Unconfined Uniaxial Compression Tests Research Articles

Exploring the mechanisms of diverging mechanical and water stability in macro‐ and microaggregates

AbstractBackgroundSoil stability is often evaluated using either mechanical or hydraulic stress. The few studies that use both approaches suggest that these two types of stability behave differently.AimsOur aim was to explore the mechanisms of aggregate stability regarding mechanical and water stability at the macro‐ and microscale, among other things, the effect of differing pore structure and soil organic matter content.MethodsSamples were taken from two adjacent plots that were expected to differ in stability due to land use, that is, cropped versus bare fallow (BF). The stability of dry‐separated macroaggregates (8–16 mm) and microaggregates (53–250 µm) was determined via wet sieving and unconfined uniaxial compression tests. To explore the mechanisms of stability, 3D pore characteristics were analyzed with microtomography scans. Furthermore, the contents of carbon and exchangeable polyvalent cations as well as contact angles were determined.ResultsWater stability of macroaggregates was much higher in the cropped plot (geometric mean diameter 0.65–2.37 mm [cropped] vs. 0.31–0.56 mm [BF]), while mechanical stability was very similar (median work 17.3 [cropped] and 17.5 N mm [BF]). The two size fractions behaved similarly regarding both types of stability, with more pronounced differences in macroaggregates. Several soil characteristics, like carbon, exchangeable calcium, and higher connectivity of pores to the aggregate exterior, contributed to water stability. Regarding mechanical stability, the destabilizing effect of lower carbon content and exchangeable calcium in the BF plot was counterbalanced by a lower porosity.ConclusionsMechanical and water stability behaved differently in the two plots due to the different deformation mechanisms.

Quasi-Static Mechanical Properties and Continuum Constitutive Model of the Thyroid Gland.

The purpose of this study is to obtain the digital twin parameters of the thyroid gland and to build a constitutional model of the thyroid gland based on continuum mechanics, which will lay the foundation for the establishment of a surgical training system for the thyroid surgery robot and the development of the digital twin of the thyroid gland. First, thyroid parenchyma was obtained from fresh porcine thyroid tissue and subjected to quasi-static unconfined uniaxial compression tests using a biomechanical test platform with two strain rates (0.005 s-1 and 0.05 s-1) and two loading orientations (perpendicular to the thyroid surface and parallel to the thyroid surface). Based on this, a tensile thyroid model was established to simulate the stretching process by using the finite element method. The thyroid stretching test was carried out under the same parameters to verify the validity of the hyperelastic constitutive model. The quasi-static mechanical property parameters of the thyroid tissue were obtained by a quasi-static unconstrained uniaxial compression test, and a constitutional model that can describe the quasi-static mechanical properties of thyroid tissue was proposed based on the principle of continuum media mechanics, which is of great value for the establishment of a surgical training system for the head and neck surgery robot and for the development of the thyroid digital twin.

Juxtaposing fresh material characterisation methods for buildability assessment of 3D printable cementitious mortars

Both industry and academia are rapidly developing processes, materials, and projects to explore the potential of extrusion-layering additive manufacturing of cementitious materials, generally known as 3D concrete printing (3DCP). Because the lack of supportive formwork makes objects prone to failure during printing, a key aspect remains the so-called ‘buildability’, a qualitative descriptor to indicate the resistance against such failures. Obviously, the material characteristics of the applied print mortar are an important (although not sole) parameter to determine buildability. However, it is not yet clear which material properties are the most suitable, and how they should be determined experimentally. In literature, a range of approaches has been suggested, but comparative studies are very few in number and limited in scope. This paper presents a juxtaposition of fresh material characterisation methods by subjecting four different mortars to a range of tests related to buildability, including rotational rheometry, unconfined uniaxial compression tests, direct shear tests, and ultrasonic wave transmission tests. For reference, some hardened state properties were also determined, and a printing trial was performed on one mixture. Significant differences between the mixtures were found, including different development characteristics, even though three of the four mixtures were composed of different proportions of the same 4 dry materials. Furthermore, it was shown that strength values from different experiments could only be correlated by assuming significant friction angles associated with Mohr-Coulomb failure behaviour. We propose this could be established relatively easily through a novel method, by combining rheometry-shear and uniaxial compression test results. The data seem to indicate this would be a valid approach. Normalized but physically different parameters, such as compressive strength and pulse velocity, could not be consistently correlated. Their proportions are time and mixture-dependent, which adds significant complexity to quality control and the development of generalized methods to characterize and compare buildability of cementitious mortars.

