Space-time evolution rules of acoustic emission location of unloaded coal sample at different loading rates

By using MTS815 rock mechanics test system,a series of acoustic emission(AE) location experiments were performed under unloading confining pressure,increasing the axial stress.The AE space-time evolution regularities and energy releasing characteristics during deformation and failure process of coal of different loading rates are compared,the influence mechanism of loading rates on the microscopic crack evolution were studied,combining the AE characteristics and the macroscopic failure modes of the specimens,and the precursory characteristics of coal failure were also analyzed quantitatively.The results indicate that as the loading rate is higher,the AE activity and the main fracture will begin earlier.The destruction of coal body is mainly the function of shear strain at lower loading rate and tension strain at higher rate,and will transform from brittleness to ductility at critical velocities.When the deformation of the coal is mainly plasticity,the amplitude of the AE ringing counting rate increases largely and the AE energy curves appear an obvious ''step'',which can be defined as the first failure precursor point.Statics of AE information shows that the strongest AE activity begins when the axial stress level was 92-98%,which can be defined as the other failure precursor point.As the loading rate is smaller,the coal more easily reaches the latter precursor point after the first one,so attention should be aroused to prevent dynamic disaster in coal mining when the AE activity reaches the first precursor point.

Experimental study on the effect of unloading rate on gneiss rockburst

Rockburst are often encountered in tunnel construction due to the complex geological conditions.To study the influence of unloading rate on rockburst,gneiss rockburst experiments were conducted under three groups of unloading rates.A high-speed photography system and acoustic emission(AE)system were used to monitor the entire process of rockburst process in real-time.The results show that the intensity of gneiss rockburst decreases with decrease of unloading rate,which is manifested as the reduction of AE energy and fragments ejection velocity.The mechanisms are proposed to explain this effect:(i)The reduction of unloading rate changes the crack propagation mechanism in the process of rockburst.This makes the rockbursts change from the tensile failure mechanism at high unloading rate to the tension-shear mixed failure mechanism at low unloading rate,and more energy released in the form of shear crack propagation.Then,less strain energy is converted into kinetic energy of fragments ejection.(ii)Less plate cracking degree of gneiss has taken shape due to decrease of unloading rate,resulting in the destruction of rockburst incubation process.The enlightenments of reducing the unloading rate for the project are also described quantitatively.The rockburst magnitude is reduced from the medium magnitude at the unloading rate of 0.1 MPa/s to the slight magnitude at the unloading rate of 0.025 MPa/s,which was judged by the ejection velocity.

Unloading and successful treatment with bioresorbable stents during percutaneous coronary intervention:A case report

BACKGROUND With the development of percutaneous coronary intervention(PCI),the number of interventional procedures without implantation,such as bioresorbable stents(BRS)and drug-coated balloons,has increased annually.Metal drug-eluting stent unloading is one of the most common clinical complications.Comparatively,BRS detachment is more concealed and harmful,but has yet to be reported in clinical research.In this study,we report a case of BRS unloading and successful rescue.This is a case of a 59-year-old male with the following medical history:“Type 2 diabetes mellitus”for 2 years,maintained with metformin extended-release tablets,1 g PO BID;“hypertension”for 20 years,with long-term use of metoprolol sustained-release tablets,47.5 mg PO QD;“hyperlipidemia”for 20 years,without regular medication.He was admitted to the emergency department of our hospital due to intermittent chest pain lasting 18 hours,on February 20,2022 at 15:35.Electrocardiogram results showed sinus rhythm,ST-segment elevation in leads I and avL,and poor R-wave progression in leads V1–3.High-sensitivity troponin I level was 4.59 ng/mL,indicating an acute high lateral wall myocardial infarction.The patient’s family requested treatment with BRS,without implanta-tion.During PCI,the BRS became unloaded but was successfully rescued.The patient was followed up for 2 years;he had no episodes of angina pectoris and was in generally good condition.CONCLUSION We describe a case of a 59-year-old male experienced BRS unloading and successful rescue.By analyzing images,the causes of BRS unloading and the treatment plan are discussed to provide insights for BRS release operations.We discuss preventive measures for BRS unloading.

