Unloading Conditions Research Articles

Thermal-electrical coupling characteristic analysis of the heat pipe cooled reactor with static thermoelectric conversion

Heat pipe cooled reactor with static conversion is gaining more and more attention because of its unique advantages such as mobility, high reliability, simple structure, long lifetime, and high-power density. A 100 kW heat pipe reactor with the three-segment type thermoelectric generators namely NUSTER-100 is designed by Xian Jiaotong University. Based on the finite element software COMSOL Multiphysics, a multi-physics coupling analysis platform is established for the static heat pipe reactor NUSTER-100. The heat transfer models of the heat pipe and thermoelectric models of the three-segment type TEGs are supplemented in the simulation model. The thermal-electrical coupling characteristics of the static heat pipe reactor are investigated under different operation conditions. For the steady state condition, the temperature difference of the heat pipe between evaporator and condenser stabilizes at around 110 K, the main temperature gradient about 850 K appears between the two side of TEGs. The total output power of the NUSTER system is 123 kW and the system thermoelectric conversion efficiency is 12.3%. For the three rows unloading condition, the maximum temperature increases to 1483 K which is 133 K higher than that on the steady state condition. The output electrical power of the reactor system is 114.5 kW and the system thermoelectric conversion still could reach 11.5%. For the whole reactor unloading condition, the reactor core temperature increases by 393 K compared with the steady state and the maximum temperature is 1743 K which is lower than the core melting temperature. The operating temperatures of the heat pipes are still within the normal operating temperature range of the sodium heat pipe. The heat transfer capacity of the heat pipe is still sufficient to ensure that the core temperature is kept within the safe limits. In summary, the unloading condition will cause the core temperature to rise significantly. Therefore, the system safety characteristics under accident transient conditions need to be focused coupling with the unloading condition in the further research.

Creep damage model of rock with varying-parameter under the step loading and unloading conditions

The creep characteristics of rock under step loading and unloading conditions were investigated in this study. Based on the generalized Burgers model, the total strain of rock was decomposed into elastic, viscoelastic, varying-parameter viscoelastic, and viscoplastic strains considering the damage. The four strains were connected in series to establish a new varying-parameter creep damage model that can characterize the creep characteristics of rock under step loading and unloading conditions as well as identify and verify the model parameters. The study results showed that the varying-parameter creep damage model could better describe the creep characteristics of rock under step loading and unloading conditions, significantly the non-linear both the strain and time of attenuation creep and accelerated creep. The model fitting curve was highly consistent with the experimental data, and the correlation coefficient R2 was greater than 0.98, which thoroughly verified the accuracy and rationality of the model. These findings can provide theoretical support for analyzing the deformation and long-term stability of rock and soil.

The time-dependent behaviour and failure mechanism of dacite under unloading condition

To obtain time-dependent responses of rock in excavation engineering projects, a series of creep tests of dacite are conducted through stepwise unloading of confining pressure. Based on the test results, time-dependent behaviour and unloading failure mechanism of dacite are studied in terms of creep rate, failure mode, long-term strength, and time-dependent damage. According to the creep rate, creep deformation can be divided into decelerating creep stage, steady creep stage, and accelerating creep stage during the last unloading failure process. Time-dependent deformation exhibits an anisotropic characteristic. Local shear bands are found in failed samples, which is markedly different from tensile failure in a short-term unloading test. Time-dependent damage of dacite under unloading condition is the result of combined effects of cracks. Creep failure of dacite under unloading condition caused by concentrating in a local zone and coalescing to form one or more macroscopic shear bands. This study gives an insight to understand the mechanism of unloading creep failure of rock and provides an experimental foundation for developing a time-dependent constitutive model under unloading condition.

