Unloading Cycles Research Articles

Experimental Research into the Evolution of Permeability in a Broken Coal Mass under Cyclic Loading and Unloading Conditions

The permeability characteristics of a broken coal mass under repeated loading and unloading conditions exert significance on spontaneous combustion of coal in goaf during the mining of coal seam groups. Considering this, by using the seepage test system for broken coal-rock mass, seepage tests under cyclic loading and unloading conditions, were carried out on broken coal masses. The test results show that the fitting curves between permeability and effective stress, strain and porosity are a logarithmic function, cubic function and power function, respectively. Besides, the permeability of a broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law. With increased cyclic loading and unloading times, the permeability loss, stress sensitivity and the crushing amount of the broken coal sample were gradually reduced, but the particle size gradation of the broken coal sample gradually became better. During one loading and unloading cycle, the stress sensitivity of the permeability of coal samples in the loading stage was far higher than that in the unloading stage. In the loading stage, the re-arrangement, breakage and compressive deformation of coal particles can lead to a reduction in porosity, consequently resulting in a decreased permeability. In the unloading stage, only the permeability reduction of coal samples due to particle deformation can be recovered.

Experimental study of permeability change of organic-rich gas shales under high effective stress

Shale permeability and its variation under high stress are vital for gas production from deep shale gas reservoirs. Most experiments of stress-dependent permeability for organic-rich shale were conducted under lower stress less than 40 MPa, therefore, shale permeability evolution under high stress is not clear. In this work, the effects of high stress on the permeability and fracture compressibility of shales were investigated experimentally. Moreover, the impact of stress cycling on permeability were also studied. Four shale samples including two intact samples and two fractured samples from Cambrian Niutitang Shale formation and Silurian Longmaxi Shale formation were used. Permeability was measured using Helium under different stress conditions, including different confining pressure, different gas pressure, and constant effective stress. The highest effective stress and gas pressure in this work was 59.5 MPa and 10 MPa, respectively. Fracture compressibilities were calculated using the stress-dependent permeability data. The results show that the permeability of the intact samples and fractured samples decreased by one order of magnitude and three orders of magnitude, respectively, with the effective stress changing from 1.5 MPa to 59.5 MPa. The shale permeability results show a two-stage characteristic and nonlinearly decreasing trend with the increase of effective stress, demonstrating that the fracture compressibility is stress dependent and decreases with stress. The permeability hysteresis occurs between the loading and unloading cycles due to the inelastic compression of the pore. The modelling results also show that the Klinkenberg constant show a positive correlation with effective stress, as effective stress reduces the fracture opening and absolute permeability.

Characteristics of Deformation and Damping of Cement Treated and Expanded Polystyrene Mixed Lightweight Subgrade Fill under Cyclic Load

To investigate the deformation and damping characteristics of cement treated and expanded polystyrene (EPS) beads mixed lightweight soils, this study conducted a series of triaxial shear tests cyclic loading for different confining pressures, cement contents, and soil categories. Through repeated loading and unloading cycles, axial accumulative strain, resilient modulus, and damping ratio versus axial total strain were analyzed and the mechanical behavior was revealed and interpreted. Results show that the resilient modulus increases with increasing confining pressure and cement content. A decreasing power function can be used to fit the relationship between the resilient modulus and the axial total strain. Although sandy lightweight specimens usually own higher resilient modulus than silty clay lightweight specimens do, the opposite was also found when the axial total strain is larger than 8% with 50 kPa confining pressure and 14% cement content. For damping ratio the EPS beads mixed lightweight soil yields a weak growth trend with increasing axial total strain and a small reduction with higher confining pressure and cement content. For more cementations, the damping ratio of the sandy lightweight soil is always smaller than the silty clay lightweight soil. Nonetheless, the differences of damping ratios that were obtained under all of the test conditions are not significant.

Constitutive Modeling of Smart Adaptive Composite with inactive SMP

Abstract Quite amazingly, materials of a certain class popularly named as smart materials are capable of responding to an external stimulus. Shape memory effect is the property of some of these material to gain back its original shape when activated with suitable stimulus. Shape memory alloys (SMAs) are metals, which exhibit two very unique properties, pseudo-elasticity, and the shape memory effect. The polymers which exhibit shape memory effect are called shape memory polymers. A shape memory composite (SMC) can be created by combining two or more materials involving at least one shape memory material. The presented work aims at the development of a mathematical constitutive model for a SMC of SMA wires in SMP matrix. The mathematical model has been developed using the rule of mixtures for composites and the material property curves have been generated for the SMC for various thermo mechanical loading and unloading cycles using Matlab.

