Influencing factors analysis of hard limestone reformation and strength weakening under acidic effect

Roof disaster has always been an important factor restricting coal mine safety production.Acidic effect can reform the rock mass structure to weaken the macroscopic strength characteristics,which is an effective way to control the hard limestone roof.In this study,the effects of various factors on the reaction characteristics and mechanical properties of limestone were analyzed.The results show that the acid with stronger hydrogen production capacity after ionization(pK_(a)<0)has more prominent damage to the mineral grains of limestone.When pKa increases from−8.00 to 15.70,uniaxial compressive strength and elastic modulus of limestone increase by 117.22%and 75.98%.The influence of acid concentration is manifested in the dissolution behavior of mineral crystals,the crystal defects caused by large-scale acid action will lead to the deterioration of limestone strength,and the strength after 15%concentration reformation can be reduced by 59.42%.The effect of acidification time on limestone has stages and is the most obvious in the initial metathesis reaction stage(within 60 min).The key to the strength damage of acidified limestone is the participation of hydrogen ions in the reaction system.Based on the analytic hierarchy process method,the influence weights of acid type,acid concentration and acidification time on strength are 24.30%,59.54% and 16.16%,respectively.The research results provide theoretical support for the acidification control of hard limestone roofs in coal mines.

Dynamic simulation insights into friction weakening effect on rapid long-runout landslides:A case study of the Yigong landslide in the Tibetan Plateau,China

This study proposed a novel friction law dependent on velocity,displacement and normal stress for kinematic analysis of runout process of rapid landslides.The well-known Yigong landslide occurring in the Tibetan Plateau of China was employed as the case,and the derived dynamic friction formula was included into the numerical simulation based on Particle Flow Code.Results showed that the friction decreased quickly from 0.64(the peak)to 0.1(the stead value)during the 5s-period after the sliding initiation,which explained the behavior of rapid movement of the landslide.The monitored balls set at different sections of the mass showed similar variation characteritics regarding the velocity,namely evident increase at the initial phase of the movement,followed by a fluctuation phase and then a stopping one.The peak velocity was more than 100 m/s and most particles had low velocities at 300s after the landslide initiation.The spreading distance of the landslide was calculated at the two-dimension(profile)and three-dimension scale,respectively.Compared with the simulation result without considering friction weakening effect,our results indicated a max distance of about 10 km from the initial unstable position,which fit better with the actual situation.

Borehole stability in naturally fractured rocks with drilling mud intrusion and associated fracture strength weakening:A coupled DFN-DEM approach

Borehole instability in naturally fractured rocks poses significant challenges to drilling.Drilling mud invades the surrounding formations through natural fractures under the difference between the wellbore pressure(P w)and pore pressure(P p)during drilling,which may cause wellbore instability.However,the weakening of fracture strength due to mud intrusion is not considered in most existing borehole stability analyses,which may yield significant errors and misleading predictions.In addition,only limited factors were analyzed,and the fracture distribution was oversimplified.In this paper,the impacts of mud intrusion and associated fracture strength weakening on borehole stability in fractured rocks under both isotropic and anisotropic stress states are investigated using a coupled DEM(distinct element method)and DFN(discrete fracture network)method.It provides estimates of the effect of fracture strength weakening,wellbore pressure,in situ stresses,and sealing efficiency on borehole stability.The results show that mud intrusion and weakening of fracture strength can damage the borehole.This is demonstrated by the large displacement around the borehole,shear displacement on natural fractures,and the generation of fracture at shear limit.Mud intrusion reduces the shear strength of the fracture surface and leads to shear failure,which explains that the increase in mud weight may worsen borehole stability during overbalanced drilling in fractured formations.A higher in situ stress anisotropy exerts a significant influence on the mechanism of shear failure distribution around the wellbore.Moreover,the effect of sealing natural fractures on maintaining borehole stability is verified in this study,and the increase in sealing efficiency reduces the radial invasion distance of drilling mud.This study provides a directly quantitative prediction method of borehole instability in naturally fractured formations,which can consider the discrete fracture network,mud intrusion,and associated weakening of fracture strength.The information provided by the numerical approach(e.g.displacement around the borehole,shear displacement on fracture,and fracture at shear limit)is helpful for managing wellbore stability and designing wellbore-strengthening operations.

