Uniaxial Cyclic Loading Research Articles

Effect of confining pressure strength on the characteristics of energy evolution and fatigue fracture of the coal‐rock structural body

AbstractThe stability of coal seam‐overlying strata is significant to the safety of coal mines. The coal‐rock height ratio and confining pressure strength affect the stability of coal seam‐overlying strata. The research purpose of this paper is to obtain the effects of confining pressure and coal‐rock height ratio on the mechanical properties, energy evolution characteristics, and damage variables of structural body through the uniaxial loading, uniaxial cyclic loading, and triaxial cyclic loading experiment. Based on the evolution characteristics of the energy, the concept of energy dissipation ratio is put forward, and the effect of height ratio and confining strength on the energy dissipation ratio are compared and analyzed. The research results of this paper can provide theoretical reference for exploring the stress and energy evolution of coal seam‐overlying strata during chamber excavation and working face mining.

Experimental Study on Mechanical Characteristics of Prefabricated Fractured Sandstone Samples under Uniaxial Cyclic Compression

The loading-unloading physical and mechanical characteristics of fractured coal-bearing strata are greatly affected by fracture occurrence. In order to study the physical and mechanical response characteristics of fractured rock under cyclic compression, this paper takes prefabricated fractured sandstone samples with different dip angles as the object, and uses uniaxial cyclic loading and unloading test to study the influence of crack dip angle on sample strength, failure mode and crack propagation law. The results show that with the increase of crack inclination angle, the cyclic loading and unloading peak strength and elastic modulus of the sample increase significantly, and the peak strain decreases first and then increases. During the cyclic loading process, the area of the hysteresis loop gradually decreases, the slope gradually increases, the energy con-sumed by the hysteresis loop gradually decreases, the cumulative plastic deformation increases, and the curve shape presents a “concave” type; when it is 15 ° and 30 °, the cracks at the crack tip are wing cracks. When it is 45 °, the cracks at the crack tip are secondary inclined cracks and wing cracks. When it is 60 °, 75 ° and 90 °, the cracks at the crack tip are secondary coplanar cracks and wing cracks. With the increase of fracture dip angle, the number of cycles and peak strength at failure are increasing.

A unified viscoelastic constitutive model for studying the mechanical behaviors of polyelectrolyte complex hydrogels with different crosslinker degrees

Bio-based hydrogels formed by the chemically or physically cross-links are promising materials in the biological system. In this paper, we propose a constitutive model within the framework of finite-strain viscoelasticity to investigate the mechanical behaviors of bio-based polyelectrolyte complex (PEC) hydrogels from the ionic complexation of sodium hyaluronate (HA)/chitosan with and without chemical crosslinkers. The structure of the viscoelastic constitutive model consists of a spring and a Kelvin element, in which two softening factors are adopted to represent the states of ionic bonds and bridges in the hydrogels. Based on some constitutive assumptions and through the conventional approach of thermodynamics, the constitutive equation system of the model is derived. Corresponding to the hydrogels with different crosslinker concentrations, the values of material parameters in the model are identified. To show the validity of the model, it is applied to simulate the mechanical behaviors of the hydrogel samples in the uniaxial tensile tests and cyclic loading–unloading tests. It is found that the model predictions can fit the experimental data quite well. Based on the modeling results, some further analyses regarding the effects of softening factors on the response of HA/chitosan hydrogels have also been conducted.

A New Constitutive Model for Hexagonal Close-Packed Sheet Metals Under Uniaxial Cyclic Loading

This study develops a new phenomenological constitutive model to capture the unique, evolving cyclic elastoplastic behaviours of hexagonal close-packed (HCP) sheet metals under uniaxial loading condition. This new constitutive model is developed by adopting the concepts of multiple-yield surface approaches and a combined isotropic-kinematic hardening rule. Three phenomenological modes, including tension ([Formula: see text]), compression ([Formula: see text]), and untwining (UT), are considered to represent the hardening evolution of the materials, including twining/untwining behaviours. Three different hardening laws are applied, and a Cazacu-Barlat 2004 (CB2004) type yield surface is assigned to each deformation mode. The UT hardening parameters are defined as functions of plastic pre-strains to mitigate interpolation errors caused by parameter determination processes of existing models. The new constitutive model is numerically implemented for finite element analysis via the UMAT subroutine available in a commonly-used commercial finite element analysis (FEA) package — Abaqus and validated by experimental data available from literature. The obtained results show that the new constitutive model can precisely capture mechanical behaviours of HCP sheet metals, e.g., stress–strain curves, compared to those experimental results.

