Showing papers on "Hydrostatic stress published in 1997"
TL;DR: In this paper, the authors investigated void growth and coalescence under physical states similar to those found in highly stressed regions ahead of a crack and showed that micro void cavitation and link-up will increasingly dominate the failure process resulting in a brittle-like ductile rupture mode in which very little energy is expended.
Abstract: Void growth and coalescence under physical states similar to those found in highly stressed regions ahead of a crack is investigated. The analysis introduces a representative material volume containing several large voids and a population of microvoids present from the very beginning, all of which are modeled as discrete entities. Plastic yielding has pervaded the material volume of interest. The underlying micromechanics of final rupture is dominated by a succession of rapidly growing microvoids. This involves the synergistic interaction between elasticity associated with high stress triaxiality, stiffness softening caused by plastic yielding and a rich supply of length scales arising from voids of vastly different sizes. A primary feature of the coalescence phase is an unstable deformation mode whereby a minute, benign void rapidly enlarges reaching a size set by the characteristic length of the locally elevated stress field. The process begins with a large void growing in concert with the plastic strain. Simultaneously, a local zone of high stress concentration emanates from the large void and spreads across the material raising the stresses at nearby microvoids. As a result, the hydrostatic stress surrounding one or more microvoids is raised to a level that activates an unstable deformation mode in which the stored elastic energy drives the plastic expansion of the microvoid. Although the overall stress decreases rapidly, small zones of high stress concentration are generated near growing voids—causing even smaller nearby microvoids to grow rapidly. This process continues until the submicron ligament fails by microcleavage or by shearing along crystallographic planes. Plastic yielding plays a crucial role in the above process by lowering the stress level required for the unstable-like growth mode of microvoids. The process outlined above appears to be the main operative mechanism in several observed failure modes in metal alloys. The morphologies of fracture surfaces dominated by flat dimpled rupture and voidsheet formation can be elucidated by the present work. For high-strength metals, our studies suggest that microvoid cavitation and link-up will increasingly dominate the failure process resulting in a brittle-like ductile rupture mode in which very little energy is expended.
168 citations
TL;DR: In this paper, a simple, 3D yield function that is quadratic in stresses was proposed to describe the plastic behavior of fiber composites, which relaxes the two usually used assumptions that hydrostatic stress does not influence plastic deformation and that the total plastic dilatation is incompressible.
Abstract: A simple, 3-D yield function that is quadratic in stresses was proposed to describe the plastic behavior of fiber composites. It relaxes the two usually used assumptions that hydrostatic stress does not influence plastic deformation and that the total plastic dilatation is incompressible. It is also general in nature to allow for composites with various fiber volume fractions and different fiber arrays. The applicability of this quadratic yield function to fiber composites was examined, and the accuracy of the elasto-plasticity model was verified by using the macro stress-strain data generated by a 3-D nonlinear micromechanics model. Because this anisotropic plasticity model is simple and is in the general form of those widely used in existing numerical plasticity codes, it can easily be incorporated into the existing codes with little effort.
67 citations
TL;DR: In this article, a nonlinear continuum damage model was used to obtain the evolution of material ductility with stress triaxiality, and the expression found relates the strain to failure in multi-axial state of stress regime only to the uniaxial strain-to failure, to the damage strain threshold, to material Poisson's ratio, and, of course, to stress triaxeiality.
Abstract: Ductility takes into account the material capability to plastically deform. This parameter is not only modified by temperature but it is strongly affected by the stress triaxiality that, in the case of positive hydrostatic stress, reduces the material strain to failure. Due to the importance of this parameter in engineering design many attempts to predict the evolution of ductility with stress triaxiality have been done. Here, a nonlinear continuum damage model, as proposed by the author, is used to obtain the evolution of material ductility with stress triaxiality. The expression found relates the strain to failure in multi-axial state of stress regime only to the uniaxial strain to failure, to the damage strain threshold, to the material Poisson's ratio, and, of course, to stress triaxiality. The proposed model was successfully verified comparing the predicted evolution of material ductility with the experimental data relative to several metals. The procedure for the damage parameters identification is also discussed in details.
39 citations
TL;DR: In this article, a technique involving sample preparation, video imaging, and image analysis has been developed to observe the kinematics of shear bands when geomaterials are subjected to a general state of combined stress.