The MRI-based 3D morphologic changes of knee meniscus under knee weight-bearing and early flexion conditions

There are few studies investigate morphologic changes of knee meniscus in vivo mechanical loading and three-dimensions (3D) deformation and displacement of the whole meniscus between in vivo mechanical loading and unloading conditions are still unclear. To investigate the displacements and 3D morphological changes of the menisci under knee weight-bearing and early flexion conditions in healthy adults using a Magnetic Resonance Imaging (MRI)-compatible loading device (a 3.0 T MR imaging system) combined with a newly developed 3D comparison technique. Fifteen healthy volunteers were recruited in this cross-sectional observational study. Each subject underwent MRIs of their dominant right knee in eight different scanning conditions using a 3.0-T MRI scanner with a custom-made MRI-compatible loading device. The knee meniscus images were 3D reconstructed, and dimensional comparisons were made for each meniscal model with baseline (0°-unloaded model). The morphologic changes of the meniscal-anterior horn (AH), body (BD), and posterior horn (PH) regions were expressed as mean positive and negative deviations. The displacements were further investigated, and the meniscal extrusions of different subregions were measured. The morphologic changing patterns of human meniscus under loading and flexions were presented using 3D chromatic maps. The bilateral menisci were generally shifting laterally and posteriorly in most flexion angles and were changing medially and anteriorly under fully extended knee loading conditions. The mean deviations were more significant with loading at 0° of knee flexion, while the PH region in the lateral side changed further posteriorly with loading in 30° flexion. Most of the differences were not significant in other flexion angles between loading conditions. The extrusion of meniscus’s medial body was greater in full extension compared to any other flexing angles. Mechanical loading can significantly deform the menisci in knee extension; however, this effect is limited during knee flexion. Current study can be used as a reference for the evaluations of the integrity in meniscal functions.

COMPUTATIONAL MODEL FOR HEART TISSUE WITH DIRECT USE OF EXPERIMENTAL CONSTITUTIVE RELATIONSHIPS

Heart wall tissue plays a crucial role in living organisms by generating the mechanical force for blood flow. This tissue has a complex internal structure comprised mostly of muscle cells, in which biochemical energy is transformed into mechanical active stress under rhythmical electrical excitation. The overall heart functioning depends, among other physiological conditions, on the mechanical properties of the tissue. Over the past centuries, experimental and theoretical investigations have been conducted in order to establish the constitutive laws governing wall tissue behavior. Regarding computational modeling, many material models have been introduced, from simple elastic anisotropic to more sophisticated ones, based on various formulations of strain potentials. We here present a novel computational model that directly employs experimental constitutive relationships. Therefore, we avoid any fitting of material parameters for a selected analytical form of the constitutive law. Hysteretic characteristics of the tissue are included, as well as either incompressibility or compressibility according to experimentally determined curves. Deformation is split into deviatoric and volumetric parts in order to handle compressibility. The correctness and accuracy of the model is demonstrated through simple cases for loading and unloading conditions. Furthermore, the model was implemented for left ventricle (LV) deformation, where the FE mesh was generated from echocardiography recordings. Here, a specific algorithm, which accounts for LV torsion, was introduced to determine trajectories of material points on the internal LV surface. Hysteresis of the constitutive curves was used to calculate mechanical energy of LV wall tissue deformation. For completeness, the fluid flow within the LV was computed as well.

Prochlorperazine Withdraws the Delayed Onset Tonic Activity of Unloaded Rat Soleus Muscle: A Pilot Study.

A gradual increase in rat soleus muscle electromyographic (EMG) activity is known to occur after 3–4 days of hindlimb suspension/unloading (HS). The physiological significance and mechanisms of such activity of motoneurons under unloading conditions are currently unclear. Since hyperactivity of motoneurons and muscle spasticity after spinal cord injury are associated with KCC2 downregulation, we hypothesized that a decrease in potassium (K+)/chloride (Cl−) co-transporter 2 (KCC2) in motoneurons would be responsible for an increase in soleus muscle EMG activity during HS. We aimed to investigate the effect of prochlorperazine (KCC2 activator) on the electrical activity of rat soleus muscle under HS. Wistar rats were divided into the following groups: (1) vivarium control (C), (2) 7-day HS group (7HS) and (3) 7-day HS group plus intraperitoneal injections of prochlorperazine (10 mg/kg, daily) (7HS + P). Expression of proteins in the motoneurons of the lumbar spinal cord was determined by Western blotting. An electromyogram of the rat soleus muscle was recorded using intramuscular electrodes. KCC2 content after 7-day HS significantly decreased by 34% relative to the control group. HS-induced decrease in KCC2 protein content was prevented by prochlorperazine administration. HS also induced a significant 80% decrease in KCC2 Ser940 phosphorylation; however prochlorperazine did not affect KCC2 phosphorylation. The treatment of the rats with prochlorperazine prevented a HS-induced increase in Na(+)/K(+)/(Cl−) co-transporter 1 (KCC2 antagonist) protein content. In parallel with the restoration of KCC2 content, prochlorperazine administration during HS partially prevented an increase in the soleus muscle tonic EMG activity. Thus, prochlorperazine administration during 7-day HS prevents a decrease in KCC2 protein expression in motoneurons and significantly reduces the level of HS-induced soleus muscle electrical activity.