Effect of water on the damage and energy dissipation feature of coal under uniaxial cyclic loading–unloading condition

AbstractTo gain insight into the reduction mechanism of water on the dynamic destabilization failure character of coal under mining disturbance conditions, the effect of water on the damage and energy dissipation feature of coal was investigated via cyclic uniaxial compressive loading–unloading test, acoustic emission (AE) monitoring technology, and theoretical analysis. It manifests that the softening effect of water on the mechanical properties of coal is still remarkable, lower uniaxial compressive strength and elastic modulus, while greater axial strain and higher Poisson's ratio are observed under the cyclic loading–unloading condition. Water‐saturated specimens dissipate more energy during the cyclic loading–unloading process, which reduces the proportion of input energy accumulated in the coal mass, thus lowering the burst proneness of coal. A greater amount of damage generates in air‐dried specimens than that in water‐saturated specimens at the first cyclic loading–unloading cycle, while lower damage increments are observed in the following cycles. This may be the reason for the minimal energy dissipation of air‐dried specimens in the following cycles. The elastic modulus behaves logarithmically versus the increasing cyclic loading–unloading cycle counts. The energy dissipated due to the damage evolution and plastic deformation of coal specimens positively correlates with the energy released by AE activities, the AE energy in air‐dried specimens has a greater concentration around the peak axial stress and a stronger correlation with the dissipated energy. The cumulative AE energy and energy dissipation density are linearly correlated during the cyclic loading–unloading process, which indicates a possibility in estimating the damage evolution and the dynamic failure risk of coal via cumulative AE energy, under the cyclic loading–unloading condition.

Effect of Biaxial Loading Path on Seismic Performance of RC Bridge Piers with Corrosion Damage

In recent years, the deterioration of seismic behavior for reinforced concrete (RC) bridges due to reinforcement corrosion has received increasing attention. Many studies have been performed to explore the seismic capacity of corroded piers by applying uniaxial cyclic loading. However, the uniaxial seismic responses of corroded piers cannot accurately reflect the true structural response influenced by corrosion condition and real multidimensional earthquake action. Pointing at this problem, in this study, the seismic performance of corroded pier columns suffering biaxial lateral cyclic loadings is investigated by conducting comprehensive numerical analyses. First, a fiber-based numerical model is built and verified by the cyclic experiment results of two RC pier columns under different displacement loading patterns and corrosion levels. Then, three corrosion levels and five uniaxial and biaxial displacement protocols are used in the verified numerical models. The simulation results show that the ultimate force, deformation capacity, and dissipated energy of the piers are significantly influenced by corrosion degrees and biaxial loading patterns compared with the uniaxially loaded uncorroded piers. The phase lag between the biaxial displacement trajectory and biaxial force trajectory has three stages: a small-range fluctuation stage, a wide-range fluctuating and rise stage, and a wide-range fluctuating and decrease stage. The RE pattern is the most detrimental pattern, which leads to the fastest damage accumulation of the same-level corroded bridge piers.

Cyclic Impact Damage and Water Saturation Effects on Mechanical Properties and Kaiser Effect of Red Sandstone Under Uniaxial Cyclic Loading and Unloading Compression

Cyclic Impact Damage and Water Saturation Effects on Mechanical Properties and Kaiser Effect of Red Sandstone Under Uniaxial Cyclic Loading and Unloading Compression

Experimental study on the effect of unloading rate on the damage of coal with boreholes

In coal mining, the gas pre-drainage boreholes in front of working faces are often repeated loading and unloading. Studying the failure characteristics of boreholes under cyclic loading has a guiding significance for clarifying the failure mechanism of gas pre-drainage boreholes. To simulate the influence of the advancing speed of the working face on the deformation instability of the borehole, a cyclic loading path under the influence of different unloading rates was designed. In this paper, the uniaxial cyclic loading test was conducted by the DYD-10 testing machine, and the variation law of mechanical strength characteristics with the unloading rate was obtained. Combined with digital image technology (XTDIC), the sample deformation characteristics were analyzed. The samples' compressive strength rose with the unloading rate. The sample was damaged in the tensile-shear composite form at different unloading rates. Crack opening displacement (COD) with the increase in the number of cycles shows a “stepwise” increase. Near the moment of destruction, COD suddenly increased. The number of surface fractures that could be seen reduced as the unloading rate increased, and reduced fragmentation when the sample failed. The various types of energy of the sample increase nonlinearly with the increase of the number of cycles, and the plastic damage energy drops as the unloading rate increases. When the external stress state enters the next stress state, the internal stress adjustment of the sample takes a particular time, which is manifested as lagging behind the change of the external stress state. The influence mechanism of stress adjustment lag on strength, crack evolution and damage under different unloading rates was proposed. The results of the study contribute to the research of destabilization control technology for gas pre-drainage boreholes.

