Showing papers on "Hydrostatic stress published in 2001"

Simulations of atomic level stresses in systems of buried Ge/Si islands

Abstract: Stress distribution in laterally ordered arrays of coherent Ge islands on Si(001) buried in Si cap layers is examined using atomistic simulations. The obtained hydrostatic stress dependence on the spacer layer thickness shows a nearly linear inverse dependence, unlike the commonly used inverse cubic dependence derived in the framework of an isolated embedded force dipole source model. Additionally, the hydrostatic stress on the spacer surface is found to scale more closely with the area of the island rather than its volume as implicit in the use of the force dipole model.

A New Static Failure Criterion for Isotropic Polymers

Abstract: A new static failure criterion for isotropic polymers with different strengths in tension and compression based on exponential dependence between the mean stress and the von Mises equivalent stress is proposed. The two material parameters introduced can be determined by two simple tests - the uniaxial tension and compression. The locus of the criterion is nearly conical for low hydrostatic pressures and tends to a cylindrical form if an increased hydrostatic pressure is applied. The validity of the criterion is demonstrated by experimental strength data taken from the literature for several polymers in the case of superimposed hydrostatic pressure.

Tetragonal-to-monoclinic phase transformation in CeO2-stabilized zirconia under multiaxial loading

Abstract: Critical stresses for the initiation of the tetragonal-to-monoclinic phase transformation in 9Ce-TZP zirconia materials with five different grain sizes have been studied. The influence of the grain size on the critical transformation stresses has been investigated in multiaxial stress states, namely, in four-point bending, biaxial bending and torsion. It was found that phase transformation occurs as a homogeneous phase transformation with a transformation strain increasing continuously with increasing applied stress and also as an autocatalytic phase transformation with the autocatalytic formation of transformation bands normal to the maximum principal stress. An investigation of the critical transformation stresses under different multiaxial loads in the tensile regime, i.e. with positive hydrostatic stress, showed that both the homogeneous and the autocatalytic transformation do not follow the shear-dilatant criterion investigated in multiaxial compressive testing. The experiments showed that under multiaxial loading the onset of both transformation types can be predicted with the maximum principal stress transformation criterion, with the difference between the critical stresses of both transformation mechanisms strongly decreasing with grain size.

Distinct Element Method for Study of Failure in Cohesive Particulate Media

Abstract: Permissible stresses or strains in pavement layers constitute the empirical part of the mechanistic-empirical method. Whereas it appears to be possible to predict the actual stresses or strains under a load (the mechanistic part) reasonably correctly, there is much uncertainty with respect to the permissible values. Some of the possibilities that the distinct element method (DEM) raises for calculating permanent deformation and failure in pavement materials are explored. Because the study is limited to two dimensions and makes a number of assumptions that may not be strictly correct, the results should be considered as indicative of only some of the capabilities of DEM. After a brief description of DEM, tests to the failure of two different samples of elements are presented, with different element shape and size distribution and thus different resulting pore volume after compaction. For one of the samples, seven different stress paths were followed to failure (pure tension, uniaxial tension, pure shear, etc.). From this study, a nonlinear relationship between the permissible shear stress and the hydrostatic stress was deduced. Using this relationship as the basis for the kinetic equation in continuum damage mechanics can lead to cracking from the top of and the bottom of a slab. Finally, DEM is used to study the effect of repeated loading on permanent deformation and failure for one of the samples. The effect on permanent deformation was pronounced, whereas the effect on failure was much more uncertain, although there could be some indication of fatigue.

Stress‐induced anisotropy of partially molten media inferred from experimental deformation of a simple binary system under acoustic monitoring

Abstract: Microstructural changes of partially molten media under deviatoric stress were investigated in a newly developed apparatus by deforming a large sample (a 70-mm cube) under a uniform pure shear stress. Borneol + melt system having a moderate dihedral angle and texturally equilibrated under hydrostatic stress was used as a partially molten rock analogue. The applied stress was small enough not to involve cataclastic-plastic deformation of the solid grains. Shear strain rate was about 10 -8 s -1 , and a stress exponent indicative of diffusion creep was obtained. During the deformation, sample microstructure was observed in situ by means of ultrasonic shear waves. The development of stress-induced anisotropy was successfully detected by shear wave splitting. The results obtained indicate that grain boundary contiguity in the direction of the least compressive stress (σ 3 ) was reduced with respect to the equilibrium texture and also that the relative values of liquid pressure and σ 3 play an essential role for development of anisotropy. The developed anisotropy persisted as long as deviatoric stress was applied, but the initial isotropic structure was recovered by releasing this stress. Several interesting phenomena were involved in the structural change; these include shear creep-induced dilatancy, strong dependence of the timescale of structural recovery on the amount of deformation (memory effect), and relaxation creep after releasing stress. Scaling considerations using the Griffith theory shows that the structural changes observed in the present experimental system are expected to occur in the Earth as well.

