Confined Compression Tests Research Articles

A fractional order theory of poroelasticity

We introduce a time memory formalism in the flux-pressure constitutive relation, ruling the fluid diffusion phenomenon occurring in several classes of porous media. The resulting flux-pressure law is adopted into the Biot’s formulation of the poroelasticity problem. The time memory formalism, useful to capture non-Darcy behavior, is modeled by the Caputo’s fractional derivative. We show that the time-evolution of both the degree of settlement and the pressure field is strongly influenced by the order of Caputo’s fractional derivative. Also a numerical experiment aiming at simulating the confined compression test poroelasticity problem of a sand sample is performed. In such a case, the classical Darcy equation may lead to inaccurate estimates of the settlement time.

Study on elastoplastic coupling mechanics model of the milled mixture of sugar cane

Abstract At present, the phenomenon of elastoplastic coupling isn’t considered in establishing the mechanics model of the milled mixture of sugar cane. In order to describe elastoplastic coupling and yield characteristics, based on uniaxial confined compression tests and direct shear tests, the evolution of the elastic parameters of the solid fiber with void ratio and the plastic deformation, and the plastic mechanics behavior of the solid fiber is described by Modified Drucker-Prager Cap model, the expressions of the parameters of the plastic mechanics model are derived and the corresponding parameters are calculated; and the elastoplastic coupling mechanics model is established. The evolution of the parameters of the model is achieved by the custom subroutine written in Fortran, the numerical simulation of the elastoplastic coupling model is realized by ABAQUS. The results of uniaxial confined compression tests and finite element simulation show that the elastoplastic coupling mechanics model is more accurate than the non-elastoplastic coupling mechanics model to predict the axial pressure. The results provide an important reference for the analysis and understanding of the milling process of sugar cane and the establishment of an accurate mathematical model of the milled mixture of sugar cane.

Constitutive modelling of the TiB2–B4C composite by experiments, simulation and neutral network

The JH-2 constitutive relations of the TiB2–B4C composite were studied by the experiments, simulations and feedforward neural network. The flattened Brazilian disc experiments were designed to obtain the strength model of intact ceramic. The strength and strain rates sensitivity of the TiB2–B4C composite were studied by the static and dynamic tests. The Hugoniot elastic limit and equation of state of the composite were analyzed by the plate impact tests and Doppler pins test system. The confined compression tests for ceramic particles were designed to model the normalized fracture strength relations. Then the feedforward neural network model was established to train samples and analyze the damage constants of the TiB2–B4C composite. The samples of the input layer and output layer were derived from the experiments and simulation of the ceramic/metal composite armor impacted by high speed ball projectile. The Bayesian regularization algorithm was used to avoid converging on local minimum value in the neural network. Lastly, the constitutive model of the TiB2–B4C composite was verified by the comparison between the experimental and simulation results which were the depth of penetration, radial and circular crack of ceramic plate.

Towards Patient-Specific Computational Modelling of Articular Cartilage on the Basis of Advanced Multiparametric MRI Techniques

Cartilage degeneration is associated with tissue softening and represents the hallmark change of osteoarthritis. Advanced quantitative Magnetic Resonance Imaging (qMRI) techniques allow the assessment of subtle tissue changes not only of structure and morphology but also of composition. Yet, the relation between qMRI parameters on the one hand and microstructure, composition and the resulting functional tissue properties on the other hand remain to be defined. To this end, a Finite-Element framework was developed based on an anisotropic constitutive model of cartilage informed by sample-specific multiparametric qMRI maps, obtained for eight osteochondral samples on a clinical 3.0 T MRI scanner. For reference, the same samples were subjected to confined compression tests to evaluate stiffness and compressibility. Moreover, the Mankin score as an indicator of histological tissue degeneration was determined. The constitutive model was optimized against the resulting stress responses and informed solely by the sample-specific qMRI parameter maps. Thereby, the biomechanical properties of individual samples could be captured with good-to-excellent accuracy (mean R2 [square of Pearson’s correlation coefficient]: 0.966, range [min, max]: 0.904, 0.993; mean Ω [relative approximated error]: 33%, range [min, max]: 20%, 47%). Thus, advanced qMRI techniques may be complemented by the developed computational model of cartilage to comprehensively evaluate the functional dimension of non-invasively obtained imaging biomarkers. Thereby, cartilage degeneration can be perspectively evaluated in the context of imaging and biomechanics.