A unified viscoelastic model of progressive damage and failure for solid propellants

A general viscoelastic model incorporating the progressive damage evolution and failure criterion is developed for solid propellants across a wide range of temperatures (223 K to 333 K) and strain rates (2.38 × 10−4 /s to 4.76×10−2/s). (i) The progressive damage evolution, characterized by continuous void cavitation and growth, is linked to a macroscopic damage variable derived from a simple damage potential threshold, making it possible to determine the microstructural damage from experimental data. (ii) The failure criterion introduces two microstructural ingredients: the strain energy density, which accounts for both the glassy and rubbery failure of the polymer matrix, capturing brittle and ductile fracture modes at low and high temperatures respectively; and the void volume limit accounts for interface debonding, a process of particle detachment from the matrix. The implementation of the model effectively predicts the damage response and failure strains under uniaxial tension, cyclic and complex loading as well as superimposed pressure. Results reveal strong temperature/strain-rate dependency in the tensile strength, volume dilatation, and failure strains. Given that the model unifies progressive damage with ultimate failure and requires only a few model parameters, it assists in establishing a fracture criterion for the nonlinear fracture behavior of viscoelastic materials.

Cumulative buckling deformation of stiffened panel under cyclic loading

In this study, the cumulative buckling deformation process is evaluated for a stiffened panel subjected to two different uniaxial cyclic loading conditions, namely compressive loading only and alternating tensile-compressive loading. The impact of the cyclic load on the ultimate strength of the panel is then estimated. It should be noted that the occurrence of permanent deformations could be simulated by finite element analysis using a suitable model capable of correctly capturing the deformation components. The results show that the structure accumulates a deformation that appears to grow almost linearly therefore, consequently, it is realistic to expect that if a large number of load cycles of this magnitude are imposed, the accumulated plastic deformation can be quite severe. Furthermore, it is found that these residual deformations, caused by an entity of the cyclic load lower than the design limit, affect the tensile strength of the stiffened panel. Finally, the effects on the numerical evaluation of the ultimate strength of the structure are shown.

Evolution of Physical and Mechanical Properties of Granite after Thermal Treatment under Cyclic Uniaxial Compression

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.

Research on high‐performance steel structure with high‐strength steel column, ordinary‐strength steel beam, and low‐yield‐point steel BRB

AbstractIn order to reasonably make full use of the advantages of different steels and then achieve a steel structure with excellent seismic behaviour, the authors proposed novel triple grades hybrid high‐performance steel structures (TGHSSs) comprising high‐strength steel (HSS) columns, ordinary‐strength steel beams, and low‐yield‐point (LYP) steel buckling‐restrained braces (BRBs). The basic concept and expected advantages were introduced. To validate this concept, eight full‐scale single‐bay two‐storey TGHSS specimens were tested under cyclic loads, in which columns are of 460 MPa, 690 MPa, and 890 MPa HSSs, beams are of 345 MPa steel, and BRBs are of 100 MPa, 160 MPa, and 225 MPa LYP steels. Meantime, nine LYP steel BRB specimens were taken out and tested under uniaxial cyclic loads. Based on the experimental study, numerical simulation and parametric analyses on TGHSSs were further conducted, and a performance‐based design method was proposed. Results indicated that the TGHSSs featured a sequential yielding mechanism with excellent seismic performance. Specifically, the LYP steel BRBs yielded at first to dissipate seismic energy. Then, the ordinary‐strength steel beams developed plastic hinges at beam ends. At last, the HSS columns kept almost elastic or presented limited plasticity at column bases. This research proves such a high‐performance structure with a reasonable combination of high, ordinary, and low strength steels, whose advantages can be fully developed.