Model of viscoplasticity for transversely isotropic inelastically compressible solids

Abstract: A transversely isotropic, viscoplasticity model is formulated as an extension of Bodner's model. Account is made of inelastic deformation that can occur in strongly anisotropic materials under hydrostatic stress. The extended model retains the simplicity of the Bodner model, particularly in the ease with which the material constants are determined. Although the proposed model is based on a scalar state variable, it is capable of representing material anisotropy under the assertion that history-induced anisotropy can be ignored relative to strong initial anisotropy. Physical limitations on the material parameters are identified and discussed. The model is applied to a W/Cu metal matrix composite; characterization is made using off-axis tensile data generated at NASA Glenn Research Center.

Shallow Flaws Under Biaxial Loading Conditions—Part II: Application of a Weibull Stress Analysis of the Cruciform Bend Specimen Using a Hydrostatic Stress Criterion

Abstract: Cruciform beam fracture mechanics specimensl have been developed in the Heavy Section Steel Technology (HSST) Program at Oak Ridge National Laboratory (ORNL) to introduce a prototypic, far- field, out-of-plane biaxird bending stress component in the test section that approximates the nonlinear biaxial stresses resulting from pressurized-thernxd-shock or pressure-temperature loading of a nuclear reactor pressure vessel (RPV). 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), shtdlow, surface flaws. 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. Two and three- parameter Weibull models have been calibrated using a new scheme (developed at the University of Illinois) that maps toughness data from test specimens with distinctly different levels of crack-tip constraint to a small scale yielding (SSY) Weibull stress space. These models, using the new hydrostatic stress criterion in place of the more commonly used maximum principal stress in the kernel of the OW integral definition, have been shown to correlate the experimentally observed biaxiaImore » effect in cruciform specimens, thereby providing a scaling mechanism between uniaxial and biaxial loading states.« less

Theory of bubble growth in crystals under stress

Abstract: In a previous paper, the theory of bubble growth in crystals was extended to include an analysis of the interaction that occurs between the diffusion mechanism that supplies gas to the bubble and the concurrent material displacement that accommodates its growth. A novel mechanism of gas supply was also identified that derives from the accommodation process itself. As material near to the bubble is diffusively consumed, the gas dissolved within it must be deposited into the bubble. A critical gas concentration was predicted, at which the gas supplied by this mechanism can lead to self-sustained bubble growth without the need for any diffusive supply through the crystal. In the present paper, these ideas are extended to cases in which external stresses are applied to the material. For bubbles in the lattice, only hydrostatic stress is important. External pressure is shown to inhibit self-sustained growth mechanism. For grain boundary bubbles, normal stresses at the boundary can dominate. Tensile str...

Stress transmission through a cohesionless material

Abstract: We apply a point load onto a bidimensional packing of elastic grains. The internal state of stress is accessed by means of photoelastic visualization. The region in which the stress response is confined exhibits a parabolic profile. This evidence supports the parabolic nature of the equations describing the stress transmission. Our results oppose classical elasto-plastic models of continuum mechanics, and other recent hyperbolic proposals.

Yielding of commercial poly(vinyl chloride) pipe material

Abstract: Tubular test specimens of commercial poly(vinyl chloride) pipe material were tested in biaxial stress systems covering the four quadrants of the plane–stress space. The test rig provided yield stresses at a constant temperature and controlled strain rate tension, compression, internal pressure, external pressure, and torsion combined loading. The experimental yield points fit the von Mises yield criterion. This result was confirmed by the ratio of compressive to tensile yield stresses; being unity, the density remaining constant with the applied hydrostatic stress, and, finally, the normality of the strain vector to the yield locus at the corresponding stressing point. Subsequent yielding loci of previously deformed specimens deviated largely from the von Mises locus. This deviation may have been caused by craze–void interaction. Finally, the viscous behavior, the dependence of yield stresses on the strain rate and temperature, is represented with a generalized phenomenological model based on an Eyring–Ward-type expression. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 991–999, 2001

Method for analyzing creep characteristic of a plastic molded substance

Abstract: A method for analyzing creep characteristic of a plastic molded substance is provided wherein the creep characteristic can be analyzed of an actual plastic molded substance having various shape and fastening conditions with high precision. The main stress value for each section of the analysis target is read out, hydrostatic stress σm of each section is calculated from the main stress value, and the stress condition is determined to be tensile stress when σm is positive and to be compressive stress when σm is negative. Tensile or compressive characteristic value is assigned based on the stress condition at each section, and creep is calculated for a predetermined time period based on the characteristic value. By employing characteristic values depending on the stress conditions of each section, precision of creep characteristic analysis is greatly enhanced.