Weathering effect on the small strains elastic properties of a residual soil

Residual soils in the northeast Argentina, southeast Paraguay and central and southern Brazil are the result of the weathering of a near-surface basalt formation. To assess the effect of weathering on the small and large strain mechanical properties of residual soils and saprolite, disturbed and undisturbed specimens from the city of Obera in Northern Argentina were collected and tested in the lab. Soil samples were tested from the surface down to 14 m in depth where a moderated weathered rock stratum was reached. Confined compression tests with complimentary shear wave velocity (S-wave) measurements were performed in all recovered samples. The degree of weathering of each specimen was evaluated by computing the weathering index (WI). The effect of degree of weathering, disturbance, effective stress, natural moisture content and saturation on soil stiffness was evaluated. The interpretation of the experimental results indicates that the mechanical behavior of residual soils at high strain levels is mainly controlled by the degree of weathering, the initial void ratio and the effective stress. However, at low strain levels, these soils behave like cemented soils and the stiffness of the skeleton is mainly controlled by the presence of weak bonds between particles caused by the presence of oxides and sesquioxides, and matric suction. To characterize this behavior, a new relationship between WI, effective stress and S-wave velocity is proposed. This relationship can be used to estimate the degree of weathering of residual soils using S-wave velocity measurements.

A thermomechanical theory for porous tissues with diffusion of fluid and micromechanical modeling of porosity

Biological tissues are multiphasic and multiconstituent materials, in which interstitial fluid and other extracellular matrix components move relative each other and interact mechanically, chemically and electrically. In this contribution a thermomechanical model of porous biological tissues is proposed. The tissue is modeled as a composite made of a compressible, porous solid saturated with a compressible fluid. The kinematics of the porous tissue is described by a single velocity field and the diffusion of fluid through the tissue is modeled by Darcy-type flow. An Eulerian formulation of the constitutive equations is used and a numerical example showcases the capability of the model in representing the case of a tissue sample undergoing a confined compression test.

Damage Evolution Characteristics of Rock Salt under Different Stress Conditions

Understanding the damage evolution characteristics of rock material is essential to the long‐term stability and safety analysis of the underground facility. In this study, a series of cyclic loading tests under tensile or compressive stresses are conducted to investigate the damage evolution, deformation, peak strength, and failure pattern of rock salt. A special attention is paid on the microcracking process by using a 3D acoustic emission (AE) test system. The laboratory tests show that the damage degree of rock salt under compression is the highest, followed by the damage in the direct tensile test. The lowest value of damage is determined by using the Brazilian test. The damage degrees where the damage rate starts to decrease are about 0.83 in the direct tensile test, about 0.75 in the Brazilian test, and about 0.91 in the compression test. The failure mode of rock salt changes from the tensile mode in the uniaxial compression test to the compression‐shear mode in the confined compression test at low confinement. But from the confining pressure of 15 MPa, the rock salt displays great plastic dilatant distortion and without appreciable macroscopic fractures. Accordingly, with increasing confining pressure, the positions where the rapid increase in cumulative AE counts occurs and where the AE event with high energy appears are changed, from the beginning of the test at low confinement to the postpeak stage of the test at high confinement.

GENETIC COUNSELING AS A WAY TO ENGAGE PEOPLE IN BEHAVIOUR CHANGE TO REDUCE RISK FOR COMMON COMPLEX DISEASE: RATIONALE FOR ITS POTENTIAL EFFECTIVENESS WHERE GENETIC INFORMATION HAS FAILED

Compressive properties of arable and forest soils are typically derived from data obtained in uniaxial confined compression tests. However, the stress field, final state of compression and thus compressive properties derived from such tests are dependent on sample dimensions, soil-cylinder wall friction and soil material properties. In this study, we analysed the stress field and bulk density distribution within a cylindrical soil sample under uniaxial compression, and how these are affected by sample dimension, soil-cylinder wall friction and elastic properties of soil. For this, we modelled a uniaxial compression test using the finite element method (FEM) and performed simulations for a range of sample diameter to height ratios (D/h), different values of soil-wall friction coefficient (μ) and different soil elastic properties (Young’s modulus and Poisson’s ratio). We use experimental data to validate the findings. The results showed a high impact of soil-cylinder wall friction on the stress field within the sample. This resulted in stress concentration at the top of sample edges (walls) and decreasing stresses at the bottom of the sample. However, the relative impact of soil-wall friction on sample average behaviour decreased with increasing D/h. These results suggest that the effect of soil-wall friction on sample-average bulk density cannot be neglected unless D/h >8. Correction of bulk density for μ and D/h could be a practical way to compare data obtained in laboratories using different sample sizes.