Effect of space flight on the behavior of human retinal pigment epithelial ARPE-19 cells and evaluation of coenzyme Q10 treatment.

Astronauts on board the International Space Station (ISS) are exposed to the damaging effects of microgravity and cosmic radiation. One of the most critical and sensitive districts of an organism is the eye, particularly the retina, and > 50% of astronauts develop a complex of alterations designated as spaceflight-associated neuro-ocular syndrome. However, the pathogenesis of this condition is not clearly understood. In the current study, we aimed to explore the cellular and molecular effects induced in the human retinal pigment ARPE-19 cell line by their transfer to and 3-day stay on board the ISS in the context of an experiment funded by the Agenzia Spaziale Italiana. Treatment of cells on board the ISS with the well-known bioenergetic, antioxidant, and antiapoptotic coenzyme Q10 was also evaluated. In the ground control experiment, the cells were exposed to the same conditions as on the ISS, with the exception of microgravity and radiation. The transfer of ARPE-19 retinal cells to the ISS and their living on board for 3days did not affect cell viability or apoptosis but induced cytoskeleton remodeling consisting of vimentin redistribution from the cellular boundaries to the perinuclear area, underlining the collapse of the network of intermediate vimentin filaments under unloading conditions. The morphological changes endured by ARPE-19 cells grown on board the ISS were associated with changes in the transcriptomic profile related to the cellular response to the space environment and were consistent with cell dysfunction adaptations. In addition, the results obtained from ARPE-19 cells treated with coenzyme Q10 indicated its potential to increase cell resistance to damage.

Percutaneous coronary intervention on combined VA-ECMO and IMPELLA (ECPELLA) support may reduce myocardial damage in cardiogenic shock patients with STEMI who required mechanical circulatory support

Abstract Background Venoarterial extracorporeal membrane oxygenation (VA-ECMO) is an effective therapeutic modality for patients with cardiogenic shock (CS) including STEMI. The VA-ECMO maintains end-organ perfusion, however, it significantly increases damaged left ventricular (LV) afterload. Combined treatment of VA-ECMO and a micro-axial Impella pump, ECPELLA, simultaneously provides systemic circulatory support and LV loading reduction (LV unloading). Studies in ischemic animal models displayed that LV unloading reduced myocardial size. However, it remains unknown whether LV unloading effect by ECPELLA during percutaneous coronary intervention (PCI) can reduce myocardial damage in human STEMI patients with severe cardiogenic shock. Purpose This study was to assess whether PCI on ECPELLA support can reduce myocardial damage in STEMI patients with severe cardiogenic shock. Methods Sixteen consecutive patients with STEMI and lethal CS (SCAI stage E) were enrolled. All patients suffered from cardiac arrest. Fifteen patients (94%) underwent extracorporeal cardiopulmonary resuscitation using VA-ECMO. From October 2018 to November 2019, PCI was carried out on VA-ECMO support and the mechanical circulatory support (MCS) modality during PCI was switched to ECPELLA since December 2019. There were 8 patients who underwent PCI on ECPELLA support (ECPELLA-PCI) and 8 patients who underwent PCI on single VA-ECMO support (ECMO-PCI). All patients in ECMO-PCI group were escalated to ECPELLA after PCI. Therefore, the difference between groups was timing of LV unloading, i.e., pre-PCI or post-PCI condition. We assessed total MCS flow during PCI, and peak and time course changes in CK-MB levels from days 0 to 3. Results There were no significant differences in age, rate of male sex, body surface area, shockable rhythm, serum lactate levels, and door to MCS time. Patients in ECPELLA-PCI had significantly longer door to recanalization time (73min in ECPELLA vs. 49min in VA-ECMO, p<0.05), and higher total MCS flow during PCI (3.1 L/min/sqM vs. 2.0 L/min/sqM, p<0.05). Peak and the area under the curves (AUC) from day 0 to day 3 of CK-MB in ECPELLA-PCI were significantly lower than ECMO-PCI group (peak CK-MB 120 vs. 524 IU/L and CK-MB AUC 143 vs. 464 IU/L*Day, respectively, p<0.05). While all patients received red blood cell (RBC) and plasma (PL) transfusions, ECPELLA-PCI required larger amount of RBC and PL transfusions (p<0.05). Conclusion The ECPELLA-PCI appeared to reduce myocardial damage shown by peak and AUC of CK-MB levels. Lager amount of blood transfusion is a major drawback of this treatment modality. Further studies are necessary whether ECPELLA-PCI is an effective revascularization approach for myocardial damage reduction with favorable mid- and long-term outcome in patients with STEMI with severe cardiogenic shock. Funding Acknowledgement Type of funding sources: None. Total MCS FLow and Ttoal MCS flow indexPeak CK-MB and CK-MB AUC