Review of Field Weakening Control Strategies of Permanent Magnet Synchronous Motors

Due to high power density,high efficiency,and accurate control performance,permanent magnet synchronous motors(PMSMs)have been widely adopted in equipment manufacturing and energy transformation fields.To expand the speed range under finite DC-bus voltage,extensive research on field weakening(FW)control strategies has been conducted.This paper summarizes the major FW control strategies of PMSMs,which are categorized into calculation-based methods,voltage closed-loop control methods,and model predictive control related methods.The existing strategies are analyzed and compared according to performance,robustness,and execution difficulty,which can facilitate the implementation of FW control.

Strength weakening and its micromechanism in water–rock interaction,a short review in laboratory tests

Water–rock interaction(WRI)is a topic of interest in geology and geotechnical engineering.Many geological hazards and engineering safety problems are severe under the WRI.This study focuses on the water weakening of rock strength and its infuencing factors(water content,immersion time,and wetting–drying cycles).The strength of the rock mass decreases to varying degrees with water content,immersion time,and wetting–drying cycles depending on the rock mass type and mineral composition.The corresponding acoustic emission count and intensity and infrared radiation intensity also weaken accordingly.WRI enhances the plasticity of rock mass and reduces its brittleness.Various microscopic methods for studying the pore characterization and weakening mechanism of the WRI were compared and analyzed.Various methods should be adopted to study the pore evolution of WRI comprehensively.Microscopic methods are used to study the weakening mechanism of WRI.In future work,the mechanical parameters of rocks weakened under long-term water immersion(over years)should be considered,and more attention should be paid to how the laboratory scale is applied to the engineering scale.

Marginal weakening promoted rift propagation of the Petermann Ice Shelf in northwestern Greenland from 2016 to 2022

Ice-shelf rifts are precursors of glacier calving, and thus they serve as indicators of ice shelf instability, especially under oceanic and atmospheric warming conditions. Therefore, understanding the dynamic processes underlying rift propagation and the associated damage mechanisms is essential to evaluate ice-shelf instability and to predict glacier calving. In this study, we investigated the effect of marginal weakening on rift propagation on the ice shelf of the Petermann Glacier, among the largest in Greenland, during 2016-2022. First, we analyzed satellite optical images to monitor rift growth(length and width) by tracking the tip trajectory of three large rifts identified on the Petermann Ice Shelf. Then, we estimated rift depth using ArcticDEM and ICESat-2 data. Our results indicated consistent increases of the rift widths and depths over the study period, with mean values of 133 m·a^(-1) and 0.3 m·a^(-1), respectively. We also combined remote-sensing observations with an ice-sheet numerical model to calculate the stress and damage fields on the Petermann Ice Shelf and to assess the ice shelf margin stability and strength. We determined that damage and lateral shear in the fracture zone degraded ice shelf integrity by decreasing the contact length with the fjord wall. In conclusion, marginal weakening effectively promoted rift propagation on the Petermann Ice Shelf, increasing the risk of future glacier calving.

The Non-Singular Fast Terminal Sliding Mode Control of Interior Permanent Magnet Synchronous Motor Based on Deep Flux Weakening Switching Point Tracking

This paper presents a novel non-singular fast terminal sliding mode control(NFTSMC)based on the deep flux weakening switching point tracking method in order to improve the control performance of permanent interior magnet synchronous motor(IPMSM)drive systems.The mathematical model of flux weakening(FW)control is established,and the deep flux weakening switching point is calculated accurately by analyzing the relationship between the torque curve and voltage decline curve.Next,a second-order NFTSMC is designed for the speed loop controller to ensure that the system converges to the equilibrium state in finite time.Then,an extended sliding mode disturbance observer(ESMDO)is designed to estimate the uncertainty of the system.Finally,compared with both the PI control and sliding mode control(SMC)by simulations and experiments with different working conditions,the method proposed has the merits of accelerating convergence,improving steady-state accuracy,and minimizing the current and torque pulsation.

The Weakening of the Asian Monsoon Circulation after the End of 1970's

The transition of the global atmospheric circulation in the end of 1970's can clearly be detected in the atmospheric temperature, wind velocity, and so on. Wavelet analysis reveals that the temporal scale of this change is larger than 20 years. Studies in this work indicate that the trend of the transition over the mid-latitude Asia is opposite to that of global average for some variables at the middle troposphere. Another finding of this research is that the African-Asian monsoon circulation is weaker and the trade wind over the tropical eastern Pacific is weaker as well after this transition. Such a signal may be found in the summer precipitation over China as well. Key words Asian monsoon circulation - Weakening - Transition This research was supported by the key project of Chinese Academy of Sciences under Contract KZCX2-203 and the key program of the National Natural Science Foundation of China under Contract 49894170.

