Hydrostatic Compression Tests Research Articles

Evaluation of the mechanical properties of porcine kidney.

With the development of medical diagnosis and treatment, knowing the mechanical properties of living tissues becomes critical. The aim of this study was to investigation material properties of the fresh porcine kidney and the parametric characterization of its viscoelastic material behavior. The material investigation included uniaxial tension tests in different strain rates, relaxation tests, as well as hydrostatic compression tests on the samples extracted from the fresh porcine kidney cortex. Tension tests and relaxation tests were performed by a planar dog-bone specimen with a micron loading testing machine. Hydrostatic compression tests were performed on the kidney cylinder sample which was placed in a compression chamber. Furthermore, a nonlinear viscoelastic model recently proposed by us was employed to characterize the tension data at different strain rates and relaxation test data. The the experimental and numerical results show that the stress-strain relations of the porcine kidney cortex at different strain rates in tension are presented for the first time and a higher strain rate results in higher ultimate strength and initial Young modulus but a lower rupture strain. A damage-dependent visco-elastic model is employed to model the tension data at different strain rates and relaxation data and exhibits a good agreement with the experimental data, which also demonstrates that the damage has an obvious influence on the stress-strain relation. Through comparison with the existing reference covering the uniaxial compression data, it seems that the mechanical behavior of the porcine kidney cortex manifests a stress state-dependent mechanical behavior. The ultimate strength and rupture strain are larger in compression than that in tension.

Hydromechanical characterization of the behaviour of Chateau-Landon chalk

This paper presents an experimental investigation on the hydromechanical behaviour of a partially saturated soft rock (a porous chalk), the Chateau-Landon chalk. Such conditions correspond to those of a chalk mine known as Royer located in the North-centre of France in Paris basin, subjected to shrinking, drying and potential flooding cycles. To study this rock, we apply a method usually used in soils whose hydromechanical behaviour is strongly modified by changes in suction, according to the degree of water saturation. Different degrees of saturation are imposed by controlled relative humidity conditions with continuous measurement of physical parameters. Hydrostatic compression and conventional triaxial compression tests are performed under drained conditions for saturation degrees up to 100% under low confining pressures. The obtained results have allowed to show fundamental aspects of the chalk behaviour. Correlations between water saturation degree, confining pressure and the mechanical behaviour of the chalk are discussed.

Thermo-mechanical characteristics of oxide-coated aluminum nano-powder

This paper presents a comprehensive study on the thermo-mechanical characteristics of oxide-coated aluminum nano-powder, which is a crucial property for their application in powder metallurgy, particularly in warm compaction process. A representative volume element (RVE) of oxide-coated aluminum nano-powder is analyzed under the hydrostatic compression test at different temperatures using the Reactive Molecular Dynamics (RMD) method that can predict the thermal conductivity of nano-powders by incorporating the interaction effects of density and temperature. The thermal conductivity is evaluated based on the Müller-Plathe's reverse non-equilibrium molecular dynamics (rNEMD) method, and the size dependence of the method is investigated. The computational model is validated with the experimental data. The results illustrate that the thermal conductivity of oxide-coated aluminum nano-powder is significantly lower than that of the bulk aluminum due to restricted surface contact between individual aluminum particles. Moreover, it is observed that increasing the density and temperature leads to an increase in the thermal conductivity of oxide-coated aluminum nano-powder.

Experimental investigation on the hydromechanical behaviour of a porous chalk

This paper deals with the impact of climate change on stability of abandoned subsurface cavities in chalk due to its indirect effect on ground water levels. It is the first part of a general work on the hydromechanical behaviour of a partially saturated soft rock (a porous chalk) and the effect of changes in water saturation degree. An experimental investigation including a laboratory-testing program is presented. Different degrees of saturation are imposed by controlled relative humidity conditions. Conventional hydrostatic and triaxial compression tests are performed under drained conditions for saturation degree up to 100% under low confining pressures. The obtained results have allowed to show up fundamental aspects of the chalk behaviour. The high sensitivity of the extracted material to water is described and a water induced plastic deformation is observed.