Energy Evolution and Mechanical Features of Granite Subjected to Triaxial Loading‐Unloading Cycles

Energy evolution varies during the whole process of rock deformation, and mechanical parameters are markedly altered under cyclic loading and unloading. In order to investigate the effects of confining pressure on energy evolution and mechanical parameters, cyclic loading and unloading experiments were performed for granite under six different confining pressures. The experiment revealed the confining pressure effect on variation and allocation pattern of energy and mechanical characteristics. Four characteristic energy parameters, namely, storage energy rock, storage energy limit, energy storage ratio, and energy dissipation ratio, were proposed to describe energy storage and dissipation properties of rock. Elastic modulus and dissipation ratio presented a downward “U” and “U”‐shaped trends, respectively, with loading and unloading cycles, while Poisson’s ratio increased linearly at the same time. Elastic energy was accumulated mainly before peak stress, while the energy dissipation and release were dominant after the peak strength. As the confining pressure increased, efficiency of energy accumulation and storage limit improved. An exponential function was proposed to express the relationship between the energy storage limit and confining pressure. Dissipation energy increased nonlinearly with the strain, and the volume dilatancy point defined the turning point from a relatively slow growth to an accelerated growth of dissipation energy. The dilatancy point can be used as an important indication for the rapid development of dissipation energy.

Full‐Scale Experimental Investigation of the Static and Dynamic Stiffness of Prestressed Concrete Girders

Cracking damage influences the stiffness of the girders. Many articles in the literatures have studied the development of stiffness of the scale‐down model; however, full‐scale model testing cannot be completely replaced by scale‐down testing because of material component characteristics and boundary effects. This paper deals with the effects of cracking damage on the structural static and dynamic stiffness based on three prestressed concrete (PC) girders which were removed from an old bridge. First, the equivalent flexural rigidity of cracked prestressed concrete girder was assessed using the measured load‐deflection response under cycles of loading and unloading. Then, after unloading, the frequencies were measured on the PC girders supported by the elastomeric bearings. Next, the development of frequency under different damage was studied, and finally, the dynamic stiffness of PC girders with cracks was assessed. The results indicate that the first frequency is more sensitive to the cracking of concrete compared with the second frequency and that the mode shapes are not sensitive to girder damage. The test girders cannot be simplified as an ideal simply supported beam for the purpose of identifying frequencies. In addition, the “final” (the end of the ultimate load case) equivalent flexural rigidity of the girders is 30% of the “initial” (the beginning of the first load case) equivalent flexural rigidity, compared with 50% in the scale‐down test; and the final dynamic stiffness is approximately 84% of the initial dynamic stiffness, whereas the scale‐down test is 72%.

Evaluation of the coefficient of earth pressure at rest (K0) of a saturated-unsaturated colluvium soil

To predict the stress-strain behaviour of soils under loading it is relevant the knowledge of its natural stress state, expressed by the coefficient of earth pressure at rest (K0). There are correlations in theliterature for K0determination that comes from researches developed considering sedimentary soils,typically from temperate or cold regions. In dealing with residual and colluvium soils, typical of tropical regions, it is not appropriate to use these correlations, since K0is affected by factors such as degree of weathering, laterization processes and suction, among others that also affect saturated sedimentary soils. This work analysed samples of a colluvium soil from a natural slope at the Pontifical Catholic University of Rio de Janeiro. For the determination of the coefficient of earth pressure at rest of the unsaturated colluvium soil, a flexible wall equipment, with a system of suction control was used. The influence of the net stress level on the value of the coefficient of earth pressure at rest was investigated under a constant suction of 10 kPa, following a loading and unloading cycle of applied vertical stress. The coefficient of earth pressure at rest of the saturated colluvium (null suction) was also determined using a stress-path, Bishop and Wesley type, servo-controlled triaxial equipment. The obtained results are presented and discussed.

