Mesoscopic fracture damage evolution and fractal damage constitutive model of heat-treated red sandstone under direct tensile impact loadings

Understanding the mesoscopic tensile fracture damage of rock is the basis of evaluating the deterioration process of mechanical properties of heat-damaged rock. For this, tensile tests of rocks under high-temperature treatment were conducted with a ϕ75 mm split Hopkinson tension bar (SHTB) to investigate the mesoscopic fracture and damage properties of rock. An improved scanning electron microscopy (SEM) experimental method was used to analyze the tensile fracture surfaces of rock samples. Qualitative and quantitative analyses were performed to assess evolution of mesoscopic damage of heat-damaged rock under tensile loading. A constitutive model describing the mesoscopic fractal damage under thermo-mechanical coupling was established. The results showed that the high temperatures significantly reduced the tensile strength and fracture surface roughness of the red sandstone. The three-dimensional (3D) reconstruction of the fracture surface of the samples that experienced tensile failure at 900 °C showed a flat surface. The standard deviation of elevation and slope angle of specimen fracture surface first increased and then decreased with increasing temperature. The threshold for brittle fracture of the heat-damaged red sandstone specimens was 600 °C. Beyond this threshold temperature, local ductile fracture occurred, resulting in plastic deformation of the fracture surface during tensile fracturing. With increase of temperature, the internal meso-structure of samples was strengthened slightly at first and then deteriorated gradually, which was consistent with the change of macroscopic mechanical properties of red sandstone. The mesoscopic characteristics, such as the number, mean side length, maximum area, porosity, and fractal dimension of crack, exhibited an initial decline, followed by a gradual increase. The development of microcracks in samples had significant influence on mesoscopic fractal dimension. The mesoscopic fractal characteristics were used to establish a mesoscopic fractal damage constitutive model for red sandstone, and the agreement between the theoretical and experimental results validated the proposed model.

Novel damage constitutive models and new quantitative identification method for stress thresholds of rocks under uniaxial compression

Four key stress thresholds exist in the compression process of rocks,i.e.,crack closure stress(σ_(cc)),crack initiation stress(σ_(ci)),crack damage stress(σ_(cd))and compressive strength(σ_(c)).The quantitative identifications of the first three stress thresholds are of great significance for characterizing the microcrack growth and damage evolution of rocks under compression.In this paper,a new method based on damage constitutive model is proposed to quantitatively measure the stress thresholds of rocks.Firstly,two different damage constitutive models were constructed based on acoustic emission(AE)counts and Weibull distribution function considering the compaction stages of the rock and the bearing capacity of the damage element.Then,the accumulative AE counts method(ACLM),AE count rate method(CRM)and constitutive model method(CMM)were introduced to determine the stress thresholds of rocks.Finally,the stress thresholds of 9 different rocks were identified by ACLM,CRM,and CMM.The results show that the theoretical stress−strain curves obtained from the two damage constitutive models are in good agreement with that of the experimental data,and the differences between the two damage constitutive models mainly come from the evolutionary differences of the damage variables.The results of the stress thresholds identified by the CMM are in good agreement with those identified by the AE methods,i.e.,ACLM and CRM.Therefore,the proposed CMM can be used to determine the stress thresholds of rocks.

Damage Mechanism of Ultra-thin Asphalt Overlay(UTAO) based on Discrete Element Method

Aiming to analyze the damage mechanism of UTAO from the perspective of meso-mechanical mechanism using discrete element method(DEM),we conducted study of diseases problems of UTAO in several provinces in China,and found that aggregate spalling was one of the main disease types of UTAO.A discrete element model of UTAO pavement structure was constructed to explore the meso-mechanical mechanism of UTAO damage under the influence of layer thickness,gradation,and bonding modulus.The experimental results show that,as the thickness of UTAO decreasing,the maximum value and the mean value of the contact force between all aggregate particles gradually increase,which leads to aggregates more prone to spalling.Compared with OGFC-5 UTAO,AC-5 UTAO presents smaller maximum and average values of all contact forces,and the loading pressure in AC-5 UTAO is fully diffused in the lateral direction.In addition,the increment of pavement modulus strengthens the overall force of aggregate particles inside UTAO,resulting in aggregate particles peeling off more easily.The increase of bonding modulus changes the position where the maximum value of the tangential force appears,whereas has no effect on the normal force.

