Mechanical Creep Research Articles

Compression Behavior of Municipal Solid Waste in Temperature-Controlled Isotropic Condition

Abstract The mechanical response of municipal solid waste (MSW) under different types of loading and testing conditions has been explored by many researchers. However, the effect of temperature on MSW compressibility has yet to be investigated. Temperature enhancement is well-known in landfill engineering, mainly when the bioreactor philosophy is employed. The generated heat gradients affect the solid particles and fluid phases. As a result, the mechanical response of MSW will be different compared with the most recent research results. Only a few investigations have been conducted on the impact of temperature on MSW’s mechanical response. This research shows how temperature enhancement impacts the compression behavior of MSW specimens with different compositions. A large-scale temperature-controlled triaxial device capable of imposing temperatures up to 85°C has been used. It is shown that, with increasing temperature, the compressive strains for MSW increase. However, the waste composition, mainly the MSW plastic content, also affects these strains. The temperature enhancements increase the mechanical creep and immediate compression indexes but decrease the samples’ elastic rebound and swelling index. Furthermore, the paper analyzes how pore water pressure affects the effective stress in such varying temperature conditions considering the waste particles’ compressibility. The achieved values show that the pore water pressure effect on effective stress is reduced up to 75 % because of the particles’ compressibility. The permeability of samples was examined based on the isotropic compression results. The results confirmed that MSW’s permeability depends on the plastic content, applied confining stress, and temperature. Increasing the temperature level showed a reduction in the permeability of samples. However, this difference was not paramount.

Stress Development Under Fast Charging and Its Effect on Degradation of Separator

This paper discusses the predicted stress when a pouch-type lithium-ion (Li-ion) cell undergoes cyclic fast charging and how the stress is related to the observed degradation of the separator. Previous experiments showed a significant degradation in the mechanical integrity of the separator after repeated fast charging. In this paper, an integrated multiphysics model is developed for prediction of stresses when the cell, under compression as in a pack assembly, is charged at a high rate. Creep and fatigue tests of the separator demonstrate a significant damage can be accumulated under the predicted stress level. The separator is one of the most crucial safety components inside the lithium-ion battery preventing contact between the positive and negative electrodes and internal short circuits. Therefore, the separator should be mechanically robust to withstand the mechanical stresses encountered during the battery assembly, battery operation, and under abuse conditions. Both high tensile and high puncture strengths are required to avoid rupture and penetration of electrode material which can cause short circuits in the battery. We reported a gradual and upto 50% decrease in ductility of the separator after the battery cell underwent 400, 800, and 1600 charge-discharge cycles involving 4C-rate charging [1]. Scanning electron micrographs (SEM) analysis of the cycled separators revealed accumulated damage in the separator in the form of pore closure along the thickness direction and fiber fracture and cracks along the machine direction. The degradation was attributed to the damage caused by mechanical creep which can accumulate over the cycles. This paper therefore studies the stress distribution in the separator when the battery cell under mechanical loading inside a battery pack is fast charged. For that purpose, a multiphysics model is developed consisting of three main components: (1) a one-dimensional (1D) electrochemical model, (2) a lumped-parameter model, and (3) a two-dimensional (2D) solid mechanics model. The 1D electrochemical model is used to predict the heat generated and lithium-concentration in the battery cell. The heat generated is used as an input for a lumped-parameter model which simulates the thermo-mechanical behavior of the cell inside a pack array. The predicted lithium-concentration and temperature are used in the 2D solid mechanics model to predict the expansions and stresses inside the battery cell. Therefore, the proposed multiphysics model allows predicting the stress distribution in the separator under various charging rates, thermal management conditions, and pack assembly conditions such as pre-compressions.A cycling test is conducted on a lithium-ion battery cell inside a custom fixture that mimics the constrained condition inside a pack assembly. The setup is equipped with temperature, load, and displacement measurements. The parameters of the lumped-parameter model are optimized based on the experimental data using a non-linear regression technique. The model predictions show good agreements with the experimental data.The developed multiphysics model is simulated to estimate the stress distribution in the separator under the 4C charging condition (see Figure 1). The maximum first principal stress is found to be 70 MPa which is considerably lower than the yield strength of the separator. Creep and fatigue tests are conducted on the pristine separators to test their behaviors under the 70 MPa stress. Both test results show a significant amount of damage can be accumulated in the separator under a repeated loading of 70 MPa as in charge-discharge cycling. These test results corroborate our theory of mechanical creep being the main degradation mechanism of the separator while simultaneously validating our multiphysics model for prediction of stress. Reference [1] Makki M, Ayoub G, Lee CW, Bae C, Colin X. Effect of battery fast cyclic charging on the mechanical and fracture behavior of lithium-ion battery separator. In Preparation.Figure 1. Predicted two-dimensional (2D) distribution of the first principal stress in the separator near the center of Li-ion battery cell. Figure 1

