Increase In Volumetric Strain Research Articles

Transient temperature and water distributions in compacted MX80 bentonite under high temperature gradients

Numerous element-scale and full-scale tests have been carried out on compacted bentonite to investigate its coupled thermo-hydro-mechanical (THM) behavior when used as a buffer material in high-level nuclear waste geological repositories, such as the Full-scale Engineered Barriers Experiment (FEBEX) and its forerunner laboratory mock-up test [5]. In these past studies the applied temperatures were below 100°C, while new repository conditions being explored in the recently started HotBENT project at the Grimsel Test Site involve waste canister temperatures up to 200°C [2]. Recently, Lu and McCartney [3] conducted a tank scale test where a temperature of 200°C was maintained by a cylindrical heating element having a diameter of 12.5 mm at the center of a compacted MX80 bentonite layer. A notable temperate drop in temperature was observed with distance from the small diameter heating element. A similar phenomenon was also reported by Gens et al. (2020) for heating tests on MX80 pellets during heating to 140°C. Measurements from the FEBEX project indicate a more distributed temperature decay away from the heater [3]. This discrepancy between laboratory and field tests provided the motivation for better understanding the temperature and water distribution near a heater under high temperature gradients to better guide the design of laboratory-scale experiments in the future.
 A one-dimensional heating test was performed in this study to investigate the coupled THM response of a MX80 bentonite layer in unconstrained conditions during basal heating. Measurements include the transient temperature and volumetric water content redistribution at different vertical distances from the flat plate heater under high-temperature gradients as well as global volumetric strains. Three dielectric sensors (Decagon 5TM) were placed into a compacted MX80 layer having an initial gravimetric water content of 12.2% and a dry density of 1.15 Mg/m3 within a PVC modified Proctor mold (inner diameter of 152 mm and height of 178 mm), as shown in Figure 1(a). Low-density polyethylene sheeting was stretched across the top and bottom of the soil layers to minimize any global loss of water from the compacted bentonite layer during heating. A temperature-controlled hot plate was used as the basal heat source, and a dial indicator was mounted on the top of the soil layer to measure volume changes. Fiberglass insulation was wrapped around the cell to minimize lateral heat loss from the side boundary. A basal temperature of 128°C was maintained during a heating stage lasting 4000 h to ensure THM equilibrium, after which three short-term (96 h for each) cooling-heating cycles were applied. Results indicate that soil temperatures increase rapidly in the first 20 h of heating then gradually stabilize (Figure 1(b)). The volumetric water content increases at the beginning of heating then slowly decreases (Figure 1(c)). This increasing-decreasing trend is due to the upward movement of a wetting front away from the heater due to coupled heat transfer and water flow. An initial increase in volumetric strain (swelling) was followed by a decrease (contraction) as heating progressed (Figure 1(d)). This is consistent with the observed thermal response of normally consolidated expansive soils (e.g., [4]). A sharp monotonic decrease in volumetric strain occurred during the three short-term cooling-heating cycles. The temperature and water content follow nearly identical trajectories in each cycle. The temperature distribution in the bentonite with distance from the heating element after 4000 h of heating is smoother than the sharp drop-off in temperature from the small-diameter heater observed by Lu and McCartney [3], as shown in (Figure 1(f)). The main implication of the results presented in this study is that the surface area of the heater in contact with the surrounding bentonite plays a major role in the temperature distribution and the associated coupled water flow. This is a critical finding that may help guide the geometric design of small-scale laboratory mock-up experiments used to study THM processes in controlled laboratory conditions.

Acoustic emission and computed tomography investigation on fatigue failure of fissure‐contained hollow‐cylinder granite: Cavity diameter effect

AbstractThis work aims to investigate the influence of cavity diameter on fracture behaviors for fissure‐contained hollow‐cylinder granite subjected to multistage cyclic loads, using acoustic emission detection, and post‐test computed tomography (CT) scanning. The influence of the cavity diameter on rock fracture was revealed in terms of volumetric deformation, stiffness degradation, AE output activities, hole spalling, and crack coalescence pattern. The testing results show the both the volumetric strain and AE b value decrease with increasing cavity diameter. An increase of volumetric strain and a decrease of AE b value are found during rock failure and their relationship was established. In addition, damage is quantificationally characterized by the released energy, and a damage evolution model is defined to characterize rock damage process. Moreover, post‐test CT visualization highlights the communication between the hole and fissures at the rock bridge segment, and it is suggested that failure severity decreases with the increase of hole diameter.