Study on a novel chemical slurry and the mechanical behavior of cemented sand

To enhance the strength and stability of the fine sandy soil in underground engineering construction, a novel chemical slurry was studied. Series of laboratory tests were performed to determine the effect of each component on the rheological properties of grout (e.g. viscosity, gel time and plastic strength). The relationship of the penetrability of grout in sand with different densities was explored by the falling-head permeability test. To assess the performance of cementation, the cemented sand specimens were prepared in the laboratory by hand mixing of grout and loose sand. A range of unconfined uniaxial compression tests, split tensile tests and direct shear tests were performed to determine the effect of several vital factors (e.g. the curing time, grout mix ratio and water content) on the mechanical properties of the cemented sand. As revealed from the results of the experiment, the slurry pertains to Newton fluid in the initial stage, which exhibits lower initial viscosity (2 mPa·s), controllable gel time (5–20 min) and higher plastic strength (35 kPa). In terms of the cemented sand, the maximal cohesion and friction angle is 403.8 kPa and 65°; the biggest compressive strength and tensile strength is 3.68 and 0.37 MPa. The slurry can enhance the strength and stiffness of the loose sand significantly, whereas the degree of improvement is dependent of the grout mix ratio and water content of cemented sand.

Mechanical characterization of porcine liver properties for computational simulation of indentation on cancerous tissue

An accurate characterization of soft biological tissue properties is essential for a realistic simulation of surgical procedures. Unconfined uniaxial compression tests with specimens affixed to the fixtures are often performed to characterize the stress-stretch curves of soft biological tissues, with which the material parameters can be obtained. However, the constrained boundary condition causes non-uniform deformation during the uniaxial test, posing challenges for accurate measurement of tissue deformation. In this study, we measured the deformation locally at the middle of liver specimens and obtained the corresponding stress-stretch curves. Since the effect of the constrained boundary condition on the local deformation of specimen is minimized, the stress-stretch curves are thus more realistic. Subsequently, we fitted the experimental stress-stretch curves with several constitutive models and found that the first-order Ogden hyperelastic material model was most suitable for characterizing the mechanical properties of porcine liver tissues. To further verify the characterized material properties, we carried out indentation tests on porcine liver specimens and compared the experimental data with computational results by using finite element simulations. A good agreement was achieved. Finally, we constructed computational models of liver tissue with a tumor and investigated the effect of the tumor on the mechanical response of the tissue under indentation. The computational results revealed that the liver specimen with tumor shows a stiffer response if the distance between the tumor and the indenter is small.

A New Viscous Potential Function for Developing the Viscohyperelastic Constitutive Model for Bovine Liver Tissue: Continuum Formulation and Finite Element Implementation

The mechanical behavior of very soft tissues, such as liver, brain, kidney, etc. is assumed to be viscohyperelastic. The conventional approaches like quasi-linear viscoelastic (QLV) are limited to low strain rates and are unable to capture the short-term memory effects. In this paper, a new viscohyperelastic constitutive model is presented in which the strain energy function is decomposed into an elastic and a viscous part. The elastic part of the strain energy is assumed as a Mooney–Rivlin model, while a new viscous potential, as a function of strain and its rate, is proposed. Unconfined uniaxial compression tests are conducted up to 10% compression at two loading velocities of 1.3 and 10.56[Formula: see text](mm/min), to determine the material constants. A numerical simulation is also used to investigate the model-predicted material behavior. It is shown that the model is more sensitive to the hyperelastic parameters than the viscous parameters. This model is also able to predict the stress relaxation and hysteresis; however, an instantaneous relaxation is observed.

