Showing papers on "Hydrostatic stress published in 1998"

Analysis of lattice strains measured under nonhydrostatic pressure

Abstract: The equations for the lattice strains produced by nonhydrostatic compression are presented for all seven crystal systems in a form convenient for analyzing x-ray diffraction data obtained by newly developed methods. These equations have been used to analyze the data on cubic (bcc α-Fe) and hexagonal (hcp e-Fe) systems. The analysis gives information on the strain produced by the hydrostatic stress component. A new method of estimating the uniaxial stress component from diffraction data is presented. Most importantly, the present analysis provides a general method of determining single crystal elastic constants to ultrahigh pressures.

Quantification of constraint effects in elastic-plastic crack front fields

Abstract: In-plane and out-of-plane constraint effects on crack tip stress fields under both small-scale and large-scale yielding conditions are studied by means of three-dimensional numerical analyses of boundary layer models and of finite size specimens, M(T) and SE(B), respectively. It is shown that the ratio of the plastic zone size over the panel thickness, rpt, plays a key role in formation of the crack-tip fields, particularly the outof-plane stress components. For a vanishingly small plastic zone around the crack tip the stress fields are dominated by the plane strain solution. With increase of the applied loads, i.e. increasing the plastic zone size, the stress fields develop towards the plane stress state. Characterization of “constraint effects” in terms of Q-stress is investigated. The “second term” in the near tip stress field, which is defined as the difference between the full three-dimensional stress fields and the plane strain reference solution, appears to depend on the distance to the tip and to the free surface of the specimen. Hence, the whole three-dimensional crack front fields cannot be correctly described by a two-parameter formulation as the load increases. However, a unique linear relationship between Q and the hydrostatic stress was found in all three-dimensional crack front fields.

Asymmetric Shielding Mechanisms in the Mixed-Mode Fracture of a Glass/Epoxy Interface

Abstract: An asymmetric increase in the apparent values of the interfacial fracture toughness with increasing mode II component of loading has been observed by several investigators. In this study, cracks were grown in a steady-state manner along the glass/epoxy interface in sandwich specimens in order to determine the mechanisms responsible for the shielding effect. Finite element analysis using a hydrostatic stress and strain rate dependent plasticity model for the epoxy and a cohesive zone model for the interface shows that plastic dissipation in the epoxy accounts for the asymmetric shielding seen in these experiments which cover a wide range of mode mix. Numerical predictions of normal crack-opening displacements yielded results that were consistent with measured values which were made as close as 0.3 μm from the crack tip.

Laboratory Study of Permeability Evolution in a Tight' Sandstone under Non-Hydrostatic Stress Conditions

Abstract: Hydrostatic and triaxial compression experiments have been conducted to investigate the evolution of fluid permeability and fluid storage capacity in Tennessee sandstone, a low-porosity 'tight' reservoir sandstone. A newly-developed single-ended transient pulse permeability measurement technique has been used for this study. Under hydrostatic stress, both permeability and specific storage are demonstrated to be dependent upon effective confining pressure. The evolution of axial permeability as a function of increasing compressive deviatoric axial stress has been investigated in triaxial compression experiments. All samples were deformed within the brittle faulting regime. Both permeability and specific storage decrease during compaction, and subsequently increase when axially-aligned dilatant microcracking becomes dominant. Brittle failure is sometimes accompanied by a transient increase in permeability by as much as 1.5 orders of magnitude. Following brittle failure the permeability is controlled by the properties of the fault rock. Permeability decreases as stable sliding occurs on the fault up to large axial strains, such that the fault may act as an impermeable seal to migrating fluids.