Abstract: A new technique involving sample preparation, video imaging, and image analysis has been developed to observe the kinematics of shear bands when geomaterials are subjected to a general state of combined stress. The technique provides an effective, low-cost, and non-invasive way to monitor the development and measure the deformations inside and outside the shear bands. Its capabilities are demonstrated through a series of drained tests in which thin hollow cylinders of sand are subjected to combinations of hydrostatic, axial, and torsional stresses. It is shown that the deformation within the shear band is different from the global one and the one in its vicinity. For sand specimens with the same configuration, density, and confining pressure, the initiation, orientation, and thickness of shear bands depend on the loading path.
39 citations
TL;DR: The thermodynamics of diffusion under hydrostatic pressure and non-hydrostatic stress for single crystals free of extended defects are presented in this article, and the results of ab initio calculations of the effect of biaxial stress on diffusion with no free parameters are discussed.
Abstract: The thermodynamics of diffusion under hydrostatic pressure and nonhydrostatic stress is presented for single crystals free of extended defects. The thermodynamic relationships obtained permit the direct comparison of hydrostatic and biaxial stress experiments and of atomistic calculations under hydrostatic stress for any proposed mechanism. Atomistic calculations of the volume changes upon point defect formation and migration, and experiments on the effects of pressure and stress on the diffusivity, are reviewed. For Sb in Si, using as input the results of ab initio calculations of the effect of hydrostatic pressure on diffusion by the vacancy mechanism, the thermodynamic relationships successfully account for the measured effect of biaxial stress on diffusion with no free parameters. For other cases, missing parameters are enumerated and experimental and calculational procedures outlined.
30 citations
ETSI1
TL;DR: In this article, a simple procedure is proposed to estimate the local strain rate at a crack tip, the relevant variable in environmentally assisted cracking processes, which is experimentally checked by applying a kinematic fracture criterion for high strength steel under hydrogen embrittlement environmental conditions.
29 citations
TL;DR: In this paper, a transient thermal stress problem of a rectangular plate due to a non-uniform heat supply is treated theoretically and, thereafter, fracture behaviors of the plate with a crack are examined for compressive stress states.
Abstract: In this study, a transient thermal stress problem of a rectangular plate due to a nonuniform heat supply is treated theoretically and, thereafter, fracture behaviors of the plate with a crack are examined for compressive stress states. Assuming that a crack located on an arbitrary position, with an arbitrary direction, is sufficiently small and is closed because of the compressive stress field, a temperature field, in a transient state, is analyzed by taking into account the effect of relative heat transfer on both surfaces of the plate. Thereafter, the corresponding thermal stress analysis is developed on the basis of the two-dimensional plane stress problem using Airy's stress function method, and the stress intensity factor is analyzed for the biaxial stress state. As an analytical model, we consider mechanical boundary conditions of prescribed displacement and estimate the stress intensity factor of a crack tip using parameters of the crack configuration such as the location, direction, lengt...
27 citations
TL;DR: In this paper, an effective stress of a ductile matrix is defined directly from the average second order stress moment, which is evaluated exactly provided that an estimation of the composite effective moduli is given.
19 citations
TL;DR: In this paper, the causes of biaxial and hydrostatic stress components in epitaxially grown thin GaN films on sapphire are analyzed, and it is observed that growth by Molecular Beam Epitaxy (MBE) and by Metal Organic Chemical Vapor Deposition (MOCVD) are governed by very similar physical principles.
Abstract: In this study, the causes of biaxial and hydrostatic stress components in epitaxially grown thin GaN films on sapphire are analyzed. It is observed that growth by Molecular Beam Epitaxy (MBE) and by Metal Organic Chemical Vapor Deposition (MOCVD) are governed by very similar physical principles. Differences in the absolute stress values are mainly due to the difference in growth temperature. It is argued that in the case of MOCVD growth the onset of plasticity for higher growth temperatures is responsible for a larger stress relaxation in the buffer layer. It is further found that either process can result in highly off-stoichiometric GaN layers, as manifested by the large variations in the a and c lattice parameters caused by intrinsic point defects.
14 citations
TL;DR: In this article, the authors analytically relate hydrostatic stress to strain in a random dense pack of identical spheres cemented at their contacts, and show that, as loading increases and cement becomes plastic, a finite (Hertzian) direct contact area between the spheres necessarily has to develop and progress.
9 citations
01 Sep 1997
TL;DR: In this article, the response of a variety of sucker rod couplings to an applied axial load was simulated using axisymmetric finite element models, and the results suggested approaches for improving the fatigue resistance of threaded connection.