Triaxial compression testing on early age concrete for numerical analysis of 3D concrete printing

In 3D concrete printing processes, two competing modes of failure are distinguished: material failure by plastic yielding, and elastic buckling failure through local or global instability. Structural analysis may be performed to assess if, and how, an object may fail during printing. This requires input in the form of transient material properties obtained from experimental testing on early age concrete. In this study, a custom triaxial compression test setup was developed, to characterize all essential parameters to assess failure by elastic buckling, and material yielding according to the Mohr-Coulomb criterion. The results of the triaxial tests were compared to simultaneously run unconfined uniaxial compression tests and ultrasonic wave transmission tests. The correlation between these experimental methods was reviewed. It was concluded that the triaxial compression test is an appropriate method to determine all relevant transient properties from one series of experiments. Subsequently, the experimental results were used for structural analyses of straight printed walls of different lengths with a Finite Element Modelling approach. These walls have been printed up to failure during print trials and the results were compared to the numerical predictions. The failure mode is predicted accurately by the numerical model, as is the critical height at which failure occurs for relatively small objects. For larger objects and/or longer printing processes, the quantitative agreement of the critical height with the print experiments could be improved. Two possible causes for this deviation are discussed.

Early age mechanical behaviour of 3D printed concrete: Numerical modelling and experimental testing

A numerical model was developed to analyse the mechanical behaviour of fresh, 3D printed concrete, in the range of 0 to 90 min after material deposition. The model was based on a time-dependent Mohr-Coulomb failure criterion and linear stress-strain behaviour up to failure. An experimental program, consisting of unconfined uniaxial compression tests and direct shear tests, was set-up and performed to obtain the required material properties. The material tests showed that the Young's modulus and cohesion linearly increase with fresh concrete age, as do the compressive and shear strength. The Poisson's ratio and angle of internal friction, on the other hand, remain constant. Subsequently, the model was validated by comparison to printing experiments. Modelling of the printed samples reproduced the experimental results qualitatively, but the quantitative agreement with the print experiments could be improved. However, the deviations can well be explained and the type of failure-deformation mode was predicted accurately.

Numerical predictions of concrete slabs under contact explosion by modified K&C material model

To precisely predict the failure performances of cratering and spallation in concrete slabs under the contact explosion, three modifications were made to the original K&C (Karagozian & Case) material model that is #72REL3 in LS-DYNA, i.e. the tensile damage accumulation, relationship between the yield scale factor and damage function, as well as the tensile DIFt. The mechanical properties of normal weight concrete simulated by the modified model were validated by a series of single finite element numerical tests, including unconfined uniaxial compression, triaxial tension and dynamic tension tests. Two field tests were then conducted to study the failure modes of concrete slabs under contact explosion, as well as to verify the modifications. A new erosion criterion referring to the tensile damage was proposed to improve the simulation of the tensile failure of the concrete slab. The modified model and erosion criterion were implemented in LS-DYNA with the user-defined material procedure. The corresponding predictions by the original K&C model were also presented to demonstrate the improved effects of the modifications. Sensitivity analysis of the proposed erosion criterion and tensile DIFt was carried out to investigate the stability of the modified model.

Pressure-core-based reservoir characterization for geomechanics: Insights from gas hydrate drilling during 2012–2013 at the eastern Nankai Trough