Damage Evolution and Damage Surface of Elastic-Plastic-Damage Materials under Multiaxial Loading

Abstract: Damage evolution and fundamental aspects of damage surface of a spheroidized graphite cast iron are observed. This paper is intended to discuss the applicability of the irreversible thermodynamics theory to the modeling of elastic-plastic-damage materials. By performing damage tests on the tubular specimens of the material under proportional and nonproportional loading, onset and development of material damage are first detected by measuring Acoustic Emission (AE) counts. The change in Young's modulus, Poisson's ratio and shear modulus is also observed in the tests to correlate them with the damage development in the material. The existence and the development of the damage surface, together with the condition of loading, unloading and neutral loading, are elucidated to facilitate the identification of the fundamental aspects of thermodynamics theory of the elastic-plastic-damage materials. The effects of the hydrostatic stress on the deformation and damage equations are also discussed.

Studies on the growth of voids in amorphous glassy polymers

Abstract: Numerical studies are presented of the localized deformations around voids in amorphous glassy polymers. This problem is relevant for polymer–rubber blends once cavitation has taken place inside the rubber particles. The studies are based on detailed finite element analyses of axisymmetric or planar cell models, featuring large local strains and recent material models that describe time-dependent yield, followed by intrinsic softening and subsequent strain hardening due to molecular orientation. The results show that plasticity around the void occurs by a combination of two types of shear bands, which we refer to as wing and dog-ear bands, respectively. Growth of the void occurs by propagation of the shear bands, which is driven by orientational hardening. Also discussed is the evolution of the local hydrostatic stress distribution between voids during growth, in view of possible craze initiation. © 1998 Kluwer Academic Publishers

Effects of thickness and precracking on the fracture toughness of particle-reinforced al-alloy composites

Abstract: The effect of specimen thickness on the fracture toughness of a powder metallurgically processed 7093 Al/SiC/15p composite was evaluated in different microstructural conditions. The fracture toughness in the underaged condition increased significantly with a decrease in specimen thickness, even at thicknesses well below the value specified by ASTM-E 813 for a valid J Ic test. The influence of thickness was considerably lower in the peak-aged (PA) condition. This relative insensitivity is believed to be due to the low strain to failure associated with severe flow localization in the PA condition. The effect of precracking on the fracture toughness of discontinuously reinforced aluminum (DRA) was also evaluated. The dependence of fracture toughness on specimen thickness and precracking is explained in terms of the hydrostatic stress, which has a strong influence on the plastic straining capability of the DRA material. The fracture toughness was modeled using a critical strain formulation, with the void growth strain dependent on hydrostatic stress through the Rice and Tracey model. The predicted toughnesses for the thick and thin specimens were in good agreement with the experimental data.

Fatigue life and change in mechanical properties of plain concrete under triaxial deviatoric cyclic stresses

Abstract: The effects of a lateral confinement on the fatigue behaviour of plain concrete were investigated experimentally. Triaxial cyclic tests on cylinders, with the confining pressure and axial load varying in phase opposition at 1 Hz, were performed in different deviatoric planes. The maximum stress deviator was 90% of the deviator at failure in a given deviatoric plane; the minimum deviator ranged between 70% and 95% of the maximum one. The residual strength and stiffness properties of those specimens that did not fail within a prescribed number of cycles were also assessed. In tests with cycles of equal amplitude (normalized to the triaxial static strength), the fatigue life and residual strength seem to be positively affected by an increase in hydrostatic stress only up to a certain level. With the maximum deviator equal, in a given deviatoric plane the fatigue life usually increases with decreasing cycle amplitude. In one of the deviatoric planes, the cycle amplitude and mean stress were found to have oppo...

The effects of particle bulk modulus on toughening mechanisms in rubber‐modified polymers

Abstract: The interaction of a blunting mode I plane-strain crack tip with a periodic array of initially spherical rubber particles directly ahead of and parallel to the crack front in the effective medium is studied by the crack tip-particle interaction model. The local stress concentrations responsible for rubber cavitation, matrix crazing and shear yielding are obtained by three-dimensional large deformation elastic-plastic finite element analysis with a sub-modeling technique to explore the relationship between these toughening mechanisms. It is shown that rubber particles can act as stress concentrators to initiate matrix crazing or shear yielding but they behave differently from voids at high triaxiality because of their high bulk modulus. Particle bulk modulus affects significantly the hydrostatic stress inside rubber particles as well as the plastic deformation in the ligament between the crack tip and particles. Rubber cavitation or interface debonding relieves the triaxial stress plane-strain condition so that extensive plastic deformation can be developed in the toughening process.