Wellbore instability mechanism of hard brittle rock under unloading condition in the ultra-deep wells

Limestone, igneous rock and dolomite are widely distributed in ultra-deep wells in Tarim Basin. Hard brittle rocks show different mechanical properties under high stress conditions in ultra-deep wells. Traditional wellbore stability theory states that the greater the rock strength the more stable the wellbore. The actual drilling in Tarim oilfield (more than 7000 m true vertical depth) shows that in the same section of hard dolomite and mudstone formation, there are more dolomite enlargement and block falling during drilling, while the caliper of mudstone formation is regular. The unloading mechanical experiment of hard brittle rock shows that the deformation and failure characteristics of rock during unloading are different from those during loading. In the process of unloading confining pressure, the axial stress and confining pressure decrease linearly with the increase of strain, and the unloading modulus increases with the increase of the decreasing rate of confining pressure. The smaller the unloading rate of confining pressure, the more obvious the axial plastic flow becomes. The failure of unloading rock occurs mainly due to its brittleness. By comparing the failure degree under different stress paths, the unloading confining pressure before peak is greater than that after peak, and the unloading confining pressure after peak is greater than that of the loading test. Most of the unloading cracks are tensile cracks, but there are shear cracks in the sample, and tensile flakes on the surface of the sample after unloading confining pressure. Within the experimental range (unloading rate), there may be an unloading rate that causes the most severe damage to the rock sample, that is, with the increase of the rate of penetration (ROP), the degree of rock damage caused by stress unloading does not increase monotonously, and there may be a certain ROP that causes the greatest damage to the wellbore. Large unloading will aggravate the damage evolution and failure of tight brittle rock. For the safe drilling of such rock formation, the drilling rate and drilling fluid density should be optimized based on complete understanding of the geo-mechanical environment to minimize the damage and instability of surrounding rock caused by drilling unloading.

Experimental study on the mechanical of deep-buried soft rock under triaxial loading and unloading

This study investigated the changes in the mechanical properties of the soft surrounding rocks in the deep-buried cavern during the excavation unloading process. The argillaceous sandstone in the main diversion tunnel of the Panlong pumped storage power station in Chongqing is the case study. Triaxial loading and unloading tests were carried out to discuss rock samples’ strength, deformation, and failure characteristics under different stress paths. The results showed that the soft rock’s peak strength, strain, deformation modulus, and cohesion in the triaxial loading test are greater than those of the unloading test. During the loading process, the soft rock’s elastic modulus and deformation modulus increase with the confining pressure. However, a more significant increase was observed in the deformation modulus. Under loading conditions, the soft rock suffers from double-shearing shear failure with a failure angle of about 65°. Under unloading conditions, single-shearing shear failure is dominant and accompanied by tensile failure. There are many tension cracks on the surface of the rock sample under low confining pressure, and lateral expansion occurs under high confining pressure.