Pressure coring and analysis technology has progressed remarkably over the last decade. This technology allows for methane-hydrate-bearing sands, silts, and/or clays to be recovered from the deep-sea gas hydrate reservoir and then maintained in the laboratory within the hydrate stability phase boundary for evaluation and characterization. In this study, a series of pressure-core-based undrained/drained confined compression tests, unconfined (uniaxial) compression tests, isotropic loading and unloading tests, and permeability tests are conducted to investigate and characterize the intact strength, compressibility, and permeability of the gas hydrate reservoir. The consolidation tests and fluid flow tests showed that clayey silt sediments have permeability values of approximately tens of microdarcies, and sandy sediments with and without hydrates have permeability values of tens of millidarcies. The sediment from the bottom of the reservoir, which is sandy in nature with fines, also has permeability values of tens of microdarcies. From previous reports, the undrained shear strength of clayey silt sediments with low gas hydrate saturation is relatively low. Results from the drained shear tests and uniaxial compression tests show the first Mohr−Coulomb failure criteria for natural gas hydrate-bearing sediments, in which both the effective cohesion c' and the friction angle ϕ’ increase with increasing hydrate saturation. The results indicate in situ existence of pore filling and patchy gas hydrate. Finally, a new failure envelope for hydrate-bearing sediment is proposed that includes both hydrate saturation and confining pressure; it shows a good relation with the experimental results.

Criteria for defining the required duration of a creep test

Time-dependent behaviour of some types of rocks is of the “creep” type, in particular in underground works, mining works, and in measuring procedures of rock properties. Tests used for defining material parameters or parameters relevant to defining a failure or behaviour of a material in the plastic state are usually of significantly shorter duration than the creep test. The duration of creep tests may vary from several hours to several years depending on the material being tested and the phenomenon that is the subject of the research. The required duration of the creep test, which provides reliable definition of the time-dependent material parameters of the rheological model, is a theoretical but also practical issue. The theoretical issue relates to establishing criteria for defining the required duration of the creep test. The practical issue relates to minimizing the duration of the creep test from which the necessary material parameters of the rock mass are obtained for correct numerical calculations. This paper proposes criteria for defining the required duration of a rock creep test, based on analysis of the results of unconfined uniaxial compression tests performed on marly rock samples.

Advanced shear tester for evaluation of asphalt concrete under constant normal stiffness conditions

This paper presents motivation and details on the development of an advanced shear tester (AST) device capable of investigating the response of 150-mm cylindrical specimens under constant normal stiffness (CNS) conditions. Based on the laboratory experience and field observations from the soil and rock engineering, CNS conditions are particularly desirable when the normal stress changes considerably during the shearing process. Such situations occur in asphalt pavement structures especially under certain loading configurations. The CNS conditions are complementary to the constant normal load conditions that are applied in the current shear-mode devices incorporated in testing of asphalt concrete (AC) and pavement interface properties. This paper demonstrates the complex state of stress in the upper AC layer of a typical pavement under moving truck wheel and shows the need for the CNS device. In addition to outlining unique AST design features, this paper presents also the verification effort conducted on the solid Ultra-High Molecular Weight Polyethylene specimens. The AST results matched very well the reference data that were obtained under the elastic regime in the stress-controlled mode from the unconfined uniaxial compression and indirect tension tests. Finally, this paper demonstrates the example results collected on the solid AC specimens prepared with two different nominal maximum aggregate sizes. The analysis confirmed the ability of the AST device to capture both practical and fundamental phenomena occurring during the shearing process in the AC. It is believed that AST-type devices that are rather simple in design yet allow to induce complex state of stress in the AC specimens should be more recognised in the future research efforts.

Uniaxial Compression Tests on Diesel-Contaminated Frozen Silty-Soil Specimens

AbstractContamination of permafrost by hydrocarbons is an important issue in northern regions. In addition to harmful environmental impacts, diesel-fuel spills in permafrost associated with vehicle-storage areas, storage tanks, and pipelines can also lead to loss of strength of the frozen soils. Investigations of the reduction in strength of contaminated frozen soils have typically been restricted to salt contamination. However, the mechanical behavior of diesel-contaminated frozen soils may vary from that of salt-contaminated frozen soils. This paper presents the results of two series of unconfined uniaxial compression tests on artificial (laboratory produced) diesel-contaminated and uncontaminated frozen silty-soil specimens at two different temperatures (−3 and −5°C). A total of 58 specimens were tested. The influence of total liquid content (water and diesel), density, freezing method, and amount of diesel contamination on the decrease in compressive strength of the specimens is examined.