Hydraulic properties and energy dissipation of deep hard rock under H-M coupling and cycling loads

The permeability of deep rock is closely related to the stability and safety of underground engineering. The rocks in deep stratum are mostly with high stress and high osmotic pressure. Therefore, it is necessary to consider the coupling effect between porewater pressure and in situ stress on rock mass. A series of triaxial cyclic loading and unloading experiments under hydraulic-mechanics coupling conditions are carried out to studied the mechanical and hydraulic properties of granite in the depth of 1300 m to 1500 m. Especially, the effect of the disturbance on the permeability of fractured rocks are investigated by unloaded the confining pressure. Tests results presented that the stress-strain curves of deep granite showed typical brittle characteristics. The principal stress of granite exhibited a linear relationship under the high confining pressure of 34-40 MPa and high osmotic pressure of 13-15 MPa. Dissipated energy of the rock decreased to a relatively low level after 2-3 loading cycles and then slowly increased. Permeability showed a decreasing trend as the loading and unloading cycles increase. Finally, acoustic emission technology was used to monitor the fracture evolution in rocks, the acoustic emission signal released as the fractures develop and energy dissipated. The results would provide basic data for the exploitation and excavation in the deep galleries.

Effects of Hydrostatic Pressure on the Surface Plasmon Resonance of Gold Nanocrystals.

The surface plasmon resonances of gold nanospheres and nanorods have been measured as a function of hydrostatic pressure up to 17 GPa in methanol-ethanol 4:1 solvent and up to 10 GPa in paraffin. Both the sphere resonance and the longitudinal rod resonance exhibit redshifts, whereas the transverse rod mode shows an extremely weak redshift or blueshift depending on the nanorod aspect ratio. Solidification of the solvent around 11 GPa causes some aggregation of the particles, readily identified through broadening of the surface plasmon band and further redshifting. Spectra collected during loading and unloading cycles exhibit only minimal hysteresis if the pressure remains below 11 GPa. The surface plasmon shifts are the result of two competing effects. Compression of the conduction electrons in the metals increases the bulk plasma frequency, which causes a blueshift. However, the increase in the solvent density under hydrostatic load leads to an increase in the solvent refractive index, which in turn leads to a redshift. We find that after accounting for the solvent contribution, we can spectroscopically determine the bulk modulus of the gold nanoparticles with a precision of 10%. The value obtained of K0 = 190 GPa is significantly higher than the value for bulk gold (167 GPa). Furthermore, we show that pressure-induced solidification causes a significant broadening and anomalous shift of the surface plasmon band that we attribute to aggregation and nanorod deformation.

Despacho Econômico de energia elétrica em Micro Redes usando o Software GAMS

Ao longo do tempo o mercado elétrico vem crescendo com a inserção de diversas fontes, além de dispositivos de armazenamento, diante desse panorama, nasce a questão de como despachar a energia gerada para atender à demanda de forma mais econômica e eficaz. Este trabalho apresenta a implementação de um modelo matemático de programação não-linear inteira mista para o problema do Fluxo de Potência Ótimo de Corrente Alternada (ACOPF) usando o software GAMS com o objetivo de resolver o problema de alocação e operação ótima de dispositivos de armazenamento de energia em micro redes, bem como o ciclo ótimo de carga e descarga dos dispositivos de maneira a minimizar o custo de compra de energia. O modelo observa as restrições da rede elétrica, como por exemplo, a manutenção das tensões dentro de limites preestabelecidos, bem como a intermitência de fontes renováveis baseadas em energia eólica e solar, representadas por despachos variáveis durante a operação. Para demonstrar a exatidão do modelo matemático desenvolvido e também a eficiência das técnicas de solução é utilizado um sistema teste de 5 barras.

Effects of mechanical stretching, desorption and isotope exchange on deuterated eucalypt wood studied by near infrared spectroscopy.