Aluminium Matrix Composites Reinforced with AlCrFeMnNi HEA Particulates: Microstructure, Mechanical and Corrosion Properties.

Novel aluminium matrix composites have been fabricated using a powder metallurgy route with reinforcement phase particles of high entropy alloy (HEA) consisting of third transition metals. These new composites are studied as far as their microstructure (SEM, XRD), basic mechanical properties (hardness, elastic modulus) and creep response using nanoindentation techniques are concerned. Wear (sliding wear tests) and corrosion behaviour (in 3.5 wt.% NaCl environment) were also assessed. It was observed that, microstructurally, no secondary intermetallic phases were formed. Hardness and wear resistance seemed to increase with the increase in HEA particles, and in terms of corrosion, the composites exhibited susceptibility to localised forms. Nanoindentation techniques and creep response showed findings that are connected with the deformation nature of both the Al matrix and the HEA reinforcing phase.

Rheological studying the effect of reduced graphene oxide nanoparticles on the creep behavior of epoxy coating of pipelines: Part I—Constitutive equations

AbstractIn this article, the creep behavior of pure and reinforced epoxy‐polyamine samples with different amounts of reduced graphene oxide (RGO) at two temperature levels of 28 and 40°C is investigated experimentally and analytically to develop the creep‐associated constitutive equations. Mechanical properties and creep behavior of samples are obtained by carrying out tensile and creep tests at different stress levels. The Burgers model and modified Burgers model are used to investigate the creep behavior of pure and reinforced epoxy. The experimental results show that the addition of 0.5% RGO increases the yield strength of the samples by 51% and the maximum strain by 20%. Also, by considering the production costs of reinforced epoxy, the sample reinforced with 0.5 wt% graphene is selected as the optimum added value of RGO nanoparticles which results in the desired failure load and creep resistance. Additionally, it is discovered that the modified Burgers model, albeit having a larger computing cost, can accurately simulate samples' creep behavior. Furthermore, Fourier transform infrared spectroscopy and scanning electron microscopy analyses are performed on the samples and it is found that the addition of RGO particles improves the microstructure of the reinforced samples.

Effect of adding Indiana red matta rice husk Si3N4 bioceramic on mechanical, wear, flammability and tensile creep behavior of corn husk fiber polyester composite

AbstractThe objective of this study was to examine the effect of adding Indian red matta rice husk Si3N4 bioceramic on flammability, wear, mechanical and thermal behavior of corn husk fiber‐reinforced polyester composites. The Si3N4 bioceramic was prepared using thermo‐chemical process and subsequently surface‐treated using silane via aqueous solution method. The composites were made using hand layup method and post cured at elevated temperature. The results of this investigation suggest that adding 30 vol% of corn husk fiber improved the mechanical properties as compared to pure epoxy, however the Si3N4 addition outperformed. Similar to this, the composite with 4.0 vol% of bioceramic particle has the highest wear resistances, with a lowest sp. wear rate of 0.008 mm3/Nm and a coefficient of friction of 0.31. According to the flammability results, the composite contains 30 vol% of corn husk fiber and 4 vol% of Si3N4 (ECS3) has the lowered combustion rate of 6.72 mm/min. Finally, the composite ECS3 produced the lowest creep strain of 0.0104, 0.0122, 0.0149, 0.0241, and 0.0444 for 2000, 4000, 6000, 8000, and 10000 s respectively. These composites with improved properties could be used in cutting edge latest technologies as functional materials in automotive, defense, food packaging, structural, and domestic goods manufacturing.

