Hydrostatic Compression Tests Research Articles

Constitutive Modeling and Material Characterization of Polymeric Foams

In this study, mechanical properties of three types of polymeric foams (polypropylene (PP), polystyrene (PS), and polyurethane (PU) foams) are investigated. Focus has been placed on the strain rate and temperature effects on these foams under large deformations. Selected experimental results from uniaxial compression, hydrostatic compression, and simple shear tests are presented. A phenomenological hydrodynamic elastoplastic constitutive law is developed to model these polymeric foams. Numerical implementation and validation of the constitutive model are also described.

Development of a Medium Pressure Flexible Boundary Cubical Triaxial Tester

ABSTRACT A medium pressure flexible boundary cubical triaxial tester has been designed and fabricated. In this tester, air pressure up to 70 MPa can be applied to all six surfaces of a 50X50X50 mm cube-shaped powder specimen via flexible rubber membranes. Pressure in a vertical direction (top-bottom faces of the powder specimen) and the pressure in a horizontal direction can be controlled independently. This tester can handle displacements as large as 50 mm in each of the three principal directions. Hydrostatic triaxial compression (HTC) tests. conventional triaxial compression (CTC) tests, and mean effective stress (MES) tests will be conducted on three powders, including a pharmaceutical powder, a ceramic powder, and an aluminum oxide powder. HTC tests will be conducted at 0 to 20 MPa, with 3 loading-unloading cycles. CTC and MES tests will be conducted at several pressure levels from 0 to 20 MPa.

Shear-enhanced compaction in sandstone under nominally dry and water-saturated conditions

Abstract To investigate the effect of interstitial water on compressive strength in the cataclastic flow regime, hydrostatic and triaxial compression tests were conducted on Berea and Darley Dale sandstones under nominally dry and saturated conditions at room temperature. Both hydrostatic and nonhydrostatic compaction were significantly enhanced in the presence of water. Inelastic behavior and failure mode of the nominal dry samples were qualitatively similar to those of water-saturated samples. At elevated pressures shear localization was inhibited, and all the samples failed by strain hardening. Initiation of shear.enhanced compaction (at the critical stress C*) was marked by a surge in acoustic emission activity. The compactive yield stress C* decreased with increasing effective mean stress, mapping out an approximately elliptical yield envelope in agreement with the critical state and cap models. An associated flow rule predicts compactive volumetric strains that are in good agreement with experimental observations. Shear.enhanced compaction in nominally dry samples initiated at stress levels higher than those in water-saturated samples. Significant weakening due to the presence of water was observed in Darley Dale sandstone. In Berea sandstone the weakening effect was relatively small. Microstructural observations indicate that shear.enhanced compaction was triggered by intragranular cracking emanated from grain contacts. In the presence of water, the critical stress intensity for crack growth in reduced, and the resultant subcritical crack growth may explain the overall reduction in stress level for the onset and development of shear.enhanced compaction.

97/00494 On the properties of washed straw

Pelletization is one of the promising technologies for biomass densification. Many studies have been conducted to determine the optimum conditions for pelletizing. However, no relationship between the mechanical properties of the ground biomass and pellet quality has been published. The goal of this research was to develop and validate a predictive relationship between mechanical properties of the ground biomass in early stages of densification (< 100 kPa) and the quality metrics of pellets. Switchgrass was ground with two different screen sizes (3.175 mm and 6.35 mm) and conditioned at two different levels of moisture content (17.5%, and 20%, w.b.). Hydrostatic triaxial compression (HTC) tests were performed using the Cubical Triaxial Tester (CTT) to determine bulk modulus, compression index, and spring back index below 100 kPa. Pellets were formed with ground biomass using a farm-scale pellet mill and diametral tensile strength, axial compressive strength, pellet density, and durability were determined. Regression equations between pellet quality and mechanical property having an R2 value higher than 0.80 were selected for prediction. The validation against quality of pellets, which were formed at an alternate condition, confirms that the strength quality metrics of switchgrass pellets can be predicted using size reduced materials' bulk modulus and spring-back index.

