Unloading Characteristics Research Articles

A study on microscopic damage characteristics of freeze-thaw sandstone cyclic loading and unloading based on DEM

A study on microscopic damage characteristics of freeze-thaw sandstone cyclic loading and unloading based on DEM

Study on the time delay failure characteristics of sandstone unloading under dynamic disturbance

The rockburst caused by underground engineering excavation exhibits a significant lag effect. Studies have shown that the occurrence of lag-type rockburst is closely related to the delayed failure of rocks. This paper focuses on the delayed failure characteristics of unloading-damaged sandstone under the combined action of static load and dynamic disturbance. Numerical simulations are utilized to analyze the delayed failure evolution characteristics and failure mechanisms of sandstone. The results indicate that in triaxial unloading delay failure tests, the duration of loading decreases exponentially with the increase of initial unloading damage. Compared to static load conditions, the duration of loading under dynamic disturbance decreases by more than 43 %, and the average strain rate significantly increases. The number of cracks at the endpoint of triaxial unloading delay failure increases as initial unloading damage decreases, with a substantial increase in the number of cracks under dynamic disturbance. These findings provide a valuable reference for the timeliness and delayed rockburst analysis and interpretation of rock damage and failure under high-stress levels.

Investigation on the responses of geotechnical materials to unloading and seepage based on the CFD-DEM coupling method

The unloading issue is prevalent in underground engineering and significantly affects construction safety and the operation of surrounding facilities, while research regarding it is still relatively scarce. A novel CFD-DEM coupling method with dynamic fluid meshes is proposed to reproduce the unloading effect and reveal its micro-scale mechanisms. It adopts uniformly distributed point sets to generate/update fluid meshes and solves with a proposed numerical mapping method. The one-dimensional linear unloading modeling method is further established. The numerically obtained excess pore pressure and rebound deformation under the one-way drained condition are compared with the analytical solutions. The coupling method demonstrates considerable reliability in forecasting the pore pressure response and its advantage for predicting rebound deformation is revealed. The evolutions of the fluid velocity field, particle displacement field, and pore pressure distribution profile are described. The influences of fluid compressibility, unloading rate, unloading volume and initial pore pressure on the unloading characteristics are carefully investigated. The results indicate that the pore pressure is obviously influenced by the selected factors. However, the impact of unloading volume on the final rebound deformation is most significant. Finally, influences of frictionless walls, two-way drained conditions, buoyancy and fluid viscosity on the unloading response are further discussed.

Experimental Investigation on Rockburst Energy Characteristics of True Triaxial Unloading in Sandstone Under High Stress

Experimental Investigation on Rockburst Energy Characteristics of True Triaxial Unloading in Sandstone Under High Stress

Experimental study on the effects of water content on the compression characteristics and particle breakage of calcareous sand

The particle breakage effect and compression characteristics of calcareous sand are related to the water content in the sand material. However, the effects of water content on the particle breakage and compression characteristics of calcareous sand have rarely been investigated. In this work, 50 sets of confined compression tests were conducted on calcareous sand specimens, and the compression characteristics and particle breakage effects of two single-particle-size groups (particle size ranges of 1–0.5 mm and 0.5–0.25 mm) of calcareous sand were investigated under five different water contents. The test results showed that with the increase in the water content, the final compression deformation of calcareous sand was positively correlated with the water content. The final compression deformation decreased when the water content reached a certain value. The water content corresponding to the peak final compression deformation was related to the gradation of the calcareous sand; the specific values were 10% and 15% for particle size ranges of 1–0.5 mm and 0.5–0.25 mm, respectively. With the increase in the water content, the slope of the loading curve of calcareous sand appeared to increase and then decrease, reaching maximum when the water content was 10%. Moreover, the slope of the loading curve was close to twice that of the loading curve of dry sand, whereas the slope of the unloading curve changed little. Under the same water content, the initial gradation had no effect on the compression and unloading characteristics of the specimens beyond a vertical pressure of 1 MPa. The effects of the variation in the water content on the particle breakage of calcareous sand were mainly reflected in the softening effect of water on the specimen particles, which reduced the Mohr strength of the particles.

