Showing papers on "Hydrostatic stress published in 1996"

Effect of stress state on the stress-induced martensitic transformation in polycrystalline Ni-Ti alloy

Abstract: The effect of stress state on the character and extent of the stress-induced martensitic transformation in polycrystalline Ni-Ti shape memory alloy has been investigated. Utilizing unique experimental equipment, uniaxial and triaxial stress states have been imposed on Ni-Ti specimens and the pseudoelastic transformation strains have been monitored. Comparisons between tests of differing stress states have been performed using effective stress and effective strain quantities; a strain offset method has been utilized to determine the effective stress required for transformation under a given stress state. Results of the tests under different stress states indicate that (1) despite the negative volumetric strain associated with the austenite-to-martensite transformation in Ni-Ti, effective stress for the onset of transformation decreases with increasing hydrostatic stress; (2) effective stressvs effective strain behavior differs greatly under different applied stress states; and (3) austenite in Ni-Ti is fully stable under large values of compressive hydrostatic stress.

First-Principles Calculation of the Pressure Dependence of Phase Equilibria in the Al-Li System

Abstract: The solid-phase portion of the Al-Li phase diagram has been computed from first principles both at zero pressure and at a hydrostatic compression of 5.4 GPa. Computation of the pressure dependence of the Al-Li phase equilibria answers two questions: (1) how important is the effect of the atomic size difference, and (2) is the stability of the ${\mathrm{Al}}_{3}$Li precipitates influenced by high hydrostatic stress. The zero-pressure first-principles phase diagram exhibits excellent qualitative agreement with experimental data. The presence or absence of solid solutions (SS), of stable and metastable intermetallic phases, and their degree of order are computed correctly. Compression is predicted to affect the phase equilibria in Al-Li as follows: (1) the solubility of Li in fcc Al-rich SS is decreased, (2) the solubility of Al in Li is increased. However, the low melting point of Li limits the range of SS, and (3) the metastable ${\mathrm{Al}}_{3}$Li Al-rich fcc SS phase equilibrium is unaffected and the stability of the precipitates is unchanged, (4) the ordering tendencies at Li-rich compositions are slightly enhanced. Although high pressure eliminates the difference in atomic volume of the pure constituents, it has almost no effect on the solid-solid phase equilibria in this alloy system. A simple method for verifying the accuracy of the cluster expansion for the configurational internal energy is presented and applied. Moreover, it has been shown that with a convenient choice of the occupation numbers, one can define correlation functions which greatly facilitate the determination of new ground state structures. \textcopyright{} 1996 The American Physical Society.

Influence of axial stress and dilatancy on rock tunnel stability

Abstract: In many existing elastoplastic plane-strain solutions for a circular tunnel excavated in a homogeneous, isotropic rock mass under a hydrostatic stress field, the longitudinal axial stress (or the out-of-plane stress) {sigma}{sub z} is not correctly evaluated. This paper describes the concepts and a numerical method to calculate this stress component and proposes analytical solutions for practical applications. The computed numerical solutions are compared with theoretical solutions where possible. The relations between rock-mass dilatancy, tunnel displacements, and the in-situ stress field are investigated. Mohr-Coulomb and Hoek-Brown yield criteria with appropriate plastic potential functions and nonassociated flow rules are used in the brittle plastic calculations. The effects of various boundary conditions on the numerical analyses are discussed and several important practical conclusions are drawn from the investigations.

Validity and Limits of the Effective Stress Concept in Geomechanics

Abstract: The effective stress concept in geomechanics advocated by Terzaghi is historically reviewed and the mechanical interpretation of this concept is clarified based on the theory of mixtures. In the interpretation of the effective stress concept, both the compressibility of the constituents and the balance of force are taken into consideration. The effectiveness of the effective stress comes from the fact that the descriptions of the effective stress in undrained and unjacketed conditions are approximately equal, although the role of the pore-water pressure under different test conditions is not the same. It is shown that the effective stress concept is also applicable to soft rock. Finally, classical interpretations and various definitions of the effective stresses are critically examined.

