Peak Stress Decrease Research Articles

Chemical Damage Constitutive Model Establishment and the Energy Analysis of Rocks under Water–Rock Interaction

The physical and mechanical properties of rocks can be reduced significantly by an acidic environment, resulting in engineering weaknesses, such as building foundation instability, landslides, etc. In order to investigate the mechanical properties of rocks after hydrochemical erosion, a chemical damage constitutive model was established and used to analyze chemical damage variables and energy transformation. It is assumed that the strength of the rock elements obeyed Weibull distribution, considering the nonuniformity of rock. The chemical damage variable was proposed according to the load-bearing volume changes in the rock under water–rock chemical interactions. The chemical damage constitutive model was derived from coupling the mechanical damage under the external load and the chemical damage under hydrochemical erosion. In order to verify the accuracy of the model, semi-immersion experiments and uniaxial compression experiments of black sandy dolomite were carried out with different iron ion concentrations. Compared with the experimental data, the chemical damage constitutive model proposed could predict the stress–strain relationship reasonably well after water–rock interaction. The effects of water–rock interaction on the rock were a decrease in peak stress and an increase in peak strain. The peak strain increased by 4.96–29.58%, and the deterioration rate of peak strength was 0.19–4.18%. The energy transformation of the deterioration process was analyzed, and the results showed that the decrease in releasable elastic energy, Ue, is converted into dissipated energy, Ud, after hydrochemical erosion.

Study on mechanical and fracture characteristics of inclined weak-filled rough joint rock-like specimens

Weak-filled rough joint (WFRJ) strongly influence the mechanical and fracture properties of the rock mass. A uniaxial compression test with acoustic emission (AE) and digital image correlation (DIC) system was first conducted on the WFRJ rock-like specimens with a fixed inclination and filling thickness. The test results showed that the increasing JRC of WFRJ caused an increase and then a decrease in peak stress (σp), a steady increase in failure strain (εf) and a steady decrease in elastic modulus (E). And the increasing roughness of WFRJ changes the fracture pattern from sliding fracture along the hard-weak interface to mixed tensile-shear fracture. The AE results demonstrate that the increasing roughness in WFRJ caused the quantity increase in total cracks and the percentage decrease in shear cracks. Then, mechanical models considering local roughness and global structure of the weak filling layer were established to analyze the force characteristic of WFRJ rock-like specimens. Finally, ANSYS/LS-DYNA was used to numerical study the roughness effect of WFRJ specimens. The stress evolution in numerical results is consistent with the X-direction strain field evolution in DIC results. And the quantity and percentage variation laws of cracks in each part were verified by the numerical study results.

Space–time evolution law of progressive failure area and mechanical behaviour of rock under different seepage conditions

In engineering geological disasters, the presence of water can accelerate rock mass fracture, and the state of the seepage field has a varying influence on the elastic behaviour of rock mass. A three-dimensional digital image correlation (3D-DIC) system is used to conduct triaxial compression tests on sandstone under different seepage conditions. The variation law of rock mechanical behaviour and the space–time evolution law of the surface radial strain field in the progressive failure process are analysed. The results show that, as the seepage pressure increases, the rock peak stress decreases and the Young's modulus increases, and the mechanical properties of the rock are easily affected by the varying seepage conditions. Meanwhile, a constitutive equation based on the Gumbel distribution, which can accurately reflect the axial stress–strain relationship in the rock failure process is established. By analysing the spatial distribution of virtual data points with large radial strain values at different times on the sample surface, a patent strain localisation phenomenon is identified in the progressive failure process of the rock. The localisation degree and extent of the failure area can be quantified using the average nearest-neighbour analysis (ANNA) and user-defined parameter S, which is used to normalise the average distance from the virtual data points to the fitted fracture surface. The results show that, with the continuous compression of samples under different seepage conditions, ANN decreases as S increases, and the large strain regional distribution is increasingly concentrated. The research results can be applied to predict surrounding rock failure areas in underground spaces and provide monitoring references with respect to engineering geological disasters under different seepage conditions.