Stress propagation in sand

Abstract: We describe a new continuum approach to the modelling of stress propagation in static granular media, focussing on the conical sandpile created from a point source. We argue that the stress continuity equations should be closed by means of scale-free, local constitutive relations between different components of the stress tensor, encoding the construction history of the pile: this history determines the organization of the grains, and thereby the local relationship between stresses. Our preferred model fixed principle axes (FPA) assumes that the eigendirections (but not the eigenvalues) of the stress tensor are determined forever when a material element is first buried. Stresses propagate along a nested set of arch-like structures within the medium; the results are in good quantitative agreement with published experimental data. The FPA model is one of a larger class, called oriented stress linearity (OSL) models, in which the direction of the characteristics for stress propagation are fixed at burial. We speculate on the connection between these characteristics and the stress paths observed microscopically.

A Viscoplastic Constitutive Theory for Monolithic Ceramics—I

Abstract: This paper, which is the first of two in a series, provides an overview ofa viscoplastic constitutive model that accounts for time-dependent material deformation (e.g., creep, stress relaxation, etc.) in monolithic ceramics. Using continuum principles of engineering mechanics, the complete theory is derived from a scalar dissipative potential function first proposed by Robinson (1978), and later utilized by Duffy (1988). Derivations based on a flow potential function provide an assurance that the inelastic boundary value problem is well posed, and solutions obtained are unique. The specific formulation used here for the threshold function (a component of the flow potential function) was originally proposed by Willam and Warnke (1975) in order to formulate constitutive equations for time-independent classical plasticity behavior observed in cement and unreinforced concrete. Here constitutive equations formulated for the flow law (strain rate) and evolutionary law employ stress invariants to define the functional dependence on the Cauchy stress and a tensorial state variable. This particular formulation of the viscoplastic model exhibits a sensitivity to hydrostatic stress, and allows different behavior in tension and compression.

Evaluation of constraint methodologies applied to a shallow-flaw cruciform bend specimen tested under biaxial loading conditions

Abstract: A technology to determine shallow-flaw fracture toughness of reactor pressure vessel (RPV) steels is being developed for application to the safety assessment of RPVs containing postulated shallow surface flaws Matrices of cruciform beam tests were developed to investigate and quantify the effects of temperature, biaxial loading, and specimen size on fracture initiation toughness of two-dimensional (constant depth), shallow surface flaws The cruciform beam specimens were developed at Oak Ridge National Laboratory (ORNL) to introduce a prototypic, far-field out-of-plane biaxial stress component in the test section that approximates the nonlinear stresses resulting from pressurized-thermal-shock or pressure-temperature loading of an RPV Tests were conducted under biaxial load ratios ranging from uniaxial to equibiaxial These tests demonstrated that biaxial loading can have a pronounced effect on shallow-flaw fracture toughness in the lower transition temperature region for RPV materials The cruciform fracture toughness data were used to evaluate fracture methodologies for predicting the observed effects of biaxial loading on shallow-flaw fracture toughness Initial emphasis was placed on assessment of stress-based methodologies namely, the J-Q formulation, the Dodds-Anderson toughness scaling model, and the Weibull approach Applications of these methodologies based on the hydrostatic stress fracture criterion indicated an effect of loading-biaxiality on fracture toughness, the conventional maximum principal stress criterion indicated no effect

Open die forging of porous materials

Abstract: It is the objective of this work to suggest and apply an extended formulation for the evolution of the material density in an open die forging of porous axisymmetric solid with Gurson’s yielding model. An admissible porous-dependent velocity field is obtained and applied to form the constitutive behavior of the compressible material. Concurrently, an admissible stress field of the yielded material is used to estimate the effect of the hydrostatic stress on the bulk flow. Such a combination of the dual bounds enables one to assess the non-steady loading path in terms of the current porous state and the geometry of the workpiece, regardless of the mechanism by which the densification process is achieved. Emphasis is given to the use of the newly porous-dependent shear boundary condition along with some classical interfacial friction models. The suggested procedure to simulate the evolution of the densification during the forging process is compared with FEM solutions and self-generated data.