Deformation behavior and damage-induced permeability evolution of sandy mudstone under triaxial stress

The mechanic and permeability behavior in sandy mudstone is crucial to hazard prevention and safety mining. In this study, to investigate the evolution and characteristic of permeability and mechanical properties of sandy mudstone subjected to loading of in-site stress, a series of triaxial compression–seepage experiments are performed. The increase in permeability and decrease in mechanical strength gradually transformed to the decrease in permeability and increase in mechanical strength responding to the increase in confining stress from 5 to 15 MPa, which corresponds to the transformation from brittleness to ductility in sandy mudstone, and the transformation threshold of 10 MPa confining stress was determined. The penetration shear fracture generated at brittle regime, while plastic flow behavior presented at semibrittle and ductile state. The critical value of the yielding stage in axial strain increases with the increase in confining stress. The relatively higher permeability corresponds to the higher pore pressure during the increase in confining stress. The increase in confining stress promoted the increase in volumetric strain, while increased pore pressure reduced the volumetric strain, and the lower permeability occurred at higher volumetric strain. In addition, an improved permeability model was developed to describe the loading-based permeability behavior considering the Klinkenberg effect.

Consolidated drained behaviour of PVA fibre reinforced cemented Toyoura Sand

ABSTRACT In this study, a series of consolidated drained (CID) compression tests were performed On unreinforced and reinforced cemented Toyoura sand specimens. Specimens were cured for 3, 7, 14, 28, 56 days, with cement contents of 0–3% (dry weight), and different Polyvinyl alcohol (PVA) fibre contents of 0, 0.5, 1, 2, and 3% (weight ratio). Samples were prepared to a target dry density value (e.g. = 1.489 g/cm3) of Toyoura sand using under-compaction moist tamping technique with 10% water content by dry weight of soil. Triaxial compression tests (CID) were carried out at a confining pressure of 100 kPa. The monotonic stress–strain, stiffness, volumetric change behaviour, dilatancy angle, peak strength, and residual strength and brittleness index of the samples were investigated in greater detail. Results show that peak strength increases due to increase in curing duration by approximately 50–700%, and 0–300% in residual strength at 30% axial strain, compared to Toyoura sand. For reinforced cemented specimens there is an increase in volumetric strain with increasing curing duration or in other words, the rate of dilation increases with the increase in curing period. The dilatancy behaviour increases due to the addition of fibres and cement, this dilative behaviour has been attributed to cemented particles forming highly interlocked clusters. Brittleness of the cemented specimens reinforced with 0.5%-1% fibres have been supressed significantly compared to the higher concentrations of fibre (2–3%). Hence, it shows that there might be a threshold fibre concentration range (0.5%-1%), where fibres are considered to be the most effective and beyond this concentration, fibre causes further adverse effects.

STUDY OF THE RADIATION SITUATION IN MOSCOW BY INVESTIGATING ELASTOPLASTIC BODIES IN A NEUTRON FLUX TAKING INTO ACCOUNT THERMAL EFFECTS

This article discusses the study of the background radiation level in Moscow and its impact on the environment. To identify the radiological impact on living conditions, a study of the effects of fast neutrons on infrastructure and biological objects was made. Large-scale research organizations carrying out technological development, scientific and materials research using nuclear materials in the city of Moscow and the region are considered. A system of environment radiation monitoring, covering an area of more than 1091 km², was examined in detail. A radioecological survey conducted in the Troitsky and Novomoskovsky administrative districts was analyzed to identify and describe the radiation situation. An analysis of background radiation in Moscow shows that the values of the controlled parameters of radioactive pollution of environmental objects are within the long-term fluctuations of the technogenic background of the capital. In this case, various thermal effects arise caused by radiation exposure to solids. The study of infrastructure and biological objects is carried out based on the determination of effects, because of which the elastic and mainly plastic characteristics of substances exposed to neutron irradiation and thermal effects change. Here, the main essential factors are radiation hardening of the material (increase in yield stress) and radiation swelling (increase in volumetric strain).

