Mechanical Properties Of Sandstone Research Articles

Impact of Moisture Content on the Brittle-Ductile Transition and Microstructure of Sandstone under Dynamic Loading Conditions

Rockburst frequently occurred in an unstable or violent manner, which posed great safety risk and economic loss in deep underground engineering. The water injection into rock stratum was one of the most effectively ways to reduce rockburst by weakening rock mechanics. However, the moisture content was an important index related to rock mechanical properties. Many previous studies focused on the relationship between the moisture contents and macromechanical properties of rock materials under static load and seldom explored the impact of moisture variation on the mechanical properties and brittle-ductile transition characteristics of rock materials under dynamic loads. In this paper, we studied the dynamic mechanical properties of sandstone with different moisture contents under the same strain rate by the Split Hopkinson Pressure Bar (SHPB) experimental system. The relationship between dynamic mechanical properties of sandstone and moisture content was studied, and a dynamic ductility coefficient was proposed, which could be determined by the ratio between the peak strain and the yield strain. Then, it was used to assess the critical moisture content of the brittle-ductile transition of the sandstone. Through scanning electron microscopy (SEM) examination, the microstructure of sandstones with different moisture contents was inspected at magnifications of 500, 2000, and 5000 times, respectively. We showed that as the moisture content increased, the dynamic peak strength and elastic modulus decreased at different degrees, whereas the dynamic peak strain and ductility coefficient exhibited a nonlinear increase, respectively. When the moisture content reached 2.23%, the variation ratio of the dynamic ductility coefficient commenced to increase obviously, indicating that the sandstone began to transit from brittle behavior to ductile behavior. When the sample magnification was 500 times, the microstructure of the sandstone samples with zero and 2.01% to 2.40% moisture content mainly displayed the step pattern and river pattern, respectively, showing that the damage mode was brittle fracture. When the moisture content ranged from 2.49% to 2.58%, the microstructure of the sample included a large number of dimple clusters with local snake patterns and belonged to ductile fracture. When the sample magnification was 2000 and 5000 times, the microstructure was mainly brittle fracture with a moisture content lower than 2.23%. The microstructure of the sample with moisture content of 2.23% exhibited brittle-ductile composite fracture form, whereas others exhibited obviously ductile fracture. These characteristics were fundamentally consistent with the results reflected by the dynamic ductility coefficient. Our findings could provide a theoretical basis for mitigating coal and rock bursts by injecting water methods in underground coal mines.

Experimental study of the effects of moisture content on the mechanical properties of sandstone under uniaxial compression

Water inside the micropores has a tremendous influence on the mechanical properties and damage patterns of rocks. To investigate the effects of different moisture contents on the cracking behavior of sandstone under different compression conditions, four groups of sandstone samples with moisture contents of 1.00%, 2.50%, 3.50% and 4.50% were used for uniaxial compression and cyclic loading-unloading experiments. The results show that the compressive strength and elastic modulus decrease with the increase of moisture content. The ultimate failure patterns for samples are all shear failure modes with different dip angles. In the unloading experiments, the dissipated energy and residual strain decreases with the increase of moisture content during the unloading stage with low stress and increases obviously during the unloading stage with high stress.

Investigation on Creep Behavior and Permeability Evolution Characteristics of Sandstone under Different Pore Pressures

To investigate the influence of pore pressure ( σ w ) on the creep behavior and permeability of red sandstone, triaxial creep test with permeability test under different pore pressures was conducted using MTS 815 testing system. The experimental results demonstrate that water has significant weakening effect on the long-term mechanical properties of sandstone, and the long-term strength of sandstone gradually decreases with increase in pore pressures. All permeability-time curves demonstrate a “decreasing-increasing” trend, but two different permeability evolution trends during the steady creep stage are observed, which are related to deviatoric stress. The permeability of both intact ( k 0 ) and fractured sandstone ( k f ) samples increases with the increase in pore pressure, which are in consistent with the failure mode analysis of fractured sandstone samples. However, while the relationship between k 0 and σ w is positive linear, it is a positive exponential function relationship between k f and σ w .