Experimental assessment on the size effects of circular concrete-filled steel tubular columns under axial compression

In this study, eight circular concrete-filled steel tubular (CFST) short columns with diameters (D) of 275–1100 mm and the column diameter-to-steel tube thickness ratios (D/t) of 55 and 69 were tested under axial compression. The bearing capacities of the steel tube and confined concrete were analyzed separately to illustrate the size effect of each component of the composite column. The experimental results show that all test specimens suffered a shear failure. The peak stress, peak strain, ductility index, and strength index decrease with the increasing D, while the composite elastic modules maintain to be relatively constant with the various specimen dimensions. Under the peak load, the vertical stress in the steel tube increases, while the hoop stress decreases with the increasing D. Compared with the specimens with a small D/t ratio, the vertical stress in the steel tube with a larger D/t ratio is lower, while the hoop stress is higher. The confined concrete strength can be divided into the unconfined concrete strength and the increased concrete strength due to lateral confinement, and the reduction of the latter is the main cause of the size effect of the CFST columns. The current design codes such as EC4, AISC, AIJ, and GB50936 overestimate the peak bearing capacity of circular CFST short columns with large diameters, compared to the experimental results. Therefore, a model considering the size effect was established to evaluate the axial bearing capacity of circular CFST short columns.

Experimental Investigation on Fracture Behavior and Mechanical Properties of Red Sandstone Subjected to Freeze–Thaw Cycles

The freeze–thaw process plays a dominant role as far as the exploration and development of natural resources in cold regions are concerned. Freeze–thaw cycles can cause frost heaving pressure in the rock matrix and result in micro cracking, which influences its physical and mechanical properties. A series of physical and mechanical tests are performed on red sandstone to investigate the fracture behavior and mechanical properties induced by freeze–thaw cycles. The testing results show that after being treated by freeze–thaw cycles, the mass, density, and P-wave velocity of rocks decrease, while the volume of rocks increases. The peak stress and elastic modulus decrease with the increase in freeze–thaw cycles, while peak strain and Poisson’s rate increase. When 30 MPa confining pressure is applied, the peak stress and elastic modulus of untreated samples reach the maximum values of 92.49 MPa and 12.84 GPa, respectively. However, after being treated by 30 freeze–thaw cycles, the peak strain and Poisson’s rate reach the maximum values of 0.631 % and 0.18, respectively. The development of micro-cracks and the growth of pores induced by frost heaving stress are the main reasons for the deterioration of the mechanical properties of rocks. Confining pressure and freeze–thaw cycles can transfer the rock’s failure mode from tensile to shear and make red sandstone show more ductility features.

Effect of interfacial angle on the mechanical behaviour and acoustic emission characteristics of coal–rock composite specimens

This paper studies the mechanical and acoustic emission (AE) evolution characteristics of coal–rock composite specimens with different interfacial angles. Uniaxial compression tests with various interfacial angle are conducted, with acoustic emissions features being monitored. The test results show that both the peak stress and elastic modulus decrease linearly as the interfacial angle increases, whereas the peak strain does not exhibit a clear trend. Meanwhile, the failure mode changes from matrix splitting to interface slip failure. The AE evolution characteristics of a coal–rock composite specimen can be categorized into three stages: quiet period, active period and remission period. With an increase in interfacial angle, the duration of the quiet period gradually decreased. With the increase of stress, the RA value first stabilizes and then rises, while the AF value has a trend of periodic fluctuation and continuous decline. The decrease of AF and increase of RA can be used as precursors of instability in coal–rock composite specimens. The failure mechanism of the coal–rock composite structure is analyzed based on the mechanical model containing the structural plane, and the theoretical analysis result is consistent with the experimental result. The paper also discussed the field AE monitoring applications, and the results in this study can provide a reference for the study of rib failures when multi-layers are presented in the roadway.

Layer Orientation Effect on Fracture Mode and Acoustic Emission Characteristics of Continental Shale

Based on the Brazilian tests of continental shale with different layer orientations, combined with AE monitoring, the influence of layer orientation on the anisotropy of mechanical properties, fracture mode, and fracture mechanism of continental shale was analyzed. The results show that the tensile strength and deformation at the peak stress decrease with the increase of layer orientation at a constant deformation loading rate of 0.06mm/min, and the splitting modulus decreases first and then increases. The tensile strength was 90° > 60° > 45° > 30° > 0°, and the maximum and minimum tensile strengths were 5.154 MPa and 0.669 MPa, respectively. Under the action of splitting load, the samples with 30°, 45°, and 60° layer orientations mainly undergo shear failure along the layer orientation, while the samples with 0° and 90° layer orientations undergo tensile failure. In addition, the crack propagation in the 0° and 30° samples penetrated the bedding. These characteristics have important reference significance for the study of the mechanism of hydraulic fracture communication, propagation, and activation of structural planes.