Deuterium exchange combined with near infrared (NIR) spectroscopy was used to study the roles of accessible and inaccessible cellulose in the load transfer of eucalyptus wood. Monitoring the drying process helped to assign NIR bands of deuterated wood samples. Polarized NIR spectra of protonated and deuterated samples confirmed that inaccessible hydroxyl groups in eucalyptus wood were preferably oriented in the longitudinal direction. The spectral changes on NIR spectra caused by mechanical strain could be highlighted by averaging loading and unloading cycles to compensate for effects of desorption and isotope re-exchange due to environmental fluctuations. After deuteration, the bands affected by mechanical strain at around 6420, 6240 and 4670 cm-1, which had been assigned to hydroxyl groups in cellulose, remained at these positions, suggesting the inaccessible cellulose fraction was the main load-bearing component in wood. A small band at around 4700 cm-1 responding to mechanical strain, becoming visible in the deuterated spectra, indicated that accessible hydroxyls also contributed to the load transfer. Furthermore, the measurements confirmed previous reports of moisture adsorption of wood under tensile stress.

The "independent breath" algorithm: assessment of oxygen uptake during exercise.

Reduction of noise of breath-by-breath gas-exchange data is crucial to improve measurements. A recently described algorithm ("independent breath"), that neglects the contiguity in time of breaths, was tested. Oxygen, carbon dioxide fractions, and ventilatory flow were recorded continuously over 26min in 20 healthy volunteers at rest, during unloaded and moderate intensity cycling and subsequent recovery; oxygen uptake ([Formula: see text]) was calculated with the "independent breath" algorithm (IND) and, for comparison, with three other "classical" algorithms. Average [Formula: see text] and standard deviations were calculated for steady-state conditions; non-linear regression was run throughout the [Formula: see text] data of the transient phases (ON and OFF), using a mono-exponential function. Comparisons of the different algorithms showed that they yielded similar average [Formula: see text] at steady state (p = NS). The standard deviations were significantly lower for IND (post hoc contrasts, p < 0.001), with the slope of the relationship with the corresponding data obtained from "classical" algorithms being < 0.69. For both transients, the overall kinetics (evaluated as time delay + time constant) was significantly faster for IND (post hoc contrasts, p < 0.001). For the ON transient, the asymptotic standard errors of the kinetic parameters were significantly lower for IND, with the slope of the regression line with the corresponding values obtained from the "classical" algorithms being < 0.60. The "independent breath" algorithm provided consistent average O2 uptake values while reducing the overall noise of about 30%, which might result in the halving of the required number of repeated trials needed to assess the kinetic parameters of the ON transient.

Sedimentary loading–unloading cycles and faulting in intermontane basins: Insights from numerical modeling and field observations in the NW Argentine Andes

The removal, redistribution, and transient storage of sediments in tectonically active mountain belts is thought to exert a first-order control on shallow crustal stresses, fault activity, and hence on the spatiotemporal pattern of regional deformation processes. Accordingly, sediment loading and unloading cycles in intermontane sedimentary basins may inhibit or promote intrabasinal faulting, respectively, but unambiguous evidence for this potential link has been elusive so far. Here we combine 2D numerical experiments that simulate contractional deformation in a broken-foreland setting (i.e., a foreland where shortening is diachronously absorbed by spatially disparate, reverse faults uplifting basement blocks) with field data from intermontane basins in the NW Argentine Andes. Our modeling results suggest that thicker sedimentary fills (>0.7–1.0 km) may suppress basinal faulting processes, while thinner fills (<0.7 km) tend to delay faulting. Conversely, the removal of sedimentary loads via fluvial incision and basin excavation promotes renewed intrabasinal faulting. These results help to better understand the tectono-sedimentary history of intermontane basins that straddle the eastern border of the Andean Plateau in northwestern Argentina. For example, the Santa María and the Humahuaca basins record intrabasinal deformation during or after sediment unloading, while the Quebrada del Toro Basin reflects the suppression of intrabasinal faulting due to loading by coarse conglomerates. We conclude that sedimentary loading and unloading cycles may exert a fundamental control on spatiotemporal deformation patterns in intermontane basins of tectonically active broken forelands.

Large electrostrain and structural evolution in (1-x)[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-xAgNbO3 ceramics

A lead-free system formulated as (1-x)[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-xAgNbO3 exhibits an electrostrain of 0.50% at 80 kV/cm and a maximum d33* of 721 pm/V at 60 kV/cm for x=0.3. The incorporation of AgNbO3 shifts the relaxor-ferroelectric phase transformation temperature (TF-R) to room temperature and lowers the energy barrier of the field-induced phase transformation. Furthermore, the in-situ electric field dependent high-energy synchrotron X-ray diffraction (SXRD) technique reveals that the sample x=0.03 transforms from dominant P4bm phase to a phase mixture of R3c + P4mm at 55 kV/cm during the electric field loading, and returns to initial dominant P4bm phase at 15 kV/cm during the unloading cycle of the electric field. Furthermore, it can be demonstrated that the electric field induced-phase transition in NBTBT-3AN occurs in the whole sample, rather than in the single direction of electric field. Therefore, the electrocstrain in NBTBT-3AN is more uniform, which would be beneficial to its actuator applications.