Impact of sample dimensions, soil-cylinder wall friction and elastic properties of soil on stress field and bulk density in uniaxial compression tests

Compressive properties of arable and forest soils are typically derived from data obtained in uniaxial confined compression tests. However, the stress field, final state of compression and thus compressive properties derived from such tests are dependent on sample dimensions, soil-cylinder wall friction and soil material properties. In this study, we analysed the stress field and bulk density distribution within a cylindrical soil sample under uniaxial compression, and how these are affected by sample dimension, soil-cylinder wall friction and elastic properties of soil. For this, we modelled a uniaxial compression test using the finite element method (FEM) and performed simulations for a range of sample diameter to height ratios (D/h), different values of soil-wall friction coefficient (μ) and different soil elastic properties (Young’s modulus and Poisson’s ratio). We use experimental data to validate the findings. The results showed a high impact of soil-cylinder wall friction on the stress field within the sample. This resulted in stress concentration at the top of sample edges (walls) and decreasing stresses at the bottom of the sample. However, the relative impact of soil-wall friction on sample average behaviour decreased with increasing D/h. These results suggest that the effect of soil-wall friction on sample-average bulk density cannot be neglected unless D/h >8. Correction of bulk density for μ and D/h could be a practical way to compare data obtained in laboratories using different sample sizes.

Confined compression behaviour of a shear thickening fluid with concentrated submicron particles

Abstract This study investigates the confined compression behaviour of a shear thickening fluid (STF) with concentrated submicron particles. The styrene/acrylate particles suspended in an ethylene glycol (EG) show a narrow size distribution, with a mean size of 0.4 µm. The confined compression test results show the significant compressibility of the STF corresponding to the compression of the liquid medium EG and the compressibility of the STF starts to lessen when the applied compression stress is greater than 4 MPa. It is also found that the confined compression results of the STF are not sensitive to the loading rate ranging from 1 to 50 mm/min. Finally, the bulk moduli of the STF and the EG are obtained.

Response of a polystyrene foam subjected to large strains and high pressures

This article presents experimental data, and their comprehensive analysis, for a polystyrene foam (with an initial density of 0.623 g/cm3) subjected to large strains and high pressures. The data were obtained from unconfined uniaxial stress tests in compression (at two different strain rates), confined compression tests with compressive strains to almost 50%, and a tension test. The confinement for the compression tests consists of a steel cylinder with external strain gages to monitor the hoop strains due to the radial stresses from the compressed foam. The procedure used to analyze the data allows for a continuous determination of axial, radial and hoop stresses, pressure, and von Mises equivalent stress. A parametric finite element analysis was used to provide for the determination of the stresses from small strains to large strains approaching 50%. There is excellent agreement between test results for similar conditions, as well as the results from data obtained from the confined experiments with two different wall thicknesses. The strength (von Mises equivalent stress) is highly dependent on the pressure, but not as dependent on the strain. The strain rate has a significant effect on strength for the uniaxial stress tests, but only limited data were obtained. Constants were obtained for an existing computational model, and there is good correlation between the experimental data and the model.

Application of Flattened Brazilian Test to Investigate Rocks Under Confined Extension

A failure envelope that ignores the intermediate principal stress is typically adequate for the design of rock structures. To obtain the complete envelope, the rock cylinders should be tested using confined compression as well as confined extension tests. While confined compression tests are common, confined extension tests are rarely carried out. Current techniques available to test rocks under confined extension conditions require shaping the cylindrical specimens to a dog-bone geometry. The limited data available in the literature indicates that the results from these dog-boned shaped tests produces strengths that are considerably greater than the confined triaxial tests carried out on traditional cylindrical specimens. Whether this increased strength is real or simply an artifact of the stress path for triaxial confined extension tests (intermediate principal stress equal to the maximum principal stress) versus confined compression tests (intermediate principal stress equal to the minimum principal stress) is unknown. We propose here a new methodology to test rocks under confined extension condition with zero intermediate principal stress. We used the flattened Brazilian test and calculated the major and minor principal stresses from the strain on the surface of the specimens. To test the rock under increased confined conditions, we tested specimens with increasing depth of flattening. We derived equations to calculate the major and minor principal stresses in a flattened Brazilian specimen considering the bi-modularity in the stress–strain equations. We tested Lac du Bonnet granite specimens for major principal stress range 15–37% of its UCS and observed that the minor principal stresses remains in the range of Brazilian strength of the material (11.6 MPa). The results suggest that the nonlinear Hoek–Brown failure criterion based on only compressive triaxial results provides a reasonable estimate of the tensile strength.