Stress sensitivity of sandstones

Abstract: Our observations made on dry-sandstone ultrasonic velocity data relate to the variation in velocity (or modulus) with effective stress, and the ability to predict a velocity for a rock under one effective pressure when it is known only under a different effective pressure. We find that the sensitivity of elastic moduli, and velocities, to effective hydrostatic stress increases with decreasing porosity. Specifically, we calculate the difference between an elastic modulus, M ( P 1, Φ), of a sample of porosity Φ at effective pressure P 1 and the same modulus, M ( P 2, Φ), at effective pressure P 2. If this difference, Δ M = M ( P 1, Φ) - M ( P 2, Φ), is plotted versus porosity for a suite of samples, then the scatter of Δ M is close to zero as porosity approaches the critical porosity value, and reaches its maximum as porosity approaches zero. The dependence of this scatter on porosity is close to linear. Critical porosity here is the porosity above which rock can exist only as a suspension—between 36% and 40% for sandstones. This stress-sensitivity pattern of grain-supported sandstones (clay content below 0.35) practically does not depend on clay content. In practical terms, the uncertainty of determining elastic moduli at a higher effective stress from the measurements at a lower effective stress is small at high porosity and increases with decreasing porosity. We explain this effect by using a combination of two heuristic models—the critical porosity model and the modified solid model. The former is based on the observation that the elastic-modulus-versus-porosity relation can be approximated by a straight line that connects two points in the modulus-porosity plane: the modulus of the solid phase at zero porosity and zero at critical porosity. The second one reflects the fact that at constant effective stress, low-porosity sandstones (even with small amounts of clay) exhibit large variability in elastic moduli. We attribute this variability to compliant cracks that hardly affect porosity but strongly affect the stiffness. The above qualitative observation helps to quantitatively constrain P - and S -wave velocities at varying stresses from a single measurement at a fixed stress. We also show that there are distinctive linear relations between Poisson's ratios (ν) of sandstone measured at two different stresses. For example, in consolidated medium-porosity sandstones ν40 = 0.018 + 0.913ν20, where the subscripts indicate hydrostatic stress in MPa. Linear functions can also be used to relate the changes (with hydrostatic stress) in shear moduli to those in compressional moduli. For example, G 40 - G 20 = 0.084 + 0.344 ( M 40 − M 20), where G = ρ VS 2 is shear modulus and M = ρ VP 2 is compressional modulus, both in GPa, and the subscripts indicate stress in MPa.

Pressure Dependence of Ferroelectric Properties in PbZrO3-PbTiO3 Solid State System under Hydrostatic Stress

Abstract: Pressure dependence of ferroelectric properties in PbZrO3-PbTiO3 solid state system (PZT) were calculated under hydrostatic stress and compared with experimental results. When the ratio of electrostrictive coefficients in PZT, Q11/Q12 was exactly equal to -2.000, the ferroelectric properties were not affected by the application of hydrostatic pressure; i.e. isotropic compressive stress. At ratios other than Q11/Q12=-2.000, the Curie points were decreased with increasing hydrostatic pressure, in particular near the morphotropic phase boundary (MPB). The piezoelectric charge and voltage coefficients, d ij and g ij, increased and decreased, respectively, near the MPB with increasing hydrostatic pressure, due to the pressure dependence of the dielectric constant and spontaneous polarization.

Impact Response of Foam: the Effect of the State of Stress

Abstract: In this study the finite element (FE) predictions of the physical response of foams during impact by a rigid body such as the Hybrid Ill headform is determined by material law equations generally approximated based on the elastoplasticity theory. However, the structural foam aspect, its discontinuous nature, makes it difficult to apply the laws of continuum mechanics and to construct constitutive equations for foam-like material. One part of the problem relates to the state of stress. In materials such as steel, the state of hydrostatic stress does not affect the stress strain behaviour under uniaxial compression or tension in plastic regime. In other words, when steel is subject to hydrostatic pressures the stress strain characteristic can be predicted from a uniaxial test. However, if the stresses acting on a section of foam are triaxial, the response of a headform may be different than predicted from uniaxial test data. The experimental data presented indicate that the state of hydrostatic stress may affect the response of a Hybrid III headform interacting with foam padding. Based on these results and using elastoplasticity theory, an idealized computational material model of foam is generated and used to quantify the state of stress effects. (A) For the covering abstract of the conference see IRRD 891635.