Abstract: The response of a variety of sucker rod couplings to an applied axial load was simulated using axisymmetric finite element models. The calculations investigated three sucker rod sizes and various combinations of the slimhole, Spiralock, and Flexbar modifications to the coupling. In addition, the effect of various make-ups (assembly tightness) on the performance of coupling was investigated. An axial load was applied to the sucker rod ranging from {minus}5 ksi to 40 ksi, encompassing three load cycles identified on a modified Goodman diagram as acceptable for indefinite service life of the sucker rods. The simulations of the various coupling geometries and make-ups were evaluated with respect to how well they accomplish the two primary objectives of preloading threaded couplings: (1) to lock the threaded coupling together so that it will not loosen and eventually uncouple, and (2) to improve the fatigue resistance of the threaded connection by reducing the stress amplitude in the coupling when subjected to cyclic loading. Perhaps the most significant finding in this study was the characterization of the coupling parameters which affect two stress measures. The mean hydrostatic stress, which determines the permissible effective alternating stress, is a function of the coupling make-up. Whereas, the alternating effective stress is a function of the relative stiffnesses of the pin and box sections of the coupling and, as long as the coupling does not separate, is unaffected by the amount of circumferential displacement applied during make-up. The results of this study suggest approaches for improving the fatigue resistance of sucker rod couplings.
TL;DR: In this article, the thermodynamics of diffusion under hydrostatic pressure and non-hydrostatic stress were developed for single crystals free of extended defects and applied to the case of boron diffusion in silicon.
Abstract: The thermodynamics of diffusion under hydrostatic pressure and nonhydrostatic stress is developed for single crystals free of extended defects and is applied to the case of boron diffusion in silicon. The thermodynamic relationships obtained permit the direct comparison of hydrostatic and biaxial stress experiments and of atomistic calculations under hydrostatic stress. Assuming various values for the anisotropy in the migration strain, a currently unknown parameter, comparison is made between various measurements under hydrostatic pressure and nonhydrostatic stress, and various atomistic calculations of the volumetrics of B and Si diffusion by an interstitial-based mechanism. An independent determination of the anisotropy of the migration strain would permit a parameter-free determination of the predominant diffusion mechanism and would permit the prediction of the ratio of the diffusivity normal to the free surface to the diffusivity parallel to the surface for biaxially strained films. Procedures for measuring and calculating the anisotropy in the migration strain are described.
TL;DR: In this article, a set of finite element computations are achieved with ABAQUS code on elasto-plastic structures with micro-voided material, and the influence of the damage material parameters is analyzed and discussed in the chapter in regard to local mechanical variables, notably, the microvoid volume fraction, the effective plastic strain and the hydrostatic stress.
Abstract: Publisher Summary A set of finite element computations are achieved with ABAQUS code on elasto-plastic structures with microvoided material. First, a tensile test is focussed on the microvoid volume fraction evolution for various random initial distributions of the porosity and two values of the initial microvoid volume fraction f i , 0.001 and 0.01. It is clearly shown that to perform realistic predictions of forging sequence for elasto-plastic damage material by finite element simulation, an accurate characterization of the initial porosity distribution isn't necessary for small values of the initial porosity. Indeed, the influence of the damage material parameters is analyzed and discussed in the chapter in regard to local mechanical variables, notably, the microvoid volume fraction, the effective plastic strain and the hydrostatic stress.
TL;DR: In this article, a bending beam technique was used to measure the stress of passivated Al- and AlSiCu lines during thermal cycling from RT to 450 r.C and isothermal relaxation at different temperatures between RT and 350r.
Abstract: Mechanical stress relaxation in passivateci metal lines generally has two components, shear relaxation at constant volume, changing the stress distribution, and volumetric relaxation changing the hydrostatic stress. In order to contribute to the understanding of the underlying mechanisms we have collected detailed data on relaxation of passivated Al- and AlSiCu-lines with different aspect ratios a (0.2 ≤ a ≤ 1) and different thicknesses. A bending beam technique was used to measure the stress of these lines during thermal cycling from RT to 450r.C and isothermal relaxation at different temperatures between RT and 350r.C. Main results are: i) During thermal cycling the stress at RT and the total stress amplitude increase with increasing aspect ratio a, while stress hysteresis formation increases with decreasing a. ii) During isothermal relaxation the relaxed stress is strongly dependent on temperature with maxima between 150 and 250r.C. The maximum shifts to lower temperatures with decreasing aspect ratio. From the temperature dependence the effective activation energy of 0.6±0.3eV for AlSiCu can be determined, iii) Finite element calculation and the method of "eigenstrains" are used to estimate seperately volumetric (voiding) and shear plasticity in these lines. Clearly voiding dominates the isothermal stress relaxation in all measured samples.