A theoretical deduction for the location of neutral axis in intestine at unloaded configuration

The function of Gastrointestinal tract including intestine is to a large degree mechanical.The mechanical properties of the intestinal wall,and the tonic (sustained) and/or phasic(short-

Mechanical properties and energy evolution of Beishan shallow-layer granite under different unloading paths

Rock has mechanical characteristics and a fracture damage mechanism that are closely related to its loading history and loading path. The mechanical properties, fracture damage features, acoustic emission(AE) characteristics, and strain energy evolution of the Beishan shallow-layer granite used in triaxial unloading tests were investigated in this study. Three groups of triaxial tests, namely, conventional triaxial compression test(Group Ⅰ), maintaining deviatoric stress synchronously unloading confining pressure test(Group Ⅱ), and loading axial pressure synchronously unloading confining pressure test(Group Ⅲ), were carried out for the cylindrical granite specimens. AE monitoring device was utilized in these tests to determine the degree to which the AE waves and AE events reflected the degree of rock damage. In addition, the crack stress thresholds of the specimens were determined by volumetric strain method and AE parameter method, and strain energy evolution of the rock was explored in different damage stages. The results show that the shallow-layer granite experiences brittle failure during the triaxial loading test and unloading test, and the rock has a greater damage degree during the unloading test. The crack stress thresholds of these samples vary greatly between tests, but the threshold ratios of all samples are similar in the same crack damage stage. The Mogi-Coulomb strength criterion can better describe the unloading failure strength of the rock. The evolution of the AE parameter characteristics and strain energy differs between the specimens used in different stress path tests. The dissipative strain energy is the largest in Group Ⅱ and the smallest in Group Ⅰ.

Effects of dry-wet cycles on the mechanical properties of sandstone with unloading-induced damage

Sandstone is the fundamental material in various engineering and construction projects.However,the mechanical integrity of sandstone can be compromised by initial unloading damage resulting from activities such as engineering excavations.Furthermore,this degradation is further exacerbated under periodic dry-wet environmental conditions.This study investigated the effects of dry-wet cycles and unloading on the mechanical properties of jointed fine sandstone using uniaxial and triaxial compression tests.These tests were performed on rock samples subjected to varying unloading degrees and different numbers of dry-wet cycles.The results demonstrate that with an increase in the unloading degree from 0%to 70%,there is a corresponding decrease in peak stress ranging from 10%to 33%.Additionally,the cohesion exhibits a reduction of approximately 20%to 25%,while the internal friction angle experiences a decline of about 3.5%to 6%.These findings emphasize a significant unloading effect.Moreover,the degree of peak stress degradation in unloading jointed fine sandstone diminishes with an increase in confining pressure,suggesting that confining pressure mitigates the deterioration caused by dry-wet cycles.Additionally,as the number of dry-wet cycles increases,there is a notable decline in the mechanical properties of the sandstone,evidencing significant dry-wet degradation.Utilizing the Drucker Prager criterion,this study establishes a strength criterion and fracture criterion,denoted as σ_(1)(m,n) and K_(T)^(Ⅱ)(m, n), to quantify the combined impacts of dry-wet cycles and unloading on jointed fine sandstone,which provides a comprehensive understanding of its mechanical behavior under such conditions.

Effects of pressure relief holes on coal burst:Insights from true-triaxial unloading tests

Drilling pressure relief is one of the methods to reduce the risk of coal bursts in deep mines.However,the effect of the drill hole orientations has not been studied well enough to understand their impact on the burst failure mechanism.In this study,we investigated two designs of drill hole orientations.The first design includes drill holes located on the upper free face of the rectangular samples and labelled as upper hole(UH)and centre hole(CH)e the long axes of the drill holes are aligned with minor principal stress,s3,direction.The second design includes drill holes at the top(TH)and the side(SH)of the rectangular samples in which the long axes of the drill holes are aligned with the maximum,s1,and intermediate principal stress,s2,directions,respectively.The coal samples with the proposed drill hole orientations were subjected to the true-triaxial unloading coal burst tests.The results show that the drill holes reduce the risk of coal bursts.However,we found that the intensity of coal burst was significantly reduced with the SH-type,followed by the CH-types.We also observed that the coal burst intensity is reduced better for the CH,UH,TH,and SH-type drilling patterns.However,it was found that the orientations of drill holes have little influence on the failure mode(splitting).The acoustic emission(AE)activities for coal with drill holes noticeably decreased,especially for the UH and CH layouts.The drill holes reduced the upper limit of the AE entropy(chaos of microcracks generation).However,regarding reducing the coal burst risk,the TH and SH are less effective than UH and CH.