Experimental study of the influence of temperature and cooling method on dynamic mechanical properties and damage of granite

AbstractThe change in mechanical properties of high‐temperature rock after cooling treatments is getting increasing attention. However, only a few studies have been conducted on the dynamic characteristic parameters. Therefore, this study aims to perform a dynamic damage analysis of high‐temperature rocks subjected to different thermal shocks. Accordingly, the specimens were grouped and heated to 200°C, 400°C, 600°C, and 800°C, and they were cooled by natural, water, and liquid nitrogen cooling. Ultrasonic detector tests, the split Hopkinson pressure bar experiments, and scanning electron microscope tests of the high‐temperature granite specimens were conducted using different cooling methods. Subsequently, the change rules of dynamic stress–strain curves and properties were calculated, and the morphology and micromorphology of fragments were compared. Based on the outcomes, the thermal shock damage and impact damage evolution were evaluated and quantified. The findings revealed that when the treatment temperature increases, the dynamic peak stress and elastic modulus decrease, while the peak strain increases, and the effect of rapid cooling was more significant in the influence of rock dynamic characteristics. Moreover, when the damage factor is more than 0.51, the change of dynamic stress‐strain is accelerated, leading to the instability of the structure under the impact load. The research results are expected to provide an adequate theoretical basis for the development and utilization of geothermal resources and underground engineering applications.

Precise distinction of mechanical behavior stages of coal under temperature-pressure constraints and its permeability and energy consumption characteristics

The mechanical behavior and permeability (K) of coal reservoirs are important research directions in the development of coalbed methane recovery. In this study, the raw coal was collected from the Baode block, China. The pseudo-triaxial tests examined the coal mechanical behavior and K under high temperature-pressure (T-P) conditions. We proposed a new micro-slope method to accurately distinguish the deformation stages of coal under high T-P conditions. The whole deformation process is divided into five stages, and the dynamic permeability is divided into six stages. The results show that the K is closely related to the deformation stage and exhibits a U-shaped change with the axial strain, and its minimum value occurs at the end of the elastic deformation. The T and P control the evolution of coal mechanical behavior and K. The peak stress (σmax) and peak strain (εmax) decrease with the T but increase with the P. The initial permeability (K0) decreases with the increase in P and T in the 10–30 MPa and 30–80 °C range, respectively. On the other side, the minimum permeability (Kmin) is least affected by the T and P. The kinetic properties of energy dissipation show that, the T and P control the dynamic changes of K by affecting the energy at different stages inside the coal. This study provides a reference for understanding the mechanical behavior of deep coal reservoirs.

Mechanical properties and acoustic emission response of cemented tailings backfill under variable angle shear

To investigate the mechanical properties and acoustic emission (AE) response of cemented tailings backfill (CTB) under the action of compression–shear, 30°, 45°, and 60° variable angle shear tests (VAST) were conducted. Based on the stress–strain law and AE parameter characteristics, the rupture law of the CTB with different shear angles was analyzed. The evolution of the fractal dimension (FD) and b-value and the relationship between them were investigated. The results indicate the following: (1) With increasing shear angle, the peak stress and normal stress show a decreasing trend, the plastic yield stage shortens or disappears, and damage evolution is accelerated; with a shear angle greater than 45°, the specimen deformation and damage mode change. (2) The AE ringing counts and energy evolution at 30° and 45°, and the ringing count evolution at 60° show an inverted U-shaped distribution; the energy evolution at 60° shows a stepped distribution. As the shear angle increases, the active and decline periods move rearward, and the proportion of ringing count and AE events before peak stress decreases. (3) With increasing shear angle, the crack evolution mode of the specimen changes from intermittent local failure repeatedly to progressive expansion. The fracture evolution is a dimensionality reduction and an orderly process; before failure, crack development is a process from disorder to order, producing a state in which large-scale crack propagation stabilizes and small-scale cracks expand. The conclusion provides a basis for analyzing the rupture evolution of CTB under different compression–shear angles and provides a reference for the stability analysis of CTB backfilled goaf.