Experimental Investigation on the Correlation between Dynamic Ultrasonic and Mechanical Properties of Sandstone Subjected to Uniaxial Compression

Ultrasonic wave velocity is effective to evaluate anisotropy property and predict rock failure. This paper investigates the correlation between dynamic ultrasonic and mechanical properties of sandstones with different buried depths subjected to uniaxial compression tests. The circumferential anisotropy and axial wave velocity of sandstone are obtained by means of ultrasonic wave velocity measurements. The mechanical properties, including Young’s modulus and uniaxial compressive strength, are positively correlated with the axial P wave velocity. The average angles between the sandstone failure plane and the minimum and maximum wave directions are 35.8° and 63.3°, respectively. The axial P wave velocity almost keeps constant, and the axial S wave velocity has a decreasing trend before the failure of rock specimen. In most rock samples under uniaxial compression, shear failure occurs in the middle and splitting appears near both sides. Additionally, the dynamic Young’s modulus and dynamic Poisson’s ratio during loading are obtained, and the negative values of the Poisson’s ratio occur at the initial compression stage. Distortion and rotation of micro/mesorock structures may be responsible for the negative Poisson’s ratio.

Experimental Study on the Mechanical Properties of Sandstone under the Action of Chemical Erosion and Freeze-Thaw Cycles

In order to study the mechanical properties of sandstone under the coupling action of chemical erosion and freeze-thaw cycles, the fine-grained yellow sandstone in a mining area in Zigong, China, is collected as the research object. The changes in mechanical properties of yellow sandstone under the coupling action of chemical solution erosion and freeze-thaw cycles are analyzed based on uniaxial compression tests (UCTs) and triaxial compression tests (TCTs). The results show that, with the increase in freeze-thaw cycles, the compressive strength, elastic modulus, and cohesion of the sandstone samples decrease with varying degrees. Under constant freeze-thaw cycles, the most serious mechanical properties of degradation are observed in acidic solution, followed by alkaline solution and neutral solution. Under different confining pressures, the compressive strength and elastic modulus of the sandstone samples decrease exponentially with the increase in freeze-thaw cycles. Under the action of the chemical solution erosion and freeze-thaw cycles, the internal friction angle fluctuates around 30°. For the cohesion degradation, 35.4%, 29.3%, and 27.2% degradation are observed under acidic, alkaline, and neutral solutions. Nuclear magnetic resonance imaging shows that the chemical erosion and freeze-thaw cycles both promote the degradation of rock properties from surface to interior; after 45 freeze-thaw cycles, the mechanical properties drop sharply. To properly design rock tunneling support and long-term protection in the cold region, the impact of both freeze-thaw cycles and chemical erosion should be considered.

Identification of changes in mechanical properties of sandstone subjected to high temperature: meso-and micro-scale testing and analysis

Material properties largely depend on their structure, and are strongly dependent on the scale of observation. Under the influence of various processes, the structure of a material can undergo evolution, which leads to major changes in the mechanical parameters and morphology of the medium. To understand the behaviour of a given material exposed to the influence of various factors, e.g. loading and temperature treatment, and to be able to modify it appropriately, it is crucial to recognize its structure both in the scale of engineering applications and at the micro-scale. The article proposes a procedure for assessing changes in the structure of sandstone exposed to the temperature treatment. The presented procedure allows the morphology of the material to be evaluated and the influence of temperature treatment on mechanical parameters of rocks to be analysed, by combining use of different laboratory techniques. The changes in rock material have been characterized using three investigative techniques, i.e. a uniaxial compression test, nanoindentation and micro-computed tomography. The uniaxial compression tests were carried out for 11 different temperature values in the range of 23–1000 °C, which enabled the determination of the change in uniaxial compressive strength and Young’s modulus of the sandstone as a function of temperature. Micro-scale laboratory tests were utilised to identify changes in the mechanical and morphological parameters of the sandstone exposed to the temperature of 1000 °C. The results were referred to those obtained for the reference samples, i.e. not subjected to heating (T = 23 °C). Comparison of the results showed an evident relation between the microstructure changes and the mesoscopic properties.