Modal Analysis of Multi Purpose Hydraulic Die Spotting Press

Multi purpose hydraulic die spotting presses can be used for both die spotting and mass production. Due to the reducing cycle time of hydraulic presses in mass manufacturing, dynamic analysis can be used to determine the oscillations caused by the structural design of the hydraulic press. Oscillations are generated during loading and unloading cycle of press operation and can affect product quality, energy consumption, product life of press etc. Resonance which is related to natural frequency and damping ratio, must be avoided in order to increase the quality of press. There must be a necessary interval between operation (cycle) frequency and the natural frequency of the hydraulic press. Thus, the resonance can be prevented. In this work, the natural frequencies of the 2000 tone multi purpose hydraulic die spotting press is firstly calculated by modal analysis by using finite element model (FEM) and secondly measured during operation.

"Work-to-Work" exercise slows pulmonary oxygen uptake kinetics, decreases critical power, and increases W' during supine cycling.

We have previously demonstrated that the phase II time constant of pulmonary oxygen uptake kinetics (τv˙o2) is an independent determinant of critical power (CP) when O2 availability is not limiting, that is, during upright cycle exercise in young, healthy individuals. Whether this causative relationship remains when O2 availability is impaired remains unknown. During supine exercise, which causes an O2 availability limitation during the exercise transition, we therefore determined the impact of a raised baseline work rate on τv˙o2 and CP. CP, τv˙o2, and muscle oxygenation status (the latter via near‐infrared spectroscopy) were determined via four severe‐intensity constant‐power exercise tests completed in two conditions: (1) with exercise initiated from an unloaded cycling baseline (U→S), and (2) with exercise initiated from a moderate‐intensity baseline work rate of 90% of the gas exchange threshold (M→S). In M→S, critical power was lower (U→S = 146 ± 39 W vs. M→S = 132 ± 33 W, P = 0.023) and τv˙o2 was greater (U→S = 45 ± 16 sec, vs. M→S = 69 ± 129 sec, P = 0.001) when compared to U→S. There was no difference in tissue oxyhemoglobin concentration ([HbO2 + MbO2]) at baseline or during exercise. The concomitant increase in τv˙o2 and reduction in CP during M→S compared to U→S shows for the first time that τv˙o2 is an independent determinant of CP in conditions where O2 availability is limiting.

Adjustable buttons for ACL graft cortical fixation partially fail with cyclic loading and unloading.

Despite good initial pullout strength, it is unclear whether adjustable button (AB) devices for anterior cruciate ligament (ACL) soft-tissue graft fixation, which are based on the Chinese finger trap technique, resist cyclic loading. Furthermore, they have never been tested in a cyclic protocol including complete unloading. It was hypothesized, that the displacement of AB devices with the Chinese finger trap technique would be greater than that of continuous suture loop devices and other available AB mechanisms in a cyclic loading with complete unloading protocol. ACL reconstruction was performed in a porcine knee model using three different types of cortical fixation devices: two different AB devices that use the Chinese finger trap design, one AB device that uses a locked suture loop mechanism and two different continuous loop devices as control groups (n = 40). Specimens were mounted in a material-testing machine (Instron Inc.) that permitted 2500 loading and complete unloading cycles to a maximum of 250N, as well as continuous elongation recording. A one-way ANOVA was performed for statistical analysis. The displacement of ABs with a Chinese finger trap loop (mean 8.1; SD 1.5mm and mean 6.1; SD 1.4mm) was significantly greater than that of AB with a locked suture loop (mean 4.7; SD 1.0mm; p < 0.05) and devices with a continuous loop (mean 4.1; SD 0.5mm and mean 4.4, SD 0.3mm; p < 0.01). No significant differences were detected between the ABs with a locked suture loop and the continuous loops. Cyclic loading and unloading of AB using the Chinese finger trap technique leads to significantly greater construct lengthening compared with other devices. Complete unloading of the ACL is very likely to occur during rehabilitation after ACL reconstruction. Lengthening of the AB device due to cyclic loading might be a potential mode of failure of the ACL graft fixation. Therefore, when using an AB femoral fixation technique, a locked suture loop design or a careful rehabilitation protocol should be considered.

