Mechanical Behavior, Energy Release, and Crack Distribution Characteristics of Water-Saturated Phyllite under Triaxial Cyclic Loading

Many geological engineering hazards are closely related to the dynamic mechanical behaviors of rock materials. However, the dynamic mechanical behaviors of phyllite are less studied. In this study, we have carried out a series of triaxial cyclic tests on dry and water-saturated phyllite by employing the MTS 815 servohydraulic testing system and AE testing equipment to reveal the mechanical behavior, energy release, and crack distribution characteristics of phyllite. Results show that phyllite is a water-sensitive rock. Water and cyclic loading substantially affect the compressive strength, crack damage stress, deformation parameters, dilatancy, energy release, and crack distribution characteristics of phyllite. Furthermore, based on the dissipated energy, a new damage variable for phyllite is established. The critical damage variable for phyllite is approximately 0.80; this variable can be used as an index to predict the failure of phyllite. The water saturation effect of phyllite is very obvious; that is, it results in the weakness of mechanical properties of phyllite and changes the AE energy release and crack distribution characteristics of phyllite. This research can provide guidance for engineering construction and disaster prevention and control.

Damage Evolution Constitutive Behavior of Rock in Thermo-mechanical Coupling Processes

For thermal and loaded rock in engineering structures for some projects, triple-shear Drucker–Prager yield criteria, compaction coefficient K, damage variable correction factor δ, and thermal damage variable DT are introduced in a new thermomechanical (TM) constitutive model for the entire process. The compaction stage of rock in uniaxial compression test and the strain softening of rock caused by thermal attack are considered in this article. The damage evolution of rocks is described by a damage variable and a constitutive equation, which are in agreement with the actual thermal experimental breakage. The uniaxial compressive strength of granite subjected to a TM coupling effect can be predicted properly by this new unified constitutive model. The new TM unified constitutive model considering the compaction stage and post-failure stage is in good agreement with the test curves throughout the entire process. The coupling effect of heat and load in the total damage of rock has obvious nonlinear properties, but the coupling effect significantly weakens the specimens. By using the new TM unified constitutive model, the whole process of changes in rock damage with strain after high temperature can be calculated. Meanwhile, the model well represents the stress–strain curve at the post-failure stage. It is expected that this model can provide references for studying the mechanical response of the rock damage propagation characteristics in the future.

The Effect of Joint Damage on a Coal Wall and the Influence of Joints on the Abutment Pressure on a Fully Mechanised Working Face with Large Mining Height

Coal wall spalling is regarded as a key technical problem influencing safe and efficient mining of large-mining-height working faces while the distribution of abutment pressure within the limit equilibrium zone (LEZ) influences coal wall spalling within a large-mining-height working face. This research attempted to explore the distribution characteristics of abutment pressure within the LEZ in a large-mining-height working face. For this purpose, the influences of the orientation of joints on mechanical characteristics of coal with joints and on the distribution of abutment pressure within the LEZ in the large-mining-height working face were analysed by theoretical analysis and numerical simulation. Research results show that the damage variable of coal with joints first rises, then decreases, and finally increases with increasing dip angle of the joints; as the azimuth of the joints increases, the damage variable first declines, then increases; the damage variable gradually declines with increasing joint spacing; an increase in the dip angle of joints corresponds to first reduction, then growth, and a final decrease of the abutment pressure at the same position in front of the coal walls; on certain conditions, the abutment pressure at the same position within the LEZ first rises, then declines as the azimuth of joints increases; with the growth of the joint spacing, the abutment pressure at the same position within the LEZ rises. The dip angle and azimuth of joints marginally affect the abutment pressure within the LEZ.