Modeling of normal faulting in the subducting plates of the Tonga,Japan,Izu-Bonin and Mariana Trenches:implications for near-trench plate weakening

The plate flexure and normal faulting characteristics along the Tonga, Japan, Izu-Bonin and Mariana Trenches are investigated by combining observations and modeling of elastoplastic deformation of the subducting plate. The observed average trench relief is found to be the smallest at the Japan Trench(3 km) and the largest at the Mariana Trench(4.9 km), and the average fault throw is the smallest at the Japan Trench(113 m) and the largest at the Tonga Trench(284 m). A subducting plate is modeled to bend and generate normal faults subjected to three types of tectonic loading at the trench axis: vertical loading, bending moment, and horizontal tensional force. It is inverted for the solutions of tectonic loading that best fit the observed plate flexure and normal faulting characteristics of the four trenches. The results reveal that a horizontal tensional force(HTF) for the Japan Trench is 33%, 50% and 60% smaller than those of the Mariana, Tonga and Izu-Bonin Trenches, respectively. The normal faults are modeled to penetrate to a maximum depth of 29, 23, 32 and 32 km below the sea floor for the Tonga,Japan, Izu-Bonin and Mariana Trenches, respectively, which is consistent with the depths of relocated normal faulting earthquakes in the Japan and Izu-Bonin Trenches. Moreover, it is argued that the calculated horizontal tensional force is generally positively correlated with the observed mean fault throw, while the integrated area of the reduction in the effective elastic thickness is correlated with the trench relief. These results imply that the HTF plays a key role in controlling the normal faulting pattern and that plate weakening can lead to significant increase in the trench relief.

Exploration of weakening mechanism of uniaxial compressive strength of deep sandstone under microwave irradiation

Traditional mechanical rock breaking method is labor-intensive and low-efficient,which restrictes the development of deep resources and deep space.As a new rock-breakage technology,microwave irradiation is expected to overcome these problems.This study examines the failure characteristics,weakening law,and breakdown mechanism of deep sandstone(depth=1050 m)samples in a microwave field.The macroscopic and microscopic properties were determined via mechanical tests,mesoscopic tests,and numerical simulations.Microwave application at 1000 W for 60 s reduced the uniaxial compressive strength of the sandstone by 50%.Thermal stress of the sandstone was enhanced by uneven expansion of minerals at the microscale.Moreover,the melting of some minerals in the high-temperature environment changed the pore structure,sharply reducing the macroscopic strength.The temperature remained high in the lower midsection of the sample,and the stress was concentrated at the bottom of the sample and along its axis.These results are expected to improve the efficiency of deep rock breaking,provide theoretical and technical support for similar rock-breakage projects,and accelerate advances in deep-Earth science.

Evaluation of the weakening behavior of gas on the coal strength and its quantitative influence on the coal deformation

The coal strength and deformation properties are key factors affecting safe coal mining and highefficiency coalbed methane(CBM)development.In this paper,reconstituted coal samples are chosen to investigate the weakening behavior of gas on coal strength,meanwhile,its effects on coal deformation are quantitatively evaluated.The results indicate that the weakening degree of gas on coal strength is closely related to the confining stress and gas pressure.Compared with non-gas-saturated coals,the maximum weakening ratios of adsorbed gas to coal strength are 10.58%,18.12%,8.55%and 14.65%under the conditions of confining stress CS=3 MPa and gas pressure GP=1 MPa,CS=3 MPa and GP=2 MPa,CS=4 MPa and GP=1 MPa,and CS=4 MPa and GP=2 MPa,respectively.Furthermore,the maximum weakening ratios of free gas to coal strength are 18.27%,36.54%,14.79%and 29.58%,respectively,under above four conditions.The maximum coal bulk strain decreases as particle sizes of coal powders increase,and it has a maximum value of 0.0227 and a minimum value of 0.0191 in particle size ranges of 0.01–0.041 and 0.5–1 mm.Under the same conditions,the coal bulk strain increases with increasing gas pressure,revealing that coal deformation properties can be enhanced by gas.

The influence of borehole arrangement of soundless cracking demolition agents(SCDAs)on weakening the hard rock

The hard roof difficult to collapse easily causes gas accumulation,which threatens the production safety of coal mine.Therefore,roof pre-cracking is required.Although blasting and hydraulic fracturing can also crack the roof,blasting can easily induce rock bursts,whereas hydraulic fracturing needs complex equipment.In contrast,soundless cracking demolition agents(SCDAs)with noise-free,dust-free,and safe characteristics have obvious advantages.The main component of SCDA is calcium oxide,which reacts with water to produce higher expansion pressure.In this paper,focused on the angles of the borehole,the effect of SCDA is analyzed by numerical simulation based on Pingdingshan coal mine.The research results showed that the azimuthal angle a(between borehole projection and the roadway direction)does not significantly affect the efficacy of SCDAs,whereas the influence of borehole elevation angle b is far more significant than that of the azimuthal angle.Therefore,the angle b is a dominant factor influencing the effect of SCDAs.Based on different effects of SCDAs at different angle of boreholes,the weakening unit was established,so the SCDAs could give full play to roof fracturing.Moreover,field tests validated the importance of borehole angle on weakening the hard roofs.