Fracture assessment of HSST Plate 14 shallow-flaw cruciform bend specimens tested under biaxial loading conditions

Abstract: A technology to determine shallow-flaw fracture toughness of reactor pressure vessel (RPV) steels is being developed for application to the safety assessment of RPVs containing postulated shallow surface flaws. Matrices of cruciform beam tests were developed to investigate and quantify the effects of temperature, biaxial loading, and specimen size on fracture initiation toughness of two-dimensional (constant depth), shallow, surface flaws. The cruciform beam specimens were developed at Oak Ridge National Laboratory (ORNL) to introduce a far-field, out-of-plane biaxial stress component in the test section that approximates the nonlinear stresses resulting from pressurized-thermal-shock or pressure-temperature loading of an RPV. Tests were conducted under biaxial load ratios ranging from uniaxial to equibiaxial. These tests demonstrated that biaxial loading can have a pronounced effect on shallow-flaw fracture toughness in the lower transition temperature region for an RPV material. The cruciform fracture toughness data were used to evaluate fracture methodologies for predicting the observed effects of biaxial loading on shallow-flaw fracture toughness. Initial emphasis was placed on assessment of stress-based methodologies, namely, the J-Q formulation, the Dodds-Anderson toughness scaling model, and the Weibull approach. Applications of these methodologies based on the hydrostatic stress fracture criterion indicated an effect of loading-biaxiality on fracture toughness; the conventional maximum principal stress criterion indicated no effect. A three-parameter Weibull model based on the hydrostatic stress criterion is shown to correlate the experimentally observed biaxial effect on cleavage fracture toughness by providing a scaling mechanism between uniaxial and biaxial loading states.

Molecular Dynamics Study on Grain Boundary Diffusion in Aluminum under Hydrostatic Stress.

Abstract: Fracture in an aluminum conductor of an LSI(large-scale integrated circuit) is often caused by stress-induced diffusion of atoms along a grain boundary, which is called"stress migration". As the conductor is subjected to tensile hydrostatic stress at a high temperature due to thermal mismatch, it is important to understand the effect of hydrostatic stress on the diffusion. However, this effect has not yet been examined since experiments under tensile hydrostatic stress are extremely difficult. In this study, the effect is investigated on the basis of a molecular dynamics simulation using the Nose-Hoover and the Parrinello-Rahman methods. The simulation enabled us to observe the motion of atoms under constant stress and temperature. The results obtained are summarized as follows. (1)The simulated coefficient of lattice diffusion under a stress-free condition agrees very well with the experimental ones. This indicates the validity of the present simulation. (2)The motion of atoms is accelerated by tensile hydrostatic stress in the region of about 5 atomic layers near the grain boundary. (3)Tensile hydrostatic stress accelerates the diffusion along the grain boundary, while compressive stress suppresses it. (4)The coefficient is proportional to exp(-(ΔE)/(k(T/Tm)))regardless of the magnitude of hydrostatic stress where T and Tm are the temperature and the melting temperature of the grain boundary under the corresponding hydrostatic stress, respectively.