TL;DR: In this paper, a model of ductile failure of a body with a crack was developed which enables predicting fracture toughness on the upper shelf of the fracture toughness temperature dependence taking into account the influence of the stress state.
Abstract: A model of ductile failure of a body with a crack has been developed which enables predicting fracture toughness on the upper shelf of the fracture toughness temperature dependence taking into account the influence of the stress state. The model is based on the physical-mechanical model of ductile failure which is controlled by the critical value ef reached by plastic strain at the crack tip eiρ. In this case it is assumed that both the eiρ value, which precedes the crack growth onset by the mechanism of pore coalescence, and the critical strain ef are functions of specific stress state parameters, namely: the critical strain is a function of the stress state triaxiality σm/σn (σm is the hydrostatic stress, σi is the stress intensity), and eiρ is a function of the parameter χ introduced, which is an explicit function of all three principal local stresses in the process zone at the crack tip and which defines the degree to which the stress state approaches the plane strain conditions for a body of specified thickness. The model developed has two modifications one of which enables predicting fracture toughness of large-size bodies from the results of testing only small cylindrical specimens without cracks (smooth and with a circular recess) and the other from the results of testing small cylindrical specimens and small specimens with a crack.
01 Jan 1997
TL;DR: In this paper, the effect of monoaxial stresses on transformation kinetics and transformation plasticity deformations is discussed. But the authors focus on cases where both transformation kinetic modifications and transformation plasminarity deformations are observed.
Abstract: When diffusive phase transformations occur under applied stresses different modifications appear depending on the nature of the applied stress. The effect of the hydrostatic stresses is essentially to modify the transformation kinetics and the composition of the phases formed. These results are briefly reviewed. For non hydrostatic stresses, different modifications are observed, changes in the transformations kinetics, in the mechanical behaviour i.e. Transformation Plasticity Deformation, and in some cases in the morphology of the product phases. The effect of monoaxial stresses will be shown considering both kinetics changes and modifications of the mechanical behaviour. The paper is focused on cases were both transformation kinetics modifications and transformation plasticity are observed. Most examples are given for ferrous alloys and namely the pearlitic transformation.
TL;DR: In this paper, the effect of Poisson's ratio on two-dimensional elastic-plastic stress is studied by using the body force method, in which the stress field caused by plastic deformation is replaced by a stress field due to force doublets.
Abstract: In this study, the effect of Poisson's ratio on two-dimensional elastic-plastic stress is studied by using the body force method, in which the stress field caused by plastic deformation is replaced by a stress field due to force doublets. It is shown that the elastic-plastic stress induced by a prescribed surface traction for a given strain hardening rate H′ E is independent of Poisson's ratio ν provided that the stress is in a state of plane stress and the vector sums of traction on holes are zero for every individual hole.
TL;DR: In this paper, the authors used polarized Raman spectroscopy to measure the axial and hydrostatic stress components along the [111] and [100] directions of a 2-in-diam polycrystalline diamond film showing mainly [111]- and [110] facets.
Abstract: Polarized Raman spectroscopy is used to measure the hydrostatic and axial stress components along the [111] and [100] direction of a 2-in.-diam polycrystalline diamond film showing mainly [111] and [110] facets. The theoretical background and the instrumentation necessary to measure stress and crystal orientation are presented. Experimental examples of a [111] oriented single crystal having uniaxial stress along the [111] direction, as well as a [100] oriented single crystal having uniaxial stress along the [100] direction, are presented. The diamond Raman peak height and shift indicating hydrostatic stress are measured at 1776 different locations across the entire film. The hydrostatic stress, the axial stress along the [100] direction, and the axial stress along the [111] direction of 441 neighboring crystals of a center portion of the film are also determined. The results show a very clustered distribution of the hydrostatic and axial stress with a variation of up to 0.8 GPa.