Forming and Springback Prediction of Strips Under Multi-square Punch Concave Forming Process Considering Partial-unloading Effects

To further investigate the forming mechanism and springback characteristics of strips under multi-square punch forming (MSPF) considering partial-unloading effects, a series of concave form ing tests of strips are conducted on the MSPF machine. This paper aims to reveal the physical mecha nism of the elastic-plastic deformation in the MSPF process considering the effect of the forming ap proaches, and derive appropriate mathematical interpretations. The theoretical model is firstly estab lished to analyse the concave forming mechanism and springback characteristics of the strip, and its accuracy is then validated by experimental data. The forming history and load evolutions are depicted to explore the required forming capacity through the proposed analytical method. Besides, the paramet ric studies are carried out to discuss their effects on the springback of the strip. The results suggest that the deformation paths of the strip are influenced by the forming approach, and the springback of the strip in convex forming is larger than that in concave forming.

Bioresorbable stent unloading during percutaneous coronary intervention:Early detection and management

In this letter,we comment on a recent case report by Sun et al in the World Journal of Cardiology.The report describes the successful management of a rare complication:The unloading or detachment of a bioresorbable stent(BRS)during percutaneous coronary intervention(PCI)in a male patient.The unloading of BRS was detected via angiography and intravascular ultrasound(IVUS)imaging of the left coronary artery and left anterior descending artery.Although this case is interesting,the authors’report lacked crucial details.Specifically,insufficient information about the type of BRS used,potential causes of BRS unloading,or whether optical coherence tomography(OCT)imaging for coronary arteries was performed before,during,or after PCI.The OCT imaging of coronary arteries before PCI can potentially prevent BRS unloading due to its higher resolution compared to IVUS.In addition,despite detecting myocardial bridging during the PCI,the authors did not provide any details regarding this variation.Here we discuss the various types of BRS,the importance of OCT in PCI,and the clinical relevance of myocardial bridging.

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.

岩石蠕变模型的二次开发

岩石的非线性蠕变本构模型的二次开发是研究岩石非线性蠕变的重点。本文对前人建立的本构模型进行三维差分公式推导,将三维公式写入FLAC^(3D)软件,进行本构模型的二次开发;对比分析数值模拟和试验数据的拟合程度,验证模型的正确性。

Failure properties of rocks in true triaxial unloading compressive test

Slabbing failure often occurs in the surround rock near a deep underground excavation. The mechanism of slabbing failure is still unclear. In order to reveal the influence of the intermediate principal stress （σ2） on slabbing failure, true triaxial unloading compressive test was carried out based on the stress path of the underground engineering excavation, i.e., unloading the minimum principal stress （σ3）, keeping σ2, increasing the maximum principal stress （σ1）. The initiation and the propagation of slabbing fracture in rock specimens were identified by examining the acoustic emission （AE） and the infrared radiation characterization. The test results show that the failure modes of the granite and red sandstone specimens are changed from shear to slabbing with the increase of σ2. The AE characteristic of rock specimen under low σ2 is swarm type which is the main shock type under high σ2. The infrared radiation properties of rock specimen under different σ2 are also different. The temperature change area is just along the shear fracture such as the uniaxial compression. With the increase of σ2, the temperature change area is planar of rock specimen which proofs that the failure mode of rock specimen turns into slabbing.

Energy conversion of rocks in process of unloading confining pressure under different unloading paths

Based on energy theory and tests of rocks with initial confining pressures of 10, 20 and 30 MPa under different unloading paths, the processes of strain energy conversion were investigated. The absorbing strain energy for axial compression, the dissipating strain energy for plastic deformation and cracks propagation, the expending strain energy for circumferential deformation, and the storing and releasing elastic strain energy were considered. Unloading paths included the condition of fixing axial pressure and unloading axial pressure, increasing axial pressure and unloading confining pressure, as well as unloading axial pressure and confining pressure simultaneously. Results show that expending strain energy for circumferential deformation has mainly evolved from absorbing strain energy for axial compression in three unloading paths during unloading processes. Dissipating strain energy is significantly increased only near the peak point. The effect of initial confining pressure on strain energy is significantly higher than that of unloading path. The strain energy is linearly increased with increasing initial confining pressure. The unloading path and initial confining pressure also have great influence on the energy dissipation. The conversion rate of strain energy in three paths is increased with increasing initial confining pressure, and the effect of initial confining pressure on conversion rate of strain energy is related with the unloading paths.