Simulation of Stress tests using a Poroelastic model to estimate the Permeability behavior of bedford limestone samples

Por su relevancia en Geociencias, el efecto de los esfuerzos sobre las propiedades de flujo de fluido en rocas ha sido estudiado experimental y teóricamente por varias décadas. Los trabajos se han orientado principalmente a rocas tipo arenisca, y relativamente poco, a pesar de su relevancia, en rocas carbonatadas. Por otro lado, la descripción del fenómeno es matemática y numéricamente complejo, ya que involucra el acoplamiento no lineal del flujo de fluido con la deformación de la roca. En este trabajo se busca incursionar en esa parte poco explorada de pruebas experimentales en carbonato y su simulación numérica.En él se presentan pruebas experimentales realizadas en núcleos de caliza Bedford, y se analiza el efecto de los esfuerzos de confinamiento y de la presión del fluido (presión de poro) sobre la permeabilidad a través de dos tipos de pruebas, conocidas como hidrostáticas (bajos esfuerzos de corte) y no hidrostáticas (altos esfuerzos de corte). En este trabajo se presenta un modelo matemático para describir el fenómeno, considerando un fluido monofásico, una respuesta elástica de la roca, y su implementación numérica en elementos finitos.El acoplamiento del flujo de fluido con los esfuerzos se modela utilizando relaciones algebraicas de porosidad y permeabilidad en función del esfuerzo. La relación permeabilidad-esfuerzo utilizada en este trabajo depende linealmente de la deformación volumétrica y la presión de poro. Los resultados experimentales muestran que en general la permeabilidad se incrementa con la presión del fluido y se reduce con el esfuerzo de confinamiento. La reducción con el esfuerzo de confinamiento en el rango de esfuerzos de confinamiento analizado es relativamente pequeña, pero es notoriamente más importante en las pruebas no-hidrostáticas (8%) que en las hidrostáticas (2%).El ajuste del modelo a los resultados experimentales, específicamente a la caída de presión en el núcleo en función del tiempo, se realiza mediante la variación de los parámetros de la relación permeabilidad-esfuerzo. El valor de estos parámetros de ajuste difiere del valor reportado para areniscas, lo cual puede ser indicativo de la diferencia en la estructura porosa y las propiedades mecánicas de las rocas.

Experimental and Numerical Investigations on the Mechanical Behavior of Basalt in the Dam Foundation of the Baihetan Hydropower Station

Abstract The mechanical behavior of basalt is critical to the stability and safety of large structures erected on basalt. In this paper, a series of tests (hydrostatic compression test, triaxial co...

Numerical modelling the influence of water content on the mechanical behaviour of concrete under high confining pressures

The present paper focuses on the hydro-mechanical behaviour of concrete under high confining pressures. By using the poromechanics approach, an elastoplastic model is adopted to describe the mechanical behaviour of concrete under different saturation conditions and high confining pressures. A hydrostatic compression test with the measurement of interstitial pore pressure is firstly used to identify the model’s parameter. After that, three quasi-oedometric tests with different water contents are simulated. The obtained numerical results exhibit that the mechanical behaviour of concrete is strongly coupled with its hydraulic responses. For instance: the saturation kinetics is accelerated by the plastic deformation related to pore collapse and volumetric compaction. The numerical predictions and discussions can help engineers to enhance their understandings on the influence of interstitial pore pressure on the vulnerability of concrete structures subjected to near-field detonations or impacts.

Hydrostatic compression tests, capillary rheometry tests, and extrusion tests performed on unvulcanized rubber confirm importance of compressibility for die swell — Arguments from dimensional analysis

This paper aims at elucidating which factors are effective in terms of influencing the die swell of unvulcanized rubber upon extrusion. To that end, compression, viscosity, and extrusion tests were performed on two types of ethylene–propylene–diene rubbers, while additional compression tests were performed on natural rubber. First, all three testing modalities were assessed separately, revealing that for the tested materials the compressibility is pressure-dependent; the viscosity is velocity-dependent; and the die swell is influenced by several, partly intertwined factors. In order to better understand the collected experimental data, we have employed the theoretical tool of dimensional analysis, showing that the compressibility of unvulcanized rubber is of great importance for the die swell. Surprisingly, the temperature dependence of the die swell turns out to be much less prominent than standardly assumed. Further key factors influencing the die swell concern the geometries of the die and of the canal.