Mechanical Properties of Sandstones under Initial Unloading Damage

To study the influence of initial unloading damage on the mechanical properties of sandstone, the repeated loading test of unloading damaged sandstone was carried out considering 8 initial unloading quantities of 100%, 93.33%, 86.67%, 80%, 73.33%, 66.67%, 40%, and 0%. The results were compared with those of the triaxial compression test of intact samples. The results show that the peak strength of intact samples is higher than that of unloading damaged samples, and the difference is more obvious when the unloading quantity is more than 80%. During the unloading process, the strain increasing rate of rock samples is obvious, and the lateral dilatation is significant, and the deformation modulus and compressive strength of the rock sample deteriorate obviously. From the energy point of view, the greater the unloading damage, the smaller the stored elastic strain energy, which leads to the decrease of peak strength. At present, the unloading fracture inside the rock sample has developed, and the failure mode of the sample gradually changes from compression shear failure to tensile shear failure. In the process of engineering rock mass excavation, the unloading area and unloading damage amount of the rock mass is a dynamic adjustment process. To ensure the safety of the engineering rock mass, it is suggested to determine reasonable reinforcement time, reinforcement area, and reinforcement measures.

Study on the Performance Degradation of Sandstone under Acidification.

In most oilfields, acid fracturing is widely used for oil production. Understanding the relationship between the individual factors (i.e., carbonate rock types, acid rock reaction kinetics, and deterioration of rock mechanical properties) can provide practical guidelines that can be used for the design and optimization of acid fracturing operation. This paper takes hydrochloric acid, acetic acid, and citric acid as the main research objects and carries out acidification experiments on sandstone in Changqing Oilfield, China. In addition, the effects of tribasic, dibasic, and monobasic acids on the mechanical properties of sandstone were studied. Results show that in this study area, the most obvious effect was seen with the use of dibasic acids (hydrochloric acid + acetic acid), which effectively reduced the sample quality, uniaxial compressive strength, and elastic modulus. Citric acid and Mg promote the conversion of amorphous calcium carbonate to high-crystallinity calcite, forming a white precipitate. Furthermore, it is found by scanning electron microscopy analysis that experimental group 5 (hydrochloric acid + acetic acid) has the most ideal rock erosion effect. Inductively coupled plasma emission spectrometry analysis shows that the acid rock is present in the solution. X-ray diffraction qualitative analysis of the composition and concentration of ions shows that the formation of white precipitates is citric acid and Mg promotes the conversion of amorphous calcium carbonate to high-crystallinity calcite, forming a white precipitate. The findings of this study can help to better understand the erosion, failure state, and failure mechanism of different acid types on sandstone, which may provide certain references and guidelines for sandstone acid fracturing oil production.

Influence of Drying–Wetting Cycle of Acid Solution on the Mechanical Properties of Sandstone

Ensuring the stability of reservoir slopes requires a comprehensive understanding of the mechanical properties of rocks after the drying–wetting cycles under acidic conditions. In this work, sandstone samples were selected and subjected to cyclic drying–wetting treatments with aqueous solutions of pH 7.0, 6.0, 4.0, and 2.0. The uniaxial compression tests and P-wave velocity measurement were carried out to evaluate the variations in the physico-mechanical properties of sandstones with different treatments. The results revealed that the internal structure of sandstones is softened by the drying–wetting cycles, which deteriorate their physico-mechanical properties. As the acidity of the aqueous solution increases, porosity and Poisson’s ratio show an increasing trend, while the strength characteristics, Young’s modulus, and P-wave velocity of sandstones present a decreasing trend. The physico-mechanical properties of sandstones significantly deteriorate from neutral to weakly acidic solutions; however, the deterioration becomes slower as the acidity continues to increase. Moreover, the physico-mechanical properties of sandstones are markedly reduced under strong acidic conditions.

Study on Long-Term Dynamic Mechanical Properties and Degradation Law of Sandstone under Freeze-Thaw Cycle

This study is based on the tunnel-face slope engineering of Dongfeng tunnel in Shanxi section of China’s Shuozhou-Huanghua Railway. The sandstone specimens in the perennial freeze-thaw zone of the slope were collected to carry out freeze-thaw cycle static physical mechanics test and split Hopkinson pressure bar (SHPB) dynamic mechanical test. Thus, the damage process of sandstone under freeze-thaw cycle and impact load is studied. Also, the dynamic compressive strength and dynamic elastic modulus of sandstone are analysed under different loading strain rates and freeze-thaw cycle based on LS-DYNA, a dynamic finite element program. The results showed that the dynamic compressive strength of sandstone subjected to multiple freeze-thaw cycles under 0.04 MPa air pressure has a greater damage ratio than that under 0.055 MPa and 0.07 MPa air pressure, which was more likely to cause damage to slope sandstone than in actual engineering; the dynamic compressive strength and elastic modulus of sandstone decrease greatly within a certain range of freeze-thaw cycles and loading strain rate, leading to significant deterioration. When the freeze-thaw cycle exceeded 200 times and the strain rate was greater than 200 s−1, the physical and mechanical properties of sandstone gradually tended to be stable.