Wave Dissipation and Energy-Absorption Characteristics of Wave-Absorbing Metal Plates with Different Aperture Sizes and Thicknesses under True-Triaxial Static-Dynamic-Coupling Loading

Deep rock masses exist in a complex environment with multi-field coupling; therefore, it is necessary to develop a true-triaxial static-dynamic-coupling loading test machine to explore their characteristics and mechanical response mechanism. To meet the test requirements of true-triaxial loading and strong disturbance, a wave-absorbing metal plate was selected as the boundary material between the granite and transmission end, and the modified SHPB was used to perform static-dynamic-coupling loading tests. In this study, two series of experiments on wave- absorbing metal plates were conducted, which were fixed aperture sizes with different thicknesses and fixed thicknesses with different aperture sizes. The static-dynamic-coupling loading tests on each aperture size and plate thickness were carried out under the condition of equal energy impact. The effects of the aperture size and plate thickness on the incident- and reflection-stress curves, reflectivity, energy consumption law, energy evolution, and other mechanical properties of the wave-absorbing metal plate materials were studied. The results show that the peak stress and reflectivity decrease with increasing aperture size and plate thickness, and the influence of the thickness is greater than that of the aperture size. The energy-absorption rate of the wave-absorbing metal plate increased with increasing thickness and aperture size and was maximized when the aperture size and thickness were 6–7 mm and 3–4 mm, respectively. The variation trend of the energy reflectance is opposite to that of the energy absorption and reaches a minimum when the aperture size is 6–7 mm and plate thickness is 3–4 mm. The energy transmittance of the wave-absorbing metal plate fluctuated in a stable range, but the variation range was less obvious compared to that of the energy-absorption rate.

Experimental Study on Dynamic Mechanical Characteristics and Energy Evolution of Sandstone under Cyclic Impact Loading

In order to study the dynamic mechanical properties of the surrounding rock of roadway in deep mines with rock burst, cyclic impact compression tests of sandstone under different impact velocities were carried out by using the split Hopkinson pressure bar (SHPB) test system, and the dynamic stress-strain curves of the samples under cyclic impact loads and the relationship between energy dissipation and impact numbers were analyzed. The results show that, as the impact loading times increase, the initial elastic modulus of sandstone samples decreases, the peak stress decreases, and the peak strain increases; the average strain rate increases. The cumulative damage of sandstone samples under impact load is significantly affected by impact velocity and times. During the cyclic impact test, the reflected energy of sandstone increases with the increase of impact numbers, while the transmitted energy decreases and the absorbed energy increases; the cumulative specific energy absorption variable is introduced, and the cumulative specific energy absorption value of the samples increases with the increase of impact numbers, which can better characterize the variation of energy accumulation and dissipation of sandstone samples under cyclic impact.

Study on Mechanical Properties of Basalt Fiber Shotcrete in High Geothermal Environment.

With the development of infrastructure, there are growing numbers of high geothermal environments, which, therefore, form a serious threat to tunnel structures. However, research on the changes in mechanical properties of shotcrete under high temperatures and humid environments are insufficient. In this paper, the combination of various temperatures (20 °C/40 °C/60 °C) and 55% relative humidity is used to simulate the effect of environment on the strength and stress–strain curve of basalt fiber reinforced shotcrete. Moreover, a constitutive model of shotcrete considering the effect of fiber content and temperature is established. The results show that the early mechanical properties of BFRS are improved with the increase in curing temperature, while the compressive strength at a later age decreases slightly. The 1-day and 7-day compressive strength of shotcrete at 40 °C and 60 °C increased by 10.5%, 41.1% and 24.1%, 66.8%, respectively. The addition of basalt fiber can reduce the loss of later strength, especially for flexural strength, with a increase rate of 11.9% to 39.5%. In addition, the brittleness of shotcrete increases during high temperature curing, so more transverse cracks are observed in the failure mode, and the peak stress and peak strain decrease. The addition of basalt fiber can improve the ductility and plasticity of shotcrete and increase the peak strain of shotcrete. The constitutive model is in good agreement with the experimental results.