Improving low-cycle fatigue life of ZK61 magnesium alloy via basal texture weakening by the compression-extrusion process

In response to the problem that intense basal texture seriously affects the low-cycle fatigue life of magnesium alloys, the compression-extrusion process for basal texture weakening was implemented in ZK61 magnesium alloys. The low-cycle fatigue life under uniaxial symmetrical tension-compression cyclic loading was dramatically improved by basal texture weakening, up to 1.5∼3 times that in an intense-textured state. Basal texture weakening promoted basal slip to dominate the cyclic deformation, thus significantly reducing the cyclic tension-compression asymmetry caused by the {10–12} twinning-detwinning. Further, the suppression of {10–12} twinning and the decrease of compressive plastic strain slowed down dislocation accumulation inhibiting the cyclic hardening and fatigue crack initiation. The Manson-Coffin equation was feasible to predict the fatigue life of the intense-textured and weak-textured ZK61 magnesium alloys. The promotion of basal slip arising from basal texture weakening led to the decrease of fatigue ductility coefficient and the increase of fatigue ductility exponent contributing to the higher fatigue life.

Influence of Inclination Angle on the Acoustic Emission Characteristics of Pre-Cracked Sandstone in Uniaxial Arc Cyclic Loading and Unloading Tests

Influence of Inclination Angle on the Acoustic Emission Characteristics of Pre-Cracked Sandstone in Uniaxial Arc Cyclic Loading and Unloading Tests

Study on the evolutionary characteristics of coal burst proneness under the action of multiple mining disturbances

AbstractCoal burst is a serious hazard in deep coal mining. To investigate the coal burst proneness evolution during multiple mining disturbances (MMD), this paper implemented a range of uniaxial cyclic loading–unloading tests coupled with acoustic emission (AE) measurement to simulate the loading–unloading effects of the MMD. The maximum load was used to simulate the highest load that the coal has endured in history, the loading–unloading rate was used to simulate the mining speed, and the loading–unloading cycles were used to simulate the number of mining disturbances. Finally, the burst proneness indexes of the damaged coal sample are measured and compared with the origin coal sample. Analysis on the experiment results suggest that, the burst proneness increases when the maximum load is 30% of the uniaxial compressive strength (UCS), interestingly, when the low loading–unloading rate is applied, 50% of the UCS enlarge the burst proneness as well. However, it decreases when the maximum load is 70% of the UCS. Sensitivity analysis indicates that the maximum load, loading–unloading rate, and loading–unloading cycles exhibit decreasing influence sensitivity to the elastic modulus, as well as to the UCS and duration of dynamic fracture. Similarly, the influence sensitivity of maximum load, loading–unloading cycles, and loading–unloading rate to the elastic strain energy index decreases, as well as to the bursting energy index. To ensure the safety of coal mining, it is crucial to avoid the disturbances that would put the coal under low loading and low loading rate.

Evolution of residual strain and strain energy in rocks under various types of uniaxial cyclic loading-unloading

In the process of roadway excavation, the rock mass around the roadway is often subjected to cyclic loads, and the rock mechanical properties and strain energy evolution under cyclic loads are obviously different from those under compression, so it is urgent to carry out research on rock mechanical properties and strain energy evolution under cyclic loads. This study aims to investigate the evolution of residual strain and strain energy in rocks under uniaxial cyclic loading-unloading experiments (UCLUE). Four types of rocks, namely coal, white sandstone, red sandstone, and granite, were subjected to uniaxial compression experiments (UCE) and various uniaxial cyclic loading-unloading experiments (UEACLUE). The findings are as follows: analysis of UEACLUE revealed a gradual decrease in residual strain with an increasing number of cycles, leading to its eventual disappearance. However, if the cyclic loading-unloading (CLU) was continued beyond this point, the rocks displayed a reappearance of residual strain. The number of cycles required to eliminate residual strain was found to be inversely proportional to the peak strength of the rocks, while directly proportional to the upper limit value of UCLUE. Among the different stages of the uniaxial cyclic loading and unloading test, the plastic stage of white sandstone exhibited the largest disparity in dissipated strain energy, followed by the plastic stage of red sandstone, with coal displaying the smallest difference. Analysis of dissipated strain energy in the four types of uniaxial cyclic loading and unloading tests revealed differences of 0.00348 mJ▪mm−3, 0.03488 mJ▪mm−3, 0.02763 mJ▪mm−3, and 0.01619 mJ▪mm−3 in the plastic stage for the respective rock types. Furthermore, examination of the input strain energy density (ISED) and dissipated strain energy density (DSED) during the CLU process showed a linear relationship between these variables. Additionally, the investigation of ISED and DSED in other types of UCLUE demonstrated adherence to the cyclic-linear dissipation law (CLDL). The study of mechanical properties and strain energy evolution under CLU is of positive significance for the development of rock fatigue damage and rock damage mechanics.