A Coupled Elastoplastic Damage Dynamic Model for Rock

Rock dynamic constitutive model plays an important role in understanding dynamic response and addressing rock dynamic problems. Based on elastoplastic mechanics and damage mechanics, a dynamic constitutive model of rock coupled with elastoplastic damage is established. In this model, unified strength theory is taken as the yield criterion; to reflect the different damage evolution law of rocks under tension and pressure conditions, the effective plastic strain and volumetric plastic strain are used to represent the compressive damage variable and the equivalent plastic strain is used to represent the tensile damage variable; the plastic hardening behavior and strain rate effect of rocks are characterized by piecewise function and dynamic increase factor function, respectively; Fortran language and LS-DYNA User-Defined Interface (Umat) are used to numerically implement the constitutive model; the constitutive model is verified by three classical examples of rock uniaxial and triaxial compression tests, rock uniaxial tensile test, and rock ballistic test. The results show that the constitutive model can describe the dynamic and static mechanical behavior of rock comprehensively.

Combined effects of sulfate attack under drying–wetting cycles and loading on the fatigue behavior of concrete

Concrete structures often undergo both fatigue loading and environmental impacts during their useful lifetime. This study aims to explore the fatigue properties of concrete subjected to sulfate attacks under drying–wetting cycles and loading. The coupled influences of major cycle number and sodium sulfate solution on the residual deformation, elastic modulus, and damage variable were investigated by uniaxial cyclic loading tests. Moreover, the phase composition of concrete samples was examined by X-ray diffraction. Results indicate that the concrete residual deformation and damage variable could be classified into initial and stable stages, while the elastic modulus fluctuated within a certain range. The fatigue strength of concrete was found to increase with an increase in the major cycle number and sodium sulfate concentration in the early stages, whereas the fatigue performance of concrete decreased as the major cycle number and sodium sulfate concentration increased in the later stage. The degree of influence of major cycle number and sodium sulfate concentration on the fatigue properties of concrete differed in each stage. These findings can contribute to understand the variation pattern of concrete properties in complicated environments and provide an important reference for associated construction projects.

Nonlinear Dynamic Analysis of a Masonry Arch Bridge Accounting for Damage Evolution

This study investigates the nonlinear dynamic response of the masonry bridge ‘Ponte delle Torri’ in Spoleto, aiming at assessing the seismic performance of the structure and evaluating the occurring damaging mechanisms. A 3D Finite Element (FE) macromechanical procedure implemented in the FE program FEAP is adopted to model the bridge. To reproduce the typical nonlinear microcracking process evolving in masonry material when subjected to external loads, an isotropic damage model is used. This is based on a scalar damage variable introduced in the stress-strain constitutive law and equally degrading all the components of the elastic constitutive operator. A nonlocal integral definition of the damage associated variable, that is the equivalent strain measure governing its evolution, is adopted to overcome the mesh dependency problems of the FE solution typically occurring in the presence of strain softening behavior. Based on the results of a recent study by some of the authors, a single equivalent pier is analyzed, whose geometry and boundary conditions are selected so that its response can provide useful information on the out-of-plane dynamic behavior of the overall bridge. To perform the seismic assessment, a set of recorded accelerograms is properly selected to simulate the seismic history of the Spoleto site. The nonlinear dynamic response of the structure is evaluated and monitored in terms of top displacement time history, evolution of the global damage index, and distribution of the damage variable. First, a set of analyses is performed by imposing the selected ground motions one by one on the initial undamaged configuration for the structure with the aim of emphasizing the damaging effects on its dynamic response. Then, the accelerograms are arranged in sequence to reproduce the seismic history of the site and analyze the influence of accumulated damage on the dynamic amplification of the response. A critical comparison of the bridge response to the sequence of accelerograms and the single records is made, and the interaction between the damaged structure dynamic response and the signal characteristic is highlighted, as well.

Evaluation of the behaviour of reinforced concrete beams repaired with glass fibre reinforced polymer (GFRP) using a damage variable

The use of fibre reinforced polymers (FRP) for increasing the strength of RC structures became a usual method. FRP presents easy application and demands low space and provide significant strength increase. Usually, the decision for FRP use is made in terms of applied loads and deflections. However, such quantities can vary significantly depending on the characteristics of the structural element e.g. span, effective depth and concrete resistance. Therefore, this paper aims to present an alternative control variable to analyse the behaviour of RC beams repaired with glass fibre reinforced polymer (GFRP), called damage. Such damage variable accounts for concrete cracking and it was experimentally measured before and after the application of GFRP. Note that the application of GFRP increased the ultimate load for all repaired beams. The damage values of such beams also increased when collapse was reached. Furthermore, it was observed that the collapse mechanism shifted to shear and did not occurred the failure of the GFRP.