Mechanical properties of cemented backfill under different unloading rates after cured at different temperatures

In the process of deep filling mining in coal mine, due to the influence of mining and other factors, the confining pressure on both sides of cemented backfill decreases. It is necessary to master the mechanical characteristics of cemented backfill under unloading conditions in different temperature environments to ensure the stability of cemented backfill. To simulate the unloading process of cemented backfill in the deep highland environment of coal mine, triaxial unloading confining pressure tests of cemented coal gangue- fly ash backfill at different unloading rates after curing at different temperatures (20 ℃, 35 ℃ and 50 ℃) were carried out by the Rock Triaxial Testing System, and the mechanical characteristics of cemented backfill unloading were systematically studied. The findings of the study show that, for the same unloading rate, the peak strength of cemented backfill specimen reduces initially before increasing with curing temperature rises. After curing at the same temperature, the peak strength of cemented backfill specimen varies exponentially with an increase in unloading rate. The deformation modulus and volume strain at different unloading rates satisfy an exponential function relationship, while the generalized Poisson's ratio and volume strain satisfy a quadratic function relationship. Under unloading conditions, there was no large fracture or fragmentation of cemented backfill, only cracks appeared on surface. At the same unloading rate, cemented backfill specimen cured at 35 ℃ and 50 ℃ exhibits significantly more shear cracks after failure compared to the material cured at 20 ℃. As the unloading rate increases, the proportion of shear cracks generated during unloading failure of cemented backfill specimen cured at 35 ℃ and 50 ℃ gradually increases. In addition, the theoretical curve of the four-stage cemented backfill unloading constitutive model established in sections is essentially compatible with experimental curve, confirming the theoretical model's reasonableness. The study's findings provide a theoretical basis for evaluating stability of cemented backfill after unloading in deep high temperature (35–50 ℃) environments.

Research on creep characteristics of loading and unloading of hard Flint limestone

The hard Flint limestone of Shuibuya hydropower station underground construction cavern is utilized as a research object to investigate the creep problem caused by excavation of rock masses such as caverns. In order to perform a triaxial compression grade-unloading creep test, the actual adjustment path of stress during excavation of underground cavern surrounding rock is used. Limestone under different confistiff pressures is then evaluated. Based on the Nishihara model, the elastic damage element considering time-dependent damage is introduced, and the unloading creep constitutive model of stiff Flint limestone is established and verified by experiments. The results show: 1) Deformation and creep strain appear at all stress levels. 2) As the unloading amount increased from 2 to 4 MPa, the quasi-destructive stresses of the samples were smaller from 83 to 79 MPa, indicating that the unloading amount affected the final creep damage strength of the rock samples. In other words, the higher the unloading amount, the lower the ultimate creep failure strength. 3) When entering the accelerated creep stage, the axial and lateral creep strains of the sample increase non-linearly, and the rupture duration of the sample is very short. Therefore, the creep deformation and creep rate characteristics of this stage should be paid attention to in practical engineering. 4) Different from the loading stress path, the failure mode of the Flint limestone rock sample is different. When the unloading amount is 2 and 4 MPa, the creep failure mode of the Flint limestone rock sample is shear failure, showing a significant oblique section crack. 5) The non-linear creep model curve of aging damage and the fitting effect of the unloading creep test curve are acceptable. The rationality of the established non-linear creep model is illustrated.

Analysis of Mechanical-Hydraulic Cooperative Response of Hydraulic Support Under Roof Rotary Impact

Hydraulic support is a collaborative bearing equipment with hydraulic transmission power and mechanical bearing load. The method of the mechanical-hydraulic co-simulation model is used to discuss the response difference of hydraulic support in the process of roof rotation. Based on this model, the variation law of hydraulic support is analyzed by changing the rotation velocities of the roof. Then, by changing the discharge flow of relief valves at the column, the energy unloading characteristics of the column are discussed. The results demonstrate that with an increase in the roof rotary velocity, the pressure and flow peaks of the column increase continuously (the increase multiples of pressure and flow peaks are 123% and 51%, respectively). The tendency of load variation at different locations of hinge points appears to be varied. Lastly, this paper increases the discharge flow of the relief valve to shorten the energy unloading time (by 1.5 s) and reduce the load at the hinge point. This approach helps reduce the probability of damage to the hydraulic support and prolongs its life.

Numerical evaluation of effective compression behaviour of spiral wound gasket under loading and unloading

ABSTRACTGaskets are used as interfacing element between the mating surfaces, to seal the flange joint. The interfacing elements are usually subjected to compression during loading and relaxation during unloading. The unloading behaviour of the gasket material, when subjected to internal fluid pressure, controls the residual deformation in gasket. A methodology is proposed to obtain the homogenised behaviour of heterogenous gasket by volume averaging the response from micromechanical analysis. The response includes both loading and unloading characteristics of the gasket material. Here, the individual constituents’ characteristics in the spiral wound gasket is modelled using their respective elasto-plastic behaviour. But the unloading behaviour is more affected by the plastic properties. Therefore, the anisotropic Hill’s plastic parameters are evaluated by analysing the representative volume element of the gasket under different loading conditions. The compressive behaviour of the spiral wound gasket from homogenisation is compared with the experimental values for both loading and unloading conditions. An improved unloading behaviour is obtained using the proposed methodology.