Experimental Study of Stress-Seepage Coupling Properties of Sandstone under Different Loading Paths

The study on hydromechanical coupling properties of rocks is of great importance for rock engineering. It is closely related to the stability analysis of structures in rocks under seepage condition. In this study, a series of conventional triaxial tests under drained condition and hydrostatic compression tests under drained or undrained condition on sandstones were conducted. Moreover, complex cyclic loading and unloading tests were also carried out. Based on the experimental results, the following conclusions were obtained. For conventional triaxial tests, the elastic modulus, peak strength, crack initiation stress, and expansion stress increase with increased confining pressure. Pore pressure weakened the effect of the confining pressure under drained condition, which led to a decline in rock mechanical properties. It appeared that cohesion was more sensitive to pore pressure than to the internal friction angle. For complex loading and unloading cyclic tests, in deviatoric stress loading and unloading cycles, elastic modulus increased obviously in first loading stage and increased slowly in next stages. In confining pressure loading and unloading cycles, the Biot coefficient decreased first and then increased, which indicates that damage has a great impact on the Biot coefficient.

Stress sensitivity of porosity and permeability under varying hydrostatic stress conditions for different carbonate rock types of the geothermal Malm reservoir in Southern Germany

In geothermal reservoir systems, changes in pore pressure due to production (depletion), injection or temperature changes result in a displacement of the effective stresses acting on the rock matrix of the aquifer. To compensate for these intrinsic stress changes, the rock matrix is subjected to poroelastic deformation through changes in rock and pore volume. This in turn may induce changes in the effective pore network and thus in the hydraulic properties of the aquifer. Therefore, for the conception of precise reservoir models and for long-term simulations, stress sensitivity of porosity and permeability is required for parametrization. Stress sensitivity was measured in hydrostatic compression tests on 14 samples of rock cores stemming from two boreholes of the Upper Jurassic Malm aquifer of the Bavarian Molasse Basin. To account for the heterogeneity of this carbonate sequence, typical rock and facies types representing the productive zones within the thermal reservoir were used. Prior to hydrostatic investigations, the hydraulic (effective porosity, permeability) and geomechanical (rock strength, dynamic, and static moduli) parameters as well as the microstructure (pore and pore throat size) of each rock sample were studied for thorough sample characterization. Subsequently, the samples were tested in a triaxial test setup with effective stresses of up to 28 MPa (hydrostatic) to simulate in-situ stress conditions for depths up to 2000 m. It was shown that stress sensitivity of the porosity was comparably low, resulting in a relative reduction of 0.7–2.1% at maximum effective stress. In contrast, relative permeability losses were observed in the range of 17.3–56.7% compared to the initial permeability at low effective stresses. Stress sensitivity coefficients for porosity and permeability were derived for characterization of each sample and the different rock types. For the stress sensitivity of porosity, a negative correlation with rock strength and a positive correlation with initial porosity was observed. The stress sensitivity of permeability is probably controlled by more complex processes than that of porosity, where the latter is mainly controlled by the compressibility of the pore space. It may depend more on the compaction of precedented flow paths and the geometry of pores and pore throats controlling the connectivity within the rock matrix. In general, limestone samples showed a higher stress sensitivity than dolomitic limestone or dolostones, because dolomitization of the rock matrix may lead to an increasing stiffness of the rock. Furthermore, the stress sensitivity is related to the history of burial diagenesis, during which changes in the pore network (dissolution, precipitation, and replacement of minerals and cements) as well as compaction and microcrack formation may occur. This study, in addition to improving the quality of input parameters for hydraulic–mechanical modeling, shows that hydraulic properties in flow zones largely characterized by less stiff, porous limestones can deteriorate significantly with increasing effective stress.

Mechanical Compaction of Crustal Analogs Made of Sintered Glass Beads: The Influence of Porosity and Grain Size