Deformation and failure analysis of surrounding rock mass in deep buried tunnels subjected to excavation

With the rapid development of underground engineering, the construction of hydraulic tunnel is faced with complex geological conditions. Rock burst and deformation have become the bottleneck of engineering construction. The study of deformation and failure mechanisms of surrounding rock mass during the excavation of deep buried tunnel has become important for the field of underground engineering. The traffic tunnel of Shuangjiangkou hydropower station is studied to identify the failure mechanism of surrounding rock mass of deep buried tunnel. We introduced microseismic monitoring technology to delineate potential risk areas and reveal the space-time evolution of microfractures. The failure type of the surrounding rock is determined by source parameters. The relationship between surrounding rock deformation and excavation dynamic response is analyzed based on the geological and microseismic data. The results show that the 3D microseismic visualization model can realize the advanced detection of the internal damage of surrounding rock. The source parameters reveal that tensile failure is the main form of failure in the tunnel surrounding rock. Strong unloading and poor geological conditions are the main factors affecting the stability of surrounding rock mass. The study combines microseismic monitoring with field investigation, and then comprehensively evaluates the stability of the tunnel surrounding rock providing a reference for similar underground engineering.

Hydromechanical behavior of unsaturated expansive clay under repetitive loading

Compacted layers of expansive soils are used in different engineering projects, such as subgrades, engineered clay barriers, and buffers for radioactive waste disposal. These layers are exposed to a variety of stresses and wetting conditions during field serviceability. Coupling between hydraulic and mechanical repeated loading provides insight understanding to the induced progressive deformation of expansive clay. This study was conducted to investigate the hydromechanical behavior of unsaturated compacted expansive clay under repeated loading–unloading (RLU) conditions. Two series of one-dimensional (1D) oedometer tests were conducted under controlled matric suction up to 1500 kPa using the axis translation technique (Fredlund soil-water characteristic curve device, SWC-150). The first test series was carried out at different levels of controlled matric suction for non-repeated loading–unloading (NRLU) cycles. RLU cycles were applied in the second test series at different repetitive-stress levels and under different levels of matric suction. The results indicated increasing axial wetting strain εa(s), axial swell pressure σs(s), compression index Cc(s), and swell index Cs(s) with suction reduction. The estimated load–collapse (LC) curves obtained from NRLU series (LCN) and RLU series (LCR) indicated increasing yield stress σy(s) with increasing suction. This is attributed to the developed apparent cohesion between soil particles, which in turn rigidifies the material response. Applying repetitive loading induced a notable reduction of compression index Cc(s) at the same level of suction, whereas swell index Cs(s) seems to be independent of repetitive loading. Finally, repetitive loading exceeding initial yield stresses results in plastic hardening and, hence, enlargement of yield stress locus (i.e. LCR curve).

Effect of posture and body weight loading on spinal posterior root reflex responses.

The posterior root muscle response (PRM) is a monosynaptic reflex that is evoked by single pulse transcutaneous spinal cord stimulation (tSCS). The main aim of this work was to analyse how body weight loading influences PRM reflex threshold measured from several lower limb muscles in healthy participants. PRM reflex responses were evoked with 1-ms rectangular monophasic pulses applied at an interval of 6s via a self-adhesive electrode (9 × 5 cm) at the T11-T12 vertebral level. Surface electromyographic activity of lower limb muscles was recorded during four different conditions, one in decubitus supine (DS) and the other three involving standing at 100%, 50%, and 0% body weight loading (BW). PRM threshold intensity, peak-to-peak amplitude, and latency for each muscle were analysed in different conditions study. PRM reflex threshold increased with body weight unloading compared with DS, and the largest change was observed between DS and 0% BW for the proximal muscles and between DS and 50% BW for distal muscles. Peak-to-peak amplitude analysis showed only a significant mean decrease of 34.6% (SD 10.4, p = 0.028) in TA and 53.6% (SD 15.1, p = 0.019) in GM muscles between DS and 50% BW. No significant differences were observed for PRM latency. This study has shown that sensorimotor networks can be activated with tSCS in various conditions of body weight unloading. Higher stimulus intensities are necessary to evoke reflex response during standing at 50% body weight loading. These results have practical implications for gait rehabilitation training programmes that include body weight support.

Effect of Microgravity on Endothelial Cell Function, Angiogenesis, and Vessel Remodeling During Wound Healing.