Anti-mutation load and error stability of a large aperture lens based on parallel collimation locking.

Based on the lens of spatial filter parallel auxiliary assembly and collimation method, aiming at the unloading and locking problem of the filter lens after collimating, a lens parallel adjustable axial locking structure was proposed. A special elastic chuck structure was designed to realize clamping axial friction locking; each branch realized interference fit, and the adjustable structure of "line-line-line" contact realized equivalent fixation through spring pre-tightening. Compared with radial locking, the abrupt error of the axial locking lens in the unloading process of a collimating auxiliary machine was small, and the posture state maintenance and structural response stability were strong. The established homogeneous transformation matrix (HTM) and lens global error showed that the pose torsion roll error of the radial locking structure was significantly suppressed. The relative stability experiment showed that the angular drift error of the lens axial locking structure was ± 0.2 µrad. The design effectively improved the unloading anti-mutation characteristics and stability of the filter lens, which provided an experimental basis for the application of intelligent collimation assisted assembly and adjustment scheme in the upgrading of a high-power laser device.

Dynamic behavior of PMMA under planar shock conditions

As a non-metallic low wave impedance material, PMMA has been widely used in experiments of blast and shock. But the dynamic performances of different kinds of PMMA vary greatly. In this paper, the plane shock-wave experiments were performed with a 57 mm bore single-stage light gas gun in order to determine the dynamic behavior of PMMA at stress levels between 0.4 and 3.8 GPa. The particle velocity and shock wave velocity were measured with electromagnetic particle velocity sensors embedded into the samples directly, and the velocity of the flyer was measured with a laser velocity measuring system. The typical wave forms of particle velocity and the Hugoniot data of the experiments were presented in this paper. The dynamic stress–strain relations of experiments at different pressure ranges were obtained by the Lagrangian analysis method based on one dimensional strain, and the loading and unloading characteristics of which were discussed.

Research on rock triaxial cyclic loading and unloading characteristics based on PFC3D

Numerical methods provide a new way for solving complex geotechnical engineering problems. Firstly, we review the emergence and development of numerical methods, and introduce the basic principles of particle flow discrete element and its application in geotechnical engineering. After that, a method of servo combination of cylinder and loading plate is used, the numerical simulation of the three-dimensional rock in triaxial loading and unloading test is carried out by PFC3D software, and the loading and unloading mechanical characteristics of rock mass under the linear elastic contact constitutive model and parallel bonded contact constitutive model are studied respectively, and we obtain the triaxial loading and unloading stressstrain curves of rock under the elastic constitutive model and elastoplastic constitutive model. The residual strains and hysteresis phenomena of the rock under the two constitutive models are analysed, and the feasibility of the particle flow discrete element method in studying the mechanical characteristics of rocks is demonstrated.

Multifield Coupling Mechanism of Unloading Deformation and Fracture of Composite Coal-Rock

The deformation and fracture evolution of coal and rock under unloading are prone to sudden instability or dynamic damage. To solve the problem, this paper combines interdisciplinary theories such as damage mechanics and electromagnetic field theory. The mathematical model of multiphysics coupling during loading and unloading of composite coal-rock is deduced. In addition, numerical simulations along with experimental verification are carried out to study multi-physical field variation and coupling mechanisms. The composite coal-rock deforms and ruptures under unloading, and the brittle failure of the rock body becomes more sudden when the confining pressure is unloaded. Macroscopically, many microcracks are generated and expanded during the loading and unloading of composite coal-rock. Microscopically, the internal old molecular chains are broken to form new molecular chains by the force. Simulation results show that, during the loading and unloading process, the three physical fields of the composite coal-rock all change regularly. During the unloading of coal and rock, there is a transition period in which the temperature increases sharply and reaches the maximum. Then, the temperature decreases due to the gradual decrease of its bearing capacity. Besides, the electromagnetic field is strongest on the surface of the coal body, and its propagation in the air decays exponentially. There are small fluctuations that appear at the junction of the coal body and the air. The experimental results show that the internal infrared radiation temperature of the composite coal-rock decreases during the initial stage of loading and unloading due to the discharge of internal gas. In the first stage of “loading and unloading,” it increases with the increase in stress, and the temperature suddenly increases in a short time after unloading. The electromagnetic radiation fluctuates in small amplitudes at the initial stage. When the stress is about to reach the peak, the electromagnetic radiation intensity increases and reaches the peak suddenly. Then, the coal-rock ruptures, the stress decreases, and the electromagnetic radiation weakens. The experiment and simulation results are consistent. The multiphysics coupling model is used to study the characteristics of coal and rock unloading under complex conditions, providing a theoretical basis and new method for the prediction and forecast of coal and rock mining dynamic disasters.