Experimental Study of the Brittle Behavior of Clay shale in Rapid Unconfined Compression

The mechanical behavior of clay shales is of great interest in many branches of geo-engineering, including nuclear waste disposal, underground excavations, and deep well drilling. Observations from test galleries (Mont Terri, Switzerland and Bure, France) in these materials have shown that the rock mass response near the excavation is associated with brittle failure processes combined with bedding parallel shearing. To investigate the brittle failure characteristics of the Opalinus Clay recovered from the Mont Terri Underground Research Laboratory, a series of 19 unconfined uniaxial compression tests were performed utilizing servo-controlled testing procedures. All specimens were tested at their natural water content with loading approximately normal to the bedding. Acoustic emission (AE) measurements were utilized to help quantify stress levels associated with crack initiation and propagation. The unconfined compression strength of the tested specimens averaged 6.9 MPa. The crack initiation threshold occurred at approximately 30% of the rupture stress based on analyzing both the acoustic emission measurements and the stress–strain behavior. The crack damage threshold showed large variability and occurred at approximately 70% of the rupture stress.

Effect of a mechanical stimulation bioreactor on tissue engineered, scaffold‐free cartilage

Achieving sufficient functional properties prior to implantation remains a significant challenge for the development of tissue engineered cartilage. Many studies have shown chondrocytes respond well to various mechanical stimuli, resulting in the development of bioreactors capable of transmitting forces to articular cartilage in vitro. In this study, we describe the production of sizeable, tissue engineered cartilage using a novel scaffold-free approach, and determine the effect of perfusion and mechanical stimulation from a C9-x Cartigen bioreactor on the properties of the tissue engineered cartilage. We created sizable tissue engineered cartilage from porcine chondrocytes using a scaffold-free approach by centrifuging a high-density chondrocyte cell-suspension onto an agarose layer in a 50 mL tube. The gross and histological appearances, biochemical content, and mechanical properties of constructs cultured in the bioreactor for 4 weeks were compared to constructs cultured statically. Mechanical properties were determined from unconfined uniaxial compression tests. Constructs cultured in the bioreactor exhibited an increase in total GAG content, equilibrium compressive modulus, and dynamic modulus versus static constructs. Our study demonstrates the C9-x CartiGen bioreactor is able to enhance the biomechanical and biochemical properties of scaffold-free tissue engineered cartilage; however, no additional enhancement was seen between loaded and perfused groups.

Breakage of drop nucleated granules in a breakage only high shear mixer

Wet granule breakage is a significant mechanism, particularly in high shear mixer granulation. This paper presents a study of the wet breakage mechanism using a Breakage Only Granulator. Granules with varying powder and liquid binder properties were created using single drop nucleation. These granules were inserted in a Breakage Only Granulator, a high shear mixer granulator with non-granulating cohesive sand as the bulk medium. Two different impellers were used at impeller speeds of 500 and 750 rpm. An 11° beveled edge impeller was used to create both impact and shear in the granulator, and a flat plate impeller was used to minimize impact and maximize shear in the granulator. The fraction of granules which broke during the granulation process was used as a measure of granule breakage within the granulator. These results were compared with Stokes deformation numbers calculated using mean dynamic peak flow stresses measured in unconfined uni-axial compression tests. Results for the beveled edge impeller blade show increasing breakage with increasing Stokes deformation number. Significant breakage was observed at high Stokes deformation number. Increasing impeller speed increased the magnitude of breakage. The Stokes deformations number appears to be a reasonable predictor for granule breakage within the granulator. Results for the flat plate impeller show very little breakage at 500 rpm, and significant breakage for only one formulation at 750 rpm. This suggests that either impact is dominant over shear for breakage within the granulator, or that the two impeller designs give substantially different collision velocities in the granulator. The impeller speed, type and shape have a profound effect on granule breakage in high shear mixer granulators.

