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The permeability and porosity of a porous sandstone under cyclic loading-unloading conditions are measured using an integrated permeability and porosity measurement system. Mineral content analysis of the rock is performed. The rock contains open microcracks and pores, which can be closed after cyclic loading-unloading; as a result, the permeability and porosity decrease. Based on the test results, the empirical relations of confining stress-dependent sandstone permeability and porosity are derived. A power law is suggested to describe the relationship between the stress-dependent porosity and permeability of the sandstone under both loading and unloading conditions within the stress range used in the experiment. The permeability of the sandstone is more sensitive to stress than to porosity. It is inferred from the test results that the evolution of sandstone porosity and permeability is related to the relative movement of particles and the closure of microcracks. [ABSTRACT FROM AUTHOR]

The failure mechanism of shale under cyclic loading is useful for understanding fatigue hydraulic fracturing in reservoirs. The anisotropic characteristics of this mechanism were investigated in this study by conducting acoustic emission (AE) monitoring tests on shale specimens with seven different bedding inclination angles (θ) under uniaxial cyclic loading and unloading. The results showed that during the cyclic loading path, except for shale specimens with bedding inclination angles of 0° and 45°, the cumulative number of AE events and cumulative AE energy at each loading level tend to decrease and then increase during failure process; the Felicity ratio decreases overall with increasing the cycle numbers. Different trends in the number of AE events and the cumulative AE energy induced by tensile or shear fracture with increasing θ were observed. The bedding plane was found to not only affect the shale fracture mechanism but to also significantly impact the magnitude of the fracture energy release rate for the same fracture mechanism. The b value and spatial evolution of AE revealed the evolution of fractures in shale. Both tensile and shear AE events under cyclic loading and unloading were mostly generated by micro- and medium-scale fractures, regardless of θ. The AE time series parameters of anisotropic shale exhibited multifractal characteristics. The multifractal spectrum width Δα effectively reflected the nonuniformity of the AE energy parameters for failure in specimens with different θ. The difference in the fractal dimension Δ f in specimens with increasing θ was found to effectively characterize the occurrence probability of large-energy AE events. [ABSTRACT FROM AUTHOR]

Based on the measurement standards of rock burst tendency of coal samples, it has been measured under different gas pressures. The methods of energy accumulation and dissipation were applied to analyze the relationship between energy accumulation and dissipation of coal samples under the conditions of uniaxial compression and uniaxial cyclic loading and unloading in different gas pressures. The results show that with the increase of gas pressure, the indexes of every rock burst tendency are constantly reduced, and their outburst-proneness are decreased. Before destruction, the elastic energy stored is reduced, while the gaining rate of dissipated energy decreases rapidly in the form of nonlinear relationship during destruction. Under cyclic loading, the dissipated energy of coal samples in each cycle increases along with the increase of gas pressure. Gas reduces the destructive impact of coalbed rockburst, but when considering the expansion energy of gas, the overall destruction can be more serious. Therefore, gas should be considered when the rockburst tendency is evaluated in high gassy coal mines. [ABSTRACT FROM AUTHOR]

Using TAW-2000 servo testing machine and Germany VallenAMSY-6 acoustic emission signal acquisition system, the acoustic emission characteristics of marble under uniaxial compression, constant amplitude cyclic loading and unloading and tiered cyclic loading and unloading were studied. In addition, using Fast Fourier Transformation (FFT),the acoustic emission spectrum characteristics of marble under uniaxial compression and Kaiser points and Felicity points in the process of tiered cyclic loading and unloading were analyzed. Test results show that: Firstly, main frequency locate mainly in two areas, which are the regions less than 200 kHz and near 300 kHz, and the main frequency varies from low frequency to high frequency and the amplitude of main frequency declines on the whole with the increasing stress. Secondly, a new indicator to reflect acoustic emission waveform information, namely SF (secondary frequency) -by-MF (main frequency) ratio α, was put forward and in the process of uniaxial compression test SF (secondary frequency)-by-MF (main frequency) shows ascendant trend. Thirdly, the change of main frequency at Kaiser point and Felicity point is not obvious and the SF (secondary frequency)-by-MF (main frequency) ratio at Felicity point is higher than the one at Kaiser point in the hierarchical cyclic loading and unloading process. At last, by regarding AE (Acoustic Emission) counts at the point of stress in the second cycle whose stress is equal to the peak stress of first cycle as the "significant increased" scale of Kaiser effect, obvious Felicity effect is showed. [ABSTRACT FROM AUTHOR]

In situ X-ray diffraction was carried out to identify and evaluate strain-induced martensitic transformation in 304 austenitic stainless steel under cyclic tensile loading and unloading. Experimental results indicated that the fraction of strain-induced martensite increases due to the alteration of the internal structure involving the internal stress and dislocation configuration when unloading occurs. It is found that the enhanced transformation-induced plasticity effect can prolong the time to neck formation to a significant extent. [Copyright &y& Elsevier]

