To investigate the effects of prior cyclic loading damage in rocks on subsequent unloading failure characteristics under true-triaxial conditions, a series of complicated unloading tests ...incorporating the damage-controlled cyclic loading path and stress σ3 unloading path was conducted using a true-triaxial test system. The experimental results reveal that the prior cyclic loading damage has an impact on the strength and deformation characteristics, energy conversion and failure mode. As the number of prior cyclic loads increases, the unloading strength and Young's modulus increase firstly and then decrease, while the peak unloading strain, as well as the ratio (η) of crack damage stress to peak unloading stress, exhibits a descending trend. The energy storage capacity of rock samples is dramatically reduced as cycle number increases to 10 and enlarged slightly with a further increase in cycle number. Both shear fracture and tensile fracture appear in each rock sample under this unloading condition, as the prior cyclic loading number increases, the dominant failure mode of rock samples changes from tensile failure to mixed tensile-shear failure, then to shear failure, while the failure angle ranging from 65° to 80° deceases firstly and then turns to rise.
In order to study the influence of cyclic mining stress on gas-containing coal seams during mining, the seepage properties, acoustic emission (AE) characteristics and energy dissipation of coal under ...tiered cyclic loading were experimentally investigated. Permeability recovery rate, damping ratio, AE energy rate and ring count rate were defined to describe this process. A new equation was established for the damage variable of coal with dissipation energy. These results indicate that, with tiered cyclic loading and unloading, the following occurred. 1) The absolute recovery of permeability first decreased gradually, and then increased while the relative recovery rate of permeability gradually increased. 2) AE ring count rate and AE energy rate all showed a volatility change in the initial stage, with significant increases in the yielding stage, and rapid increases in the failure stage. When the stress level was low, the AE signal disappeared after unloading to the last peak stress. When the stress level was high, the AE signal appeared in the whole unloading process. 3) The cumulative dissipation energy increased with an exponential function, and the damping ratio of coal first decreased and then increased. The damage variable also increased and reached its critical damage variable value of Dc≈0.9 at coal failure
•Permeability, energy dissipation and AE characteristics of coal under tiered cyclic loading•Definition of permeability recovery rate for quantitative analysis of permeability change•A new evolution equation of coal damage variable based on dissipation energy•AE ring count rate for quantitative analysis of the average intensity of AE signal in each cycle
Judgement of rock burst proneness of rock materials is one of the basic problems in the field of rock burst research. In this study, a peak-strength strain energy storage index is proposed for ...estimating and classifying the rock burst proneness of rock materials. The method for determining this index is also introduced in this paper. The peak-strength strain energy storage index is defined as the ratio of the elastic strain energy density to the dissipated strain energy density corresponding to the peak compressive strength of rock specimen. In order to obtain the elastic strain energy density and the dissipated strain energy density at the peak strength, a series of single cyclic loading-unloading uniaxial compression tests on nine rock materials were conducted. The relationships between the total input energy density and the elastic strain energy density at different unloading stress levels were investigated. The results show that, for every rock material, the elastic strain energy density increases linearly with the increase of the total input energy density. Based on this linear storage energy law, the elastic & dissipated strain energy density at the peak strength can be calculated for each specimen, and the peak-strength strain energy storage index can be obtained accordingly. A new criterion for rock burst proneness of rock materials is proposed. The rock burst proneness of nine rock materials estimated with the proposed criterion agreed well with the laboratory test results.
To simulate the cyclic behaviour of rock comprehensively, relevant mechanical features were analysed sufficiently, including the massing effect, ratcheting effect, Kaiser effect and Felicity effect. ...A constitutive body was proposed based on the deformation mechanism and implemented by strain increment method. This body comprised three elements (elastic element Ee, plastic element Ep and friction element Ef), in which Ee and Ep were connected in series, and Ef was connected with them in parallel. Two kinds of triaxial cyclic loading experiments were conducted to determine the constitutive laws of these elements and the Poisson’s ratios of Ee and Ep; moreover, the simulation effect was validated against the experimental data. Results showed that the tangential modulus of Ee was nonlinear due to damage development and variation of particle dense state, the backwards movement of the yield surface involving Ep during reloading induced the ratcheting effect and Felicity effect, and the opposite role of Ef during loading and unloading caused the massing effect. The constitutive laws of Ee and Ep could be expressed by quadratic polynomial function and logistic equation, respectively; the Poisson’s ratio variations in Ee and Ep with axial plastic strain could be expressed by linear function and exponential function, respectively.
This work aims at investigating the fracture evolution and energy characteristics of marble subjected to fatigue cyclic loading and confining pressure unloading (FC-CPU) conditions. Although rocks ...under separated fatigue cyclic loading and triaxial unloading conditions have been well studied, little is known about the dependence of the fatigue damage accumulation on the subsequent confining pressure unloading condition that influences the rock fracture behaviors. In this work, the servo-controlled GCTS 2000 rock mechanical system combined with the post-test X-ray computed tomography (CT) scanning technique were used to reveal the fracture behaviors of the marble samples. The samples were tested at three stages: the static loading stage, the fatigue cyclic loading stage, and the confining pressure unloading stage. Results show that the damage index-cycle number curve shows a different pattern—the damage increasing rate is different for the samples experiencing different fatigue damage. The damage accumulation at the fatigue cyclic stage influences the final failure mode and energy conversion. In addition, post-test CT scanning further reveals the effects of fatigue cycles on the crack pattern, as well as the stimulated crack scale and density after FC-CPU testing depending on the fatigue cycle. Furthermore, the stored elastic energy decreases and the dissipated energy increases with increasing fatigue cycle at the fatigue loading stage, and the energy conversion is consistent with the crack pattern analysis. By investigating the failure mechanism of marble under FC-CPU conditions, a theoretical basis for rock dynamic disaster prediction can be created.
