Search Results (26)

The surrounding rocks of roadways are typically subjected to cyclic loading–unloading stress states in underground engineering. In addition, cyclic loads are discontinuous under real working conditions, usually while loading rock mass in a cycle–intermission–cycle manner. Based on the XTDIC 3D (XTOP Three-dimensional Digital Image Correlation) full-field strain measurement system and AE (Acoustic Emission) system, the work performed uniaxial cyclic loading–unloading tests with constant-pressure durations of 0, 0.5, 2, and 6 h. The purpose was to investigate the damage degradation mechanism of sandstone under multi-stage equal-amplitude intermittent cyclic loading and unloading. The results are as follows. (1) As the constant-pressure duration increased, the uniaxial compressive strength of sandstone samples decreased, along with a decline in elastic modulus and a deterioration in stiffness and deformation recovery capacity. (2) The evolution of deformation localization zones became more intense in sandstone samples during cyclic loading and unloading with the increased constant-pressure duration. The maximum principal strain field became more active at failure. Sandstone samples exhibited shear failure accompanied by spalling failure and an increased failure degree. (3) As the constant-pressure duration increased, the damage variable of sandstone samples increased, indicating that the constant-pressure stage promoted the damage degradation of sandstone samples. The above results reveal the damage degradation mechanism of sandstone under multi-stage equal-amplitude intermittent cyclic loading and unloading, which is of significant importance for maintaining the safety of underground engineering. Full article

Additive Manufacturing (AM) has revolutionized the production of intricate geometries tailored to customized functional mechanical properties, making it widely adopted across various industries, including aerospace, automotive, and biomedical sectors. However, the fabrication of mechanical springs has remained largely constrained by conventional manufacturing techniques, which limit their cross-sectional geometries to regular shapes, thereby restricting their mechanical performance and energy absorption capabilities. This limitation poses a significant challenge in applications where enhanced load-bearing capacity, energy absorption, and tailored stiffness characteristics are required. To address this issue, this study investigates the influence of coil shape on the mechanical properties of wave springs, specifically focusing on load-bearing capacity, energy absorption, stiffness, and compression behavior during cyclic loading and unloading. Nine contact-type wave springs with distinct coil shapes—square, rectangular, pentagonal, hexagonal, heptagonal, octagonal, quadro, circular (4 waves per coil), and circular (6 waves per coil)—were designed and fabricated using MultiJet Fusion (MJF) technology. Uni-axial compression testing was conducted over ten loading–unloading cycles to evaluate their mechanical performance and deformation characteristics. The results indicate that wave springs with square and rectangular coil shapes exhibit the highest energy absorption while maintaining the lowest stiffness and minimal energy loss during the first ten loading–unloading cycles. Furthermore, experimental findings were validated using finite element analysis (FEA) under identical boundary conditions, demonstrating close agreement with a deviation of only 2.3% compared with the experimental results. These results highlight AM’s potential for customizing wave springs with optimized mechanical performance. Full article

During the formation of deep rock bodies, such as hot dry rock, they are frequently exposed to high temperatures and repeated stress perturbations. The prolonged interaction of these two factors is a potential cause of deep underground rock instability. To investigate the effects of high temperature and cyclic loading–unloading modes on rock mechanical properties, cyclic tests were conducted on granite under real-time high-temperature conditions using a multifunctional high-temperature testing machine. By comparing uniaxial compression test results with scanning electron microscopy (SEM) observations, the following was found: (1) The uniaxial compressive strength and elastic modulus of granite under real-time high-temperature conditions initially increase and then decrease as the temperature rises, while the peak strain consistently increases with temperature. (2) Under both cyclic loading–unloading modes, the mechanical properties of granite first improve and then deteriorate as the temperature increases. (3) As the temperature rises, microcracks in granite under both cyclic loading–unloading methods evolve from intracrystalline to intergranular cracks. The fracture surfaces of granite exhibit a significant increase in fracture severity, along with a noticeable rise in both the number and width of cracks. Crack propagation and crystal integrity degradation are more severe and complex in specimens subjected to variable lower limit cyclic loading–unloading than in those under constant-limit cyclic loading–unloading. These findings are of significant theoretical value for studying rock stability under simultaneous high-temperature and cyclic stress conditions. Full article