AbstractThe fundamentals of our understanding of the mechanical compaction of porous rocks stem from experimental studies. Yet, many of these studies use natural materials for which microstructural parameters are intrinsically coupled, hampering the diagnosis of relationships between microstructure and bulk sample behavior. To probe the influence of porosity and grain size on the mechanical compaction of granular rocks, we performed experiments on synthetic samples prepared by sintering monodisperse populations of glass beads, which allowed us to independently control porosity and grain diameter. We conducted hydrostatic and triaxial compression tests on synthetic samples with grain diameters and porosities in the ranges 0.2–1.15 mm and 0.18–0.38 mm, respectively. During hydrostatic compaction, sample porosity decreased suddenly and substantially at the onset of inelastic compaction due to contemporaneous and extensive grain crushing, a consequence of the monodisperse grain size. During triaxial tests at high confining pressure, our synthetic samples failed by shear‐enhanced compaction and showed evidence for the development of compaction bands. Critical stresses at the onset of inelastic compaction map out linear‐shaped yield caps for the porosity‐grain diameter combinations for which the critical stress for inelastic hydrostatic compaction is known. Our yield caps reinforce the first‐order importance of porosity on the compactive yield strength and show, all else being equal, that grain size also exerts a first‐order control and should therefore be routinely measured. Our study further reveals the suitability of sintered glass beads as analogs for crustal rocks, which facilitate the study of the deconvolved influence of microstructural parameters on their mechanical behavior.

Calibration of KCC concrete model for UHPC against low-velocity impact

As an innovative material, UHPC (ultra-high performance concrete) is regarded as a promising material to improve dynamic resistant performance of civil engineering structures suffered from extreme loadings such as blast, shock and impact loading. The mechanical properties of UHPC members suffered from static/dynamic loading have been systematically investigated through many physical experiments. However, the rationality of the constitutive model utilized in numerical simulations (especially the finite element simulations) to precisely describe behaviors of UHPC has been little emphasized. The objective of this study, therefore, is to calibrate the KCC (Karagozian & Case Concrete) model to effectively predict behaviors of UHPC suffered from low-velocity impact loading. At the beginning, the properties of UHPC under triaxial compression were experimentally investigated by a series of triaxial compression tests to obtain the parameters of three strength surfaces by experimental data fitting. Meanwhile, EoS (equation of state) was derived by the combination of hydro-static compression test, “p-alpha equation” and Mie–Grueneisen equation of state. Moreover, suitable damage parameters were given by repeated trials, and the suitable model of strain-rate effect for UHPC was also suggested. The single element analysis results indicate that the calibrated KCC model can reasonably reflect the behaviors of UHPC under the uniaxial and triaxial stress state. Eventually, numerical results of low-velocity impact tests of UHPC columns and beams based on the calibrated KCC model agree very well with the tests results, which indicates the rationality of the calibrated KCC model for numerical simulations on low-velocity impact.

Pressure-dependent bulk compressibility of a porous granular material modeled by improved contact mechanics and micromechanical approaches: Effects of surface roughness of grains

The change of the mechanical properties of granular materials with pressure is an important topic associated with many industrial applications. In this paper we investigate the influence of hydrostatic pressure (Pe) on the effective bulk compressibility (Ceff) of a granular material by applying two modified theoretical approaches that are based on contact mechanics and micromechanics, respectively. For a granular material composed of rough grains, an extended contact model is developed to elucidate the effect of roughness of grain surfaces on bulk compressibility. At relatively low pressures, the model predicts that the decrease of bulk compressibility with pressure may be described by a power law with an exponent of -1/2 (i.e., Ceff ∝Pe−1/2), but deviates at intermediate pressures. At elevated pressures beyond full contact, bulk compressibility remains almost unchanged, which may be roughly evaluated by continuum contact mechanics. As an alternative explanation of pressure-dependent bulk compressibility, we suggest a micromechanical model that accounts for effects of different types of pore space present in granular materials. Narrow and compliant inter-granular cracks are approximated by three-dimensional oblate spheroidal cracks with rough surfaces, whereas the equant and stiff pores surrounded by three and four neighboring grains are modeled as tubular pores with cross sections of three and four cusp-like corners, respectively. In this model, bulk compressibility is strongly reduced with increasing pressure by progressive closure of rough-walled cracks. At pressures exceeding crack closure pressure, deformation of the remaining equant pores is largely insensitive to pressure, with almost no further change in bulk compressibility. To validate these models, we performed hydrostatic compression tests on Bentheim sandstone (a granular rock consisting of quartz with high porosity) under a wide range of pressure. The relation between observed microstructures and measured pressure-dependent bulk compressibility is well explained by both suggested models.