Enhanced damage mechanism of reinforced concrete targets impacted by reactive PELE: An analytical model and experimental validation

Compared with PELE with inert fillings such as polyethylene and nylon,reactive PELE(RPELE)shows excellent damage effects when impacting concrete targets due to the filling deflagration reaction.In present work,an analytical model describing the jacket deformation and concrete target damage impacted by RPELE was presented,in which the radial rarefaction and filling deflagration reaction were considered.The impact tests of RPELE on concrete target in the 592-1012 m/s were carried out to verify the analytical model.Based on the analytical model,the angle-length evolution mechanism of the jacket bending-curling deformation was revealed,and the concrete target damage was further analyzed.One can find out that the average prediction errors of the front crater,opening and back crater are 6.8%,8.5%and 7.1%,respectively.Moreover,the effects of radial rarefaction and deflagration were discussed.It was found that the neglect of radial rarefaction overestimates the jacket deformation and concrete target damage,while the deflagration reaction of filling increases the diameter of the front crater,opening and back crater by 25.4%,24.3%and 31.1%,respectively.The research provides a valuable reference for understanding and predicting the jacket deformation and concrete target damage impacted by RPELE.

Constitutive model of viscoelastic dynamic damage for the material of gas obturator in modular-charge howitzer

In order to investigate the mechanical response behavior of the gas obturator of the breech mechanism,made of polychloroprene rubber(PCR), uniaxial compression experiments were carried out by using a universal testing machine and a split Hopkinson pressure bar(SHPB), obtaining stress-strain responses at different temperatures and strain rates. The results revealed that, in comparison to other polymers, the gas obturator material exhibited inconspicuous strain softening and hardening effects;meanwhile, the mechanical response was more affected by the strain rate than by temperature. Subsequently, a succinct viscoelastic damage constitutive model was developed based on the ZWT model, including ten undetermined parameters, formulated with incorporating three parallel components to capture the viscoelastic response at high strain rate and further enhanced by integrating a three-parameter Weibull function to describe the damage. Compared to the ZWT model, the modified model could effectively describe the mechanical response behavior of the gas obturator material at high strain rates. This research laid a theoretical foundation for further investigation into the influence of chamber sealing issues on artillery firing.

Temperature dependence of mechanical properties and damage evolution of hot dry rocks under rapid cooling

Understanding the differences in mechanical properties and damage characteristics of granitoid under high temperatures is crucial for exploring deep geothermal resources.This study analyzes the evolution of the acoustic emission(AE)characteristics and mechanical parameters of granodiorite and granite after heating and water cooling by uniaxial compression and variable-angle shear tests under different temperature gradients.We identify their changes in mesostructure and mineral composition with electron probe microanalysis and scanning electron microscopy.Results show that these two hot dry rocks have similar diagenetic minerals and microstructure,but show significantly different mechanical and acoustic characteristics,and even opposing evolution trends in a certain temperature range.At the temperatures ranging from 100℃to 500℃,the compressive and shear mechanical properties of granodiorite switch repeatedly between weakening and strengthening,and those of granite show a continuous weakening trend.At 600℃,both rocks exhibit a deterioration of mechanical properties.The damage mode of granite is characterized by initiating at low stress,exponential evolutionary activity,and intensified energy release.In contrast,granodiorite exhibits the characteristics of initiating at high stress,volatile evolutionary activity,and intermittent energy release,due to its more stable microstructure and fewer thermal defects compared to granite.As the temperature increases,the initiation and propagation of secondary cracks in granodiorite are suppressed to a certain extent,and the seismicity and brittleness are enhanced.The subtle differences in grain size,microscopic heterogeneity,and mineral composition of the two hot dry rocks determine the different acoustic-mechanical characteristics under heating and cooling,and the evolution trends with temperature.These findings are of great significance for the scientific and efficient construction of rock mass engineering by rationally utilizing different rock strata properties.