Strength weakening effect of high static pre-stressed granite subjected to low-frequency dynamic disturbance under uniaxial compression

This study aimed to elucidate the strength weakening effect of high static pre-stressed rocks subjected to low-frequency disturbances under uniaxial compression.Based on the uniaxial compressive strength(UCS)of granite under static loading,70%,80%,and 90%of UCS were selected as the initial high static pre-stress(σ_(p)),and then the pre-stressed rock specimens were disturbed by sinusoidal stress with amplitudes of 30%,20%,and 10%of UCS under low-frequency frequencies(f)of 1,2,5,and 10 Hz,respectively.The results show that the rockburst failure of pre-stressed granite is caused by low-frequency disturbance,and the failure strength is much lower than UCS.When theσp or f is constant,the specimen strength gradually decreases as the f or σ_(p) increases.The experimental study illustrates the influence mechanism of the strength weakening effect of high static pre-stress rocks under low-frequency dynamic disturbance,that is,high static pre-stress is the premise and leading factor of rock strength weakening,while low-frequency dynamic disturbance induces rock failure and affects the strength weakening degree.

Experimental investigation on the reformation and strength weakening of hard limestone by acidizing

Several derivative disasters such as ground pressure disasters and methane explosions can be caused by the hard roof in coal mines.For limestone roofs with fine integrity and extreme hardness,collapse is difficult and the effect of conventional roof control methods is limited.Acidizing reformation is an effective way to weaken the strength of roof strata based on acid-rock reaction.In this study,the rock strength damage law and acid reaction characteristics were tested by the limestone acidification experiment.Besides,the strength degradation mechanism of limestone under the acidity effect was analyzed.The results show that the acid corrosion characteristics of limestone are obvious,as numerous mineral grains generate voids under the effects of acid corrosion,and more defects are formed inside.The acid-rock reaction is the most intense at the early stage and then gradually reaches dynamic equilibrium,and the acid corrosion rate of limestone is 4.24%(10%HCl,360 min).The hard limestone is damaged after acidification.Furthermore,the internal cracks can be induced to rapid initiation and unstable propagation under load,which reduces the strain required for rock failure by 33.33%.The failure morphology is more complicated,and the uniaxial compressive strength and elastic modulus decrease by 52.42%and 34.44%respectively.The strength weakening of hard roof after acidification is due to the defects such as intergranular cracking caused by the corrosion of rock crystals under acidity effect,which accelerate the initiation and propagation of internal cracks with external force.Macroscopically,acidification induced the deterioration of rock mechanical properties by reforming the roof structure.The feasibility of acidizing reformation method to control hard roof is confirmed in this study.

COMPARATIVE ANALYSIS OF THE INTENSIFYING AND WEAKENING LANDFALL TROPICAL CYCLONES DURING EXTRATROPICAL TRANSITION OVER CHINA

Based on the Tropical Cyclone(TC) Yearbooks data and JRA-25 reanalysis data from the Japan Meteorological Agency(JMA) during 1979-2008, dynamic composite analysis and computation of kinetic energy budget are used to study the intensifying and weakening TCs during Extratropical Transition over China. The TCI shows strong upper-level divergence, strengthened low-level convergence and significantly enhanced upward motion under the influence of strong upper-level troughs and high-level jets. The TCI is correspondingly intensified after Extratropical Transition(ET); TCW exhibits strong upper-level divergence, subdued low-level convergence and slightly enhanced upward motion under the influence of weak upper-level troughs and high-level jets. It then weakens after ET. The increase(decrease) of the generation of kinetic energy by divergence wind in TCI(TCW) at low level is one of the major reasons for TCI's intensification(TCW's weakening) after transformation. The generation of kinetic energy by divergence wind is closely related to the development of a low-level baroclinic frontal zone. The growth of the generation of kinetic energy by rotational wind in TCI at upper level is favorable for TCI's maintenance, which is affected by strong upper-level troughs. The dissipation of the generation of kinetic energy by rotational wind in TCW at upper level is unfavorable for TCW's maintenance, which is affected by weak upper-level troughs.

