Damage Characteristics of Rocks by Uniaxial Compression and Cyclic Loading-Unloading Test

IntroductionIn order to reduce the impact of secondary disasters caused by the instability of rock and soil mass (RSM) during engineering construction on the environment, and to achieve safe and efficient engineering construction. Therefore, investigating the mechanical properties (M.P.), energy evolution laws, and damage characteristics of limestone with different water saturation (w) under cyclic loading-unloading (CLU) conditions is of significant engineering significance.MethodsThis study conducted uniaxial compression (UC) and cyclic loading-unloading tests on limestone samples with different w values (i.e., 0%, 25%, 50%, 75%, 100%) to elucidate their mechanical properties and energy dissipation. The influence of w on the degradation of limestone was examined based on damage variables.ResultsThe results indicated that (1) as w increases, both the compressive strength (fc) and elastic modulus (E) of the samples gradually decrease, while the peak axial strain gradually increases. When the w exceeded 0.4%, the failure characteristics transitioned from brittleness to ductility. (2) For limestone samples with the same w, the fc and E under CLU conditions were greater than those under uniaxial compression conditions, while the peak axial strain was smaller than that under UC conditions. Analysis using the DRA method confirmed that w did not significantly affect the deformation memory effect of limestone. (3) As the axial strain and number of cycles (N) increased, both the input energy and dissipated energy gradually increased, while the elastic energy initially increased before rapidly declining. The proportion of elastic energy first increased and then decreased, while the proportion of dissipated energy first increased, then decreased, and finally suddenly increased. Compared with UC, CLU significantly enhanced the rock’s capacity to store elastic energy. (4) For the same N, limestone with higher w exhibited greater damage than that with lower w. Moreover, samples with high w always failed earlier than those with low w under both the UC and CLU conditions.DiscussionThe research results provide a theoretical basis for understanding the dynamic response behavior and stability analysis of limestone slopes under disturbance and rainfall effects.

Characterization of nonlinear response in quasi-unidirectional E-glass fabric reinforced polypropylene composites under off-axis tensile loading

In this paper, uniaxial cyclic compression and shear test was carried out for lignite samples. The effects of inclination angle (θ) and upper limit of cyclic stress (σmax) on mechanical properties of coal samples were analyzed, and the damage variables of coal samples were studied based on energy dissipation theory. The results show that the uniaxial compressive strength (UCS) of coal samples after uniaxial cyclic compression and shear tests decreases with the increase of the upper limit of cyclic stress and inclination angle. The shear stress component generated by the increase of inclination angle can effectively reduce the UCS and increase the damage degree of coal samples. With the increase of inclination angle, the failure mode of coal samples is changed from tensile failure (θ = 0°), the combined tensile failure and shear failure (θ = 5°) to shear failure (θ = 10°). The peak axial and radial strain of coal samples first increases rapidly and then stagnates. The peak volume strain rapid increases and then stagnates (θ = 0° and θ = 5°). When the inclination angle is 10°, the peak volume strain first decreases rapidly and then stagnates. Even if the upper limit of cyclic stress is lower than its UCS, it will still promote the propagation of micro cracks and the generation of new cracks and increase the internal damage of coal samples. With the increase of the cycle number, damage variables of coal samples after uniaxial cyclic compression and shear tests nonlinearly increase, and the growth rate decreases gradually.

Fatigue-thermal damage characteristics of red sandstone with a hole under high-temperature cyclic loading coupling and microstructural degradation

Experimental study on thermo-mechanical properties of Beishan granite under mild temperature

A multifrequency ultrasonic approach to extracting static modulus and damage characteristics of rock