Damage and fracture behavior and spatio-temporal evolution of acoustic emission of sandstone before and after laser radiation

Laser technology holds significant promise for enhancing rock-breaking efficiency.Experimental investigations were carried out on sandstone subjected to laser radiation,aiming to elucidate its response mechanism to such radiation.The uniaxial compressive strength of sandstone notably decreases by 22.1%–54.7%following exposure to a 750 W laser for 30 s,indicating a substantial weakening effect.Furthermore,the elastic modulus and Poisson ratio of sandstone exhibit an average decrease of 33.7%and 25.9%,respectively.Simultaneously,laser radiation reduces the brittleness of sandstone,increases the dissipated energy proportion,and shifts the failure mode from tensile to tension-shear composite failure.Following laser radiation,both the number and energy of acoustic emission events in the sandstone register a substantial increase,with a more dispersed distribution of these events.In summary,laser radiation induces notable damage to the mechanical properties of sandstone,leading to a substantial decrease in elastic energy storage capacity.Laser rock breaking technology is expected to be applied in hard rock breaking engineering to significantly reduce the difficulty of rock breaking and improve rock breaking efficiency.

Effect of neutral polymeric bonding agent on tensile mechanical properties and damage evolution of NEPE propellant

Introducing Neutral Polymeric bonding agents(NPBA) into the Nitrate Ester Plasticized Polyether(NEPE)propellant could improve the adhesion between filler/matrix interface, thereby contributing to the development of new generations of the NEPE propellant with better mechanical properties. Therefore,understanding the effects of NPBA on the deformation and damage evolution of the NEPE propellant is fundamental to material design and applications. This paper studies the uniaxial tensile and stress relaxation responses of the NEPE propellant with different amounts of NPBA. The damage evolution in terms of interface debonding is further investigated using a cohesive-zone model(CZM). Experimental results show that the initial modulus and strength of the NEPE propellant increase with the increasing amount of NPBA while the elongation decreases. Meanwhile, the relaxation rate slows down and a higher long-term equilibrium modulus is reached. Experimental and numerical analyses indicate that interface debonding and crack propagation along filler-matrix interface are the dominant damage mechanism for the samples with a low amount of NPBA, while damage localization and crack advancement through the matrix are predominant for the ones with a high amount of NPBA. Finally, crosslinking density tests and simulation results also show that the effect of the bonding agent is interfacial rather than due to the overall crosslinking density change of the binder.

Experimental and numerical study of hypervelocity impact damage on composite overwrapped pressure vessels

Ground-based tests are important for studying hypervelocity impact(HVI)damage to spacecraft pressure vessels in the orbital debris environment.We analyzed the damage to composite overwrapped pressure vessels(COPVs)in the HVI tests and classified the damage into non-catastrophic damage and catastrophic damage.We proposed a numerical simulation method to further study non-catastrophic damage and revealed the characteristics and mechanisms of non-catastrophic damage affected by impact conditions and internal pressures.The fragments of the catastrophically damaged COPVs were collected after the tests.The crack distribution and propagation process of the catastrophic ruptures of the COPVs were analyzed.Our findings contribute to understanding the damage characteristics and mechanisms of COPVs by HVIs.

Analysis of the damage threshold of the GaAs pseudomorphic high electron mobility transistor induced by the electromagnetic pulse

An electromagnetic pulse（EMP）-induced damage model based on the internal damage mechanism of the Ga As pseudomorphic high electron mobility transistor（PHEMT） is established in this paper. With this model, the relationships among the damage power, damage energy, pulse width and signal amplitude are investigated. Simulation results show that the pulse width index from the damage power formula obtained here is higher than that from the empirical formula due to the hotspot transferring in the damage process of the device. It is observed that the damage energy is not a constant, which decreases with the signal amplitude increasing, and then changes little when the signal amplitude reaches up to a certain level.