Wound healing is a complex phenomenon that involves different cell types with various functions, i.e., keratinocytes, fibroblasts, and endothelial cells, all influenced by the action of soluble mediators and rearrangement of the extracellular matrix (ECM). Physiological angiogenesis occurs in the granulation tissue during wound healing to allow oxygen and nutrient supply and waste product removal. Angiogenesis output comes from a balance between pro- and antiangiogenic factors, which is finely regulated in a spatial and time-dependent manner, in order to avoid insufficient or excessive nonreparative neovascularization. The understanding of the factors and mechanisms that control angiogenesis and their change following unloading conditions (in a real or simulated space environment) will allow to optimize the tissue response in case of traumatic injury or medical intervention. The potential countermeasures under development to optimize the reparative angiogenesis that contributes to tissue healing on Earth will be discussed in relation to their exploitability in space.

Phase-Specific Verbal Cue Effects on Countermovement Jump Performance.

Krzyszkowski, J, Chowning, LD, and Harry, JR. Phase-Specific Verbal Cue Effects on Countermovement Jump Performance. J Strength Cond Res 36(12): 3352-3358, 2022-The aim of this study was to determine whether countermovement vertical jump (CMVJ) phase-specific cues can improve jump performance and phase-specific force-time characteristics. Twenty-nine subjects (14 males and 15 females) performed 15 total CMVJ trials (5 per condition) while being provided with a control and phase-specific (unloading phase and eccentric braking phases) foci of attention. Jump height, reactive strength index-modified, countermovement depth, time-to-takeoff, and CMVJ subphase force-time characteristics were compared between each phase-specific verbal cues and the control condition using paired samples t-tests ( α = 0.05) and Cohen's d effect sizes ( d ; large >1.2). Female ( d = 0.242; p = 0.012) and male ( d = 1.96; p = 0.047) subjects achieved greater jump heights in the control condition compared with the unloading phase condition. Females demonstrated a faster unloading phase, less unloading force, greater unloading yank, and greater braking force during the unloading condition, as well as greater eccentric braking force during the eccentric braking condition compared with the control condition ( p ≤ 0.014; d ≥ 0.242). Males exhibited less body mass unloading, greater unloading yank, faster eccentric braking time, greater eccentric braking force, and greater eccentric braking yank for both the unloading and eccentric braking conditions compared with the control condition ( p ≤ 0.047; d ≥ 0.196). Collectively, these results suggest that phase-specific foci of attention do not acutely improve jump performance but can enhance phase-specific force-time characteristics in recreationally active individuals. Specifically, practitioners should consider using an eccentric braking phase instruction for individuals need to improve eccentric braking force generation.

Influences of True Triaxial Loading-Unloading Stress Paths on Mechanical Properties and Wave Velocity of Coal Samples subject to Risk of Rock Burst

To study fracture evolution and peak stress in burst risk coal samples (BRCSs) under true triaxial loading and unloading conditions, experimental and numerical research was applied to BRCSs under true triaxial stress paths entailing “x-direction displacement fixed, y-direction loading, z-direction unloading.” Both the experimental and the numerical results demonstrated that the peak stress borne by the BRCSs was not only affected by the initial stress but also had a negative exponential relationship with the ratio of the unloading rate and the loading rate (RURLR); therefore, peak stress equations of BRCSs under true triaxial loading and unloading conditions were established. The triaxial stress-time curves obtained by experiments and simulations exhibited an “elasticity-yield-destruction” phase, and the characteristics of the yield phase were determined by the RURLR. A typical BRCS was selected for velocity tomographic imaging to analyze the fracture evolution characteristics under true triaxial loading and unloading. The results showed that when the BRCS was subjected to a triaxial state of stress, the high- and low-velocity regions existed alternately due to the presence of the crack; during the elastic phase, the crack closed during loading in the previous phase was reopened upon unloading, so that the velocity of the sample decreased and a wide range of low-velocity regions could be formed; when entering the yield phase, the original crack continued to expand into a hole-through crack, leading to wider extreme values and ranges of these low- and high-velocity regions; at the breaking phase, multiple microcracks were generated around the hole-through cracks, decreasing the overall velocity, and showing point distributions characteristics of high- and low-velocity regions. Overall, many low-velocity regions with similar normal directions to the unloading direction were formed; these correlated well with macrofractures (postfailure).