Viscoplastic Analysis of Adhesive Joints

Abstract: The uniaxial constitutive law for an adhesive is studied by constant strain rate tensile, creep and relaxation tests. The S-D effect of the adhesive is taken into account by using the Raghava yielding criterion in a three dimensional constitutive formulation. The obtained constitutive law is then used to analyze a single lap joint and a butt joint by a finite element method. Constant cross head speed tensile and creep loading cases are examined. For a butt joint, the results show that the viscous effect and the influence of the hydrostatic stress must be taken into account due to the variation of the hydrostatic stress and of the loading rate in the adhesive layer as function of its thickness. A comparison with experimental results is also given. A good agreement between viscoplastic calculations and experimental results is obtained for single-lap joints. A reasonable result is obtained for butt joints and the discrepancy is attributed to interfacial debonding.

Hydrogen Embrittlement Characterization by Disk Pressure Tests: Test Analysis and Application to High Chromium Martensitic Steels

Abstract: The paper reports and discusses the results of an experimental and numerical activity, aimed to the characterization of the influence of hydrogen on the mechanical properties of a few high chromium martensitic steels which are candidates for fusion reactor and chemical applications. Experiments were conducted with the Disk Pressure Test technique, according to which a circular thin specimen is loaded up to rupture by an uniform pressure. As a detailed analysis of the stress/strain distributions in the specimen was not available, this kind of test being mainly used to obtain comparative information about different materials, a nonlinear numerical (Finite Element) model of the specimen was set up, by which stress and strain could be accurately evaluated as a function of the applied pressure. This model was employed both for interpreting experimental results and to achieve a more general understanding of the capabilities of the Disk Pressure Test for the characterization of hydrogen embrittlement effects. The calculated strain at failure showed the typical dependence on hydrogen content, falling to very low levels as a threshold concentration is exceeded.

Analysis of axisymmetric tube extrusion

Abstract: A comprehensive investigation of an axisymmetric steady-state tube extrusion through a streamlined die is carried out by the finite element method (FEM) to study the influence of process variables on tool design and final product quality for a strain hardening material. The process variables considered are: the reduction in area; coefficient of friction; mandrel radius; die-length; the hardening capacity of the material. The extrusion parameters studied are: the extrusion pressure; die pressure; mandrel pressure; hydrostatic stress distribution; strain-rate distribution; strain distribution. The mixed (pressure-velocity) formulation is used along with the Householder method to solve the resulting ill-conditioned algebraic equations. The extrusion pressure predicted by the present model is in reasonably good agreement with the published experimental results. The trends predicted for other parameters also conform with the published results.

Introduction to Fracture Mechanics

Abstract: When an engineer designs a structure or piece of equipment, there are two basic forms of mechanical failure which must be considered. These are brittle fracture and, if the material employed is metallic, yielding. The second is by far the easier to take into account: it is necessary to know only the state of stress at every point in order to assemble the appropriate yield parameter. The maximum value of this quantity is found within the structure and set equal to the yield stress, which is taken as a true material property, i.e. it is independent of geometry. Usually the loading or stresses are reduced by a so-called factor of safety, which allows for unexpected overloads during the life of the structure. The load level corresponding to the onset of first yield is known as the elastic limit. There are considerable reserves of strength in any real structure if the elastic limit is moderately exceeded, partly because most real structures exhibit a high degree of redundancy, partly because cyclic loading will induce beneficial residual stresses, promoting shakedown, and partly because most common metals and alloys exhibit work hardening to a greater or lesser degree. By far the most important characteristics of yield from the point of view of design are: (a) that the yield stress is a highly repeatable true material property, being very insensitive to the geometry of the component under consideration. (b) that given the yield stress under uniaxial loading, the combination of stresses which will cause local failure under multiaxial conditions is well defined — assumptions of isotropy, independence of yield from hydrostatic stress and convexity of the yield surface (Paul, 1968) being necessary to obtain excellent bounding values for physically acceptable yield criteria.