Abstract: This paper deals with elasto-plastic coupling solutions for the prediction of displacements around circular openings in elasto-brittle-plastic rock mass subject to hydrostatic stress. Both linear Mohr–Coulomb criterion and nonlinear Hoek–Brown criterion on the basis of a non-associated flow rule are considered. The restrictive condition between strength parameters and Young’s modulus in post-failure region for rock materials is theoretically established. Meanwhile, the proposed restrictive condition is validated by FLAC3D code that the advanced elasto-plastic coupling model (EPCM) based on Mohr–Coulomb criterion is merged in. Finally, the elasto-plastic coupling dimensionless displacements of circular opening in elasto-brittle-plastic rock mass are analyzed with different deterioration degree of Young’s modulus using two kinds of rocks. The results show that the deformation of rock mass increases obviously with the decrease of Young’s modulus in the plastic region, but Young’s modulus has little influence on the plastic region radius and stresses distribution form. [Copyright &y& Elsevier]

For many rock engineering projects, the stress of surrounding rocks is constantly increasing and decreasing during excavating progress and the long-term operation stage. Herein, the triaxial creep behavior of dolomitic limestone subjected to cyclic incremental loading and unloading was probed using an advanced rock mechanics testing system (i.e., MTS815.04). Then, the instantaneous elastic strain, instantaneous plastic strain, visco-elastic strain, and visco-plastic strain components were separated from the total strain curve, and evolutions of these different types of strain with deviatoric stress increment were analyzed. Furthermore, a damage variable considering the proportion of irrecoverable plastic strain to the total strain was introduced, and a new nonlinear multi-element creep model was established by connecting the newly proposed damage viscous body in series with the Hookean substance, St. Venant body, and Kelvin element. The parameters of this new model were analyzed. The findings are listed as follows: (1) When the deviatoric stress is not more than 75% of the compressive strength, only instantaneous deformation, transient creep, and steady-state creep deformation occur, rock deformation is mainly characterized by the instantaneous strain, whereas the irrecoverable instantaneous plastic strain accounts for 38.02–60.27% of the total instantaneous strain; (2) Greater deviatoric stress corresponds to more obvious creep deformation. The visco-elastic strain increases linearly with the increase of deviatoric stress, especially the irrecoverable visco-plastic strain increases exponentially with deviatoric stress increment, and finally leads to accelerated creep and delayed failure of the sample; (3) Based on the experimental data, the proposed nonlinear creep model is verified to describe the full creep stage perfectly, particularly the tertiary creep stage. These results could deepen our understanding of the elasto-visco-plastic deformation behavior of dolomitic limestone and have theoretical and practical significance for the safe excavation and long-term stability of underground rock engineering. [ABSTRACT FROM AUTHOR]

• Cyclic loading and unloading had obvious influence on the mechanical properties of the CPB, which was closely related to the number of layers and confining pressure. • The change of deformation modulus caused by cyclic loading and unloading is the direct cause of the change UCS. • The different maintenance time of each layer of CPB with multiple layers results in different pore distribution and structure in each layer. At the same time, the confining pressure affects the stress state of the pore. In this paper, an experimental investigation was conducted to study mechanical properties and microstructural characteristic of layered cemented paste backfill (CPB) under triaxial cyclic loading and unloading condition. A number of CPB specimens were prepared with different number of layers (1, 2, 3, and 4). Four confining pressures (0, 0.2, 0.5, and 0.8 MPa) and three cycles were designed. The results indicated that cyclic loading and unloading had obvious influence on the mechanical properties of the CPB, which was closely related to the number of layers and confining pressure. Compared with the conventional triaxial compression test results, when the confining pressure is 0 and 0.2 MPa, the cyclic loading and unloading can increase the uniaxial compressive strength (UCS) of CPB, and when the confining pressure is 0.8 MPa, it decreased the UCS. When the confining pressure is 0.5 MPa, it increased the strength of the CPB 1 and 2 layers, but decreased the strength of CPB with 3 and 4 layers. The change of UCS had a positive linear correlation with the change of secant modulus of unloading curve caused by the cycle loading and unloading. The results of SEM and CT scanning showed that there were more pores in the post filling layers due to the difference of curing time which were compacted easily under small confining pressure. Under the action of cyclic loading and unloading, the CPB was repeatedly compacted, resulting in the increase of UCS. With the increase of confining pressure, the difficulty of pore compaction increases gradually and stress concentration was easy to form around the pores, which resulting in local fatigue damage and reduce the strength under cyclic loading and unloading. [ABSTRACT FROM AUTHOR]