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. Full article

In order to investigate freeze–thawed red sandstone failure processes under cyclic loading and unloading conditions, real-time acoustic emission (AE) and scanning electron microscopy (SEM) techniques were used to reveal the fracture process of the saturated red sandstone after cyclic loading and unloading tests using uniaxial compression. The results show that the stress–strain curves of the freeze–thawed sandstones show signs of hysteresis and exhibit a two-stage evolution of “sparse → dense”. In the cyclic loading and unloading process, the modulus of elasticity in the loading process is always larger than that in the unloading process, while the Poisson’s ratio is the opposite, and the radial irreversible strain and cumulative irreversible strain are larger than those in the axial direction. As the number of freeze–thaw cycles increases, the rock specimens need more cycles of loading and unloading to make the crack volume compressive strain $\Delta {\epsilon }_{cv}^{+}$ reach the maximum value and tend to stabilize, while the crack volume extensional strain $\Delta {\epsilon }_{cv}^{-}$ tends to decrease gradually. This study also shows that the growth phase of the cyclic loading and unloading process has more ringing counts and a shorter duration, while the slow degradation phase has more ringing counts with loading and less with unloading. In addition, the F-T cycle gradually changes the internal microcracks of the red sandstone from shear damage, which is dominated by shear cracks, to tensile damage, which is dominated by tensile cracks. This study’s findings contribute to our knowledge of the mechanical characteristics and sandstone’s degradation process following F-T treatment, and also serve as a guide for engineering stability analyses conducted in the presence of multiphysical field coupling. Full article

With the depletion of shallow resources, deep resource mining has become a trend. However, the high temperature and complex stress environment in deep mines make resource extraction extremely challenging. This paper developed a thermal insulation grouting material made of glazed hollow beads, sodium silicate, and cement and tested the compressive strength, gelation time, and stone rate under various curing days in light of the issue of high temperature heat damage in high ground temperature mines and the impact of mining on roadway grouting bolt support. Fatigue strength, fatigue deformation, load-residual strain, energy evolution and microscopic features were studied and analyzed in relation to the damage law of graded cyclic loading and unloading under the number of varying cycles. The findings demonstrate that cyclic loading and unloading strength is lower than uniaxial compressive strength. The fatigue strength is significantly decreased when the number of cycles reaches its limit. Residual strain is less sensitive to changes in stress than load strain. The fitting correlation coefficients of total output energy and elastic energy are higher than 0.71. Full article

In this study, ground polymers were prepared from mudstone and slag. NaOH and water glass were used as alkaline exciters and mine waste rock aggregate was used as the aggregate for mudstone slag-based waterproof composites (MSWCs). A series of laboratory tests, including a uniaxial compression test, uniaxial cyclic loading and unloading test, scanning electron microscope test, and rock penetration test were conducted for macrostructural and microstructural analysis. The effect of the coupling between the mudstone proportion and the number of uniaxial cyclic loading and unloading tests was investigated. The results showed that it is feasible to use mudstone and slag to synthesize geopolymers, and that MSWCs fulfil the conditions for use as a reconstituted water barrier. The permeability of MSWCs with the different mudstone proportions set in this study fulfils the requirement of being used as a material, and the permeability and uniaxial compressive strength of the MSWCs gradually decreased with increases in the mudstone proportion. Considering the UCS and permeability of the MSWCs, the optimal mudstone proportion of the MSWC is r = 0.6. In this test, cyclic loading and unloading times of 0, 25, 50, and 100 were set, and with an increase of cyclic loading and unloading times, the UCS of the MSWCs showed a tendency of increasing first and then decreasing. In the SEM test, with an increase of cyclic loading and unloading times, microfractures and pores appeared in the MSWCs, which led to a gradual increase in its permeability and a decrease in its waterproofness. Full article

Influenced by the anisotropy and water-softening characteristics of gently inclined layered shale, many tunnels have encountered bottom deformation issues during construction and operation, which severely impact the safety of tunnel structures. The energy evolution law during rock deformation and damage can provide support for the assessment and prediction of structure deformation. However, most studies have been conducted on enstatite, granite, and sandstone with limited research on shale. In this study, both conventional and single-cyclic loading-and-unloading uniaxial compression tests were conducted on shale specimens with varying dip angles of the structural plane (Dφ) and water content (Wc) in addressing the most typical layered shale in the Chaoyang Tunnel. The energy evolution features of rock samples at each stage of the tests were analyzed to determine the discriminating indicator (SC) for tunnel bottom deformation tendency. The indicator was based on the elastic strain energy (U^e_i) and the post-peak dissipation energy (U^d_i). The results demonstrated that the Dφ and Wc directly affected the energy storage and dissipation process of rock specimens, which in turn enabled them to exhibit different damage evolution features. The U^e_i and the total input energy (U^l_i) satisfied a linear relationship, which was determined by the Dφ and Wc of rock specimens. The energy evolution-based indicator SC can accurately characterize the bottom deformation of the tunnel constructed in a gently inclined layered shale stratum. The findings can offer a scientific foundation for rational evaluation of the structure deformation of tunnels under construction. Full article

