Triaxial Cyclic Loading Research Articles

Effects of Fatigue Load Amplitude on the Mechanical and Energy Evolution Characteristics of Stratified Rock Mass under Triaxial Cyclic Loading and Unloading Conditions

Effects of Fatigue Load Amplitude on the Mechanical and Energy Evolution Characteristics of Stratified Rock Mass under Triaxial Cyclic Loading and Unloading Conditions

Damage index correlation in massive granite-porous backfills under hydromechanical triaxial cyclic loading using acoustic emissions and X-ray computed tomography

The characterization accuracy of damage indices in massive and porous composite biomaterials under complex osmotic conditions has long been a challenge in engineering materials science. Establishing correlations between damage indices calculated using different methodologies is crucial to validate and improve defects characterizations. In this study, we investigated the damage index correlation of cylindrical massive granite-porous backfill structural composites under both dry and seepage conditions. A series of triaxial cyclic loading and unloading tests were conducted on specimens under hydromechanical conditions with varying osmotic pressures (0, 1, 3, 5, 7, and 9 MPa) and a confining pressure of 9 MPa involving acoustic emission monitoring and phase-based 3D nondestructive CT scanning. The results revealed the AE energy was lower under osmotic conditions compared to dry conditions. The AE Damage Index (DAE), defined by separating tensile and shear microcracking signals, significantly increased from 0.32 to 0.55 with rising osmotic pressure. 3D mesoscopic fracture visualization provided detailed insights into the fractures in granite and backfill interfaces. The number of small-angle cracks increased with osmotic pressure, and the crack inclination distribution range expanded from 63°- 85° to 49°–80°. The average crack angle showed a significant monotonic decrease with increasing osmotic pressure. An Energy Dissipation Damage Index (DED) was defined, and its correlation with the DAE under dry conditions was modeled using a reciprocal function DED=a/(DAE+bσ3)+c. This novel approach offers insights into the transition points between tensile and shear-dominated failure stages. Furthermore, an exponential function DV=aDAEb(b<0) was used to model the correlation between AE Damage Index and Visual Damage Index (DV) under osmotic conditions. The damage indices derived from different methods can be mathematically correlated, providing a comprehensive evaluation of material damage states. The study found the flow transition between different natural-human medias results in different damage mechanisms, which are all obey the correlation law. These damage mechanisms offered constructive recommendations for engineering applications based on varying ratios of osmotic pressure to confining pressure.

Damage evolution and acoustic emission characteristics of sandstone under different true triaxial cyclic loading and unloading modes

In the process of deep coal mining, the cyclic mining disturbance behavior produces significant cyclic loading and unloading effects, to ensure safe production in the mining area, it is necessary to study the damage characteristics and acoustic emission characteristics of the rock body under different cyclic loading and unloading effects. This study developed a TAWZ-5000/3000 rock true triaxial (disturbance) test system and used it to carry out true triaxial cyclic loading and unloading tests in different modes, with acoustic emission (AE) monitoring of the deformation and damage processes. The test results proved that a large number of cycles weakens the rock’s ability to control deformation, and this weakening ability is also related to the types of cyclic loading and unloading modes. The hysteresis loop characteristics and mechanical parameters show obvious path dependence. Based on the damage equivalence method, it is found that the absolute damage parameter increases with the increase of the number of cycles in the same mode, and the accumulation rate of the cumulative damage parameter is faster; the cumulative damage parameter in the equal amplitude cyclic loading and unloading (EAC) mode is always higher than that in the other two modes. The normalized AE ring count rate in the rock samples under cyclic loading and unloading has a significant relationship with the damage percentage, and the crack extension scale function (b-value) shows the “steady increase – strong fluctuation − significant decline” trend. By analyzing the AE characteristic parameters, namely rise time/amplitude (RA) and ringing count/duration (AF), it is found that the sandstones in EAC, graded cyclic loading and unloading (GC), and stepped cyclic loading and unloading (SC) modes are mainly damaged by tension-shear and shear, respectively. By analyzing the values of the acoustic emission parameters RA-AF, it is found that the sandstone is mainly damaged by tensile shear, tensile tension, and shear under EAC, GC, and SC modes, respectively, corresponding to the rock samples’ damage patterns. The study results have important guiding significance for preventing and controlling disasters in deep mines.

Characteristics of Energy Evolution and Failure Mechanisms in Sandstone Subject to Triaxial Cyclic Loading and Unloading Conditions

Characteristics of Energy Evolution and Failure Mechanisms in Sandstone Subject to Triaxial Cyclic Loading and Unloading Conditions

Mechanical Properties of Latex-Modified Cement Stone under Uniaxial and Triaxial Cyclic Loading.