Atomistic simulation of the effect of trace elements on grain boundary of aluminum

Abstract: Molecular statics simulations are used in conjunction with the Embedded Atom Method to study the segregation tendency of magnesium and the effect of doping on aluminum symmetric tilt grain boundaries (STGB). It is observed that a state of hydrostatic stress exists predominantly near the grain boundary and influences the segregation energy of magnesium. The origin of hydrostatic stress is explained in terms of local geometrical arrangement of atoms at the grain boundary. The energy of a given grain boundary is found to increase with addition of Mg atoms and the increase depends on the normal distance of the atom from the grain boundary.

SECTION 9.9 – Hydrogen Transport and Interaction with Material Deformation: Implications for Fracture

Abstract: This chapter presents two models: non-hydride-forming systems and hydride-forming systems. It discusses; the validity, background, description of the model, identification of model, and how to use this model. The non-hydride-forming systems applies to a variety of systems, such as fcc, bcc, hcp, pure metals, solid solutions, precipitation-strengthened systems, and intermetallics. It is used to calculate the local hydrogen concentration qualified by hydrostatic stress and trapping induced by plastic straining. Then by using information on whether the fracture is strain or stress-controlled, one can predict the location of the first microcracking event in a specimen. The hydride-forming system applies to systems that fail predominantly by hydride formation and cleavage. These are systems in which hydrides are either stable or can be stabilized by the application of a stress field. Examples are the IVb and Vb metals and their alloys, as well as a number of other metals such as Mg and A1. This hydride mechanism is supported by microscopic observations and thermodynamic calculations.

Bulk Modulus and Density Measurements of Small Compressible and Incompressible Samples

Abstract: A novel system is described for determining physical properties, including bulk modulus and density, of small solid samples. The system consists of a chamber in which the sample weight is measured by a weighing device immersed in a gas of controllable density. Thus, the method of measuring density is based on Archimedes’ principle where the weight of an object is reduced by the weight of the displaced fluid. This particular device has been designed for examining the density of disk-shaped samples 3 mm in diameter and 0.4 mm thick. The weighing device has a repeatability of 4 nN and sample densities can be determined to 0.5%. A significant feature of this device is the ability to measure buoyancy forces at a plurality of gas densities, which allows one to capture nonlinear behaviors associated with closed-cell compressible media. Results are presented for a quasi-closed cell foam that experiences volume reduction as the gas pressure is increased. Volumetric strains are determined as the difference between the observed behavior and the linear behavior of incompressible media. Plots of hydrostatic stress versus volumetric strain are initially linear, as described by the bulk modulus, and exhibit a “kink” at high pressures, presumably due to the complete compression of internal cells.

Experimental and numerical aspects of cavitation

Abstract: Rubber isolation systems represent an effective method to isolate the superstructure from seismic ground accelerations. In this study, two sets of high-damping natural rubber bearings are subjected to characterization tests to determine the corresponding stiffness parameters. Bearings with flat steel laminae represent the first set, while the second one has inclined steel plates necessary to obtain a transversely non-isotropic response. During shear deformation a negative hydrostatic stress state and irreversible material damage in the form of internal rupture develops. The cavitation region is not visible from the outside and thus not recognized or accounted for during experimentation. Numerical simulation using a cavitation damage model based on a variable bulk modulus quantifies the amount of stiffness reduction attributed to internal rupture in the material. It is shown that the amount and extension of damage is much more severe for elastomeric bearings when inclined steel plates are used to generate anisotropy in the in-plane response.

Stress Pattern Analysis by Thermal Emission of Plain Weave Carbon Fiber/Epoxy Resin Composite Iosipescu Specimens with Three Different Notch Angles

Abstract: Iosipescu specimens were developed to examine the shear properties of isotropic materials. In the past decade, there has been much effort made to analyze this specimen configuration for measuring anisotropic materials, particularly laminated and woven composites. Some authors have used notch angles ranging from 90 to 110° in these materials and other authors have performed finite element analysis (FEA) to determine the notch angle and depth with the most uniform shear stress zone based on the elastic moduli. The object of this study was to verify the preferred angle of woven fiber composites through the use of stress pattern analysis by thermal emission (SPATE), which is a technique that can image the hydrostatic stress state in a material under load. SPATE images were collected for various specimens at different notch angles (90, 100, and 110°) and different weave lay ups. Mean load was held constant while the SPATE image was collected but later increased to examine the stress state of the Iosipescu specimens at high loads. The results indicate that for a plain woven carbon fiber/epoxy resin composite, the 100° notched specimen is the best specimen of the three conditions examined due to the delay in occurrence of damage and the large size of the pure shear zone.