• Cyclic loading and unloading tests of sandstone with different lower limit of cyclic stress are carried out. • The effects of different lower limit of cyclic stress on the mechanical properties and AE spectrum of samples are compared. • The damage accumulation and deformation characteristics of the sample during cyclic loading and unloading are investigated. This study is aimed at exploring the effect of the lower limit of cyclic stress (LLCS) on the loading and unloading failure process of sandstone. First, cyclic loading and unloading experiments were performed on sandstone samples under different stress paths. Furthermore, the variations of mechanical properties and acoustic emission (AE) signals of samples during cyclic loading and unloading were analyzed. The results show that the equivalent residual strain produced by a single loading and unloading with a low LLCS is larger than the residual strain produced by that with a high LLCS. Under cyclic loading and unloading, the loading elastic moduli of samples grow as the number of cycles increases. In the loading failure stage, the loading elastic modulus under a high LLCS is greater than that under a low LLCS. In the 2nd and 3rd stages, the AE counts, main frequency signals and high-amplitude signals generated under loading and unloading with a high LLCS all account for smaller proportions than those generated under loading and unloading with a low LLCS. Low-frequency and high-amplitude signals appear in both the third stage and the loading failure stage under a low LLCS, while they only appear in the loading failure stage under a high LLCS. In the 2nd and 3rd stages, the b value first grows and then decreases under a low LLCS, while it keeps rising under a high LLCS. A high LLCS is conducive to suppressing the axial deformation of sandstone and reducing the amount of damage caused during a single loading and unloading. This study boasts theoretical reference value for the monitoring of coal rock stability and the prevention of dynamic disasters. [ABSTRACT FROM AUTHOR]

To analyse the effect of loading rate on the energy evolution of rocks under cyclic loading and unloading, tests on saturated limestone were conducted at loading rates of 0.15, 0.2, and 0.3 mm/min, and the evolution characteristics of plastic, elastic, dissipation, and input energies were examined under different loading rates. The results indicated that the plastic strain in the entire test was directly proportional to the loading rate. In addition, strength, residual stress, plastic energy, and dissipation energy under residual resistance were inversely proportional to the loading rate. The plastic strain exhibited a decreasing–stabilising–increasing trend, and the smaller loading rate delayed the "increasing" trend. The increasing extent of each energy exhibited the following trend: input > elastic > plastic > dissipation energy. Furthermore, the first three types of energy exhibited a slow–fast–slow–fast increase trend. The dissipation energy exhibited a fast–steady–fast–slow–fast increase trend. Additionally, the elastic energy index exhibited a large increase–steady increase–decrease trend, which was proportional to the loading rate. The damping ratio exhibited a decrease–increase–decrease–increase–decrease trend which was proportional to the loading rate in the compaction stage and inversely proportional to the plastic stage. [ABSTRACT FROM AUTHOR]

• Increasing the frequency by reducing the duration of loading cycles can harden rock. • Increase in frequency induced by increase in loading cycles causes rock more compact. • Cumulant of irreversible deformation can greatly degrade rock strength. To explore the mechanism of frequency on the evolution of deformation parameters, triaxial compression tests under multi-level loading were carried out at different frequencies. The results show that increasing the frequency by reducing the duration of loading cycles can inhibit the development of primary and new fractures in a rock in the compaction and plastic stages, thus hardening the rock and improving the strength. The increase in frequency induced by an excessive increase in loading cycles causes the rock more compact. The cumulant of irreversible deformation greatly increased to degrade the strength. Additionally, loading cycles plays a more important role. [ABSTRACT FROM AUTHOR]

The Kaiser effect reflects the memory of the loaded rock to the irreversible damage and deformation. The stress level, loading rate and lithology are the main factors affecting the Kaiser effect of the rock. To identify the accurate stress point of the Kaiser effect, the MTS 816 rock mechanics testing system and the DS5-A acoustic emission testing and analysis system were adopted. The uniaxial cyclic loading–unloading and acoustic emission characteristic test of 90 rock specimens from three types of rocks under different stress level and loading rate was carried out. The evolution of acoustic emission under uniaxial compression of the rock corresponds to the compaction stage, elastic stage, yield stage and post-peak stress drop stage of the rock deformation and failure process and is divided into the quiet period, transition period, active period and decay period of the acoustic emission. The larger the hardness of rock is, the earlier the stress point of the Kaiser effect appears. The loading stress level (σA) has appreciable influence on the Kaiser effect of the rock. When σA ≥ 0.7σc, the Kaiser effect disappears. Usually, the dilatancy stress (crack initiation stress) does not exceed 70% of the uniaxial compressive strength (σc) of the rock, and the stress point can be the threshold to determine whether the Kaiser effect occurs. The influence of loading rate (lr) on Felicity rate (FR) is relatively large when lr < 0.01 mm/s, and FR rapidly grows with increase of the loading rate. When lr ≥ 0.01 mm/s, the influence of the loading rate on FR is relatively small. The findings facilitate the future application of the Kaiser effect and improvement of the accuracy of the acoustic emission data interpretation. [ABSTRACT FROM AUTHOR]