Identifying crack initiation stress threshold in brittle rocks using axial strain stiffness characteristics

As an estimate for the in-situ spalling strength around massive underground excavations to moderately jointed brittle rocks, crack initiation stress marks the initiation of rock micro fracturing. It is crucial to accurately identify crack initiation stress level by proper method. In this study, confined compression tests of sandstone samples are used to examine the validity/applicability of proposed axial strain stiffness method. The results show that by highlighting the minuscule changes in stress-strain curve, the axial strain stiffness curve provided further insight into rock failure process and revealed five stages: (a) irregular fluctuation, (b) nearly horizontal regular fluctuation, (c) irregular fluctuation gradually decreasing to zero, (d) extreme fluctuation, and (e) near zero, which mainly correspond to five stages of stress–strain curve. The ratio of crack-initiation stress to peak strength determined using this approach is 0.44–0.51, similar to the ranges previously reported by other researchers. In this method, the key is to accurately detect the end point of the stage (b), “nearly horizontal regular fluctuation” characterized by a sudden change in axial strain stiffness curve, and the sudden change signifies crack initiation in rock sample. Finally, the research indicates that the axial strain stiffness curve can provide a mean to identify the crack-initiation stress thresholds in brittle rocks.

Transparent confined compression tests on particle beds: Observations and implications

Particle breakage is quite common in a variety of engineering circumstances, and the underlying mechanism as well as its effects on overall responses of particle assembly still demand further investigation mainly due to the difficulty in monitoring the progressive size reduction process. This paper presents a series of confined compression tests for two kinds of quasi-brittle spherical particles, namely Al2O3 and ceramic granules. A broad range of particle sizes and feed gradations were considered, and a transparent sample container was purposely adopted for a direct observation of particle disintegration. Typical macro- and micro-scale test results are presented, including the development of overall load-bearing capacities versus the compaction displacement, disintegration modes of single particles as well as the description of fragment size distributions. Two characteristic indexes tsqrt and Br′ are proposed to quantify the fragment size distribution and particle breakage degree, respectively. It is also found that under the same specific input energy, distributions of fine products from samples of different feed gradations remain basically the same. The findings facilitate a better understanding of the macro- and micro-scale progressive particle breakage behaviour under one-dimensional compression, and some concluding remarks are provided in the final part of the paper.

Mealiness in ‘Forelle’ pears: relationship between TSS and prediction based on FT-NIR spectroscopy

'Forelle' pears are prone to mealiness, a dry textural disorder characterised by loss of free juice and development of a floury texture when fruit softens. There is currently no method of determining mealiness in intact fruit and no instrumental index which correlates with mealiness. Presently, mealiness determination is done by sensorial panels on cut fruit. This study investigated physicochemical attributes associated with mealiness and the potential of Fourier transform-near infrared (FT-NIR) spectroscopy for non-destructive detection of mealiness in 'Forelle' pears. Spectra were acquired on 400 fruit harvested from four farms over two seasons, stored for 8 weeks at -0.5°C and then for 7 d shelf life at 20°C. Physicochemical measurements were taken using standard destructive techniques. Mealiness was evaluated on each individual fruit by a panel of three trained evaluators and confirmed using the confined compression test. Principal component analysis (PCA) was done on physicochemical parameters to identify parameters that relate to mealiness. Orthogonal partial least squares discriminant analysis (OPLS-DA) of FT-NIR spectra was used to discriminate mealy from non-mealy fruit. PCA results showed high positive correlations between mealiness and total soluble solids (TSS). The results showed potential for discriminating between mealy and non-mealy fruit using FT-NIR spectroscopy. Classification accuracies for the two-class OPLS-DA models ranged from 51-95%. Through discriminant analysis, FT-NIR was shown to be a potential non-destructive method for determining mealiness and for the first time TSS was shown to be linked with mealiness.

DEM simulation and experimental validation for mechanical response of ellipsoidal particles under confined compression

The representation of non-spherical particles in discrete element method (DEM) has not been addressed adequately. Although the multiple sphere method (MSM) is the most popular approach to describe non-spherical particle shape, the validity of the MSM has not been established yet. The purpose of this study is to examine the validity and adequacy of the MSM. A uni-axial confined compression test was designed and set up to study the mechanical behaviour of an ellipsoidal granular assembly under vertical loading and the load transfer to the contacting boundary. Four levels of multi-sphere approximation for an axi-symmetric ellipsoidal particle were employed in DEM simulation to investigate the adequacy of multi-sphere approximation. A comparison on compression characteristics between the numerical and experimental results was made and discussed in this paper. Most of the compared physical properties showed reasonable agreement, indicating that capturing the key linear dimensions of a non-spherical particle may be sufficient to predict reasonable results. A small number of sub-spheres (say, N ≥ 5) for representing an axi-symmetric ellipsoidal particle can give plausible results. However, the DEM simulations also produced a certain extent of discrepancy in loading stiffness with experiments. Plausible explanations are provided and require further investigation.