Weakening effect of plastic yielding inception in thin hard coating systems

Hard coatings have been widely applied to enhance tribological performance of mechanical components.However,it was predicted that thin hard coatings may have a weakening effect which could reduce the coating/substrate system’s resistance to plastic yielding compared with the uncoated substrate material.In this paper,analytical simulation is utilized to investigate the origin of weakening effect.The functions of material mechanical properties and coating thickness on the weakening effect are theoretically investigated.Partial-unloading spherical nanoindentation tests are performed on tungsten coated single crystalline silicon and copper to acquire the stress-strain curves and compared with the uncoated cases.The experimental results are in consistence with the analytical solutions,demonstrating the presence of weakening effect.

Using the Fractional Order Method to Generalize Strengthening Buffer Operator and Weakening Buffer Operator

To reveal the relationship between a weakening buffer operator and strengthening buffer operator, the traditional integer order buffer operator is extended to one that is fractional order. Fractional order buffer operator not only can generalize the weakening buffer operator and the strengthening buffer operator, but also results in small adjustments of the buffer effect.The effectiveness of the grey model(GM(1,1)) with the fractional order buffer operator is validated by six cases.

Hybrid assessment of pre-blasting weakening to horizontal section top coal caving (HSTCC) in steep and thick seams

Horizontal section top-coal(HSTCC)caving offers a powerful method to efficiently excavate rude coal in steep and thick seams,and pre-blasting weakening has a profound effect on pursuing great production,high efficiency and good benefit under particular conditions like a small-scale working face with large-scale sectional caving height.+564-level HSTCC working face in B3–6coal seams of Jiangou Colliery in Urumqi was taken as study case for in situ industrial experiment.Total thickness of seams in the study case is about 50.0 m and average angel here is over 83°.In the industrial experiments,at first we adopted continuous charge machine and emulsion matrix explosive to substitute for traditional blasting schemes for specific geological settings in the study case.Hybrid analyses and assessments with blasting crack propagation analysis,abutment pressure monitoring prediction and economical benefit assessment were attributed to be able to attest pre-blasting weakening effects practically.Meanwhile crack propagation analysis after pre-blasting weakening showed that in all triple monitoring bore holes rock masses of top-coal would be fallen into three stages from the bottom up:fracture zone,plastic zone and elastic zone generally,and fracture toughness respectively in correspondent zones was calculated by the analytical formula:0.5616–0.8806,0.6403–0.9541 and0.7535–1.1900 MPa m1/2after pre-blasting weakening.Pressure monitoring prediction and economical benefit assessment also indicated that it was necessary to introduce the pre-blasting weakening with predominant blasting scheme from both views.For excavation in extremely steep and thick coal seams,relevant results would be a useful tool to study the mechanism of pre-blasting weakening both qualitatively and quantitatively.

A STUDY ON THE MECHANISM OF RAPID WEAKENING OF TYPHOON XANGSANE (0020) OVER THE EAST CHINA SEA

： Using the National Center for Environmental Prediction reanalysis data on 1.0°×l.0° grids and data from theTropical Cyclone yearbook （2000）, a diagnostic analysis and numerical simulation were performed to investigate the characteristics and mechanism underlying the rapid weakening of typhoon Xangsane. The results show that a sharp decline in the intensity of typhoon Xangsane resulted from its movement into the cool sea surface temperature area in the East China Sea, the intrusion of cold air from the mainland into the typhoon, and a rapid increase of the vertical wind shear in the surrounding environment. An important factor that led to the demise of the typhoon was a significant decrease in the moisture transport into the typhoon. Furthermore, the results of the numerical simulation and sensitivity experiments indicate that sea surface temperature largely modulated the rapid weakening of typhoon Xangsane.

Weakening Mechanisms of Platelike σ Phase on Ni-base Superalloys

On the basis of an investigation on σ phase in Ni-base cast superalloy K24 and the results about σ phase in other Ni-base superalloys,an embrittl- ing mechanism and a softening mechanism,by which platelike σ phase weakens the Ni-base superalloys,have been proposed.It is considered that the platelike morphology and the habit precipi- tation along{111}of σ phase are necessary condi- tions for both mechanisms.The embrittling mecha- nism is dominant at room temperature and high strain rate,and the softening mechanism is domi- nant at high temperature and low strain rate.Ac- cording to the idea of the softening mechanism and the analyses of σ phase and alloy compositions,it is considered that Nb,Mo and W in the alloys may be resistant to the detrimental effect of σ phase on the stress-rupture properties of the alloys.