Characteristics of energy accumulation, evolution, and dissipation in uniaxial cyclic loading and unloading compression of 30 sandstone rock specimens under six different loading rates were explored. ...Stress–strain relations and acoustic emission characteristics of the deformation and failure of rock specimens were analyzed. The densities and rates of stored energy, elastic energy, and dissipated energy under different loading rates were confirmed, and an effective approach for the equivalent energy surface was presented. The energy evolution of rock deformation and failure were revealed. It turns out that the rock deformation behavior under uniaxial cyclic loading and unloading compression remained almost unchanged compared with that of uniaxial compression. The degree of match between reloading stress–strain curves and previous unloading curves was high, thereby demonstrating the memory function of rock masses. The intensity of acoustic emission fluctuated continually during the entire cyclic process. Emissions significantly increased as the stress exceeded the unloading level. The peak of acoustic emission increased with increasing loading stress level. Relationships between energy density and axial load indicate that the rock mass possesses a certain energy storage limitation. The energy evolution of rock masses is closely related to the axial loading stress, rather than to the axial loading rate. With increasing axial loading stress, stored energy varied most rapidly, followed by that of the elastic energy, then dissipated energy. Energy accumulation dominates prior to the axial load reaching peak strength; thereafter, energy dissipation becomes dominant. The input energy causes the irreversible initiation and extension of microcracks in the rock body. Elastic energy release leads to sudden instability of rock bodies and drives rock damage.
Intermittently jointed rocks, widely existing in many mining and civil engineering structures, are quite susceptible to cyclic loading. Understanding the fatigue mechanism of jointed rocks is vital ...to the rational design and the long-term stability analysis of rock structures. In this study, the fatigue mechanical properties of synthetic jointed rock models under different cyclic conditions are systematically investigated in the laboratory, including four loading frequencies, four maximum stresses, and four amplitudes. Our experimental results reveal the influence of the three cyclic loading parameters on the mechanical properties of jointed rock models, regarding the fatigue deformation characteristics, the fatigue energy and damage evolution, and the fatigue failure and progressive failure behavior. Under lower loading frequency or higher maximum stress and amplitude, the jointed specimen is characterized by higher fatigue deformation moduli and higher dissipated hysteresis energy, resulting in higher cumulative damage and lower fatigue life. However, the fatigue failure modes of jointed specimens are independent of cyclic loading parameters; all tested jointed specimens exhibit a prominent tensile splitting failure mode. Three different crack coalescence patterns are classified between two adjacent joints. Furthermore, different from the progressive failure under static monotonic loading, the jointed rock specimens under cyclic compression fail more abruptly without evident preceding signs. The tensile cracks on the front surface of jointed specimens always initiate from the joint tips and then propagate at a certain angle with the joints toward the direction of maximum compression.
Composites with 3D-textile reinforcement present several engineering advantages. However, their intricate yarn architecture also creates a material with a number of nonlinear behaviours and features, ...which need to be understood in order to enable their efficient use. To demonstrate the anisotropic development of such non-linear behaviours, and how they depend on loading mode, tensile samples of a 3D-woven layer-to-layer angle interlock carbon-fibre reinforced epoxy composite are tested experimentally (data shared publicly). More specifically, specimens are cut and tested at orientations of 0°, 15°, 30°, 45° and 90° relative to the direction of the warp yarns. The samples are tested cyclically by loading and unloading them at progressively higher displacement values. By monitoring the reduction in stiffness and the development of permanent strains it is possible to identify material parameter values used to calibrate an anisotropic macroscale elasto-plastic damage model. The model shows promising agreement with the experimental results.
•Experimental testing campaign to understand and characterise in-plane non-linear behaviours of 3D-textile composites.•The test data has been made publicly available.•Cyclic testing the composite material shows a clear development in progressive damage and permanent strains as the tests progressed.•A macroscale elasto-plastic damage model is calibrated and validated against off-axis testing. The model shows promising agreement to the experimental results.
Fibre-reinforced polymer (FRP) is a promising composite to be used in construction in coastal and marine environments to resist seawater corrosion that deteriorates the properties of conventional ...civil engineering materials. A classical ground or seabed − structure interaction problem that is involved in the design of FRP structures, is however less understood when the soft nature of FRP is in subjection to the cyclic loadings from traffic, wind, wave and currents, causing penetration and abrasion at the soft FRP − soil interface. This study has downscaled the preceding problem into a micromechanical study at a benchmark sand grain − FRP interface. A large number of cyclic loading is applied, for the first time, at the sand − FRP composite interface, focusing on the development of the elastoplastic behaviour in the normal direction and the evolution of friction and energy dissipation in the tangential direction. The study combines the understanding from the tribology with the knowledge of civil engineering involved in the sand − FRP interaction, suggesting that a larger stick zone at the contact subjected to cyclic shearing is a key triggering of the simultaneous occurrence of the increased coefficient of friction and reduced damping ratio.