During the cyclic injection and extraction process in underground storage wellbores, the cement sheath undergoes loading and unloading stress cycles. In this study, we investigated the mechanical properties of latex-modified cement stone (LMCS), widely used in oil and gas wells, through uniaxial and triaxial cyclic loading and unloading tests. The aim of the study was to determine the effect of various loading conditions on the compressive strength and stress-strain behavior of LMCS. The results show that the stress-strain curve of LMCS exhibits a hysteresis loop phenomenon, with the loop intervals decreasing throughout the entire cyclic loading and unloading process. As the number of cycles increases, the cumulative plastic strain of the LMCS increases approximately linearly. Under uniaxial cyclic loading and unloading conditions, the elastic modulus tends to stabilize. However, under triaxial conditions, the elastic modulus increases continuously as the number of cycles increases. This result provides data for engineering predictions. Furthermore, a comparison of the uniaxial and triaxial cyclic loading and unloading of LMCS shows that its cumulative plastic strain develops rapidly under uniaxial conditions, while the elastic modulus is larger under triaxial conditions. These findings provide a valuable reference for constructing underground storage wellbores.

Characteristics and Association Mechanism of Energy Dissipation and Crack Development in Siltstone Under Triaxial Cyclic Loading and Unloading

Characteristics and Association Mechanism of Energy Dissipation and Crack Development in Siltstone Under Triaxial Cyclic Loading and Unloading

Strain energy dissipation during liquefaction of fibre-reinforced sand under undrained cyclic triaxial loading

Strain energy dissipation during liquefaction of fibre-reinforced sand under undrained cyclic triaxial loading

The Experimental Characterization of Iron Ore Tailings from a Geotechnical Perspective

The mining industry produces large amounts of tailings which are disposed of in deposits, which neglects their potential value and represents important economic, social and environmental risks. Consequently, implementing circular economy principles using these unconventional geomaterials may decrease the wide-ranging impacts of raw material extraction. This paper presents an experimental characterization of iron ore tailings, which are the most abundant type of mining waste. The characterization includes various aspects of behavior that are relevant to different types of use as a building material, including physical and identification properties, compaction behavior and stress–strain properties under undrained monotonic and cyclic triaxial loading. The tailings tested can be described as low-plasticity silty sand materials with an average solids density of 4.7, a maximum dry unit weight close to 3 g/cm3 and a higher angle of friction and liquefaction resistance than common granular materials. The experimental results highlight the particular features of the behavior of iron ore tailings and emphasize the potentially promising combination of high shear resistance and high density that favors particular geotechnical applications. Overall, the conclusions provide the basis for promoting the use of mining wastes in the construction of sustainable geotechnical works and underpin the advanced analysis of tailings storage facilities’ safety founded on an open-minded geotechnical approach.

Correlating ballast resilient modulus with particle movement through SmartRock sensing

The resilient modulus of ballast is a key parameter in assessing track quality and is influenced by various loading conditions, such as deviator stress, bulk stress, loading frequency, and confining pressure. These factors, when incorporated into different models, can sometimes complicate their application in track maintenance planning. Therefore, a simplified model that collectively considers these factors would be beneficial in practical applications for estimating the ballast resilient modulus. This study utilizes smart sensing technology, SmartRock, to track particle movement during large-scale triaxial cyclic loading tests, exploring the potential correlation between ballast resilient modulus and particle motion. The findings indicate that the settling acceleration of ballast particles significantly impacts the ballast resilient modulus. This effect is dependent on the applied cyclic loading pulses, an aspect previously overlooked in research. Consequently, an index termed the Cyclic Loading and Acceleration Index (CLAI) has been introduced. CLAI integrates the effects of deviator stress, loading frequency, loading pulses, and settling acceleration. Based on CLAI, a simplified model for ballast resilient modulus has been developed and validated.