Variation Law of Infrared Radiation Temperature of Unloading Fracture of Composite Coal-Rock

For composite mining coal-rock dynamic disaster, combining the theory of thermodynamics, damage mechanics, and other disciplines, the thermodynamic coupling mathematical model of composite coal-rock under an unloading condition is deduced, and the simulation model of composite coal-rock is established for numerical simulation. And the variation law of thermal infrared radiation under triaxial loading and unloading of composite coal-rock is analyzed and verified by experiments. The results show the following findings: (1) The distribution of thermal infrared radiation temperature of composite coal-rock is different in different stages of stress. The overall temperature of the temperature field is lower than the initial temperature field in the three-dimensional stress loading stage and the stage of stress-keeping pressure, but the internal temperature of the coal body is the highest. In the first stage of “loading and unloading,” the temperature of a coal seam increases slightly, and the temperature of other parts of the rock layer increases except for the circular low-temperature zone. In the second stage of “loading and unloading,” an alternating zone of high and low temperatures appears in the rock mass, and the temperature field is enhanced, among which the temperature field reaches the strongest after unloading the confining pressure. After jumping over the maximum stress, the temperature field decreases as a whole at the instability and rupture stage. (2) The variation of surface average thermal infrared radiation temperature ( T ave ) of composite coal-rock can be divided into the initial fluctuation stage, the linear heating stage, the local decline stage, the temperature sudden increase stage, and the fracture decline stage. At three different unloading rates of 0.003 MPa/s, 0.03 MPa/s, and 0.05 MPa/s, the T ave of coal body, floor rock, and roof rock reach the maximum before composite coal-rock instability and fracture, and the temperature change of the coal body is the most obvious. (3) Under different confining pressure unloading rates, the T ave of roof rock, coal body, and floor rock shows a strong linear relationship with stress after linear fitting. And the correlation between simulation and experimental results after fitting is above 0.89. The larger the confining pressure unloading rate is, the shorter the peak time of T ave arrives, and the larger the peak value. The comparison between the experimental results and the simulation analysis shows that the two results are consistent, and the research results can provide a theoretical basis for the prevention and control of dynamic disasters in coal and rock mining.

Surrounding rock effect on coal burst under unloading condition: a numerical study

Surrounding rock effect on coal burst under unloading condition: a numerical study

Investigating the Macro-Micromechanical Properties and Failure Law of Granite under Loading and Unloading Conditions

The excavation of rock significantly changes the initial stress state of rock slopes, which makes rock in complex loading and unloading conditions. However, the failure mechanisms and macro-micromechanical properties of rock under loading and unloading conditions are not very clear. This study investigates the macro-micromechanical properties and failure law of granite under loading and unloading conditions through traditional laboratory tests and particle flow simulations. Granite specimens are taken from Shuichang iron mine, and stress unloading experiments are designed based on the engineering practices. The stress-strain curves and failure modes under different confining pressures and unloading paths are obtained to analyze the granite properties from the viewpoint of the macroscopic mechanism. Moreover, numerical models are established in PFC software. The microcracks developments, failure characteristics, and energy evolutions under loading and unloading conditions are obtained and discussed. Results show that compared with the loading tests, the brittle failure characteristics of specimens under unloading tests are more obvious. When the confining pressure reduces to about 66% of the initial confining pressure, the specimen loses its load-bearing capacity and destroyed due to the lateral expansion. For loading tests, an inclined plane can be produced as the main failure surface. While for confining pressure unloading tests, there are many damage zones parallel to or intersecting with the main failure surface.

An innovative test equipment with rate-dependence in interface damage and its operation under cyclic loading.

An innovative test equipment for analyzing the dynamic damage at the interface of geosynthetic materials is introduced. It provides a result of shear damage on the structural surface of a geosynthetic material that has been subjected to cyclic loading of different amplitudes. First, the experimental data of geosynthetics under cyclic loading are obtained based on the innovative test equipment mentioned above, and a mathematical identification method for the relationship between cyclic loading parameters and shear mechanical behavior of geosynthetics is proposed based on the Mohr-Coulomb criterion, assuming that the micro-element strength of geosynthetics follows Weibull distribution. Moreover, the proposed model, which takes into account repeated loading-unloading conditions, enables this simulated damage process to have properties of reoccurring load cycles, and the proposed method was evaluated by experimental datasets from this study and the existing literature and proved to have a satisfying performance in predicting mechanical behavior of shear stress in geosynthetics under different cyclic loadings. Second, the self-developed test instrument can well solve the direct shear parameters of geosynthetics under dynamic load and can be obtained as a new dynamic statistical model with rate-dependence in interface damage.