The effect of diatomite dosage on physical, mechanical, and rheological creep properties of poplar wood/polypropylene composites

AbstractThe present study investigates the influence of micron‐sized diatomite dosage on the physical, mechanical, rheological, and creep properties of poplar wood/polypropylene (PW/PP) composites to enhance their toughness and practicality. The diatomite‐PW/PP composites were prepared using the melt extrusion method, replacing different proportions of PW with diatomite, and subsequently evaluated for their properties. The results indicate that adding diatomite enhances the density while reducing the brightness and water absorption of the composites. Scanning electron microscopy, analysis reveals a uniform distribution of diatomite in the range of 5–10 wt%, which effectively serves as a filler and reinforcing agent, consequently improving the toughness of the composite system. Specifically, the impact strength, tensile strength, and elongation at break are enhanced by 25.9%, 9.5%, and 35.7%, respectively. Furthermore, creep rheological analysis demonstrates improved processability and creep resistance in the composite system.

Mechanical Properties and Creep Behavior of Undoped and Mg-Doped GaN Thin Films Grown by Metal–Organic Chemical Vapor Deposition

This paper investigates the mechanical properties and creep behavior of undoped and Mg-doped GaN thin films grown on sapphire substrates using metal–organic chemical vapor deposition (MOCVD) with trimethylgallium (TMG) and bis(cyclopentadienyl)magnesium (Cp2Mg) as the precursors for Ga and Mg, respectively. The Mg-doped GaN layer, with a [Mg]/[TMG] ratio of 0.33, was systematically analyzed to compare its mechanical properties and creep behavior to those of the undoped GaN thin film, marking the first investigation into the creep behavior of both GaN and Mg-doped GaN thin films. The results show that the incorporated [Mg]/[TMG] ratio was sufficient for the transition from n-type to p-type conductivity with higher hole concentration around 4.6×1017 cm−3. Additionally, it was observed that Mg doping impacted the hardness and Young’s modulus, leading to an approximately 20% increase in these mechanical properties. The creep exponent is also affected due to the introduction of Mg atoms. This, in turn, contributes to an increase in pre-existing dislocation density from 2 × 108 cm−2 for undoped GaN to 5 × 109 cm−2 for the Mg-doped GaN layer. The assessment of the creep behavior of GaN and Mg-doped GaN thin films reveals an inherent creep mechanism governed by dislocation glides and climbs, highlighting the significance of Mg doping concentration in GaN thin films and its potential impact on various technological applications.

Nano-Mechanical Behavior of Ti6Al4V Alloy Manufactured Using Laser Powder Bed Fusion.

The microstructure of Ti6Al4V alloy, manufactured using laser powder bed fusion (L-PBF), is affected by process parameters and heat treatment. However, their influence on the nano-mechanical behavior of this widely applicable alloy is still unknown and scarcely reported. This study aims to investigate the influence of the frequently used annealing heat treatment on mechanical properties, strain-rate sensitivity, and creep behavior of L-PBF Ti6Al4V alloy. Furthermore, the influence of different utilized L-PBF laser power-scanning speed combinations on mechanical properties of annealed specimens has been studied as well. It has been found that the effect of high laser power remains present in the microstructure even after annealing, resulting in increase in nano-hardness. Moreover, the linear relation between the Young's modulus and the nano-hardness after annealing has been established. Thorough creep analysis revealed dislocation motion as a dominant deformation mechanism, both for as-built and annealed conditions of the specimens. Although annealing heat treatment is beneficial and widely recommended, it reduces the creep resistance of Ti6Al4V alloy manufactured using L-PBF. The results presented within this research article contribute to the L-PBF process parameter selection, as well as to understanding the creep behavior of these novel and widely applicable materials.