Experimental and Numerical Simulation Investigation of Cement Sheath Integrity during Multi-Stage Fracturing in Offshore Tight Oil Reservoir

The integrity of the cement sheath is susceptible to failure during multi-stage fracturing. In this study, the failure mechanisms of cement sheath integrity during multi-stage fracturing in the A offshore tight oil reservoir wells were investigated. The cement samples were subject to triaxial compression test (TCT), triaxial cyclic loading test (TCLT), and permeability test. A full-scale device was constructed for cement sheath integrity experiments. Additionally, a 3-D finite element model was developed to simulate the interface debonding and the subsequent growth of micro-annuli throughout multi-stage fracturing. The results revealed that TCLT induced cumulative plastic deformation in the cement samples, resulting in a 10.7% decrease in triaxial compressive strength, an 8.3% decrease in elastic modulus, and a 150% increase in permeability. Despite these significant variations, no serious damage was caused to the cement sheath matrix. It was observed that gas leakage occurred at the 8th, 10th, and 14th cycles under cyclic loading with upper limits of 70 MPa, 80 MPa, and 90 MPa, respectively. After 15 cycles, the experimentally measured widths of micro-annuli were 117 μm, 178 μm, and 212 μm, which were in good agreement with simulation results of 130 μm, 165 μm, and 205 μm, respectively. These findings elucidate the causes of cement sheath integrity failure, providing insights into the failure mechanisms of cement sheath integrity during multi-stage fracturing.

Evaluation of large axial strain of granular sand in post-liquefaction stage using DEM simulation

Field investigations in recent studies have revealed that significant damage to foundations often occurs in the post-liquefaction stage, during which sand undergoes substantial deformation. As a result, numerous researchers have dedicated considerable effort to studying the evolution of the axial strain at this stage. However, the accurate measurement of the exact value of this strain is challenging mainly owing to the limitations of the triaxial apparatus. In this study, the numerical method of discrete element method (DEM) is used to simulate undrained cyclic triaxial loading tests according to a large number of loading cycles. The mechanical behaviour of granular sand under different cell pressures was reproduced using the DEM method, wherein the liquefaction resistance increased with the cell pressure. The post-liquefaction axial strain exhibited a similar pattern of variation among the calculated cases at different cell pressures. The value of the double amplitude of the axial strain stabilised at a maximum value after a certain loading cycle. The microscopic characteristics derived from the DEM calculations, including the contact number and force chain network, were analysed to provide reasonable explanations. The sand fabric was re-established after a large number of loading cycles and controlled by cyclic loading, which helped stabilise the accumulated axial strain.

Correlational fractal analysis of the Kaiser effect in rock: implications for salt-cavern engineering under triaxial cyclic loading and unloading

ABSTRACT The Kaiser effect exhibits stress-induced changes in the acoustic properties of rock, so accurately identifying the Kaiser point of rock under complex conditions is essential for understanding the damage and failure mechanisms. In this paper, triaxial loading and unloading tests were conducted on salt rock using MTS815 material testing machine and PCI-II acoustic emission (AE) system. By introducing the Kaiser point calculation approach involving AE signal optimisation, Grassberger-Procaccia (GP) algorithm, K-means clustering, and random sample consensus (RANSAC) algorithm, we achieved enhanced accuracy in analysing the Kaiser effect for salt rock. The cumulative AE energy exhibited a distinct surge point during the cycle number (N) was less than 21, closely aligned with the optimised key AE energy point. For N > 21, the cumulative energy displayed stable changes, rendering conventional method ineffective. By utilising screened AE signals (8796 samples) within the GP algorithm, the surge point of cumulative energy consistently aligned with the sudden drop point of the correlation dimension when N<21, with a maximum error of 6 s. Remarkably, the GP algorithm identified the prominent Kaiser point even when N > 21, suggesting its applicability throughout the loading cycle.

Mechanical Behavior and Energy Dissipation of Sandstone–Shotcrete under Monotonic and Cyclic Triaxial Loading

Mechanical Behavior and Energy Dissipation of Sandstone–Shotcrete under Monotonic and Cyclic Triaxial Loading

Experimental study on damage evolution process, seepage characteristics, and energy response of columnar jointed basalt under true triaxial cyclic loading

The physical response of Columnar Jointed Basalt (CJB) under a stress–seepage coupling field is of great significance for dam safety and oil and gas reservoir evaluation. A true triaxial experimental machine was used for investigating the deformation, damage evolution characteristics, seepage properties, and energy response of the CJB. Before the start of the test, a series of uniaxial compression tests were used to determine the ratio of similar materials, and the cyclic loading test results indicate that: The strength characteristics of CJB decreases slowly and then increases rapidly as the dip angle β varies from 0° to 90° under two loading conditions of the tiered cyclic loading (T–C) and the constant amplitude cyclic loading (CA–C), showing an obvious U–shaped trend, and four different loading stages can be distinguished by characteristic stress. In the yield stage, the stress–strain curve shows an obvious plastic hysteresis loop, there is a strong correlation between the failure modes of the CJB and β of the specimen, the joint surface gradually deteriorates owing to loading and unloading conditions, and structural control failure occurs widely in specimens. The damage evolution process is discussed in combination with the Load–Unloading Response Ratio, and the relative damage amount (Df) is defined by cycle index. The permeability–axial stress relationship of the specimen conforms to the power exponential relationship, and the permeability evolution characteristics are strongly correlated with the degree of damage. The energy response of the CJB were investigated by tress–strain curve, under two loading modes, T–C and CA–C, the cumulative curves of dissipation energy Ed can be fitted by an exponential function and a logarithmic function, respectively. The findings of this research hold immense importance for clarifying the mechanical and seepage properties of CJB under a stress–seepage coupling field.