Nonlinear isolation performance of 6 × 19 wire rope of different lengths under compression

Stainless steel wire rope isolator is widely used in engineering. To optimize design of the isolator, loading, and unloading characteristics of the 6 × 19 6 mm wire rope under compression are investigated. Ropes of different lengths are tested to get the force-displacement relations. The stiffness, the equivalent damping ratio, and the hysteresis loop of the wire rope are derived. The stiffness decreases with both the length of the rope and the vibration amplitude. It has an approximate linear relationship with the reciprocal of length and amplitude. The equivalent damping ratio has an approximate quadratic relationship with the reciprocal of length and amplitude. The hysteresis loop of the wire rope is described using the proposed quadrilateral model. The loading stage is found to be determined by the length of the rope. The unloading stage is influenced by both the vibration amplitude and the length of the rope. Influences of the excitation amplitude and the frequency on the isolation performance for both steady-state vibration and transient impact vibration are revealed based on the models. The work would help engineers to design the isolators and predict responses of the structures.

Particle Flow Simulation of Failure Characteristics of Deep Rock Influenced by Sample Height-to-Width Ratios and Initial Stress Level under True-Triaxial Unloading

The micromechanism of the effects of different height/width ratios (H/W) and initial stress levels on unloading characteristics of deep rock was investigated based on PFC3D true-triaxial unloading simulation. The results show that the increase of H/W will increase the movement speed of rock particles and intensify the acoustic emission (AE) activity inside the rock. With the increase of H/W, the failure mode of rock changes from splitting failure to tensile-shear failure. With increasing initial stress level, the particle velocity and overall fragmentation degree of rock increase. However, the increase of lateral stress will limit the coalescence of microfractures and weaken AE activity in the rock. Under unloading condition, the bonds between particles generally crack along the unloading direction, and the tensile effect is more pronounced under the condition of low initial stress level and high H/W. Under unloading condition, the variable energy of rock increases with increasing H/W and initial stress level, and the kinetic energy of rock particles increases with increasing H/W. The increase of initial stress level will increase the kinetic energy of rock particles when H/W is high.

Research on the Mechanical Characteristics of Cyclic Loading and Unloading of Rock Based on Infrared Thermal Image Analysis

In the operations of underground rock engineering, such as mining, the formation of goafs is often accompanied by unloading and energy effects. In this study, a cyclic loading and unloading stress test is carried out to analyze the strength characteristics of the loaded samples under different loading and unloading ranges as well as different numbers of cycles. The rock force is accompanied by substantial energy changes. To better fit the energy analysis under cyclic loading and unloading conditions, thermal infrared radiation characteristic analysis is performed during rock loading and unloading. An infrared radiation camera is adopted to detect the infrared characteristics of the rock force process after cyclic loading and unloading. Multiangle detection is implemented on the temperature, temperature field, and frequency histogram. The analysis shows that cyclic loading and unloading first strengthen and then weaken the rock. Moreover, the failure caused by the local stress concentration leads to a sharp increase in the temperature. There are significant temperature fluctuations before and after failure, and the temperature field after failure can be divided into three zones, namely, the normal temperature zone, heating zone, and mutational temperature zone, to comprehensively reflect that rock compression failure which is accompanied by the process of energy accumulation and release. On the basis of infrared energy analysis, the index of the energy release rate is introduced, and the loading and unloading analysis model is constructed. The research results reveal that rock failure is accompanied by the process of energy accumulation and release, which provides evidence for the analysis of the spatial stability of the rock mass under cyclic loading and unloading conditions and engineering excavation.

Extracorporeal life support for cardiogenic shock during pregnancy and postpartum: a single center experience.

The use of veno-arterial extracorporeal membrane oxygenation (VA ECMO) for cardiogenic shock in pregnant and postpartum patients remains limited by concerns of bleeding, hemolysis, and fetal risks. This case series examines the underlying characteristics and management strategies for this high-risk population. All pregnant and post-partum patients who underwent VA ECMO in the cardiovascular intensive care unit between January 1, 2016 and November 1, 2019, were included in this retrospective study. Management of maternal and fetal O2 delivery, left ventricular (LV) unloading, anticoagulation, and ECMO circuit characteristics were evaluated. Five patients required veno-arterial ECMO for restoration of systemic perfusion. Three patients developed peripartum cardiomyopathy, one septic cardiomyopathy, and one acute right ventricular (RV) failure. The median age was 30.6 years, with median gestational age in pregnant patients of 31 weeks. Maternal and fetal survival to discharge was 80%. Bleeding was the primary complication, with two patients requiring blood transfusions; one requiring interventional radiology (IR) embolization and the other requiring surgical intervention to control bleeding. One patient was successfully delivered on VA ECMO. No fetal complications were directly attributed to VA ECMO. VA ECMO can be employed successfully in obstetric patients with cardiogenic shock with appropriate patient selection. Further research is needed to determine if VA ECMO provides a survival advantage over traditional management strategies in this vulnerable population.