Effect of Water-to-Cement Ratios on Performance of Concrete with Prewetted Lightweight Aggregates

Lightweight aggregates (LWAs), as a popular material for internal curing (IC), have been extensively utilized in internally cured concrete (ICC). This paper investigated the development of mechanical properties, restrained shrinkage, quantitative stress relaxation, and tensile creep of ICC with prewetted LWAs under the condition of diverse w/c ratios (0.33, 0.40, and 0.50) by experimental investigations. Besides, the mixture proportion without IC was designed to investigate the effect of IC on the performance of the concrete mixture with a w/c ratio of 0.50. Corresponding results show that increasing the w/c ratio diminished the mechanical properties of ICC. A decrease in residual stress and stress rate was related to an increase in the w/c ratio. Relaxed stress and tensile creep increased with the w/c ratio increasing in ICC in the initial days. The introduction of IC decreased the mechanical properties, restrained shrinkage, stress rate, relaxed stress, and tensile creep of the concrete mixture with a w/c ratio of 0.50. The relationship between creep and relaxation coefficients was established, which contributed to the accurate evaluation of tensile stress development of ICC with prewetted LWAs under restrained conditions.

Moving research direction in the field of metallic bioresorbable stents-A mini-review.

In contrast to polymer bioresorbable stents (BRS) that exhibited suboptimal performance in clinical trials due to their deficient mechanical properties, metallic BRS with improved mechanical strength have made their way into the clinic and have demonstrated more promising results. In the roadmap of research and development of metallic BRS, magnesium and iron based biodegradable metal stents had been clinically used, and the zinc based biodegradable metal stents had been trailed in Mini-Pigs. In this mini-review paper, we demonstrate the current technology levels and point out the future R&D direction of metallic BRS. Magnesium based BRS should target for decreasing struct thickness meanwhile balancing with enough supporting strength. Iron based BRS should move towards high efficient absorption, conversion, metabolism, elimination of its degradation products. Zn based BRS should strive to improve mechanical stability, creep resistance and biocompatibility. Future R&D directions of metallic BRS should move towards new materials such as Molybdenum, intelligent stent integrated with degradable biosensors, and new stent with multiple biofunctions, such as NO release.

Effects of Bi and Cu addition on mechanical properties of Sn9Zn alloy and interfacial intermetallic growth with Ni substrate

Sn9ZnxBixCu (x = 0, 1.0, 2.0, 3.0 and 4.0 wt%) solder alloys were prepared by adding different contents of Bi and Cu elements, and the effects of Bi and Cu addition on the mechanical properties, wettability, and interfacial intermetallic compound (IMC) layer were researched. The addition of Bi and Cu can significantly improve the mechanical properties and creep resistance of the alloy. When the addition of Bi and Cu reaches 2 wt%, small Bi phases are dispersed in the matrix, and moderate Cu5Zn8 phases appear, which strengthens the solder matrix. The ultimate tensile strength (UTS) of the Sn9Zn2Bi2Cu alloy reaches 51.01 MPa, which is 149.32% higher than that of the Sn9Zn alloy, and the elongation reaches 26.56%. Nanoindentation results show that the holding displacement of the Sn9Zn4Bi4Cu alloy decreased by 28.48% compared with that of the Sn9Zn alloy at a loading rate of 0.1 mN/s. In addition, proper addition of Bi and Cu can improve the wettability of the solder alloy and inhibit the growth of interfacial intermetallic compounds (IMCs) between the solder and Ni substrate. When the reflow time reaches 40 min (min), the interfacial IMC thickness of the Sn9Zn2Bi2Cu/Ni solder joint is 1.93 µm, which is 46.09% smaller than that of the Sn9Zn alloy. Moreover, the shear strength of the Sn9Zn2Bi2Cu/Ni solder joint increases by 33.3%, reaching the maximum value (38.76 MPa). When the Bi and Cu addition reaches 2 wt%, the wetting area reaches a larger value (13.525 mm2), which is 28.35% higher than that of Sn9Zn alloy.