Experimental Investigation of Porous and Mechanical Characteristics of Single-Crack Rock-like Material under Freeze-Thaw Weathering

Freeze-thaw weathering changes the pore structure, permeability, and groundwater transportation of rock material. Meanwhile, the change in rock material structure deduced by frost heaving deteriorates mechanical properties of rock material, leading to instability and insecurity of mine slopes in cold regions. In this paper, rock-like specimens containing prefabricated cracks at different angles and having undergone various freeze-thaw cycles are used as the object. Their pore structure, compressive mechanical properties, strain energies, failure characteristics, and the connection between pore structure and mechanical properties are analyzed. Results show that the porosity, spectrum area of mesopores, and spectrum area of macropores increase with the increase in freeze-thaw cycles, while crack angle shows no obvious influence on pore structure. Peak stress and elastic modulus drop with the increase in freeze-thaw cycles, while peak strain shows an increasing trend. Peak stress and elastic modulus decrease in the beginning, and then increase with the increase in crack angle, while peak strain shows a reverse trend. Elastic strain energy and pre-peak strain energy drop with the increase in freeze-thaw cycles. Elastic strain energy decreases first, and then increases with the increase in crack angle. The correlation between the spectrum area of macropores and elastic modulus is the strongest among different pores. Elastic modulus and peak stress decrease with the increase in macropore spectrum area, and peak strain increases with the increase in macropore spectrum area.

An Experimental Investigation of the Effect of Grain Size on “Dislocation Creep” of Ice

AbstractThe creep behavior of ice is believed to be dominated by dislocation creep at high stresses (1 MPa) and high homologous temperatures (0.9). Dislocation creep of ice is often described by the Glen law, , with a canonical value of 3, and is independent of grain size. Laboratory studies of ice deformation at elevated pressures, however, suggest that dislocation creep of ice is characterized by a value of 4. Here, we deformed ice samples with initial grain sizes of 0.23 and 0.63 mm, at 263 K and confining pressures of 10 or 20 MPa. The experiments yield a value of 3.6 using the peak stresses obtained at a strain 0.02, and reveal a marked dependence of the stress on initial grain size. The nominally constant flow stresses at larger strains, up to 0.2, yield a value of 3.9, and no influence of the initial grain size on the stress is observed. The lower value of at peak stresses and the decrease in peak stress with decreasing initial grain size suggest a contribution to the strain rate from a grain‐size‐sensitive process, such as grain boundary sliding. Our data yield two flow laws, one for isotropic ice and one for highly anisotropic ice deformed to large strains, the latter of which may be used to model flow in high‐strain natural environments.

Characteristics of rock damage variable under cyclic loading

In order to study the mechanical characteristics of rock under dynamic load, shock and P-wave velocities were measured by split Hopkinson (SHPB) testing system and NM-4b non-metallic ultrasonic testing analyser. In the test, the specimens were subjected to single impact with different air pressure and cyclic impact with the same air pressure. Acoustic velocity was measured before and after impact to observe the morphological characteristics of rock samples after impact. The results show that: under different air pressure, the rock fracture morphology is closely related to the air pressure. The higher the air pressure, the greater the peak strength of the sample, the more concentrated the damage range to the center of the sample; The results show that the peak stress decreases, the cumulative damage increases and the wave velocity decreases with the increase of cyclic impact times.

Tensile deformation behavior of coarse-grained Ti-55 titanium alloy with different hydrogen additions

The effect of hydrogen addition on the deformation behavior of coarse-grained Ti-55 alloys (~ 20 μm) was studied by uniaxial tension tests at high temperature. The elongation of hydrogenated Ti-55 titanium alloy firstly increases and then decreases with hydrogen content increasing at 875 °C. The highest elongation of 243.8% is obtained in the hydrogenated alloy with 0.1 wt% H, and the peak stress reaches a minimum value of 29.0 MPa in the hydrogenated alloy with 0.3 wt% H. Compared with that of the unhydrogenated alloy, the elongation of the hydrogenated alloy with 0.1 wt% H increases by 41.3% and its peak stress decreases by 40.6% at 875 °C. Hydrogen addition can promote the transformation of β phase and the dislocation movement. Appropriate hydrogen content can evidently improve the deformation properties of coarse-grained Ti-55 titanium alloy.