Measurement of anisotropic behavior of dry cohesive and cohesionless powders using a cubical triaxial tester

Anisotropy of dry cohesive and cohesionless powders and the effect of particle shape and sample deposition method on the anisotropy were investigated using a cubical triaxial tester. Hydrostatic triaxial compression (HTC) tests and conventional triaxial compression (CTC) tests were conducted on four test powders: two cohesive, wheat flour (irregular-shaped particles) and potato starch (rounded); and two cohesionless, glass beads (spherical and milled glass fibers (cylindrical). The loading-unloading cycle for HTC tests was from 0-190-0 kPa. The CTC tests were conducted at 34.5 kPa confining ( σ 2 = σ 3) pressure, where σ 2 and σ 3 are intermediate and minor principal stresses, respectively. Two different deposition methods, plunging and tapping, were used to prepare cubical (50 mm) test samples. Test results showed that wheat flour and glass beads were basically isotropic materials in the stress range used in this study. Potato starch showed some anisotropy which tended to increase with applied stress. Significant anisotropy was observed for milled glass fibers (0.05 level of significance). The sample deposition method influenced the anisotropy of powder depending on the particle shape and cohesion. The anisotropy of powder was strongly dependent upon the particle shape. Cohesion was found to minimize the effect of sample deposition method. The material parameter values for the linear elastic isotropic and anisotropic constitutive equations were determined based on test data. No large difference between material parameter values was found for wheat flour, potato starch and glass beads. However, the percent difference between elastic modulus for milled glass fibers reached 37.2%, whereas for the Poisson ratio this difference was 800%.

Micro-mechanisms of pore collapse in limestone

Many poorly-consolidated reservoir rocks undergo irreversible deformation (compaction) as a result of an increase in effective confining pressure during production of hydrocarbon. Such compaction is usually the result of pore collapse caused by changes at the microscopic level within the rock mass. Pore collapse is considered to be a potential problem in many producing reservoirs. The main goal of this study was to gain an in-depth understanding of the micro-mechanisms operative in a calcite-cemented, porous, clastic limestone during compaction. To achieve this goal, an experimental program was designed and pursued including both macroscopic and microscopic aspects. Hydrostatic compression tests were performed on cubical samples of Cordoba Cream limestone (24.5% porosity) in High Capacity Cubical Device (HCCD). The samples were loaded to three different levels of confining pressures representing three stages of pore collapse, namely: pre-pore collapse, pore collapse, and post-pore collapse. The samples were unloaded after the desired stress levels were reached. Load deformation response of limestone was observed from these test results. Standard thin sections were prepared from both tested and untested samples. Microscopic changes that accompany different stages of pore collapse were examined by an optical microscope. Microscopic changes are correlated with macroscopic responses (stress-strain behavior), and are illustrated to identify the micro-mechanisms and their sequences during the progression of pore collapse. Although cataclastic flow and crystal plasticity are found to be the dominant micro-mechanical processes that are operative throughout the pore collapse, the contribution of a single mechanism to the total deformation mechanism causing pore collapse varies during specific collapse stages. The findings of this study can be effectively used in understanding the macro-micro correlations of deformation and in developing realistic constitutive models and accurate prediction methods for calculating reservoir subsidence because these require an in-depth understanding of the actual phenomena.