Eigendeformation-Based Homogenization of Concrete

Abstract : A two-scale approach based on eigendeformation-based homogenization is explored to predict the behavior of concrete targets subjected to impact loading by high speed projectiles. The method allows accounting for micromechanical features of concrete at a computational cost comparable to single scale phenomenological models of concrete. The inelastic behavior of concrete is modeled using three types of eigenstrains. The eigenstrains in the mortar phase include pore compaction (or lock-in), rate-dependent damage and plasticity eigenstrains, whereas the inelastic behavior of aggregates is assumed to be governed by plasticity only. Material parameters were identified using inverse methods against unconfined compression and uniaxial compression tests. A unit cell was constructed from a 3D digital image of concrete. The eigendeformation-based homogenization approach was validated for the projectile penetration into concrete target. The simulation results were found to be in reasonable agreement with the experimental data. Attention is restricted to non-reinforced concrete.

Experimental study on the rate dependent strength of ice‐silica mixture with silica volume fractions up to 0.63

We conducted deformation experiments of ice‐1 μm silica beads mixture to clarify the effects of silica beads volume fraction and temperature on the strength. The silica beads volume fraction was changed from 0 to 0.63 to simulate the surfaces of icy bodies. Unconfined uniaxial compression tests were made in a cold room at the temperatures from −10°C to −25°C and the constant strain rates ranged from 2.9 × 10−3 to 8.5 × 10−7 s−1. We determined the rate dependent strength of the mixture written by = A · σmaxn from the relationship between the maximum stress, σmax, on the stress‐strain curve and the applied strain rate, . At −10°C, the mixtures with silica volume fractions of 0.004–0.04 had almost the same strength with pure ice and the stress exponent, n, is about 3. On the other hands, at the silica volume fractions more than 0.15, the mixture became harder as the beads were more included, and it had the same stress exponent, about 6. This high stress exponent might be caused by crack generation. Also, we found that the A for silica volume fractions more than 0.15 was written by an exponential equation related to the silica volume fraction, ϕ, A = 6.86 × 10−8exp(−6.35ϕ). Furthermore, we found that the n of ϕ = 0.15 was independent of the temperature, and the brittle‐ductile boundary of ϕ = 0.29 and 0.63 was more than 30°C higher than that of pure ice.

Engineering Properties of Grouted Sands

A comparison of the behavior of uncemented and grouted sands is presented. Four sands ~Fontainebleau sand and three types of alluvial deposits of the Seine River! were tested. Specimens of grouted sands were prepared in the laboratory by injection of very fine cement or mineral grouts. An initial series of unconfined uniaxial compression tests and tensile tests was performed to highlight the effect of some key factors ~mainly the cement-to-water ratio of the grout and the relative density of the granular skeleton ! on the strength of the grouted sands. Subsequent triaxial tests showed that when a soil is impregnated by either a very fine cement grout or a mineral grout, both stiffness ~secant stiffness or small-strain stiffness ! and strength of the soil improve. Similar trends were observed for the behavior of both uncemented and grouted sands. The behavior of grouted sands can be roughly reproduced by applying a linear elastic, perfectly plastic model with a nonassociated Mohr-Coulomb yield criterion whose parameters can be easily determined. Finally, preliminary recommen- dations are proposed relative to improvements ratios of the parameters of this simple constitutive model that is still commonly used in geotechnical engineering.

Estimation of the Compressive Strength and Modulus of Elasticity of Cement-Treated Aggregate Base Materials

Experimental study on the development of strength and modulus of elasticity of cement-treated aggregate base (CTAB) materials was undertaken. Unconfined uniaxial compression tests were conducted with 189 samples for 16 CTAB mixtures at different ages. Two different aggregates, conventional crushed limestone base and recycled concrete materials, were used in the test program. Using the test results, equations were proposed to estimate the development of compressive strength and modulus of elasticity of CTAB materials with time. Test results indicated that the relationship between the compressive strength and elastic modulus of CTAB materials could be expressed in a single equation regardless of aggregate type and mixture proportions.