Application and Analysis of Measurement Model for Calibrating Spatial Shear Surface in Triaxial Test

Discrete element method has great advantages in simulating the contacts, fractures, large displacement and deformation between particles. In order to analyze the spatial distribution of the shear surface in the three-dimensional triaxial test, a measurement model is inserted in the numerical triaxial model which is generated by weighted average assembling method. Due to the non-visibility of internal shear surface in laboratory, it is largely insufficient to judge the trend of internal shear surface only based on the superficial cracks of sheared sample, therefore, the measurement model is introduced. The trend of the internal shear zone is analyzed according to the variations of porosity, coordination number and volumetric strain in each layer. It shows that as a case study on confining stress of 0.8 MPa, the spatial shear surface is calibrated with the results of the rotated particle distribution and the theoretical value with the specific characteristics of the increase of porosity, the decrease of coordination number, and the increase of volumetric strain, which represents the measurement model used in three-dimensional model is applicable.

Triaxial experiments on adsorption deformation and permeability of different sorbing gases in anthracite coal

The gas ad-/de-sorption induced deformation of coal is one of the key parameters that influences the coal permeability and the long-term coalbed methane (CBM) production. Three coal samples were collected from No.3 coal seam of Permian Shanxi Formation in the southern Qinshui Basin at which is the most productive CBM field in China. Triaxial experiments of CO2, CH4 and N2 adsorption induced deformations in coal were carried out by a laboratory desorption-seepage testing system. The law of adsorption induced deformations of different gases and their influences on the coal permeability were comprehensively analyzed. For anthracite, the experimental results indicate that axial, radial and volumetric sorption induced strains increase as increasing of adsorption equilibrium pressure if the core was kept under a constant effective stress defined as the difference between the average external stress and pore pressure. And the strain perpendicular to the bedding plane is higher than that parallel to the bedding plane; the axial strain, radial strain and volumetric strain induced by CO2 adsorption is higher than that by CH4 and N2. At a constant effective stress, the permeability of N2 in coal is higher than that of CH4 and CO2, with the increase of adsorption equilibrium pressure, the adsorption swelling strain of coal matrix increases. With the increase of volumetric strain, the permeability declines by a negative exponential function during the adsorption process; the behaviors of adsorption deformation and adsorption capacity of different gases in coals follow similar trends.

Evolution of irradiation-induced strain in an equiatomic NiFe alloy

The evolution of irradiation-induced atomic strain in an equiatomic NiFe concentrated solid-solution alloy is investigated using both atomistic simulations and x-ray diffraction analysis. The irradiations are performed using 1.5MeV Ni ions to fluences ranging from 1×1013 to 1×1014cm−2. The irradiation simulations are carried out by overlapping 5keV Ni recoils cascades up to a total of 300 recoils. An increase of volumetric strain is observed at low dose, which is associated with production of point defects and small clusters. Relaxation of strain occurs at higher doses, when large defect clusters, e.g., dislocation loops, dominate.

Effect of Stress Triaxiality on the Flow and Fracture of Mg Alloy AZ31

The microscopic damage mechanisms operating in a hot-rolled magnesium alloy AZ31B are investigated under both uniaxial and controlled triaxial loadings. Their connection to macroscopic fracture strains and fracture mode (normal vs shear) is elucidated using postmortem fractography, interrupted tests, and microscopic analysis. The fracture locus (strain-to-failure vs stress triaxiality) exhibits a maximum at moderate triaxiality, and the strain-to-failure is found to be greater in notched specimens than in initially smooth ones. A transition from twinning-induced fracture under uniaxial loading to microvoid coalescence fracture under triaxial loading is evidenced. It is argued that this transition accounts in part for the observed greater ductility in notched bars. The evolution of plastic anisotropy with stress triaxiality is also investigated. It is inferred that anisotropic plasticity at a macroscopic scale suffices to account for the observed transition in the fracture mode from flat (triaxial loading) to shear-like (uniaxial loading). Damage is found to initiate at second-phase particles and deformation twins. Fracture surfaces of broken specimens exhibit granular morphology, coarse splits, twin-sized crack traces, as well as shallow and deep dimples, in proportions that depend on the overall stress triaxiality and fracture mode. An important finding is that AZ31B has a greater tolerance to ductile damage accumulation than has been believed thus far, based on the fracture behavior in uniaxial specimens. Another finding, common to both tension and compression, is the increase in volumetric strain, the microscopic origins of which remain to be elucidated.

Postcyclic behavior of low-plasticity silt with limited excess pore pressures

This paper investigates the postcyclic behavior of low-plasticity silt with excess pore pressure ratio (Ru) less than 1. The testing specimens were prepared from Mississippi River Valley (MRV) silt. Full and no reconsolidation were allowed after specimens were subjected to various excess pore pressure ratios due to cyclic loading in a cyclic triaxial cell, and then monotonic shear tests were conducted. The effect of the Ru on shear strength and stiffness at small and large deformation was investigated. It was found that a Ru greater than 0.70 is a prerequisite of large increase in volumetric strain and undrained shear strength for specimens with full reconsolidation. In contrast, a significant decrease in yield shear strength and initial stiffness was noted for specimens without reconsolidation. In comparison to published data for sands, the silt experienced significant volumetric strain due to reconsolidation at lower Ru, indicating that the specimen fabric was modified or strained at lower Ru.