Evolution of bulk compressibility and permeability of granite due to thermal cracking

Granite samples were subjected to heating at different temperatures and rapid water cooling. Hydrostatic compression tests were performed on the thermally treated samples. The initial tangent bulk modulus was measured. The critical value of hydrostatic stress for closing thermally induced micro-cracks was identified and the residual volumetric strain after unloading was determined. Gas permeability of granite samples was also determined under different levels of hydrostatic stress. It is found that the density of thermally induced micro-cracks can be correlated with the residual volumetric strain, both increasing when the pre-treatment temperature is higher. There is a significant increase of gas permeability up to several orders of magnitude as a consequence of micro-cracks induced by the heating and water-cooling process.

Volumetric Deformation, Ultrasonic Velocities and Effective Stress Coefficients of St Peter Sandstone During Poroelastic Stress Changes

A suite of hydrostatic compression tests in a St Peter sandstone core was performed to characterize its static and dynamic poroelastic response. The pore pressure cycles under constant confining pressure simulated scenarios of reservoir depletion and injection. The volumetric deformation and axial ultrasonic velocities (Vp and Vs) were recorded and the corresponding effective stress coefficients were derived. As expected, the variations of pore pressure under constant confining pressure induce poroelastic deformation and velocity changes, which is generally consistent with the exact effective stress law. Data discrepancy between depletion and injection was observed throughout the cycles, which is considered to be a result of material hysteresis. The dynamic elastic moduli were calculated based on the velocities by assuming isotropy, which are in qualitative agreement with the static values. The static and dynamic effective stress coefficients were derived with respect to volumetric deformation and Vp and Vs. It is found that the static effective stress coefficient is consistently less than unity, and decreases from an average of ~ 0.6 with pressure difference to as low as ~ 0.35. The pore pressure has a measurable positive influence on the coefficient for the same pressure difference. The dynamic effective stress coefficient is generally around unity when the pressure difference is low (< 30 MPa), but deviates significantly from unity as the pressure difference further increases. The trend of such deviation highly depends on the wave type and loading path. Despite possible experimental artifacts, the derived dynamic effective stress coefficient is quite distinct from the static counterpart. This corroborates previous findings that the effective stress coefficients of different physical quantities are fundamentally different. It emphasizes that the estimation of static effective stress coefficient based on correlation with dynamic data is not fully warranted and can incur significant errors.

Modification and verification of the Deshpande–Fleck foam model: A variable ellipticity

The ellipticity involved in the Deshpande–Fleck foam model describing the constitutive behaviour of cellular materials was usually considered to be constant, but some very different values were suggested in the literature. A cell-based finite element model of closed-cell foam under uni-/multi-axial compression is employed to verify the Deshpande–Fleck foam model. The ellipticity is determined by applying uniaxial and hydrostatic compression tests and it is found to vary with the equivalent plastic strain, i.e., the ellipticity decreases with the equivalent plastic strain and then increases sustainably before full densification. According to the understandings from the numerical results, a fitting relation between the ellipticity and the equivalent plastic strain is suggested. The ellipticities of an open-cell foam and a closed-cell foam studied experimentally in the literature are fitted well with this relation. A modification to the Deshpande–Fleck foam model with a variable ellipticity is thus proposed. The modified Deshpande–Fleck foam model brings much accurate predictions with using a rigid–plastic hardening (R-PH) idealisation model, which describes the stress–strain relation of cellular material under uniaxial compression. Good agreement is also observed between the experimentally measured stress–strain responses and the predictions of the modified Deshpande–Fleck foam model, especially when considering the effect of the plastic Poisson's ratio with a non-associated flow rule. The findings herein are helpful to improve the prediction accuracy of the Deshpande–Fleck foam model.

Research and development of compressible foam for pressure management in casing annulus of deepwater wells

Casing annulus pressure build-up owing to fluid thermal expansion in sealed annuli severely threaten the casing safety and wellbore integrity of deepwater wells. To mitigate the annular pressure with stronger effect and lower cost, a new type of syntactic foam with high compression ratio, which consists of hollow glass microspheres and a resin matrix system, was developed in this study. It can provide extra space for the annular fluid to expand when the annular pressure exceeds the specific foam crush pressure, allowing the foam to collapse and consequently, mitigate the annular pressure. The foam compression behavior is divided into three stages according to a hydrostatic compression test, and the annular pressure is mainly influenced by the microspheres volume and temperature. To optimize the parameters of the compressible foam, a novel model considering fluid thermal expansion, casing elastic deformation, and foam compression behavior in multi-annuli is established. The results indicate that the compressible foam can reduce the trapped annular pressure effectively and permanently. In a case study well, the annular pressure no longer exceeds the maximum acceptable annular pressure in each annulus with optimized design of the foam wraps and a suitable number of installed foam casing joints.