Compression model for cohesionless soils

A simple four-parameter elasto-plastic model describes the non-linear volumetric behaviour of freshly deposited cohesionless soils in hydrostatic and one-dimensional compression. It expresses the tangent bulk modulus as a separable function of the current void ratio and mean effective stress using natural strains. Specimens compressed from different initial formation densities approach a unique response at high stress levels—the limiting compression curve (LCC)—which is linear in a double logarithmic void ratio-effective stress space. The model describes irrecoverable, plastic strains which develop throughout first loading and represent mechanisms ranging from particle sliding and rolling at low stresses to crushing—the principal component of deformation for LCC states. The three input parameters describing plastic deformation can be readily estimated from a hydrostatic or one-dimensional compression test loaded to high stress levels; the elastic bulk modulus requires accurate small strain measurements in unloading. The model can be further simplified at low stress levels where compressive behaviour is controlled primarily by the formation density. Input parameters for a wide range of cohesionless soils are presented from which it is possible to infer the effects of particle mineralogy, size, grading and shape on compressibility. The model gives excellent predictions of measured compressive behaviour over a wide range of stresses and densities, and provides a useful basis for the construction of hardening rules for generalized constitutive models. KEYWORDS: compressibility; constitutive relations; plasticity; sands. Un modèle élastoplastique simple à quatre paramétres permet de décrire le comportement volumique non-linéaire de sols fraîchement déposés, sous des conditions de compression hydrostatique ou unidimensionelle. Il exprime, à partir des déformations naturelles, que le module matriciel tangent est fonction de l'indice des vides courant et de la contrainte effective moyenne. Les échantillons, comprimés avec différentes valeurs initiales de den-sité, tendent vers une même réponse aux fortes contraintes—la `limiting compression curve' (LCC) —réprésentée par une droite dans le diagramme biologarithmique indice des vides-contrainte effective'. Le modéle décrit les déformations plastiques irréversibles qui se développent tout au long de la premiére mise en charge, et les mécanismes présents, depuis le glissement et la rotation des particules, sous faibles contraintes, jusqu'au broyage (mécanisme principal de déformation aux états LCC). Les trois paramétres d'entrée décrivant la déformation plastique peuvent 7eacute;tre estimés à partir d'essais de compression hydrostatiques ou unidimensionels réalisés à fortes contraintes. La determination du module élastique demande une mesure trés précise des petites déformations lors du déchargement. Ce modéle peut étre simplifié pour les faibles contraintes pour lesquelles le comportement en compression est essentiellement contrôlé par la masse volumique de la formation. L'article présente, pour une large gamme de sols pulvérulents, plusieurs paramétres d'entrée permettant d'étudier l'influence de la minéralogie, de la forme et de la granulométrie des particules sur la compressibilité. Le modéle présenté permet de prévoir avec grande précision le comportement en compression mesuré pour une large gamme de contraintes et de densités. II fournit également une base indispensable à l'établissement de lois de durcissment pour des modeles constitutifs généralisés.

Laboratory velocities and attenuation of P‐waves in limestones during freeze‐thaw cycles

The velocity and the attenuation of compressional P‐waves, measured in the laboratory at ultrasonic frequencies during a series of freezing and thawing cycles, are used as a method for predicting frost damage in a bedded limestone. Pulse transmission and spectral ratio techniques are used to determine the P‐wave velocities and the attenuation values relative to an aluminum reference sample with very low attenuation. Limestone samples were water saturated under vacuum conditions, jacketed with rubber sleeves, and immersed in an antifreeze bath (50 percent methanol solution). They were submitted to repeated 24-hour freezing and thawing cycles simulating natural environment conditions. During the freeze/thaw cycles, P‐wave velocities and quality factor Q diminished rapidly in thawed rock samples, indicating modification of the pore space. Measurements of crack porosity were conducted by hydrostatic compression tests on cubic rock samples that had been submitted to these freeze/thaw cycles. These measurements are used as an index of crack formation. The hydrostatic compression tests confirmed the phases of rock damage that were shown by changes in the value of Q. Furthermore, comparisons between Q values and crack porosity demonstrated that the variations of P‐wave attenuation are caused by the creation of new cracks and not by the enlargement of pre‐existing cracks.