Mechanical response and microscopic damage mechanism of pre-flawed sandstone subjected to monotonic and multilevel cyclic loading:A laboratory-scale investigation

This study aims to investigate the mechanical response and acoustic emission(AE)characteristic of pre-flawed sandstone under both monotonic and multilevel constant-amplitude cyclic loads.Specifically,we explored how coplanar flaw angle and load type impact the strength and deformation behavior and microscopic damage mechanism.Results indicated that being fluctuated before rising with increasing fissure angle under monotonic loading,the peak strength of the specimen first increased slowly and then steeply under cyclic loading.The effect of multilevel cyclic loading on the mechanical parameters was more significant.For a single fatigue stage,the specimen underwent greater deformation in early cycles,which subsequently stabilized.Similar variation pattern was also reflected by AE count/energy/b-value.Crack behaviors were dominated by the fissure angle and load type and medium-scale crack accounted for 74.83%–86.44%of total crack.Compared with monotonic loading,crack distribution of specimen under cyclic loading was more complicated.Meanwhile,a simple model was proposed to describe the damage evolution of sandstone under cyclic loading.Finally,SEM images revealed that the microstructures at the fracture were mainly composed of intergranular fracture,and percentage of transgranular fracture jumped under cyclic loading due to the rapid release of elastic energy caused by high loading rate.

Micro Model of Carbon Fiber/Cyanate Ester Composites and Analysis of Machining Damage Mechanism

Machining damage occurs on the surface of carbon fiber reinforced polymer (CFRP) composites during processing. In the current simulation model of CFRP, the initial defects on the carbon fiber and the periodic random distribution of the reinforcement phase in the matrix are not considered in detail, which makes the characteristics of the cutting model significantly different from the actual processing conditions. In this paper, a novel three-phase model of carbon fiber/cyanate ester composites is proposed to simulate the machining damage of the composites. The periodic random distribution of the carbon fiber reinforced phase in the matrix was realized using a double perturbation algorithm. To achieve the stochastic distribution of the strength of a single carbon fiber, a novel method that combines the Weibull intensity distribution theory with the Monte Carlo method is presented. The mechanical properties of the cyanate matrix were characterized by fitting the stress-strain curves, and the cohesive zone model was employed to simulate the interface. Based on the model, the machining damage mechanism of the composites was revealed using finite element simulations and by conducting a theoretical analysis. Furthermore, the milling surfaces of the composites were observed using a scanning electron microscope, to verify the accuracy of the simulation results. In this study, the simulations and theoretical analysis of the carbon fiber/cyanate ester composite processing were carried out based on a novel three-phase model, which revealed the material failure and machining damage mechanism more accurately.

Damage characteristics and catastrophic failure mechanism of coal rock induced by gas adsorption under compression

To reveal the damage characteristics and catastrophic failure mechanism of coal rock caused by gas adsorption,physical tests and theoretical methods are employed.The results show that adsorption swelling can damage coal rock,which can be distinguished by fractal dimension.A fitting relationship between the adsorption damage and fractal dimension is proposed by experimental testing and theoretical analysis.High gas adsorption pressure proves to be the dominant factor that leads to coal failure softening and gas outburst disasters.Three main parameters concerning adsorption damage include the change rate of released energy density,the transition difference in the post-peak acoustic emission(AE)b value and the change rate of cumulative AE energy.Results show that all the three parameters present a step-type decreasing change with the increase in fractal dimension,and the fractal dimension shows a linear relationship within the same failure mode.Finally,a method is proposed to evaluate coal rock disaster transformation,based on the aforementioned three main parameters of adsorption damage.

Study on the critical stress threshold of weakly cemented sandstone damage based on the renormalization group method

During the microstructural analysis of weakly cemented sandstone,the granule components and ductile structural parts of the sandstone are typically generalized.Considering the contact between granules in the microstructure of weakly cemented sandstone,three basic units can be determined:regular tetrahedra,regular hexahedra,and regular octahedra.Renormalization group models with granule-and pore-centered weakly cemented sandstone were established,and,according to the renormalization group transformation rule,the critical stress threshold of damage was calculated.The results show that the renormalization model using regular octahedra as the basic units has the highest critical stress threshold.The threshold obtained by iterative calculations of the granule-centered model is smaller than that obtained by the pore-centered model.The granule-centered calculation provides the lower limit(18.12%),and the pore-centered model provides the upper limit(36.36%).Within this range,the weakly cemented sandstone is in a phase-like critical state.That is,the state of granule aggregation transforms from continuous to discrete.In the relative stress range of 18.12%-36.36%,the weakly cemented sandstone exhibits an increased proportion of high-frequency signals(by 83.3%)and a decreased proportion of low-frequency signals(by 23.6%).The renormalization calculation results for weakly cemented sandstone explain the high-low frequency conversion of acoustic emission signals during loading.The research reported in this paper has important significance for elucidating the damage mechanism of weakly cemented sandstone.