Comparative Evaluation of Oxidation Resistance of Detonation Gun-Sprayed Al2O3–40%TiO2 Coating on Nickel-Based Superalloys at 800 °C and 900 °C

High-temperature oxidation is the major degradation mechanism of boiler and gas turbine components. Superalloys are used in these applications because of their superior mechanical properties and creep resistance, but they lack resistance to oxidation under aggressive environment. In the present investigation, Al2O3–40%TiO2 coating was deposited on Superni718 and Superni600 superalloys by a detonation gun spray process. The as-sprayed coating was characterized by SEM analysis. The high-temperature oxidation behavior of Superni718 and Superni600 and Al2O3–40%TiO2 coating on these superalloys was evaluated in air under cyclic conditions at 800 °C and 900 °C for a total duration of 50 cycles. Each cycle consisted of keeping the samples for one hour at the elevated temperature followed by 20 min cooling in ambient air. Oxidation kinetics were studied using a thermo-gravimetric technique and found to follow the parabolic rate law. The coating was successfully formulated using a detonation gun spray process. The coating was found to be adherent to the substrate superalloys. It was found that among the two alloys, uncoated and coated Superni600 superalloys have much better oxidation resistance as compared to uncoated Superni718 superalloy.

Global void ratio of municipal solid waste for compression indices estimation.

Compressibility is one of the important engineering properties of municipal solid waste (MSW) affecting the stability and functionality of a landfill. Although the correlations between MSW properties and compression parameters have been established, they either have low accuracy and small datasets or are only limited to a few specific landfills in a region. In this study, a new method using the initial global void ratio (e0*) of MSW to estimate the compression indices is developed based on a comprehensive MSW dataset. The dataset consists of 124 sets (91 laboratory and 33 field) of MSW compression results obtained from 44 studies in 13 countries with different income levels and climate conditions. We categorized MSW as a ternary mixture with biodegradable (B), reinforcing (R), and inert (I) fractions, and suggested average specific gravity values (Gs,B=1.20, Gs,R=1.07, and Gs,I=2.64), respectively. The e0* values were calculated using the initial dry unit weight (γd,0) and ternary composition of MSW. The correlations between the e0* and the immediate compression index, secondary compression index induced by mechanical creep, and secondary compression index induced by bio-compression of MSW were evidently established. The results are applicable to the MSW with B=0-79.2%, R=0-54.0%, I=2.8-100.0%, and γd,0=2.0-14.2 kN/m3. A simple flowchart was established to estimate the compression indices and strains of MSW disposed on in landfills and dumpsites in countries with different income levels.

Enhanced nanocrystalline stability of BCC iron via copper segregation

Nanocrystalline alloys are known for their high density of grain boundaries (GBs), which results in low thermal stability and a tendency for coarsening. In this work, we investigated Fe–Cu alloys to explore the effects of Cu concentration on the thermal stability of nanocrystalline samples. Using hybrid molecular dynamics (MD) and Monte Carlo (MC) simulations, annealing treatments of nanocrystalline samples with varied Cu concentrations were conducted. The simulation results revealed that Cu atoms tended to accumulate at GBs. Subsequently, the model with Cu segregation was subjected to mechanical creep loading at various temperatures. Through the analysis of atomic structure evolution during creep deformation, it was found that Cu segregation efficiently stabilizes GBs and restricts their movement. The present findings highlight that the thermal stability of nanocrystalline Fe–Cu alloys can be effectively improved by introducing suitable Cu segregation at GBs.