Effect of induction unloading on weakening of rock mechanics properties

The effects of induction unloading such as drilling, blasting, lancing and water-infusion softening on weakening of rock mechanics properties were investigated. Three stress paths were chosen as test schemes corresponding to the triaxial compressive test, pre-peak and post-peak unloading the confining pressure tests. The results show that compression deformation is the main cause of rock failure under loading condition. However, the strong dilatation leads to the rock failure along unloading direction. Rock failure happens even under little axial stress with confining pressure unloading. Poisson ratio increases with the decrease of confining pressure during the process of unloading. Elastic modulus increases slowly along with the decline of confining pressure, but decreases rapidly when unloaded to yielding strength. It shows that the weakening rate of rock intensity tends to be faster with easily failure under the unloading condition.

Intercellular Symplastic Connection and Isolation of the Unloading Zone in Flesh of the Developing Grape Berry

The ultrastructure and intercellular connection of the sugar unloading zone (i.e. the phloem in the dorsal vascular bundle and the phloem_surrounding the assimilate sink_cells) of grape ( Vitis vinifera× V. labrusca cv. Jingchao) berry was observed via transmission electron microscopy. The results showed that during the early developmental stages of grape berry, numerous plasmodesmata were found in the phloem between sieve element (SE) and companion cell (CC), between SE/CC complexes, between SE/CC complex and phloem parenchyma cell and in between phloem parenchyma cells, which made the phloem a symplastic integration, facilitating sugar unloading from sieve elements into both companion cells and phloem parenchyma cells via a symplastic pathway. On the contrary, there was almost no plasmodesma between phloem and its surrounding flesh photoassimilate sink_cells, neither in between the flesh photoassimilate sink_cells giving rise to a symplastic isolation both between phloem and its surrounding flesh photoassimilate sink_cells, as well as among the flesh photoassimilate sink_cells. This indicated that both the sugar unloading from phloem and postphloem transport of sugars should be mainly via an apoplastic pathway. During the ripening stage, most of the plasmodesmata between SE/CC complex and the surrounding phloem parenchyma cells were shown to be blocked by the electron_opaque globules, and a phenomenon of plasmolysis was found in a number of companion cells, indicating a symplastic isolation between SE/CC complex and its surrounding parenchyma cells during this phase. The symplastic isolation between the whole phloem and its surrounding photoassimilate sink_cells during the early developmental stages shifted to a symplastic isolation within the phloem during the ripening phase, and thus the symplastic pathway of sugar unloading from SE/CC complex during the early development stages should be replaced by a dominant apoplastic unloading pathway from SE/CC complex in concordance.

微重力导致骨质疏松的机制

背景:在微重力环境中骨吸收和骨形成的失衡,导致航天员出现严重的骨质流失。现有研究表明,微重力环境下的骨质流失是由多种细胞、组织和系统共同作用的结果。目的:综述微重力对不同的细胞、组织或系统产生不同的生物学效应,总结微重力导致骨质疏松发生的机制。方法:在Pub Med、Web of Science和Cochrane图书馆数据库检索2000-2023年的相关文献,纳入标准为与微重力导致骨质疏松相关的组织工程研究和基础研究的所有文章,最终对85篇文献进行综述。结果与结论:(1)在微重力环境下,骨髓间充质干细胞更倾向于向成脂细胞分化,减少向成骨细胞分化,而微重力环境中的造血干细胞则更倾向于分化为破骨细胞,减少向红系分化;同时,微重力抑制成骨细胞增殖和分化、促进成骨细胞凋亡、改变细胞形态及降低成骨细胞矿化能力,显著增加破骨细胞数量和活性并且阻碍成骨细胞向骨细胞分化,促进骨细胞的凋亡。(2)在微重力环境下,人体会出现骨骼肌萎缩、微血管重塑、骨微循环障碍及内分泌紊乱等组织或系统的变化,这些变化会导致骨骼微环境下机械应力卸载,血液灌注不足以及钙循环紊乱等,对骨质疏松的发生产生了重要的影响。(3)目前,微重力导致骨质疏松的机制较为复杂,通过更深入的研究这些生理机制对于确保航天员在长期太空任务中的健康至关重要,并为预防和治疗骨质疏松症提供了理论依据。