Influence of unloading and loading stress cycles on the creep behavior of Darley Dale Sandstone

We report uniaxial experiments on Darley Dale Sandstone designed to reveal the effect of stress perturbations during deformation at a constant stress. To guide the target creep stress to be used in our unloading and loading creep tests, we first performed a series of uniaxial compression tests and conventional creep tests. Our unloading and loading creep tests show that cycling the stress transiently increases the strain rate and the rate of AE activity. We interpret this behavior as the result of a sharpening of microcracks during the stress cycling, thereby increasing the efficiency of stress corrosion cracking. After several hours, however, the microcracks return to their “blunt” state and the rates of deformation return to their initial values. Since the loading cycles reload the sample back to the initial creep stress, our experiments do not therefore comply with the Kaiser “stress memory” effect, which stipulates that additional microcrack damage is imparted only when a sample is exposed to a stress higher than previously experienced. Our experiments also suggest that stressing rate (in the range 0.05 and 5 MPa/s) does not influence the increase in strain rate and AE rate following a particular stress cycle. We conclude that the stress perturbations that commonly occur during mining practices may promote the failure of the pillars and/or surrounding rock in active mines.

Subsurface fracture distribution and its correlation with the shape and thickness of the Lac du Bonnet batholith

The Lac du Bonnet batholith (LDBB) is an Archean pluton in the Superior Province in southeastern Manitoba and formerly the site of the Underground Research Laboratory operated by Atomic Energy of Canada Limited. Batholith-scale investigations were conducted there between the late 1970s and 2005 into the Canadian concept of used nuclear fuel disposal in crystalline rocks of the Canadian Shield. A review of legacy data from this period has revealed that the distribution of the fracture network can be related directly to the thickness of the intrusion, as inferred from the model of the base of the batholith. Fractures have preferentially developed in the shallow and thinner sill-like western remnant of the batholith and, in comparison, they are suppressed within and over its deep root. Fracture orientations, frequency and continuity as seen at the surface are not characteristic of the fractures within the batholith interior, even at relatively shallow depths. A substantial volume of unfractured granite has been preserved despite the LDBB’s great age and its exposure to several cycles of loading and unloading, as revealed by the regional stratigraphic record and a unique apatite fission track dating profile. The LDBB is not a one-of-a-kind anomaly, and similar fracture/host rock relationships can be found in the literature. Based on the data reported here, it can be recommended that site models for any continuance of underground nuclear waste storage define the volume distribution of the host rock and its internal structure as these may serve as guides for subsurface exploration and the development and incremental testing of a conceptual fracture model.

Elastic-plastic fracture resistance of carbon steel for cyclic load (prediction of J-R curve assuming fatigue crack growth)

It is well known that the elastic-plastic fracture toughness for a cyclic load is smaller than that for a monotonic load. This study was aimed at investigating the root cause for the reduction in the fracture toughness of carbon steel due to application of a cyclic load. First, fatigue crack growth tests were conducted using plate specimens for a fully-reversed load of various stress amplitudes. The crack growth equation, in which the effective strain intensity factor range was quoted as the crack driving force, was derived to predict the crack growth up to 0.5 mm/cycle. Then, the same material was used to conduct the fracture toughness tests for monotonic and cyclic loading conditions. Plate specimens were used to enable the nominal strain to be measured during the tests. It was shown that the J-R curve and J0.2 fracture toughness were reduced due to application of the cyclic load. To investigate why the fracture toughness was reduced, it was assumed that the ductile crack extension was enhanced by the cyclic load. It was concluded that the reduction in the fracture toughness due to application of the cyclic load could be attributed to the fatigue crack growth. The fatigue crack growth was brought about not only by the unloading cycles but also by monotonic loading. The growth of the ductile crack extension was well predicted by considering the fatigue crack growth and the apparent crack growth caused by crack tip blunting.