Dynamic Properties and Dynamic Response Model of Jointed Granites by Cyclic Loading

The present study investigates the dynamic properties of granite samples with varying degrees of defects through triaxial cyclic loading experiments conducted under different conditions, including varied confining pressures, loading frequencies, dynamic stress amplitudes, and number of cycles, and the dynamic response model of granite samples influenced by the confining pressure and frequency are constructed. The results show that the dynamic elastic modulus of granite increases, but its dynamic damping ratio decreases as the confining pressure increases. The dynamic elastic modulus and dynamic damping ratio of the granite increase as increasing frequency. The dynamic elastic modulus of granite increases with the increasing dynamic stress amplitude while its dynamic damping ratio decreases. The dynamic elastic modulus and dynamic damping ratio of granite decreases with an increasing number of cycles. The modified Duncan–Chang model can well describe the dynamical behavior of granite influenced by the confining pressure and frequency. The correlation coefficients of the modified model reached 0.98. It is worth saying that the correlation coefficient of the model is low at 20 Hz frequency. It indicates that frequency has a strong effect on the dynamic response of granite compared with the confining pressure. These data and models will be applied to the next step of detection and prediction of the tunnel rock stress state.

Evolution characteristics of fracture volume and acoustic emission entropy of monzogranite under cyclic loading

The volume evolution behavior of rock fissures and the characteristics of acoustic emission under cyclic loading are critical for rock stability analysis. To study the volume change behavior of monzogranite fissures and the characteristics of acoustic emission signals under cyclic loading, we selected samples of monzogranite at − 1600 m from a gold mine located in the Jiaodong Peninsula at a depth of − 1600 m and investigated the samples using triaxial cyclic loading—unloading tests and acoustic emission monitoring. As the volume change behavior of the monzogranite fissures and acoustic emission signals were monitored and recorded, the calculated fracture volume strain ratio coefficient and acoustic emission entropy value were proposed to describe the evolution process of fissures inside the rock. The research results showed that the volume strain ratio curve of the rock fractures exhibited a logarithmic variation characteristic during the cyclic loading and unloading, and the fracture volume strain ratio better reflected the relative scale of the internal fracture strain in the rock to the total volume strain. The acoustic emission entropy value reflected the crack evolution behavior during the loading and failure processes, which was a rapid decline in the initial stage of loading and a rapid upward trend in the failure stage. The observed “V”-shaped change in the acoustic emission entropy can be used as an early warning for rock failure. The research results can provide theoretical guidance for rock stability analysis.

Plastic and damage energy dissipation characteristics and damage evolution of Beishan granite under triaxial cyclic loading

The damage and failure of rock under triaxial stress is an irreversible process of energy dissipation, which mainly includes plastic energy dissipation and damage energy dissipation. However, it is difficult and rare to separate plastic energy dissipation and damage energy dissipation and to conduct relevant research on this basis. In this work, a loading control method with a combination of load (0.5 kN/s) and circumferential deformation (0.01 mm/min) was used, and triaxial cyclic loading and unloading tests under five different confining pressures were performed on granite from the Beishan site area of China's high-level radioactive waste repository. The results indicate that the angle between the main fracture surface and the longitudinal axis increases with the increase of confining pressure. According to the characteristics of the stress–strain curve and energy dissipation, the test process can be divided into five stages: the initial compaction and elastic stage, the stable crack growth stage, the unstable crack growth stage, the postpeak unstable fracture stage and the residual stress stage. Importantly, a separate determination method of plastic energy dissipation and damage energy dissipation was proposed, and the evolution characteristics of plastic energy dissipation and damage energy dissipation were revealed. The ratio of plastic energy dissipation to input energy decreases slightly in the initial compaction and elastic stage, stabilizes at a low level in the stable and unstable crack growth stages, increases significantly in the postpeak unstable fracture stage and remains at a high level in the residual stress stage. Meanwhile, the ratio of damage energy dissipation to input energy increases slightly before the unstable crack growth stage, increases abruptly at the end of the unstable crack growth stage and the initial part of the postpeak unstable fracture stage, then declines rapidly, and finally remains at a low level. Furthermore, the qualitative and quantitative research results of damage evolution show that the plastic shear strain and damage variable established by damage energy dissipation can qualitatively and quantitatively describe the damage evolution of Beishan granite respectively. Finally, the verification results of Beishan granite and two other rock types show that the separation method for calculating plastic energy dissipation and damage energy dissipation proposed in this work has good adaptability, and the qualitative damage development analysis based on plastic shear strain can be verified and supplemented with the quantitative damage development analysis based on energy theory.