The effect of Cr additions and oxidation on densification, microstructure, phase constituents and mechanical properties of TiAlCr alloys produced by SPS

Ternary Ti-47.5Al-(1–3)Cr (at. %) and reference quaternary Ti-48Al-2Cr-2Nb (at. %) titanium aluminide alloys were processed from elemental powders utilizing mechanical milling, mechanical alloying and spark plasma sintering. The effect of Cr additions and oxidation on the densification behavior, microstructure, phase constituents and mechanical properties of the sintered alloys were investigated. The results point out that Cr additions and the presence of O play a significant role on the densification process i.e., compaction rates and sintering temperatures of the alloys. The microstructure and phase constituents of the alloys were studied based on historical experimental and theoretical data. Furthermore, the compression properties and fracture morphologies at experimental temperatures were found to be determined by a combination of response to densification, microstructural and phase evolutions effects as opposed to composition only. High additions of Cr coupled with the high level of oxidation impaired the compression properties of the ternary alloys, resulting in a 9% decrease in peak stress at higher testing temperatures. At these temperatures, the quaternary alloy showed good compression properties compared to the ternary alloys.

Geomechanical behaviors of shale after water absorption considering the combined effect of anisotropy and hydration

Geomechanical behaviors of shale in many geotechnical engineering areas, such as shale gas extraction, tunnel excavation, slope engineering, are highly influenced by anisotropy and hydration. In this work, a number of ϕ25 mm × 50 mm shale specimens were prepared with three levels of moisture contents and seven bedding plane orientations (β, in degrees) relative to the axial direction. A series of compression tests for these specimens was then conducted under four different confining pressures to investigate their geomechanical behaviors considering the combined effect of anisotropy and hydration. The results demonstrate the following points. 1) During the water absorption process, the shale specimens swell and suffer damage, which are primarily observed along the bedding plane. 2) The curve of compressive strength (peak deviatoric stress) vs. β is “U-shaped,” with the lowest value at β = 30°. The apparent elastic modulus decreases with the increase in β and then remains nearly unchanged when β > 45°. As β increases, the strains at peak stress and average apparent Poisson's ratio increase first and then decrease when β > 30°. 3) The plastic deformation stage and residual stress stage, which negatively relate to the elastic-brittle properties of shale, become more obvious with increasing moisture content, and the specimen for which β is closer to 30° exhibits a more significant change of that. 4) With increasing moisture content, the compressive strength and axial strain at peak stress decrease, apparent elastic modulus decreases nonlinearly, and average apparent Poisson's ratio increases approximately linearly. 5) The effect of anisotropy on the compressive strength decreases with increasing confining pressure and this weakened effect by confining pressure becomes less with increasing moisture content. 6) With the increase in β, the effect of water absorption on the compressive strength first strengthens a little and then weakens when β > 30° or 45°, and this change with β becomes more significant under higher confining pressure. 7) The effect of confining pressure on the compressive strength strengthens a little when β < 30° and then weakens with the increase in β, and this change with β weakens with increasing moisture content. 8) The anisotropy of shale could be the major controlling factor for geomechanical behaviors, and water absorption along the bedding plane can enhance the effect of anisotropy. The results can not only guide the design and construction of geotechnical engineering structures related to shale, but can also be used as a reference for geological research on the deformation and failure of shale formations affected by water intrusion and geological stresses.

Brittle Characteristic and Microstructure of Sandstone under Freezing‐Thawing Weathering

As an important property of rock material, brittleness plays a vital role in rock engineering. This paper raised the concept of elastic strain energy release rate and proposed an elastic strain energy release rate based brittleness index based on the most acceptable definition of brittleness. Mechanical and Nuclear Magnetic Resonance parameters of sandstone under various Freezing‐Thawing (F‐T) cycles are also acquired and analyzed. Then, the proposed brittleness index is used to compare with two recently proposed brittleness indices to verify its correctness and applicability. Finally, the brittleness index is applied to evaluate the brittle behavior of F‐T cycles treated sandstone under uniaxial compression. The results show that elastic modulus, value of the postpeak modulus, and peak stress decrease with F‐T cycles, and the porosity and microstructure develop with F‐T cycles. The proposed brittleness index is highly related to F‐T cycles, peak stress, porosity, and elastic modulus of sandstone that suffered recurrent F‐T cycles. It declines exponentially with F‐T cycles and porosity increase while growing exponentially with peak stress and elastic modulus increase.