The primary objective is to examine combined loading effects on the correlation of fracture behavior for the analyzed crack configurations using the Q [1, 2] and h [3] parameters. The Q parameter measures the deviation of the stress field of the studied geometry from a reference stress solution (i.e. the boundary layer model [4]). On the other hand, the h parameter is defined as the ratio of the hydrostatic stress level ahead of the crack front over the effective Von Mises stress and it characterizes the growth of micro voids in a triaxial stress field. Furthermore, simulations of ductile tearing in a surface cracked plate under biaxial loading using the computational cell methodology allow the study of biaxial effects on tearing behavior. The constraint study and the crack growth

Analysis of fracture limit curves and void coalescence in high strength interstitial free steel sheets formed under different stress conditions

Abstract: Void formation, which is a statistical event, depends on inhomogeneities present in the microstructure. The analysis on void nucleation, their growth and coalescence during the fracture of high strength interstitial free steel sheets of different thicknesses is presented in this article. The analysis shows that the criterion of void coalescence depends on the d-factor, which is the ratio of relative spacing of the ligaments (δd) present between the two consecutive voids to the radius of the voids. The computation of hydrostatic stress (σm), the dominant factor in depicting the evolution of void nucleation, growth and coalescence and the dimensional analysis of three different types of voids namely oblate, prolate and spherical type, have been carried out. The ratio of the length to the width (L/W) of the oblate or prolate voids at fracture is correlated with the mechanical properties, microstructure, strains at fracture, Mohr’s circle shear strains and Triaxiality factors. The Lode angle (θ) is determined and correlated with the stress triaxiality factor (T), ratio of mean stress (σm) to effective stress (σe). In addition, the Void area fraction (Va), which is the ratio of void area to the representative area, is determined and correlated with the strain triaxiality factor (To).

Approximation of Nonlinear Unloading Effects in the Strain Rate Dependent Deformation Analysis of Polymer Matrix Materials Utilizing a State Variable Approach

Abstract: An experimental and analytical program is carried out to explore key behaviors in the loading and unloading behavior of polymers. Specifically, the effects of strain rate and hydrostatic stresses on the nonlinear portions of the deformation response are examined. Tension, compression, and shear load only and load/unload tests are conducted on a representative polymer across a range of strain rates, and key features of the experimental results are identified. To conduct a preliminary exploration of how the key features of the deformation response could be simulated analytically, a previously developed set of constitutive equations, which were developed to analyze the strain rate dependent, nonlinear deformation of polymers including the effects of hydrostatic stresses, were modified in order to approximate key features of the nonlinear unloading behavior observed in the polymer. The constitutive relations are based on state variable constitutive equations originally developed for metals. The nonlinear unloading observed in the experiments is approximated by reducing the unloading modulus of the material as the effective inelastic strain is increased. The effects of the hydrostatic stress state on the unloading modulus are also simulated analytically. To examine the revised formulation, the loading and load/unload responses of the representative polymer in tension, compression, and shear are examined at several strain rates. Results computed using the developed constitutive equations were found to correlate reasonably well with the experimental data.

Effects of patterned, stressed SiN overlayers on Si solid phase epitaxy

Abstract: Striking nonuniformities are observed in the solid phase epitaxy (SPE) of blanket amorphized Si layers recrystallized in the presence of stress distributions induced by a patterned SiN overlayer. Measurements conducted for a range of SiN feature sizes and intrinsic stress values allowed us to isolate the effects of stress on the crystallization front. It is concluded that SiN-induced variations in SPE rates arise both from line-edge stresses, which scale with feature stress and increase SPE rates where the hydrostatic stress is compressive, and a SiN body effect, which suppresses SPE rates under the SiN features, independent of SiN stress state.