Numerical modelling of settlement of municipal solid waste in landfills coupled with effects of biodegradation

This paper presents the development of a new settlement model to predict the long-term settlement of municipal solid wastes (MSWs). The total settlement of the MSWs results from the direct loss of solids due to decomposition and compression due to stress variation induced by loss of solids, flow of gas and liquid and mechanical creep. The geotechnical properties of MSWs are considered as functions of degrees of biodegradation (DOB). To validate the model, two settlement profilers (2.5 years) and three geodetic monitoring networks (2 years) were installed at Mugga Lane Landfill, ACT, Australia to monitor the settlement of an MSW lift and three closed landfill cells, respectively. The settlement rates of the landfill cells with the ages of 7, 8, 14, and 8, 9 and 15 years are 0.53, 0.35, 0.06 and 0.46, 0.36, 0.05 mm/day, respectively. In addition, 5 large scale and 3 small scale bioreactors were set up in the field and laboratory to address the long-term physical, mechanical and biochemical behaviours of the MSWs under different stress levels. The predicted settlement is compared to the test results from the bioreactors, in-situ monitoring data and the settlement predicted using an existing model, which has been improved by coupling the geotechnical properties of MSWs with DOBs. The predicted settlement using the proposed model well fits the test results and monitoring data. The settlement strain is predicted to be 28.2 % during the filling stage of the landfill and 5.9 % in 5 years after the closure.

Evaluation of Microstructure High Chrome Austenitic Stainless-Steel grade 253MA after Creep Test at Temperature of 700°C

High Chrome Austenitic Stainless-Steel grade 253 MA is a material that widely used for high temperature. This is due the fact this material has excellent mechanical properties and creep resistance. However, changes in microstructure can occur in long-term use, which will affect the creep resistance (shortened service life of the material). The microstructure of High Chrome Austenitic Stainless-Steel 253 MA creep test specimens was investigated. Creep testing at a temperature of 700 °C with a loading of 150 MPa was carried out. The cold rolling process with 53% reduction in thickness was applied followed by annealing at 900°C, 1000°C, and 1100°C for 3600s to obtain different grain size. Grain size after annealing and after creep test was measured to see the effect of annealing temperature on the grain size of tested steel and to see its effect on creep resistance based on the creep test conducted. Grain size and morphology of the phase after creep test were observed by scanning electron microscope and optical microscope

INFLUENCE OF THE MAGNETIC FIELD WITH AN INDUCTION OF 0.4 TL ON THE PLASTIC PROPERTIES OF C2 LEAD

The paper studies the dynamics of deformation characteristics of technically pure C2 lead under the influence of a constant magnetic field with magnetic induction of 0.4 T. Mechanical creep tests were performed, the microhardness of lead samples was measured in the initial state (without the influence of a magnetic field) and under the influence of external magnetic field. Based on the experimental results, creep deformation curves and the dependence of microhardness on the exposure time in a magnetic field are constructed. The stages of creep are determined. A linear stage was identified, at the site of which the creep rate of lead was calculated before and after the application of an external magnetic field. Quantitative changes in the studied characteristics were established. Under the influence of the magnetic field, there was a significant decrease in the creep rate. The relative residual elongation of samples destroyed during creep under the influence of a magnetic field decreased. The initial effect of the influence of a magnetic field with an induction of 0.4 T on the lead microhardness is investigated: the microhardness of the studied samples increases. An increase in the holding time in the magnetic field does not lead to significant changes in the lead microhardness. The maximum magnification is achieved after holding in the magnetic field for 1 hour. The correlation of changes in the creep rate and changes in the microhardness of the material exposed to a magnetic field with magnetic induction of 0.4 T was revealed.