High-pressure uniaxial confined compression tests of mortars

The paper presents the development of an experimental setup to perform confined compression tests of cementitious paste, mortar, and concrete specimens at high pressures up to 1 GPa as well as the validation of the multi-scale model for the equation of state (EOS) of mortars. The test results for the EOS show that the secant bulk modulus of the loading branch monotonically increases with the content of fine aggregate. The comparison of the loading branches of the EOS for unsaturated specimens of two different specimen sizes indicates that there is negligible size effect for all types of the mixture compositions. Validation of the multi-scale model proposed previously for the EOS of a cement paste shows good agreement with the test results at the high-stress range with slight deviation at the lower stresses. It was demonstrated on mortars with a water to cement ratio of 0.50 that the Hirsch mixture model shows a good agreement with the test results for all levels of fine aggregate content in mortars.

The role of structural viscoelasticity in deformable porous media with incompressible constituents: Applications in biomechanics.

The main goal of this work is to clarify and quantify, by means of mathematical analysis, the role of structural viscoelasticity in the biomechanical response of deformable porous media with incompressible constituents to sudden changes in external applied loads. Models of deformable porous media with incompressible constituents are often utilized to describe the behavior of biological tissues, such as cartilages, bones and engineered tissue scaffolds, where viscoelastic properties may change with age, disease or by design. Here, for the first time, we show that the fluid velocity within the medium could increase tremendously, even up to infinity, should the external applied load experience sudden changes in time and the structural viscoelasticity be too small. In particular, we consider a one-dimensional poro-visco-elastic model for which we derive explicit solutions in the cases where the external applied load is characterized by a step pulse or a trapezoidal pulse in time. By means of dimensional analysis, we identify some dimensionless parameters that can aid the design of structural properties and/or experimental conditions as to ensure that the fluid velocity within the medium remains bounded below a certain given threshold, thereby preventing potential tissue damage. The application to confined compression tests for biological tissues is discussed in detail. Interestingly, the loss of viscoelastic tissue properties has been associated with various disease conditions, such as atherosclerosis, Alzheimer's disease and glaucoma. Thus, the findings of this work may be relevant to many applications in biology and medicine.

Evaluation of viscoelastic Poisson’s ratio of asphalt mixtures

The Poisson’s ratio (PR) of asphalt mixtures is an important input parameter in the constitutive models for pavement design and performance analysis. The design and performance analysis of asphalt pavements depend upon measuring proper material properties such as complex modulus and PR. The PR is often assumed to be time independent or it is calculated as the negative ratio of transverse to axial strains in the time domain under uniaxial loading when the time dependency is considered. This study used accurate methods to calculate the viscoelastic Poisson’s Ratio (VPR) under various loading conditions and demonstrated the error associated with using inaccurate methods for calculating the PR. The results demonstrated that the VPR increased with time in uniaxial unconfined tension and compression tests where shear relaxation was generally faster than dilatational relaxation. While the VPR slightly decreased with time in uniaxial confined compression tests where the dilatational relaxation was faster than the shear relaxation. In addition, the results demonstrated that asphalt mixtures with coarse aggregate gradations had lower VPR compared to asphalt mixtures with finer aggregate gradations. Also, the VPR was found to be sensitive to the ageing and moisture conditions. The VPR decreased with ageing and increased with the level and time of moisture conditioning of the asphalt mixtures. Finally, the error introduced by incorrectly calculating the PR as the negative ratio of transverse to axial strains in the time domain was found to be significant in some cases, while it was insignificant in others.

A Constitutive Relationship for Gravelly Soil Considering Fine Particle Suffusion.

Suffusion erosion may occur in sandy gravel dam foundations that use suspended cutoff walls. This erosion causes a loss of fine particles, degrades the soil strength and deformation moduli, and adversely impacts the cutoff walls of the dam foundation, as well as the overlying dam body. A comprehensive evaluation of these effects requires models that quantitatively describe the effects of fine particle losses on the stress-strain relationships of sandy gravels. In this work, we propose an experimental scheme for studying these types of models, and then perform triaxial and confined compression tests to determine the effects of particle losses on the stress-strain relationships. Considering the Duncan-Chang E-B model, quantitative expressions describing the relationship between the parameters of the model and the particle losses were derived. The results show that particle losses did not alter the qualitative stress-strain characteristics of the soils; however, the soil strength and deformation moduli were degraded. By establishing the relationship between the parameters of the model and the losses, the same model can then be used to describe the relationship between sandy gravels and erosion levels that vary in both time and space.