Damage and deterioration mechanism and curing technique of concrete structure in main coal cleaning plants

Concrete structures in main coal cleaning plants have been rebuilt and reinforced in the coal mines of the Shanghai Datun Energy Sources Co. Ltd., the first colliery of the Pingdingshan Coal Co. Ltd. and the Sanhejian mine of the Xuzhou Mining Group Co. Ltd. In these projects, the operating environment and reliability of concrete structures in the main plants of the three companies were investigated and the safety of the structures inspected. Qualitative and quantitative analyses were made on the special natural, technological and mechanical environments around the structures. On the basis of these analyses, we discuss the long-term, combined actions of the harsh natural (corrosive gases, liquids and solids) and mechanical environments on concrete structures and further investigated the damage and deteriorating mechanisms and curing techniques of concrete structures in the main coal cleaning plants. Our study can provide a theoretical basis for ensuring the reliability of concrete structures in main coal cleaning plants.

Damage and fracture mechanism of 6063 aluminum alloy under three kinds of stress states

To study the damage and fracture mechanism of 6063 aluminum alloy under different stress states,three kinds of representative triaxial stress states have been adopted,namely smooth tensile,notch tensile,and pure shear.The results of the study indicate the following.During the notch tensile test,a relatively higher stress triaxiality appears in the root of the notch.With the applied loading increasing,the volume fraction of microvoids in the root of the notch increases continuously.When it reaches the critical volume fraction of microvoids,the specimen fractures.During the pure shear test,the stress triaxiality almost equals to zero,and there is almost no microvoids but a shear band at the center of the butterfly specimen.The shear band results from nonuniform deformation constantly under the shear stress.With stress concentration,cracks are produced within the shear band and are later coalesced.When the equivalent plastic strain reaches the critical value（equivalent plastic fracture strain）,the butterfly specimen fractures.During the smooth tensile test,the stress triaxiality in the gauge of the specimen remains constant at 0.33.Thus,the volume of microvoids of the smooth tensile test is less than that of the notch tensile test and the smooth specimen fractures due to shearing between microvoids.The G-T-N damage model and Johnson-Cook model are used to simulate the notch tensile and shear test,respectively.The simulated engineering stress-strain curves fit the measured engineering stress-strain curves very well.In addition,the empirical damage evolution equation for the notch specimen is obtained from the experimental data and FEM simulations.

Effects of earthquake on damage evolution and failure mechanism of key rock pillars in underground engineering

Rock pillar is the key supporting component in underground engineering.During an earthquake,the key rock pillar must bear both the seismic load and the load transferred from other damaged pillars.This paper attempts to reveal the influence of the mainshock on damage evolution and failure characteristic of the key rock pillar during aftershocks by cyclic loading test of marble.Four levels of pre-damage stress(i.e.,10,30,50 and 70 MPa)in the first cycle were used to simulate the mainshock damage,and then cyclic stress with the same amplitude(namely 10 MPa)was conducted in the subsequent cycles to simulate the aftershock until rock failure.The results indicate that the presence of pre-damage has an obvious weakening effect on the bearing capacity and deformation resistance of rock materials during the aftershock process.Besides,the increase of pre-damage significantly changes the final failure pattern of the key rock pillar,and leads to an increase in the proportion of small-scale rock fragments.This study may contribute to understanding the seismic capacity of the unreinforced rock pillar during mainshock-aftershock seismic sequences and to optimizing the design of the key rock pillar in underground engineering.