Experimental study on changes of pore structure and mechanical properties of sandstone after high-temperature treatment using nuclear magnetic resonance

Variations of the mechanical properties and the pore structures of rocks after high-temperature treatment play a significant role in ensuring the operation of underground coal gasification and prevent the pollution of groundwater. In this study, to quantitatively analyse the pore structure and mechanical properties of sandstone after treatment at different temperatures (25 °C, 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, and 900 °C), an experiment was conducted based on nuclear magnetic resonance. The variation laws of the size and number of pores in sandstone after the high-temperature treatment were determined to investigate the development of cracks and the changes in the mechanical properties of the rock. At low temperatures, micropores account for over 90%, and the connectivity between pores and cracks is poor. In the temperature range of 200–500 °C, macropores and microcracks were found to increase the most, followed by mesopores, while micropores exhibited the smallest increase. Above 600 °C, the porosity and permeability of the sandstone improved significantly. Furthermore, the porosity and permeability increased sharply by 1.62 and 1.27 times, respectively, at 500–600 °C compared with those at room temperature. After the high-temperature treatment, the stress-strain curves of sandstone gradually became smoother, the post-peak residual stress intensified, and the brittleness weakened. The inflection point of sandstone strength occurs at 800 °C, which lags behind the porosity, permeability, and peak strain, because the trans-granular cracks penetrate the whole granule and connect with the surrounding fracture network.

Experiment study of physical and mechanical properties of sandstone after variable thermal cycles

To study the effects of temperature cycles on the mechanical properties, thermal damage, and fracturing of sandstone, uniaxial compression tests and Brazilian splitting tests were performed using a hydraulic pressure servo rigidity rock mechanics experiment system. Before the tests, the sandstone samples were treated with cyclic (repeated from 0 to 8 cycles) temperatures from 100 to 600 °C. In addition, the chromaticity (color parameters L*, a*, and b*), thermal conductivity, and P wave velocity of the sandstone samples were measured using a color meter, thermal conductivity tester, and acoustic wave detection device, respectively. The results show that the peak strength, tensile strength, thermal conductivity, and P wave velocity decrease rapidly when the samples are exposed to higher temperatures (> 400 °C). Below 300 °C, the decrease is mainly due to the escaping of the adhered, bound, and structural water. Between 300 and 600 °C, the thermal response of the minerals in sandstone increases the development of microcracks and weakens the sandstone. At these same temperatures, when the number of cycles increases, the peak strength and tensile strength of the sandstone decrease, which is more obvious at higher temperatures. After multiple cycles, the plastic characteristics of the sandstone increase. The results obtained in this paper are good for predicting the thermo-physical properties of sandstone when exposed to high temperatures.

Macro-micro mechanical properties of building sandstone under different thermal damage conditions and thermal stability evaluation using acoustic emission technology

Changes in the internal structure and mechanical stability of building sandstone under fire and high-temperature have been the focus of many studies. We aimed to study the macro and micro mechanical properties of building sandstone under different thermal damage conditions and evaluate its thermal stability. Consequently, in this work, changes in mechanical properties of sandstone are evaluated after subjecting the material to heat treatment at different temperatures. Besides, the causes of mechanical deterioration of sandstone after high-temperature treatment are analyzed and the damage evolution of sandstone is characterized by acoustic emission (AE) technology. Following the high-temperature treatment, the macro and micro properties of building sandstone show an obvious change, and the wave velocity decreases as a cubic polynomial. The peak stress of sandstone under load decreases gradually after the action of temperature, and the impact of temperature change on the internal composition of sandstone, especially the decrease of SiO2 content and the increase of porosity, is mainly responsible for mechanical degradation. The AE characteristics of sandstone under unconstrained heating, uniaxial loading at room temperature, and heating loading conditions were tested. The accumulative ring ratio of AE increased linearly with the increase of temperature or deformation, and the correlation coefficient was higher. The correlation coefficient under temperature and loading was more than 0.94. The magnitude of thermal stress in sandstone during the heating process was analyzed theoretically, and static damage factor and dynamic damage factor were calculated quantitatively with wave velocity and AE as characteristic parameters. A compound damage factor is proposed, that is, composite damage includes both thermal damage and loading damage. Our results are of great significance to the stability evaluation of building sandstone subjected to fire and high temperatures.