Deformation state effects on the Jc of BSCCO tapes

Abstract: Developing long-length, high- J c superconducting tapes has been a major world-wide effort in recent years because of their potential applications in power-transmission lines, motors, and other devices. The superconducting tape is usually produced by co-deforming a ductile silver sheath containing a superconducting oxide. Since the conventional thermomechanical process has failed to yield sufficient J c values for most liquid-nitrogen temperature applications, new approaches are needed to improve the criticalcurrent density, J c . This study investigated the feasibility of improving J c by increasing the shear and compressive stresses in the silver-sheathed Bi 2 Sr 2 Ca 2 Cu 3 O 10 + x (BSCCO-2223) tapes during the rolling. To investigate the effects on the J c of the stress state during rolling, specific stress states were imposed by rolling the BSCCO tapes embedded at different locations within thick steel blocks. Pure compression loading was achieved in the center plane of the blocks, while a combined compression-shear loading state was produced away from the center plane. Higher compressive hydrostatic stress at the tape edge was obtained by confining the tape width. Tapes deformed with a shear stress component exhibited higher J c values than tapes subjected to pure compression. In addition, the compressive hydrostatic stress reduced the porosity in the oxide near the tape edge and, as a consequence, increased the J c value.

On the Coupling of Pressure and Deviatoric Stress in Isotropic Hyperelastic Materials.

Abstract: : For isotropic elastic materials, we study the coupling of pressure and shear stress that results from the dependence of shear stress on volumetric strain and the dependence of pressure on shear strain. (MM)

Influences of non-singular stresses on plane-stress near-tip fields for pressure-sensitive materials and applications to transformation toughened ceramics

Abstract: In this paper, we investigate the effects of the non-singular stress (T stress) on the mode I near-tip fields lbr elastic perfectly plastic pressure-sensitive materials under plane-stress and small-scale yielding conditions. The T stress is the normal stress parallel to the crack faces. The yield criterion for pressure-sensitive materials is described by a linear combination of the effective stress and the hydrostatic stress. Plastic dilatancy is introduced by the normality flow rule. The results of our finite element computations based on a two-parameter boundary layer formulation show that the total angular span of the plastic sectors of the near-tip fields increases with increasing T stress for materials with moderately la~rge pressure sensitivity. The T stress also has significant effects on the sizes and shapes of the plastic zones. The height of the plastic zone increases substantially as the T stress increases, especially for materials with large pressure sensitivity. When the plastic strains are considered to be finite as lbr transformation toughened ceramics, the results of our finite element computations indicate that the phase transformation zones for strong transformation ceramics with large pressure sensitivity can be approximated by those lbr elastic-plastic materials with no limit on plastic strains. When the T stress and the stress intensity factor K are prescribed in the two-parameter boundary layer formulation to simulate the crack-tip constraint condition tor a single-edge notch bend specimen of zirconia ceramics, our finite element computation shows a spear shape of the phase transformation zone which agrees well with the corresponding experimental observation.

超大形軸材の鍛造における内部空げき圧着に関する研究 : 第3報, 表面冷却鍛錬法の改善による空げき圧着条件の達成

Abstract: A hydrostatic stress ratio of greater than 0.9 was proposed as one of the most significant criterion for void consolidation after closing during open die forging, on the basis of some physical modeling tests on 3.5NiCrMoV-rotor steel billet with a hole at the centerline and numerical simulation using the three-dimensional rigid-plastic finite-element method. To satisfy the criterion for void consolidation in a forging process of large rotor shafts, it was found that the use of a precooling ingot forging method for a rectangular cross-section-ingot is highly effective since the hydrostatic stress ratio at the center of the ingot remarkably increases with the increase of aspect ratio in the cross section from 1.0 to 1.8. The enlargement of width in the rectangular cross section enabled the development of the optimum maufacturing process of an ultralarge low-pressure turbine rotor shaft with 2800 mm and over diameter.