In vivo measurement of volumetric strain in the human brain induced by arterial pulsation and harmonic waves

Motion-sensitive phase contrast magnetic resonance imaging and magnetic resonance elastography are applied for the measurement of volumetric strain and tissue compressibility in human brain. Volumetric strain calculated by the divergence operator using a biphasic effective-medium model is related to dilatation and compression of fluid spaces during harmonic stimulation of the head or during intracranial passage of the arterial pulse wave. In six volunteers, phase contrast magnetic resonance imaging showed that the central cerebrum expands at arterial pulse wave to strain values of (2.8 ± 1.9)·10(-4). The evolution of volumetric strain agrees well with the magnitude of the harmonic divergence measured in eight volunteers by magnetic resonance elastography using external activation of 25 Hz vibration frequency. Intracranial volumetric strain was proven sensitive to venous pressure altered by abdominal muscle contraction. In eight volunteers, an increase in volumetric strain due to abdominal muscle contraction of approximately 45% was observed (P = 0.0001). The corresponding compression modulus in the range of 9.5-13.5 kPa demonstrated that the compressibility of brain tissue at 25 Hz stimulation is much higher than that of water. This pilot study provides the background for compression-sensitive magnetic resonance imaging with or without external head stimulation. Volumetric strain may be sensitive to fluid flow abnormalities or pressure imbalances between vasculature and parenchyma as seen in hydrocephalus.

Three-dimensional large-strain tensile deformation of neat and calcium carbonate-filled high-density polyethylene

The large-strain tensile deformation of high-density polyethylene and high-density polyethylene filled with two volume fractions ( f) of calcium carbonate particles is studied via an optical method. Digital image correlation is used to determine the local displacement gradients and full-field displacements during uniaxial tension tests on specimens of rectangular cross-section. A novel technique measures simultaneously, with a single camera, the deformation in all three dimensions. Full-field strain contours, macroscopic true stress-true strain behavior, and local volumetric strains are reduced from the raw test data. The true stress-true strain data shows an increase in modulus, but a decrease in yield stress and subsequent strain hardening, with increasing f. These results are strong evidence of particle debonding and are corroborated by an increase in volumetric strain with increasing f.

Micromechanical Modelling of Granular Materials: Effect of Particle Size and Gradation

In this paper the effects of maximum particle size, particle gradation/sorting and fabric on bulk mechanical behaviour of granular materials such as coarse grained soils and rockfills are investigated" from micromechanical considerations starting from the grain scale level, using numerical" simulations based on Discrete Element Modelling (DEM). Hydrostatic compaction and biaxial tests on 2-dimensional assemblies of discs with varying particle sizes and gradations were modelled using DEM. An examination of the constitutive behaviour of granular media considering" the particulate nature of the medium has been attempted to explain the effect of particle size and gradation. Simulation results on perfectly parallel graded assemblies indicate that with increase in the size of the particles, a marginal increase (or no increase) in the angle of internal friction is observed during biaxial loading conditions. A change to a wider gradation (keeping the minimum grain size the same) results in a decrease in the angle of internal friction and an increase in volumetric strain to a considerable extent. Based on micromechanical force and fabric parameters, the basis for the physical behaviour was established. This helps in understanding the physics of parallel gradation techniques.

Creep rupture of a linear polyethylene: 1. Rupture and pre-rupture phenomena

The creep rupture behaviour of a high-density polyethylene was studied for material prepared at two different cooling rates. The rupture phenomena were dependent upon the morphology. The slow-cooled specimens had a degenerated spherulitic structure and all underwent brittle rupture. The fast-cooled specimens had a banded spherulitic structure and exhibited either ductile or brittle rupture depending upon the applied stress. Ductile creep rupture was associated with a large tertiary creep strain, with macro-necking and a stress whitening which corresponded to an increase in volumetric strain. Brittle creep rupture was associated with an absence of macro-necking, and either an absence or a small amount of tertiary creep. Stress whitening corresponding to a decrease in volumetric strain was a feature of both forms of failure above a threshold value of stress.