先导液控单向阀动态特性研究

介绍了一种用于液压支架的先导液控单向阀。首先,对比了先导液控单向阀与二级卸载液控单向阀的结构差异,说明了先导液控单向阀提高卸载速度的原理;然后,通过试验对这2种液控单向阀的动态特性进行了测试。基于AMESim软件搭建了先导液控单向阀和试验系统的仿真模型,模拟了降柱过程2种液控单向阀的动态特性,试验数据和AMESim仿真结果一致,证明先导液控单向阀能够缩短从小阀芯开启到大阀芯开启的时间差,在提高大缸径支架移架速度方面具备显著优势。提供了一种新的思路来提高液压支架的移架速度和采煤效率,并为立柱液控单向阀的动态特性研究提供了方法。

Research on unloading nonlinear mechanical characteristics of jointed rock masses

Geological environments of rock mass projects are always very complicated, and further investigations on rock mechanical characteristics are needed. There are considerable distinctions in rock mechanical characteristics under unloading and loading conditions. A series of tests are conducted to study the stress-strain relationship of rock masses under loading and unloading conditions. Also, the anisotropy, the size effect, and the rheological property of unloading rock mass are investigated. The tests presented in the paper include model test and granite rheological test, which are conducted considering geological condition, rock mass structure, in-situ stress field of the permanent shiplock of the Three Gorges Project. The main differences between loading and unloading rock masses are stress paths, yield criteria, deformation and strength parameters, etc.. Different structural plane directions affect unloading rock mass evidently. With increasing size, the tensile strength, the compressive strength, the deformation modulus, the Poisson’s ratio and the anisotropy of rock mass all decrease. For sandstone samples with parallel bedding planes, the cohesion c increases but the internal friction angle ? decreases under unloading condition when compared with the values under loading condition. While for samples with vertical bedding planes, the trend is adverse. The rheological property of rocks has close relationship with the tensile stresses of rock masses. When the sandstone samples are tested under high stress condition, their rheological properties are very obvious with the unloading of confining pressure, and three typical rheological stages are shown. Rheological rate changes with the variations in axial stress and confining pressure.

Mechanical property and permeability of raw coal containing methane under unloading confining pressure

Based on domestic-developed triaxial servo-controlled seepage equipment for thermal-hydrologicalmechanical coupling of coal containing methane,an experimental study was carried out to investigate mechanical property and gas permeability of raw coal,under the situation of conventional triaxial compression and unloading confining pressure tests in different gas pressure conditions.Triaxial unloading confining pressure process was reducing confining pressure while increasing axial pressure.The research results show that,compared with the peak intensity of conventional triaxial loading,the ultimate strength of coal samples of triaxial unloading confining pressure was lower,deformation under loading was far less than unloading,dilation caused by unloading was more obvious than loading.The change trend of volumetric strain would embody change of gas permeability of coal,the permeability first reduced along with volumetric strain increase,and then raised with volume strain decrease,furthermore,the change trends of permeability of coal before and after destruction were different in the stage of decreasing volume strain due to the effect of gas pressure.When gas pressure was greater,the effective confining pressure was smaller,and the radial deformation produced by unloading was greater.When the unloading failed confining pressure difference was smaller,coal would be easier to get unstable failure.

Directivity effect of unloading bedding coal induced fracture evolution and its application

In the coal mining process, the gas contained in the coalbed is one source of the most serious accident hazards. Stress releasing from the coal deposit is the main controlling factor that leads to such accidents.Based on the bedding of coal samples, the gas permeability well describes the evolution of fracture, so the paper carries out research on the permeability properties of coal under different unloading directions.The research obtains that when the stress unloading direction is perpendicular to bedding, more penetrating fractures and bedding fractures occur, and the permeability significantly increases. Although the axial stress reduced, the confining pressure makes the permeability of the bedding plane fracture exist under constant. The permeability obtained when the unloading direction was perpendicular to the bedding is 52 times larger than that when it is parallel to the bedding. The results show that the efficiency of gas drainage is impacted by the relative direction of gas drainage drilling in relation to the bedding orientations. The maximum amount of gas extraction when drilling is perpendicular to the bedding is 1.3 times than that when drilling is oblique to the bedding, and 1.75 times than that when drilling is parallel to the bedding.