01 Jan 1997
TL;DR: In this paper, an early suggestion by Schleicher (1926) for the plastic yield condition of porous materials reads {fy(1)|335-1} where τ = ((3/2)S··S)1/2 is the Mises effective stress, S = σ − pI is the stress deviator, σ is the Cauchy stress tensor, I = (1/3)I··σ is the hydrostatic stress and (YC,YT) are the uniaxial yield stresses in compression and in tension
Abstract: An early suggestion by Schleicher (1926) for the plastic yield condition of porous materials reads {fy(1)|335-1} where τ = ((3/2)S··S)1/2 is the Mises effective stress, S = σ − pI is the stress deviator, σ — the Cauchy stress tensor, I — the 2nd order unit tensor, p = (1/3)I··σ is the hydrostatic stress and (YC,YT) are the uniaxial yield stresses in compression and in tension, respectively.
TL;DR: In this article, the effect of rubber particle volume fraction on the constitutive relation and fracture toughness of polymer blends was studied using elastic-plastic finite element analysis (FEA).
Abstract: The effect of rubber particle volume fraction on the constitutive relation and fracture toughness of polymer blends was studied using elastic-plastic Finite Element Analysis (FEA). The effect of rubber particle cavitation on the stress-strain state at a crack tip was also investigated. Stress analysis reveals that because of the high rubber bulk modulus, the hydrostatic stress inside the rubber particle is close to that in the adjacent matrix material element. As a result, the rubber particle imposes a severe plastic constraint to the surrounding matrix and limits its plastic strain. Rubber particle cavitation can effectively release the constraint and enable large scale plastic strain to occur. Different failure criteria were used to determine the optimum rubber particle volume fraction for the polymer blends studied in this paper.
TL;DR: In this article, the authors used a damage mechanics approach to determine the multiaxial stress parameter that most accurately characterizes the stresses driving crack tip damage in 304 stainless steel specimens.
Abstract: High-temperature crack growth experiments have been conducted with 304 stainless steel specimens under mode I, mode II, and mixed-mode conditions. Crack growth rate and direction data for three different mixed-mode loadings have been analyzed to investigate the factors that control crack growth under mixed-mode conditions. The value of C* was calculated for mode mixities ranging from pure tensile to pure shear loading at the crack tip using a reference stress approach. Effective values of C* based on a damage mechanics approach were then calculated in an attempt to determine the multiaxial stress parameter that most accurately characterizes the stresses driving crack tip damage. The hydrostatic stress was found to be the stress parameter that best correlates the crack growth rate data for mode I, mode II, and mixed-mode loading conditions. The angle of growth for the mode I and mixed-mode conditions appears to be governed by both the maximum principal stress and the hydrostatic stress. However, the lack of tensile loads for mode II loading results in crack growth that is nearly collinear with the notch corresponding to the position of the maximum effective stress. Overall, the present results indicate that the hydrostatic stress is the most valid multiaxial stress parameter for predicting high-temperature crack growth in the present material under mixed-mode conditions.
TL;DR: In this article, a composite aluminum/PZT shear transducer has been tested as a sensor during prototype development, and the mechanically amplified flexi-distortional system may improve the dhgh figure of merit by transferring the compressional stress on the device to a lateral shear on the electroceramic element.
Abstract: A composite aluminum/PZT shear transducer has been tested as a sensor during prototype development. The mechanically amplified flexi-distortional system may improve the dhgh 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, therefore, without volume change. By transforming the hydrostatic stress to transverse strain, as in the flexi-extensional devices, transverse forces can be used to develop shear strain and activate the d15 coefficient. Flexi-distortional devices are tested in order to check for their low frequency sensitivity. Initial Berlincourt tests indicate performance of up to 1200 pC/N for the driver configuration.
TL;DR: In this article, the authors presented theoretical expressions giving the effective elastic moduli of a cubic or isotropic homogeneous solid under hydrostatic stress as functions of strain and temperature, and derived the temperature dependence of these expressions within the fourth-order quasi-harmonic approximation of lattice dynamics.
TL;DR: In this article, a finite element solution to problems of electromigration and stress-driven diffusion is presented based on a formulation that assumes bulk diffusion, so that diffusion along grain boundaries and interfaces is not explicitly taken into account.
Abstract: Finite element solutions to problems of electromigration and stress-driven diffusion are presented. The numerical procedures are based on a formulation that assumes bulk diffusion, so that diffusion along grain boundaries and interfaces is not explicitly taken into account. Two cases are considered: (1) a tungsten via/aluminum line junction, and; (2) a one-dimensional case with a blocking boundary at one end of the line. The results show that line geometry can significantly affect the magnitude of the stresses that develop, although in each case the maximum hydrostatic stress in the line was proportional to the square root of time.