赤平炭を用いた室内試験からの考察 坑道引立面付近の炭層状況と破壊挙動に関する研究

To invesigate the behavior of coal seam around roadway faces, the laboratory experiments were taken effect by drilling test, hydrostatic compression test, gas permeability test, triaxial compression test with and without pore gas pressure and outburst fracture test.Based on results of above experiments together with field test data from a large diameter operations for stress relief at Akabira Coal Mine, the following considerations are obtained;Coal seam ahead of a roadway face can be devided into two regions. One is fractured region in the vicinity of the face where the stress is already relieved. Most of cracks in this region may be open so that the ductile fracture is dominant and the coal seam is relatively permeable to gas. Another is intact region under high stress ahead of the fractured region. Cracks in this region may be mostly closed so that the brittle fracture manner is dominant and the coal seam is relatively unpermeable. There is, therefore, a possibility that coal in the boundary region bursts out toward the roadway.

Influence of stress-dependent elastic moduli on stresses and strains around axisymmetric boreholes : Rock Mech Rock Engng V22, N3, July–Sept 1989, P189–203

Porous or clastic rocks often have elastic moduli which are not constant but increase with increasing minor principal stress. The use of classical constant modulus linear elasticity in these cases can lead to erroneous predictions of the deformations and of the initiation and extent of failure around underground excavations. To illustrate these effects, solutions are developed for axisymmetric excavations in infinite media having power law and exponential variations of elastic modulus with minor principal stress. The maximum stress concentrations do not occur at the excavation boundaries and are less than the constant value of 2.0 given by constant modulus elasticity. When modified slightly to allow for test boundary conditions, the theory gives predictions that are consistent with aspects of the results obtained in hydrostatic compression tests on thick walled cylinders of three sedimentary rocks.

Influence of stress-dependent elastic moduli on stresses and strains around axisymmetric boreholes

Porous or clastic rocks often have elastic moduli which are not constant but increase with increasing minor principal stress. The use of classical constant modulus linear elasticity in these cases can lead to erroneous predictions of the deformations and of the initiation and extent of failure around underground excavations. To illustrate these effects, solutions are developed for axisymmetric excavations in infinite media having power law and exponential variations of elastic modulus with minor principal stress. The maximum stress concentrations do not occur at the excavation boundaries and are less than the constant value of 2.0 given by constant modulus elasticity. When modified slightly to allow for test boundary conditions, the theory gives predictions that are consistent with aspects of the results obtained in hydrostatic compression tests on thick walled cylinders of three sedimentary rocks.

Effect of Density on the Drained Deformation Behavior of Shirasu (Volcanic Sandy Soil) Under Three-Dimensional Stresses

The effect of density on the drained deformation properties of pyroclastic flow deposits, so called Shirasu, distributed in the Southern Kyushu, Japan, was investigated by using a triaxial testing apparatus with independent stress-control systems. Hydrostatic compression tests and radial shear tests maintaining a constant mean principal stress were performed on Shirasu specimens prepared by pluviation through water. The test results indicate that the reconstituted specimens of Shirasu were characterized by an inherent cross-anisotropy and a density-dependence of their deformation properties. In general, the specimens had a low compressibility and a high expansibility in the direction of specimen-deposition compared with the direction along the bedding plane. The maximum shear strength in the identical stress state having different stress-path occurred when the major principal stress was applied to the direction of the specimen-deposition. The change of the deformation-strength properties was caused by the change of strain increment ratio which depends on the dilatancy occurring under the different stress path.

Constants for an elastic–plastic cap model for limestone

AbstractTriaxial fracture tests with overburden pressures ranging from 0.69 to 136 MPa were performed to determine the material parameters involved in a cap model for limestone. Three material parameters A, B and C, were evaluated through Powell's optimization method and the remaining parameters through simulation‐curve fitting procedure. The resulting cap model was tested against separate triaxial fracture and hydrostatic compression tests. Modification of the original hardening function, as proposed by DiMaggio and Sandler, provided an improved fit for the experimental data.