The mechanical behaviour of hydraulic fractured, possibly saturated materials

Abstract: The influence of a fluid pressure load on the extensile fracturing of mortar and sandstone has been investigated. A fluid pressure in the (initiating) fracture stimulates both fracture initiation and propagation and may be as effective as a directly applied uniaxial tensile stress. The efficiency of the fracture pressure depends on the degree of saturation For the saturated materials, a pore pressure is generated under loading which counteracts the applied stress. The effective stresses governing the mechanical behaviour of the saturated materials then are lower, giving rise to an apparent stiffer behaviour, a lower effective fracture pressure and higher fracture initiation and propagation stresses. The magnitude of the pore pressure depends, among others, on the permeability of the materials. For impermeable materials, the pore pressure increases linearly with the hydrostatic stress, for permeable materials it is equal to the applied fluid pressure. With the increase in pore pressure, the effective stresses reduce and may even become zero when cohesion of the material is low.

Computing Petrophysical Properties in Porous Rocks Using a Boundary-Element Technique

Abstract: An efficient numerical technique has been used to compute the deformation of pores of arbitrary shape embedded in a homogeneous elastic solid under the influence of applied stresses. The scheme is based on the boundary-element method, where single linear elements are used to generate solutions that satisfy prescribed boundary conditions. These solutions can be employed to describe the behavior of elastic moduli and other petrophysical properties in porous rocks. The numerical algorithm allows computation of the stress field induced by the pores in the solid. In this way, the effect of the interactions between pores caused by stress concentrations can be studied from a quantitative point of view. To test the algorithm, some interesting results are compared with existing models, for special cases available in the literature. Also, a model for the compressibility and porosity of sedimentary rocks, as a function of applied hydrostatic stress, was generated by mixing some realistic pore geometries generated with the numerical algorithm. Results were in good agreement with data obtained from selected samples of sandstones.

Constitutive Theory Developed for Monolithic Ceramic Materials

Abstract: With the increasing use of advanced ceramic materials in high-temperature structural applications such as advanced heat engine components, the need arises to accurately predict thermomechanical behavior that is inherently time-dependent and that is hereditary in the sense that the current behavior depends not only on current conditions but also on the material's thermomechanical history. Most current analytical life prediction methods for both subcritical crack growth and creep models use elastic stress fields to predict the time-dependent reliability response of components subjected to elevated service temperatures. Inelastic response at high temperatures has been well documented in the materials science literature for these material systems, but this issue has been ignored by the engineering design community. From a design engineer's perspective, it is imperative to emphasize that accurate predictions of time-dependent reliability demand accurate stress field information. Ceramic materials exhibit different time-dependent behavior in tension and compression. Thus, inelastic deformation models for ceramics must be constructed in a fashion that admits both sensitivity to hydrostatic stress and differing behavior in tension and compression. A number of constitutive theories for materials that exhibit sensitivity to the hydrostatic component of stress have been proposed that characterize deformation using time-independent classical plasticity as a foundation. However, none of these theories allow different behavior in tension and compression. In addition, these theories are somewhat lacking in that they are unable to capture the creep, relaxation, and rate-sensitive phenomena exhibited by ceramic materials at high temperatures. The objective of this effort at the NASA Lewis Research Center has been to formulate a macroscopic continuum theory that captures these time-dependent phenomena. Specifically, the effort has focused on inelastic deformation behavior associated with these service conditions by developing a multiaxial viscoplastic constitutive model that accounts for time-dependent hereditary material deformation (such as creep and stress relaxation) in monolithic structural ceramics. Using continuum principles of engineering mechanics, we derived the complete viscoplastic theory from a scalar dissipative potential function.

Transport Properties of Bi2212 Single Crystal under Uniaxial Stress

Abstract: The dependence of transport properties, such as electrical conductivity, AC susceptibility and Hall effect, both on hydrostatic and on uniaxial stress are investigated for specimens, which were cut from a single crystal of Bi2Sr2CaCu2Ox, was investigated. The dependence of T c on uniaxial stress was not expected one from the viewpoint of a simple charge transfer model. Hall coefficient was measured to investigate the peculiar behavior of T c , resulted in that the change in a density of carriers (hole) plays an important role in the system.