Experimental Research on Physical Indices and Mechanical Properties of Sandstone under Combined Action of Chemical-Freeze-Thaw Cycles

In order to study the effects of chemical solution erosion and freeze-thaw cycles on the physical parameters and mechanical properties of rocks, sandstone samples from central Shandong Province were taken as the research objects. The freeze-thaw cycle test and uniaxial compression test were carried out for rock samples eroded by 0.01mol/L H2SO4 solution, 0.01mol/L NaCl solution and 0.01mol/L NaOH solution, respectively. The results show that the mass loss and porosity of rock samples eroded by different chemical solutions increase with the increase of freeze-thaw cycles, and the peak strength and elastic modulus decrease exponentially with the increase of freeze-thaw cycles. The damage degree of acidic solution to freeze-thaw cycling sandstone samples is the greatest, that of alkaline solution is the next, and that of neutral solution is the smallest. The conclusions can be used for reference in support and design of underground engineering.

Influence of Real‐Time Heating on Mechanical Behaviours of Rocks

The rock mechanical properties under the effect of high temperature present a great significance on underground rock engineering. In this paper, the mechanical properties of sandstones, marbles, and granites under real‐time heating were investigated with a servo‐controlled compression apparatus. The results show that mechanical behaviours of all the three types of rocks are influenced by real‐time heating to different degrees. Due to thermal cracking, the uniaxial compressive strengths decrease as the heating temperature rises from room temperature to 400°C. Above 400°C, the sandstone exhibits a significant increase in UCS because of the sintering reaction. The sintering enlarges the contact area and friction between crystal grains in the sandstone, which strengthens the bearing capacity. For marbles, the UCS continues to decrease from 400°C to 600°C due to thermal cracking. However, the carbonate in the marble begins to decompose after 600°C. The generated particles would fill the cracks in the marble and increase the strength. For granites, their UCS presents a sharp decline after 400°C because of thermal cracking. For all rock elastic modulus, they present a decreasing trend, and this indicates that the rock’s ability to resist deformation gradually weakens under the effect of temperature. In general, rock mechanical behaviours under real‐time heating differ from those in normal situations, and use of the parameters presented here is important for underground rock engineering related to high temperature and can improve the precision in theoretical and numerical analysis.

Experimental Research on Mechanical Properties of High‐Temperature Sandstone with Different Cooling Methods

Various tests including the longitudinal wave velocity tests and uniaxial compression tests have been conducted to evaluate the impact of cooling methods (including natural cooling, water cooling, and cooling by liquid carbon dioxide) on mechanical properties of sandstone under the natural status and high temperature. The acoustic emission signals were also monitored during the tests. According to the tests conducted, the sandstone sample density attenuation rate and the longitudinal wave velocity attenuation rate are higher than those of the specimen under natural status while the uniaxial compressive strength and Young’s modulus are lower. Comparing with the sandstone under the natural status, the compression sections of the stress‐strain curves of the high‐temperature sandstone samples treated by three cooling methods are longer with lower strain peak values. The order of the acoustic emission is revealed as follows: the sample cooled by liquid carbon dioxide < the sample cooled by water < sample cooled naturally < the sample under natural status, which suggests that the rapid cooling (cooled by liquid carbon dioxide) produces the severest damage on the sample, followed by the water cooling and natural cooling methods. In addition, the relationship between the sample strength weakening coefficient and the cooling rate is defined based on the statistical data of the cooling time of the high‐temperature specimen under the three cooling methods.