A model for agricultural soil compaction

The compactness of agricultural soil may vary greatly during a crop season due to tillage and other machine operations that apply forces to the soil. Thus, an adequate model for agricultural soil compaction must be capable of predicting soil density over a wide range of compactness, from a very loose state to a very compacted state. The traditional logarithmic model does not adequately predict soil density at low levels of compactness and low stress levels. A three-parameter multiplicative model of soil compaction is proposed. This model satisfies certain boundary conditions for natural volumetric strain at low and high stress levels which are not satisfied by the traditional model. The model was fitted to hydrostatic compression test data for two agricultural soils, a sandy loam and a clay. The model was capable of predicting soil density at low stress levels with less error than the traditional model. Data from three levels of moisture content of the clay showed that all three coefficients of the proposed model were influenced by moisture content.

A Nondestructive Hydrostatic Bulk Compression Test for Fruit

ABSTRACT AN experiment test chamber complete with appro-priate instrumentation was constructed to force a rapid (<1 s) small volume (2 to 3 percent) decrease in individual peaches and apples submerged in water. The resulting pressure rise established a hydrostatic bulk compression (H.B.C.) test value that was corre-lated with results from non-destructive and destructive evaluation methods. Pressures of about 38 MPa for 3 s duration were non-destructive for Rio Oso Gem peaches. Correlation coefficients of -0.602 and 0.412 were found between H.B.C. and non-destructive firmness (N.D.F.) tests and between H.B.C. and Magness-Taylor (M-T) tests, respectively. A coefficient of -0.714 was found between the N.D.F. and M-T tests.

Determination of the transverse elastic coefficients of bone

A method for determining the elastic coefficients of bone as a transversely isotropic material is described. The constitutive equations relating the stresses to strains were solved simultaneously for tension and hydrostatic compression tests. Cylindrical specimens with symmetry axes oriented with the long axis of the fresh bovine Haversian femur were tested in tension, torsion and hydrostatic compression, and the results were used to calculate the five independent elastic coefficients.

Acoustic emissions in stressed soil samples

Although acoustic emission techniques have been used in a wide variety of materials, scarcely any attention has been given to particulate systems such as soils. Yet, it is in such particulate materials where the concept of strain is quite fictitious and an alternate parameter is greatly needed. The results of this study suggest that acoustic emission might be such a parameter. Included herein is a laboratory study of the acoustic-emission response of a series of soil samples at different water contents and under different stress states. The stress states are hydrostatic compression and triaxial shear compression tests. In each type of test the soil samples are laterally confined by pressure which simulates the soil in its natural state. The results show that high confinement pressure and high water content both produce high levels of acoustic emission. The emissions begin instantaneously upon the application of the stress. In all cases, the emissions cease when the soil samples reach an equilibrium position under the applied stress state. Some insight as to the wide range of problems which might be solved using acoustic-emission techniques is also noted.

Unified plastic yield criterion for ductile solids.

Introduction T basic hypothesis in the theory of plasticity is that there exists a scalar function of stresses which characterizes the yielding of materials. This scalar function, also called the yield function,/(crfj.) generates a closed surface in the stress space by a relation /(o^) = 0. When a material is subjected to increasing forces and torques, it describes a path in the stress space. When this path intersects the yield surface /(afj-) = 0, the material yields and becomes plastic. All the classical failure theories, e.g., those of Tresca, Von Mises, and Reuss, give conflicting predictions of the yield stresses of metals even for such simple cases of pure torsion or elongation. The reason for the conflicts in these theories is that these theories essentially form one-parameter yield surfaces. So fitting these criteria on tensile tests alone make these incompatible with the torsion test data, and vice versa. These theories also do not predict the failure under hydrostatic compression. Detailed references on these theories can be obtained in Ref. 1. The proposed theory, on the other hand, forms a multiparameter yield surface. Here a start is made from the basic yield point data from both tensile and torsion test experiments, as well as the hydrostatic compression test. Thus the present theory can predict yielding under hydrostatic pressure while posing no conflict between theory and, at least, the basic tension and torsion experiments. The formulation is flexible enough to satisfy a large number of the available experimental data on the yield point and subsequent flow for ductile materials, and includes both ideally plastic and strain-hardening solids. The yield surface generated is invariant with respect to rotation of coordinate axes, a necessary requirement for all such criteria.