Study on Mechanical Properties and Damage Characteristics of Red Sandstone under Freeze-thaw and Load

To analyze the effects of freeze-thaw cycles and confining pressure on the mechanical properties of red sandstone, through freeze-thaw cycles and triaxial compression tests, full stress-strain curves of different freeze-thaw cycles and different confining pressures were obtained. The degradation degree of red sandstone was quantitatively considered from different mechanical parameters of ultimate stress, elastic modulus, and Poisson’s ratio. Based on summarizing the characteristics of rock under freeze-thaw and load, the total damage variable of rock was determined by the reasonable measurement of freeze-thaw damage variable and load damage variable, and a damage constitutive model under freeze-thaw and load was established. The research showed that the freeze-thaw cycles aggravate the degree of rock damage deterioration, the rock stiffness and strength were reduced, and the characteristics of plastic deformation and ductile failure were more obvious. The confining pressure inhibited red sandstone internal damage, and with the increase of confining pressure, the stiffness and strength and the plastic characteristics were increased. In the overall trend, the mechanical parameters had different sensitivity to the degradation effect of freeze-thaw cycles and confining pressure. Regardless of the increase in the number of freeze-thaw cycles or confining pressure, the strain softening modulus tended to decrease gradually, and red sandstone plastic damage became more obvious after the stress peak. The total damage evolution path of red sandstone reflected the nonlinear influence of freeze-thaw and load on the total damage propagation. The research results provide theoretical support for the improvement of the technology of the effluent coal rock in Balasu Coal Mine.

Study on the Damage Model of Coal Rock Caused by Hydraulic Pressure and Electrical Impulse in Borehole

The purpose of this study is to study the damage and fracturing effects of high-voltage discharge on coal-rock mass under the action of hydrostatic pressure. Based on the traditional damage evolution model and the change of the effective bearing area and stress around the microcrack of coal-rock mass on a microscopic scale, the expression of macroscopic damage variation of coal-rock mass under the action of electrohydraulic coupling was derived, and its damage constitutive relation was established. The high-voltage pulse water shock stress test of coal-rock mass under the conditions of different water pressures and voltages was carried out, and its damage numerical model was established by the ABAQUS/XFEM. The damage variable values of coal-rock mass with different electrohydraulic parameters were compared to characterize the generation and expansion of internal macroscopic cracks. From the results, it is shown that the damage variable based on the effective bearing area and stress around the crack can represent the morphology and expansion change of crack of coal-rock mass with different electrohydraulic parameters clearly and quantitatively. Different hydrostatic pressures and discharge voltages have influence on the damage variables of coal-rock mass, but different influencing degrees cause different damages on coal-rock mass. The damage of coal rock can be characterized by the change of the effective bearing area and stress around microcracks in coal body. So, stress testing and numerical simulation have verified the correctness of the damage model.

Energy Dissipation and Damage Evolution Characteristics of Salt Rock under Uniaxial Cyclic Loading and Unloading Tension

Salt rock has been regarded as the optimal surrounding rock for underground gas storage (UGS), and it is occasionally subjected to cyclic tension because of the gas injection and production of salt cavern, which leads to the change in mechanical properties of salt rock. In this paper, a laboratory study is conducted to investigate the energy dissipation and damage evolution characteristics of salt rock under uniaxial cyclic tension monitored by acoustic emission (AE) machine. Compared to monotonic tension, both tensile strength and deformation capacity of salt rock are enhanced under cyclic tension. The fracture crack is approximately a single linear crack with large elliptical plastic deformation zone, which is consistent with the spatial distribution of AE events. In yield stage, the proportion of dissipative energy increases first but decreases subsequently. The relationship between AE energy-based damage variable and displacement is established. It is concluded that the damage variable is a piecewise power correlation with displacement while the growth rate of damage variable increases in the pre-peak stage but decreases in post-peak stage.