Thermal fatigue behavior of a SUS304 pipe under longitudinal cyclic movement of axial temperature distribution

Abstract: In a structural thermal fatigue test which imposed an oscillating axial temperature distribution on a SUS 304 pipe specimen, different crack initiation lives were observed between the inner and the outer surfaces, although the values of the von-Mises equivalent strain range calculated by FEM inelastic analysis were almost the same for both surfaces. The outer surface condition was an in-phase thermal cycle and an almost uniaxial cyclic stress (low hydrostatic stress). The inner surface condition was an out-of-phase thermal cycle and an almost equibiaxial cyclic stress (high hydrostatic stress). A uniaxial thermal fatigue test was performed under the simulated conditions of the outer and inner surfaces of the pipe specimen. The in-phase uniaxial thermal fatigue test result was in good agreement with the test result of the pipe specimen for the outer surface. The out-of-phase uniaxial thermal fatigue test which simulated the inner surface condition, showed a longer life than the in-phase uniaxial test, and thus contradicted the result of the structural model test. However, the structural model test life for the inner surface agreed well with the uniaxial experimental measurement when the strain range of the inner surface was corrected by a triaxiality factor.

The use of isostatic pressing to improve the strength of TLP diffusion bonds in aluminium-based composites

Abstract: Transient Liquid Phase (TLP) diffusion bonding of aluminium-SiC composites, using copper interlayers, was carried out under low bonding pressure to minimize plastic deformation. This was followed by solid-state diffusion bonding under relatively high pressure as a complementary process to improve joint strength and reliability. In the high pressure stage, plastic deformation was avoided by lateral constraint of the sample in order to build up a hydrostatic stress state, simulating hot isostatic pressing (hipping). The bonding temperature in a TLP process is usually determined by the temperature at which the liquid phase forms, e.g., the Al-Cu eutectic formation temperature in this case. In theory, it should be possible to vary the applied pressure in order to optimize bonding. However, the superplastic behavior of the material used in this work led to excessive deformation at the bonding temperature, with consequent restrictions on the bonding pressure and on the resulting bond strengths. The subsequent use of higher bonding pressures with minimal plastic deformation in the second stage of the process resulted in considerable improvements in bond strength. Bonds with shear strengths as high as 70% and 92% respectively of the shear strengths of two aluminium composites, 8090 Al/SiC and 359 Al/SiC (given themore » same thermal cycles including post solution treatment and ageing), have been achieved.« less

Bone tensile strength and hydrostatic stress

Abstract: A model making use of an elastic-plastic material law and continuous damage mechanics has been developed to define the strength of trabecular bone. With the aid of this model, the differences in the behaviour of bone under tensile and compressive stresses can be simulated numerically. It is also possible to calculate the loss of strength of bone via a damage variable. The calculated results are in good agreement with stress-strain curves determined experimentally.

Flexi-distoritional piezoelectric composites

Abstract: An alternative form of a flexural amplificaiton type sensor can be proposed using and amplified d 15 response. The mechanically amplified flexi-distortional system may improve the d h g h figure of merit by transferring the compressional stress on the device to a lateral shear on the electroceramic element. This type of amplified system is mechanically classified as distortional and, there fore, without volume change. By transforming the hydrostatic stress to trancsverse strain, as in the flexi-extensional devices, transverse forces can be used to develop shear strain and activate the d 15 coefficient. Electrodes are placed along the sides of the sensing element rather than along the top and bottom surfaces. Flexi-distortional devices are modeled in order to check for the limits of sensitivity in their performance.

The energies of microvoid formation in Si as a function of applied hydrostatic stress

Abstract: Using a Tersoff‐type empirical potential energy function, the free energy of formation for microvoids in silicon containing 24 and 57 vacancies was calculated at 1000 K as a function of externally applied hydrostatic stress in the range of −10 kbar (tensile) to 10 kbar (compressive). The results indicate that bigger microvoids are more sensitive to the applied stress than the smaller ones.

The Influence of the Back Stress (X) and the Hardening Rate (dX/Xεpqe) on Void Nucleation in α/β Titanium Alloys

Abstract: The study of void nucleation in four α/β titanium alloys has provided nucleation criteria corresponding to voids at the α/β interface. This macroscopic nucleation criterion, written as Σ m =f(e peq ), was explained with the help of microscopic observations. Microscopic parameters such as plastic strain in the α-phase or local hydrostatic stress σ m at the α/β interface have been linked to the damage initiation. Besides, the influence of the different heterogeneity levels on the macroscopic nucleation criterion was demonstrated using the macroscopic mechanical parameters: back stress (X) and hardening rate (dX/de peq ) which express the plastic strain incompatibilities and their evolution in such alloys.