Multiscale Study of Physical and Mechanical Properties of Sandstone in Three Gorges Reservoir Region Subjected to Cyclic Wetting–Drying of Yangtze River Water

Natural rock often suffers from cyclic wetting–drying involving different water types, and the resulting deterioration may differ from laboratory tests using distilled water or salt solutions. An inappropriate estimation of this deterioration effect may lead to fatal geological hazards and engineering failures. A multiscale study is conducted to investigate the physical and mechanical features of sandstone in Three Gorges Reservoir region (TGR sandstone) subjected to cyclic wetting–drying of Yangtze River water. During this study, three types of water, i.e., Yangtze River water, ionized water having similar ion compositions as the Yangtze River water, and distilled water, are used for comparison. The results show that the multiscale physical properties including mineral compositions (especially calcite and albite), micro-pore parameters, computed tomography values, and macro-mechanical parameters (i.e., Young’s modulus, uniaxial compression strength and tensile strength) are remarkably altered during the cyclic wetting–drying process. Significant correlations are found between these numerous multiscale properties. The results indicate that changes of mineral compositions and microstructure are the primary reasons for the deterioration of sandstone strength. The deterioration effect of distilled water on TGR sandstone is the least, while the effect of ionized water is the greatest, and that of river water being intermediate. These differences are ascribed to different chemical interactions, together with possible microorganism effects for river water, as microorganisms in river water potentially weaken the deterioration of cyclic wetting–drying of river water. In situ water is recommended for studying how rock properties are affected by water–rock interactions in real settings.

Результати аналізу фізико-механічних властивостей пісковиків що залягають у покрівлі розроблюваних вугільних пластів в умовах шахт Західного Донбасу

Результати аналізу фізико-механічних властивостей пісковиків що залягають у покрівлі розроблюваних вугільних пластів в умовах шахт Західного Донбасу

Effect of void ratio to the strength parameters of sedimentary rocks

This paper is to study the influence of void ratio on the compressive and shear strength of sedimentary rocks at different degree of weathering. As void ratio is related to porosity therefore the present of void ratio should influence the mechanical properties of the porous sedimentary rocks such as sandstone and limestone. In order to determine the microstructures including pores and cracks of the samples, sandstone and limestone from different weathering grades were analyzed using photomicrograph analysis method. From the image of photomicrograph, the void ratio of the sample was calculated using image processing technique. The uniaxial compression and triaxial tests were carried out to determine the compressive and shear strength of the same samples by using GCTS Testing System. Results for void ratio, compressive and shear strength of the four different weathering grades of rocks were correlated to see the trend of relationships between the void ratio and the mechanical properties. In relation to weathering grades, the void ratio of the sedimentary rocks increases as the degree of weathering increases. The results also show that the void ratio and mechanical properties of sandstone are closely related. With the increase of void ratio, the compressive strength and shear strength decrease.

Experimental study on characteristics of surface potential and current induced by stress on coal mine sandstone roof

Rocks generate voltage and current when subjected to load. Hence, the study of their characteristics is critical to the prevention and management of mine dynamic disasters and their secondary impacts. To investigate the characteristics of surface potential and current induced by stress on coal mine sandstone roof, an experimental scheme for determining the mechanical–electrical characteristics of rocks is constructed. The aforementioned system tests the evolution rules of current and voltage generated by the coal mine sandstone roof at different loading rates; the formation mechanism and coupling rules with mechanical properties of sandstone are also analysed. Experimental results show that although loading rates may differ, the variations of pressure stimulated current (PSC) and pressure stimulated voltage (PSV) curves are similar. Throughout the loading process, the PSC is divided into three stages: rapid growth, slow growth, and peak; on the other hand, the PSV curve exhibits a ‘double peak’ characteristic. The loading rate changes the current and voltage growth rates and peak values. When combined with mechanical properties, it can be observed that the PSC is related to the effective elastic modulus, whereas the PSV is related to micro-crack and macro-crack events in rocks. The current and voltage generated by the coal mine sandstone roof subjected to load are consistent with stress. The mechanical–electrical properties of sandstone indicate that the change processes in the PSC and PSV result from the coupling effects of various mechanisms. In the compaction stage, these are mainly caused by the piezoelectric effect of sandstone. In the elastic–plastic deformation stage, they are primarily generated by the electric dipoles formed by micro-cracks. In the sandstone fracture deformation stage, the change processes are mainly caused by the crack tip discharge. This study provide an experimental basis and practical results that reflect the microscale process and electrical precursors of sandstone fracturing.