Study on Carbonation Damage Constitutive Curve and Microscopic Damage Mechanism of Tailing Recycled Concrete

To improve the resource utilization of recycled aggregate concrete(RAC)and make use of the unique pozzolanic activation characteristics of iron ore tailing(IOT),the constitutive curves of tailing recycled concrete(TRC)before and after carbonization were analyzed theoretically,experimentally and microscopically.Firstly,according to the experimental data,the damage constitutive and related damage parameters of TRC were theoretically established by Weibull probability distribution function.Secondly,the comprehensive damage parameter b under different working conditions was studied.Finally,the damage mechanism was formed by EDS and SEM.The results showed that the damage constitutive model based on Weibull probability distribution function was in good agreement with the experimental results.Under each carbonization period,the b first decreased and then rose with the increase of tailings content.When its content was 30%,the b values of TRC were minimized,which were 22.14%,20.99%,25.39%lower than those of NAC,and 41.09%,34.89%,35.44%lower than those of RAC,indicating that IOT had a relatively good optimization effect on the constitutive curve of RAC.The microscopic analysis results also proved that the IOT addition with a proper amount would improve the matrix structure of RAC and increased its compactness,but when the content was higher,it would also cause harmful cracks in its matrix structure and reduced its density.Therefore,the optimal tailing content was about 30%.This paper provided a new method for damage constitutive calculation and analysis of TRC before and after carbonization.

Analysis of the damage mechanism of strainbursts by a global-local modeling approach

Strainburst is the most common type of rockbursts.The research of strainburst damage mechanisms is helpful to improve and optimize the rock support design in the burst-prone ground.In this study,an improved global-local modeling approach was first adopted to study strainburst damage mechanisms.The extracted stresses induced by multiple excavations from a three-dimensional(3D)global model established by fast Lagrangian analysis of continua in 3 dimensions(FLAC3D)are used as boundary conditions for a two-dimensional(2D)local model of a deep roadway built by universal distinct element code(UDEC)to simulate realistic stress loading paths and conduct a detailed analysis of rockburst damage from both micro and macro perspectives.The results suggest that the deformation and damage level of the roadway gradually increase with the growth of surrounding rock stress caused by the superposition of mining-or excavation-induced stresses of the panel and nearby roadways.The significant increase of surrounding rock stresses will result in more accumulated strain energy in two sidewalls,providing a necessary condition for the strainburst occurrence in the dynamic stage.The strainburst damage mechanism for the study site combines three types of damage:rock ejection,rock bulking,and rockfall.During the strainburst,initiation,propagation,and development of tensile cracks play a crucial role in controlling macroscopic failure of surrounding rock masses,although the shear crack always accounts for the main proportion of damage levels.The deformation and damage level of the roadway during a strainburst positively correlate with the increasing peak particle velocities(PPVs).The yielding steel arch might not dissipate kinetic energy and mitigate strainburst damage effectively due to the limited energy absorption capacity.The principles to control and mitigate strainburst damage are proposed in this paper.This study presents a systematic framework to investigate strainburst damage mechanisms using the global-local modeling approach.

Effect of stress paths on failure mechanism and progressive damage of hard-brittle rock

During deep buried hard-brittle rock tunnel excavation,the surrounding rock experiences a complicated stress path and stress adjustment process.Once the adjusted stress exceeds the ultimate bearing capacity of rockmass,a rock failure mode defined as stress-cracking type will occur.In order to investigate the effect of stress paths on failure mechanism and progressive damage of deep-buried rockmass,the cyclic loading-unloading,loading-unloading,uniaxial,conventional and unloading triaxial compression tests on samples of hard-brittle sandstone were conducted.According to the experimental results,increase in the confining pressure was beneficial to improve the mechanical parameters of rock,but it will reduce the brittle failure features.Compared with conventional triaxial compression,the sandstone under unloading state had more remarkable stress drop and unstable failure characteristics.Meanwhile,it was found that the energy dissipation and energy release in the whole process of rock deformation were the internal power of driven rock progressive damage.With the increase of confining pressure,the energy hardening and energy accumulation features of rock were weakened,while the progressive damage evolution characteristics could be enhanced.In unloading state,more energy could be converted into elastic energy in the energy softening phase(σeb-σP),so that the prepeak damage rate of rock was lower than that of conventional triaxial compression state.Thus,the energy dissipation rate of rock after peak strength decreased linearly with the increase of confining pressure under conventional triaxial compression state,while in unloading state it showed the opposite law.