Strength degradation of sandstone and granodiorite under uniaxial cyclic loading

The determinations of the effects of cyclic loading on the deformation failure characteristics and ultrasonic properties of granite are significant to accurate the evaluations of stability and safety of excavation damage zones. However, previous experimental studies regarding granite under cyclic loading have not achieved adequate results. This study conducted a series of cyclic tests investigating the mechanical properties and ultrasonic velocities of granite specimens. The results indicated that the stress-strain curves had presented only minimum crack growth phase and no residual strength in the post-peak phase due to the strong brittle properties of the granite. Also, the residual strain of the granite specimens under uniaxial cyclic loading-unloading compression conditions was observed to decrease with the cyclic number as a result of pre-existing crack closure. However, with triaxial cyclic loading, it was found that the residual strain had initially decreased, and then increased after several cycles. It was also was observed that the elastic strain of the granite had displayed increasing trends in both the uniaxial and triaxial cyclic loading-unloading compression experiments. The failure modes in uniaxial cyclic compression tests are known to be complicated, and more failures will be evident with increased damage to the thermoplastic membranes. After confining pressure was applied, the failure mode was determined to mainly be shear fracture and to be less serious than failures observed in the uniaxial cyclic compression tests. The P and S wave velocities showed nonlinear characteristics during the loading processes. The relationship between the dynamic Young’s modulus and the static Young’s modulus could be expressed as a linear function. However, the static Young’s modulus was determined to be larger than the dynamic Young’s modulus. In this present study, both the P and S wave velocities showed that stress related properties first increased and then decreased with the stress.

To analyze the damage characteristics of rocks during high-voltage pulse fragmentation (HVPF), two kinds of loads, shockwave and cavity, are determined by optical observation, and the pressure–time characteristics of these two and their mechanism of damage to rocks in mesoscopic view are analyzed. A model of dynamic damage characteristics of brittle rock under multiple loads is established, which includes numerical calculation and discrete element simulation. In the discrete element simulation, the rock is simplified as a circular region without reflection boundary with a certain size of the circular hole inside, and the grains in the region are discretized as rigid spheres with a definite bonding relationship. The shockwave is considered the time-varying pressure loaded to the grains of the circular hole, and the cavity is considered the quasi-static pressure loaded to the grains on both sides of the fracture. The results of the model show that shear cracks and tensile cracks are produced during the shockwave action, but tensile cracks are predominant. The shockwave acts as a preload for the expansion of cracks, and the damage radius is small. Most of the cracks in HVPF are caused by the cavity. A comparison of the numerical calculation results with the discrete element simulation results shows that the model can describe the distribution characteristics of cracks under multiple loads, which lays a foundation for further analysis of the internal mechanism of HVPF.

We estimate effects of ply thickness in mechanical properties and damage evolution of CFRP angle-ply laminates under various tensile loading. It should be noted that the laminate thickness is almost the same, but the ply thickness are quite different. Monotonic tensile tests , cyclic loading-unloading tensile tests and stress relaxation tensile tests are performed on [(±)_12]_s(t-O.O5prepregx48plies), [(±)_4]_s(t-O.15prepregxl6plies) and [(+θ_4/(-θ)_4]_s (t-0.15prepregxl6plies) T700SC/2500 carbon/epoxy laminates with various fiber directions (θ=30, 40, 45, 50 and 67.5°) .Ply thickness is expressed as t-0 05, t-0 15 and t-0 6, respectively. Damage evolution can be determined by cyclic loading-unloading tests. We use mesoscale damage model to investigate damage evolution in CFRP laminates. We discuss effect of ply thickness on mechanical property of angle-ply laminates. Laminates have thin ply thickness showed high strength and fracture strain.

Suitability of the damage-plasticity modelling concept for concrete at elevated temperatures: Experimental validation with uniaxial cyclic compression tests

The tensile strength of crystalline rocks in Brazilian tests (BT) shows a more significant size dependency than the uniaxial compression strength in uniaxial compression tests (UCT), while the micro-mechanism is still unclear. This study seeks to elucidate the micro-mechanism by comparing different microcracking processes under UCT and BT. We adopt a grain-based model in the discrete element method (DEM) to reproduce the interlocked microstructure of crystalline rocks and perform UCT and BT with different sizes ranging from 0.5 to 3 times the standard sample size. It was found that peak stresses in UCT are nearly insensitive to sizes, which is consistent with experimental results measured at very low loading rates. Due to the homogenous stress distribution, no clear cracking paths are observed; instead, randomly distributed microcracks appear at failure. In contrast, numerical results indicate that the tensile strength measured in BT decreases as size increases and continuous cracking paths are observed at failure. Larger specimens have a higher possibility to include weaker cracking paths, hence tensile strength shows stronger size dependency. The findings of this work offer microscopic explanations for different size effects observed in UCT and BT, which can bridge the gap between laboratory-scale strength data and field-scale applications. INTRODUCTION Rock engineering applications often require assessing the strength and failure characteristics of rock masses. These rock masses are composed of intact blocks across various scales. However, the strength and cracking mechanism of these field-scale intact blocks are usually indirectly measured and studied through laboratory-scale tests on intact specimens (Duan et al., 2017; Duan and Kwok, 2015; Fei et al., 2021; Mahabadi et al., 2014; Martin and Chandler, 1994; Wong et al., 1996). Uniaxial compression test (UCT) and Brazilian tests (BT) are standard tests that measure the uniaxial compression strength (σf) and tensile strength (σt) of rocks (Eberhardt et al., 1999; Li and Wong, 2013; Martin, 1994). The sizes of UCT and BT specimens in the laboratory usually vary from 5-10 cm in diameter, much smaller than in fields. However, a dependence of strength on size is widely known in rocks (Bažant, 1984; Choo et al., 2023; Paterson & Wong, 2005). The strength determined by these laboratory-scale tests may overestimate the loading capacity in fields, and the cracking process observed in the laboratory-scale specimens may also not accurately manifest the failure mechanism in the field scale. Therefore, to apply laboratory-scale experimental data to practical applications, it is vital to investigate the influence of specimen size on strength and cracking patterns.