The mechanical behaviors of three distinct lattice structures—Diamond, Gyroid, and Schwarz—synthesized through vat polymerization, were meticulously analyzed. This study aimed to elucidate the intricacies of these structures in terms of their stress–strain responses, energy absorption, and recovery characteristics. Utilizing the described experiments and analytical approaches, it was discerned, via the described experimental and analytical procedure, that the AM lattices showcased mechanical properties and stress–strain behaviors that notably surpassed theoretical predictions, pointing to substantial disparities between conventional models and experimental outcomes. The Diamond lattice displayed superior stiffness with higher average loading and unloading moduli and heightened energy absorption and dissipation rates, followed by the Gyroid and Schwarz lattices. The Schwarz lattice showed the most consistent mechanical response, while the Diamond and Gyroid showed capabilities of reaching larger strains and stresses. All uniaxial cyclic compressive tests were performed at room temperature with no dwell times. The efficacy of hyper-elastic-graded models significantly outperformed projections offered by traditional Ashby–Gibson models, emphasizing the need for more refined models to accurately delineate the behaviors of additively manufactured lattices in advanced engineering applications. Full article

The combined effects of thermal and cyclic loading result in complex mechanical behavior in engineering rock masses. The study of the physical and mechanical properties of these rock masses is of great importance for improving the stability and sustainability of structures built on thermally treated rock masses. In order to understand the failure mechanism, uniaxial compression tests and cyclic loading and unloading tests were conducted on granite specimens that had undergone thermal treatment at various temperatures. The test results indicate that the density and P-wave velocity of the specimens decrease while the degree of damage increases after thermal treatment. The compressive strength and elastic modulus of the specimens generally decrease as a result of thermal treatment, although thermal hardening does occur within the temperature range of 200–400 °C. The dilatancy characteristics of the specimens change with the treatment temperature, and they are more prone to shear dilation under external loading. Furthermore, the failure mode of the specimens transitions from brittle to ductile failure as the treatment temperature increases. The combination of thermal treatment and cyclic loading causes the rock fragments to become looser and finer following specimen failure. Full article

In this study, the uniaxial compression and cyclic loading and unloading experiments were conducted on the non-water reactive foaming polyurethane (NRFP) grouting material with a density of 0.29 g/cm³, and the microstructure was characterized using scanning electron microscope (SEM) method. Based on the uniaxial compression and SEM characterization results and the elastic-brittle-plastic assumption, a compression softening bond (CSB) model describing the mechanical behavior of micro-foam walls under compression was proposed, and it was assigned to the particle units in a particle flow code (PFC) model simulating the NRFP sample. Results show that the NRFP grouting materials are porous mediums consisting of numerous micro-foams, and with the increasing density, the diameter of the micro-foams increases and the micro-foam walls become thicker. Under compression, the micro-foam walls crack, and the cracks are mainly perpendicular to the loading direction. The compressive stress–strain curve of the NRFP sample contains the linear increasing stage, yielding stage, yield plateau stage, and strain hardening stage, and the compressive strength and elastic modulus are 5.72 MPa and 83.2 MPa, respectively. Under the cyclic loading and unloading, when the number of cycles increases, the residual strain increases, and there is little difference between the modulus during the loading and unloading processes. The stress–strain curves of the PFC model under uniaxial compression and cyclic loading and unloading are consistent with the experimental ones, well indicating the feasibility of using the CSB model and PFC simulation method to study the mechanical properties of NRFP grouting materials. The failure of the contact elements in the simulation model causes the yielding of the sample. The yield deformation propagates almost perpendicular to the loading direction and is distributed in the material layer by layer, which ultimately results in the bulging deformation of the sample. This paper provides a new insight into the application of the discrete element numerical method in NRFP grouting materials. Full article

Underground rock engineering often encounters long-term cyclic loading and unloading. Under the influence of this effect, the mechanical characteristics of rocks will inevitably change, which will affect the stability and safety of underground engineering. Therefore, it is necessary to study the fatigue characteristics of rocks under a certain period of action. With an RDL series electronic creep relaxation testing machine, fatigue loading and unloading tests of sandstone at different loading rates were carried out, followed by uniaxial compression on the samples. The study shows that the stress–strain curves of the uniaxial compression specimens have three stages: a compaction pore fracture stage, an elastic deformation stage, and an unstable fracture developing to failure stage. The stress–strain curves of the samples with a certain number of cycles of loading and unloading give the thinning and dense phenomenon, and the axial upper limit strain and axial cumulative residual strain gradually decrease as the loading rate increases. With the increase, the uniaxial compressive strength of the reloaded samples increases gradually, which is higher than the ordinary uniaxial compressive strength. In the process of cyclic loading and unloading, the internal particles of the sample present fracture and reorganization of the fragile structure and, at the same time, compaction stability. Full article