Laboratory investigations of inert gas flow behaviors in compact sandstone

The seepage evolution behavior of compact rock is significant for the stability and safety of many engineering applications. In this research, both hydrostatic and triaxial compression tests were conducted on compact sandstone using an inert gas, namely argon. A triaxial compression test with a water permeability measurement was carried out to study the difference between the gas permeability and water permeability evolutions during the complete stress–strain process. Based on the experimental data, the hydrostatic stress-dependent gas permeability was discussed firstly. A second-order function was proposed to predict and explain the gas slippage effect. The mechanical properties and crack development of the sandstone samples were discussed to better understand the permeability evolution with crack growth during the complete stress–strain process. The results show that the gas permeability evolution can be divided into five stages according to the different crack growth stages. Then, the permeability changes in the crack closure stress $$ \sigma_{\text{cc}} $$, crack initiation stress $$ \sigma_{\text{ci}} $$, crack damage stress $$ \sigma_{\text{cd}} $$ and peak stress $$ \sigma_{\text{p}} $$ with confining pressures were analyzed. Finally, we found that the difference between the corrected gas permeability and water permeability can be attributed to the interaction between the water and sandstone grains.

Comparison of mechanical properties of ground corn stover, switchgrass, and willow and their pellet qualities

ABSTRACTMechanical and physical properties of ground corn stover, switchgrass, and willow were measured and compared in addition to the quality of pellets. Biomass was size-reduced with two different screen sizes (3.175 and 6.35 mm) and conditioned to obtain samples at two different moisture contents (17.5 and 20% on wet basis). Ground switchgrass had the smallest and willow had the highest D50 when size-reduced with the same screen size. Hydrostatic triaxial compression tests were performed using the cubical triaxial tester to determine the bulk modulus, compression index, and spring-back index at specific unloading pressures (20, 45, 70, and 95 kPa). The trends of pressure vs. volumetric strain and void ratio vs. natural log of pressure were similar for all three materials; however, the magnitudes were different. Willow, size-reduced with 3.175 mm screen size at 17.5% wet basis, had the highest bulk modulus among different conditions of all the three biomass. Pellet durability values for all the three materials were higher than 80%. Corn stover pellets formed with 3.175 mm screen size at 20% wet basis had the highest diametral tensile and axial compressive strengths among different conditions for all the three biomass, however the values were not significantly different (p > 0.05).

Compressibility of unvulcanized natural and EPDM rubber: New experimental protocol and data evaluation in the framework of large strain elasticity theory

Despite considerable ongoing efforts, accurate computational simulation of the flow behavior of (unvulcanized) rubber remains an open challenge. There is growing evidence that one of the reasons for that is insufficient consideration of, or knowledge on, the mechanical compressibility of the material. As a contribution to tackle this open question, we here report on a series of hydrostatic compression tests performed on natural rubber, as well as on two types of EPDM (ethylene-propylene-diene-monomer) rubber compounds. These materials were filled into a capillary rheometer with closed extrusion canal, and then compressed and released through volume changes realized at different speeds, while monitoring the corresponding hydrostatic pressures acting on the investigated rubbers. The volume changes then entered various relevant strain measures (Green-Lagrange strains, Hencky strains, linearized strains), while the pressures were mathematically transformed into energetically conjugate stress measures (second Piola-Kirchhoff stress, Kirchhoff stress, Cauchy stress). Insertion of these measures into the dissipation inequality resulting from the two fundamental laws of thermodynamics, revealed that the investigated materials behave purely elastically under hydrostatic pressure, albeit in a non-linear fashion. Irrespective of the format chosen for elasticity theory, the elastic bulk modulus appears as a power function of the hydrostatic pressure; the former increasing under-linearly, but non-asymptotically, with the latter. Careful statistical evaluation of the corresponding power law coefficients allows for derivation of upper and lower bounds of the bulk modulus, as functions of the hydrostatic pressure, an information which may prove essential for improving the accuracy of rubber extrusion simulations.