The western part of our country is mostly alpine regions. The rock and soil have been in a strong natural freeze‐thaw environment for a long time, and their physical and mechanical properties are easily affected by external loads and external surroundings. Changes due to the influence of the environment will inevitably produce freeze‐thaw cycles, damage and destruction, expansion and fracture, etc., resulting in more stable factors than usual. However, there is a lack of theoretical and practical experience in freeze‐thaw rocks, especially freeze‐thaw hard rocks. Therefore, studying the physical and mechanical properties and damage characteristics of rocks in alpine regions under freeze‐thaw cycles has important significance. This paper uses dacite in the alpine region to carry out a freeze‐thaw cycle experiment in a variable temperature range. Freezing and thawing cycle test, uniaxial compression test, triaxial compression test, and electron microscope scanning of the rock in the indoor saturated state were carried out. Combining theory with experimental mechanics, freeze‐thaw mechanics, and damage mechanics, we studied freeze‐thaw cycle in three variable temperature ranges (−20°C–15°C; −30°C–15°C; −40°C–15°C), along with the physical and mechanical properties and damage characteristics of freeze‐thaw dacite in the alpine region under cycling. The damage curve of the final theoretical model gradually approaches 1.0 with the increase of strain during the actual test. The rock sample after the medium failure still has a certain bearing capacity, and the rock sample is often destroyed before it reaches the theoretical failure strain.

Proper characterization of the material properties of pulmonary arterial tissue is needed for many medical applications. The objective of this study was to investigate the stress-strain relationship and characterize the nonlinear elastic behavior of porcine pulmonary arteries; thus, uniaxial tension tests and cyclic loading-unloading tests were conducted on healthy porcine pulmonary arterial tissue. In these experiments, pulmonary arteries from different piglets and a commercial pulmonary valved conduit, called “Contegra 200”, were subjected to uniaxial tension. Results demonstrated a higher stiffness along the circumferential direction than the axial direction. The “Contegra 200” was much suffer than real pulmonary arterial tissue along the axial direction and had a similar stiffness to natural tissue along the circumferential direction within physiological stretch ranges, which is less than 40% strain. Elastic hysteresis was observed from cyclic loading-unloading tests, which indicates that more energy was required during the loading than the unloading. A nonlinear hyperelastic model based on second order polynomial constitutive equation was derived from average values of the test data along both axial and circumferential directions. The material model could be used in numerical analysis of pulmonary arterial response and facilitate the design of intravascular devices.

Deep underground engineering often utilizes cyclic loading. To understand the deformation and damage characteristics of rock under cyclic loading conditions, cyclic loading tests of three different specimens with varying lithology were performed. The dissipated energy method was used to analyze the magnitude of damage and rock failure characteristics during the energy evolution process of cube specimens. The results indicated that the modulus of elasticity of three lithologies were stable prior and subsequent to cyclic loading. While the cyclic loading profile improved the rock’s resistance to deformation, it increased the internal mesostructure defects. Rock damage caused by the cyclic loading reduced the uniaxial compression strength. This was especially true for coal samples. For coal samples, these observations were consistent with internal coal damage calculated by analysis. Under the same lithology and different loading modes, rock damage was caused by cyclic action, and the elastic strain energy released by instantaneous unloading of rock samples with significant damage was reduced. The rupture magnitude for cyclic loading was observed to be less than that of uniaxial compression. Under cyclic loading and varying different lithologies, sandstone absorbed the most energy, resulting in a larger final fracture magnitude compared to other lithologies.