Elastomer elastic element visco-elastic properties’ impact on damping in a vehicle suspension

Suspension structure, its elastic elements made of elastomers, is described in the paper. This allows to considerably decrease a vehicle’s unsprung weight improving its ride quality. The research is aimed at determining elastomer elastic element’s damping impact on a vehicle’s smooth ride quality. The impact estimate was carried out by comparing simulation modelling results of GAZelle NEXT vehicle’s front suspension and an equivalent suspension with an elastomer elastic element. Dual-mass single-axle suspension model was chosen as a calculation model. The modelling was performed in MATLAB/Simulink software package. Standard suspension key parameters, that is spring rate and shock absorber resistance characteristic, were experimentally obtained for each vehicle. Elastomer element elastic response was obtained at static necking and averaged with respect to loading and unloading characteristics. Damping capacity was estimated basing on the damped oscillations process realized on a special pendulum impact testing machine. “Relative” elastomer damping coefficient was calculated. All the measured parameters were reduced “awheel” by grapho-analytical methods. Vibration acceleration root-meant-square values (RMS) on the car frame and normal awheel response of a vehicle’s motion under various speed conditions over smooth cobbled road served as assessment criteria. Roadbed microprofile mathematic model was realized basing on superposition harmonic function method with various oscillations frequency and amplitude. Analytical findings reveal that a standard suspension with a cylindrical spiral spring and a hydraulic shock absorber outperforms the elastomer elastic element structure in all the given motion modes with respect to the ride quality criteria. The research findings bring about the conclusion that elastomers lack visco-elastic characteristics necessary to provide better ride quality compared to the standard suspension structure.

Behaviour of flange joints in Steam Generator under thermal loads

Pressure vessels such as steam generators are subjected to high temperature, in addition to high pressure during the operating condition. Flanges and bolts are made up of different materials whose coefficient of thermal expansion varies. Usually, thermal expansion in bolts is greater than that of flanges. At elevated temperatures bolts expand more than that of flanges, resulting in decrease of compression in connected members achieved during assembly stage, which in turn decreases the contact stress in gasket. This can lead to leakage of internal fluid. The loss in gasket contact stress due to differential thermal expansion can be nullified by using sleeves of higher thermal expansion between the flange-nut and flange-bolt head interfaces. At higher temperatures sleeves expand more than bolts and flanges, pushing the flanges closer towards each other, thus decreasing gap created due to differential thermal expansion. The behaviour of gasketed blind flange joint with and without sleeves is analysed and the performances are compared under thermal loads. The non-linear behaviour of gaskets is included by specifying the loading and unloading characteristics with hysteresis.

Unloading Mechanics and Energy Characteristics of Sandstone under Different Intermediate Principal Stress Conditions

The TRW‐3000 true triaxial rock testing machine was used to conduct loading and unloading tests of sandstone under different σ2, and the true triaxial lateral unloading mechanics and energy characteristics of sandstone under different σ2 were studied. The experimental results show the following: (1) compared with the results of the loading test, the peak strength of the sandstone under the unloading σ3 path is reduced, the unloading direction has obvious expansion and deformation, and the amount of expansion increases significantly with the increase of σ2; sudden brittle failure occurs at the end of unloading. E gradually decreases with the increase of H, and it performs well to use the cubic polynomial to fit the curve of E-H. (2) The Mogi–Coulomb strength criterion can accurately describe the true triaxial strength characteristics of sandstone under loading and unloading conditions. Compared with the results of the loading test, the values of c and φ obtained based on this criterion under the unloading σ3 path are reduced. (3) Under the condition of unloading σ3, U, Ue, and Ud, when the specimen is broken, are all linearly positively correlated with σ2. Ud increases nonlinearly with the increase of H, and as σ2 increases, the slope of the Ud -H curve becomes larger, and the specimen consumes more energy under the same unloading amount. Most of the energy absorbed by the specimen under the unloading σ3 path is converted into Ue, but as σ2 increases, Ud /U increases, and the energy consumed when the specimen is broken is greater.