Effects of Loading and Compaction Conditions on Permanent Strains of Unbound Lateritic Soils from Senegal and Burkina Faso

Permanent strains of granular materials result from the repetitive passage of heavy trucks on the pavement. Pavement design relies mainly on permanent strains to predict rut depths. In order to contribute to the improvement of technical documents used for the design and modelling of pavements in tropical African countries, the researchers have focused on the determination of the permanent deformation parameters of laterites (axial (εp,1) and radial (εp,3) permanent strains). The Repeated Triaxial cyclic Loading (RTL) apparatus was used according to the European standard (EN 13286-7: 2004, Multistage method). Four sites of gravelly lateritic soils chosen, two from Burkina Faso (Badnogo2 and Dédougou) and two from Senegal (Sindia and Lam-Lam) were tested in this research. They are compacted to 95% and more of the optimum with a variation in water content of ±2% of the optimum. The permanents deformations are influenced by the water content, the compaction rate and the stress level. The axial permanent deformation ε1,p (max) decreases by 83.4% when the water content decreases from 12.10% to 7.59%. Moreover with a compactness rate that varies from 95.58% to 100.06%, respectively from a water content that varies from 10.04% to 9.36%, we observe a decrease in deformation ε_(1,p) from 224.340*10-4 to 148.150*10-4 from the 50,000 cycles, which shows the effect of the compactness rate and the stress level on the lateritic gravels materials soils. however, for a good control of permanent deformations on flexible pavements using gravel materials, it is necessary to compact at water contents lower than the optimum added to average compaction rates. Keywords: permanent strains (p), lateritic soils, Repeated Triaxial cyclic Loading (RTL), Sindia, Badnogo2, Dedougou, Lam-lam.

An improved strain-softening constitutive model of granite considering the effect of crack deformation

This paper presents an improved strain-softening constitutive model considering the effect of crack deformation based on the triaxial cyclic loading and unloading test results. The improved model assumes that total strain is a combination of plastic, elastic, and crack strains. The constitutive relationship between the crack strain and the stress was further derived. The evolutions of mechanical parameters, i.e. strength parameters, dilation angle, unloading elastic modulus, and deformation parameters of crack, with the plastic strain and confining pressure were studied. With the increase in plastic strain, the cohesion, friction angle, dilation angle, and crack Poisson's ratio initially increase and subsequently decrease, and the unloading elastic modulus and the crack elastic modulus nonlinearly decrease. The increasing confining pressure enhances the strength and unloading elastic modulus, and decreases the dilation angle and Poisson's ratio of the crack. The theoretical triaxial compressive stress-strain curves were compared with the experimental results, and they present a good agreement with each other. The improved constitutive model can well reflect the nonlinear mechanical behavior of granite.

Triaxial experimental study of mechanical behavior of non-persistent jointed granite subjected to slight cyclic dynamic disturbances

A slight disturbance can induce fault-slip burst failure in deep rock masses. In this paper, a series of triaxial tests were conducted to investigate the strength, deformation characteristics, and failure behavior of non-persistent jointed granite specimens subjected to slight cyclic loading. A representative stress–strain curve for non-persistent jointed rock was generated and used to study the crack evolution process (initiation → propagation → coalescence). The effects of varying the joint angle and frequency and amplitude of the disturbance were then analyzed. A theoretical (fracture mechanics) model for a non-persistent joint subject to triaxial cyclic loading was also established and used to investigate the combined effects of static confining stress and slight disturbances. Finally, the mechanical mechanism by which a slight dynamic disturbance induces fault-slip burst was discussed. Our results show that the failure mode of a non-persistent jointed rock mass depends more on the joint angle than the amplitude and frequency of the slight disturbance. The disturbance induces the initiation and extension of cracks. It can also induce fracture-slip bursting once the disturbance exceeds a certain threshold. The value of this threshold depends on the initial stress state, joint angle, length, dynamic and static friction coefficients, and fracture toughness of the rock. The study thus enriches our mechanistic understanding of the crucial ingredients involved in the triggering of fault-slip rock bursts in deep rock masses by slight disturbances.