With the rapid development of the economy and urbanization, the construction of the urban rail transit system has had a great impact on the work, life, and health of residents in buildings along the rail transit line. Thus, it is particularly urgent and necessary to develop base isolation technologies to control and reduce the impact of vibrations of rail transit systems on building structures. High-damping rubber isolation bearings have shown significant effectiveness in the reduction of this impact, and their isolation performance mainly depends on the mechanical and damping energy dissipation characteristics of the high-damping rubber material. This paper aims to investigate the hyper-viscoelastic properties of the high-damping rubber material used for high-damping rubber isolation bearings during the cyclic tension and compression process in the vertical direction. These properties include hyperelastic parameters, viscoelastic coefficients, and the relaxation times of the material. For this purpose, uniaxial cyclic tension and compression tests were conducted. A three-element Maxwell rheological model combining a strain energy density function was proposed for modeling the hyper-viscoelastic behaviors of the materials during the cyclic tension and compression process. Based on the obtained results, an iterative identification procedure was used to determine the constitutive parameters of the material for each loading-unloading cycle. The aforementioned parameters were further expressed as a function of the number of cycles. New insights into hyper-viscoelastic property changes in this high-damping rubber material during the cyclic tension and compression process were gained in this work. These investigations could facilitate the development of computational tools, which would regulate fundamental guidelines for the better controlling and optimization of the isolation performance of the high-damping rubber material used for high-damping rubber isolation bearings, which have a wider perspective of applications in the urban rail transit system. Full article

The void compression stage causes porous cement mortar to present special mechanical properties. In order to study the compaction behavior and the damage evolution of the porous material, cement mortar specimens with an average porosity of 26.8% were created and cyclic uniaxial compression tests were carried out. The irreversible strain accumulated in the tests was obtained by cyclic loading and unloading. As the secant modulus of the porous cement mortar increases with stress in the pre-peak deformation stage, its damage variable is defined according to the accumulated irreversible strain instead of modulus degradation. The strain-based damage indicator fitted with the damage evolution law is characterized by linear accumulation at the beginning and has an acceleration rate of about 0.3 in the pre-peak deformation stage, and the damage value converges to 1 at failure. Based on the Weibull distribution, a constitutive damage model of porous cement mortar is improved by considering both the damage evolution during the plastic deformation stage and the mechanical behavior in the compaction stage. The theoretical envelope curves obtained by the constitutive model are in good agreement with the experimental envelope curves of cyclic uniaxial compression in the compaction and pre-peak stages, and the average absolute error is about 0.54 MPa in the entire pre-peak stage, so the proposed damage constitutive model can characterize the damage-induced mechanical properties of porous cement mortar in the compaction and pre-peak stages. Full article

Using the rock mechanics test (RMT) and acoustic emission acquisition system (DS9), based on the energy principle, uniaxial compression, uniaxial cyclic loading, and unloading tests are used to study the energy transformation characteristics of the process of sandstone absorbing axial strain energy, accumulating and releasing elastic strain energy, plastic deformation, and crack extension dissipation energy. The study results show the increase of loading rate, rock fracture surface increases, number of fragments increases, and size of fragments decreases; the sandstone damage process causes: shear damage to tensile shear damage and then splitting damage for change; the input energy and elastic energy increase nonlinearly with an increase of stress, and the dissipative energy is larger at the beginning of loading. After a small decrease, it enters the nonlinear growth stage, and the input energy density grows the fastest. The elastic energy density is the second fastest, and the dissipative energy density grows the slowest; with an increase of loading rate, in any deformation stage, the elastic energy density and dissipation energy density are increased, proportion of elastic energy density is decreased, and proportion of dissipation energy density is increased. Near the peak stress stage, the proportion of elastic energy decreases, and the proportion of dissipative energy increases; the damage variable stress curve of sandstone is “weakly concave”, which is consistent with the logistic function, and the damage evolution process has chaotic dynamics properties; the acoustic emission energy of sandstone in cyclic loading and unloading test has a similar variation with the theoretical calculation of dissipated energy. The cumulative energy curve shows a step-up law, and the stress corresponding to the step point is near the historical maximum stress. Full article