Improved strength, creep resistance and recyclability of polyisoprene vitrimers by bottom-up construction of inhomogeneous network

Vitrimer elastomers bring huge chance to solve the recycling problem of conventional rubbers. However, their application is extremely limited by their lack of creep resistance. Although the crosslinking density can significantly affect the relaxation behavior of dynamic covalent networks, but limited by the tradeoff between mechanical strength and creep resistant properties. Here, we introduce inhomogeneous dynamic covalent network with regions of different crosslinking densities, in which the low crosslinking density mainly contribute to the creep resistance and the high crosslinking density contribute to the mechanical strength. In detail, block copolymers with polymyrcene and polyisoprene blocks and its control sample polyisoprene were prepared and epoxidized. The polymyrcene block is a double bond-rich block compared to polyisoprene block, which will become an epoxy group-rich block after epoxidation and a high cross-link density phase region after crosslinking with a dicarboxylic acid agent. Therefore, the cross-linked block copolymer will have an inhomogeneous dynamic covalent network, which were confirmed by the Multi-quantum (MQ) NMR tests, AFM images and tanδ measurement. And the heterogeneous structure becomes more obvious with the increased length of polymyrcene block. WAXD results show that cross-linked block copolymers have lower strain-induced crystallization onset strain and higher crystallinity, making them far stronger and tougher. In addition, block copolymer samples demonstrate lower creep and lower permanent set due to slow relaxation of the low crosslinking density regions. The recycling efficiencies of mechanical strength of block copolymer samples were higher than that of homopolymer. This work provides a useful approach for improving the mechanical properties and dimensional stability of vitrimer elastomers simultaneously.

Study on mechanical parameters and creep seepage characteristics of different coal and rock combination samples

Based on the fact that the production of deep coalbed methane (CBM) in Yushe-Wuxiang Block is lower than expected, the paper is carried out uniaxial strength test and triaxial rheological test on coal measure strata reservoirs (mudstone, coal, sandstone and their composite sample) deeper than 1300 m in the block. In the uniaxial strength test, it is found that the stress–strain process of deep coal rock sample was similar to that of shallow one. The brittleness of deep coal rock sample is more obvious. It can also serve as described by the four stages of shallow coal rock sample in the compression process. The compressive strength of the coal in the combined samples will be improved in a small range, while the mudstone will be driven to produce radial tensile failure at the moment of failure. Creep characteristics of the composite sample are similar to those of the single one, and the axial deformation characteristics under lower stress (14 MPa, 18 MPa, 22 MPa) are close to those of coal. The axial deformation characteristics under extreme stress (26 MPa, 30 MPa, 34 MPa) are close to those of sandstone. The established stress-seepage coupled creep damage model can be used to describe the deformation law of each rock sample under different stresses. The permeability characteristics of the composite sample are determined by the poor permeability of the composite formation. There is a great correlation between the volume deformation and permeability evolution of the sample in the test. The permeability model based on the theory of porous media can be well fitted the relationship between volumetric strain and permeability. It is of great significance to predict the influence of creep effect on permeability under different in-situ stresses in engineering.

Mechanical Responses and Damage Model of Anchored Jointed Rock Mass under Fatigue Shear Load

Cyclical impact loads will transform into low-frequency fatigue loads during propagation and will induce creep deformation of far-field rock under long-term disturbance loads. Laboratory tests of specimens under different fatigue shear loads were conducted. The mechanical responses and creep laws of the specimen were analyzed in detail. Based on the experimental results, a new constitutive model was explored. The results reveal that the stress‒strain curves of the specimen show a trend of step-path increasing under stable and fatigue load circularly. With an increase in fatigue load, the shear strength and elastic modulus of the specimens decrease gradually, while the corresponding failure strain increases gradually. The damage index composed of peak time and peak shear stress can reflect the failure of rock under fatigue loading, and the fatigue amplitude has the greatest influence on specimen deterioration. A fatigue stage in the shear creep curve occurs and is caused by the fatigue load. The fatigue creep rate increases gradually as the fatigue parameter increases. A combined nonlinear damage creep model based on the Nishihara model, elastic body, nonlinear creep body, and fatigue damage function (termed the NENF model), describing the creep responses of rock, anchor, accelerated creep, and fatigue load, respectively, was established. Compared with test data, the creep equation of the model has good accuracy in describing the whole shear creep process of specimens under fatigue loading and provides a new framework for the stability of anchoring rock masses.