Uniaxial ratcheting behavior and microstructure evolution of 316H stainless steel under the random cyclic loads

In this study, the Computational Fluid Dynamics (CFD)-Finite Element Method (CFD-FEM) were used to simulate the transient stress response of the structure surface under thermal striping environment, and a uniaxial random cyclic loading (RCL) method was used to reveal the mechanical properties and initial microstructure evolution of 316H austenitic stainless steel (SS) under random stress induced by thermal striping. The experimental results showed that 316H exhibited a lower saturation ratcheting strain rate at 334.44 MPa, along with stronger cyclic hardening effects and resistance to plastic accumulation at loading rate of 100 Hz. Random loading tests with different cycles were performed at σpeak=334.44MPa and fRCL=100Hz, and ratcheted specimens were characterized to investigate the effect of the micro-mechanism on cyclic softening/hardening. The geometrically necessary dislocation (GND) density increases and then decreases with cyclic loading, which is mainly attributed to the deformation twin (DT) and multi-dislocation slip mechanisms. The deformation mechanism during the ratcheting cycle from 1000 to 3000 cycles is mainly dominated by the twinning reduction, where the dislocation migration accelerated the transformation from dislocation walls to subgrain boundaries; the twinning increase dominates from 3000 to 9000 cycles, where the subgrain boundary grew into boundaries with high angle misorientations.

Energy evolution and macro-micro failure mechanisms of frozen weakly cemented sandstone under uniaxial cyclic loading and unloading

Further research is needed on the deformation behaviour and the macro-micro failure mechanisms of frozen soft rock driven by energy under cyclic loading and unloading conditions. Combined with specific engineering projects of freeze shaft construction, taking frozen weakly cemented sandstone (−10 °C) as the research object, uniaxial cyclic loading and unloading (UCLU) tests at different rates and microscopic observations of the fracture surface by scanning electron microscopy (SEM) were conducted. Then the failure mechanisms of frozen weakly cemented sandstone during UCLU were driven by energy, which was revealed by the proposed method for calculating rock damage based on damage energy. The results showed that the peak strength of sandstone was much smaller under UCLU conditions than the uniaxial compression test, proving that the mechanical properties of the frozen sandstone were affected by UCLU. The dissipation energy and damage energy of sandstone grew with increasing rates of loading and unloading, but the elastic energy and damping energy showed opposite trends. The energy evolution eventually shifted from being dominated by damping energy to being dominated by damage energy as the loading and unloading rates increased. The cumulative damage of sandstone grew logarithmically with increasing loading and unloading rates, and tensile cracking considerably increased at fast loading and unloading rates. Hence, the fast loading and unloading rates caused the rock dissipation energy and damage energy to be larger, which was the main reason why damage was more significant at fast than slow rates.

Self-sensing study of stress in low-doped carbon fiber reinforced hydraulic concrete

As an intrinsically self-sensing material, carbon fibre concrete has promising applications in structural health monitoring. Investigating the long-term stability of carbon fibre concrete in terms of resistivity changes under the influence of factors such as ageing effects and environmental conditions can help assess its feasibility for long-term monitoring applications. In this paper, the specimens were designed based on the variables of carbon fibre admixture (0.05 vol%, 0.08 vol%, 0.1 vol%) and carbon fibre length (3 mm, 6 mm, 9 mm, 12 mm), and the DC four-electrode method was used to study the self-sensing of low admixture carbon fibre reinforced hydraulic concrete containing coarse aggregate (5–20 mm) under long age (180d) in order to analyze the resistivity change law when it was loaded under long age condition. The results show that the fraction of resistivity change during loading (in compression and flexure) is low for all specimens in the long-age condition, but the resistivity change is consistent with its internal change. Uniaxial compressive cyclic loading in the elastic phase, each specimen exhibits a piezoresistivity under each loading cycle, while the material's own resistance under bending loading increases irreversibly as the loading progresses. In addition, for the flexural toughness evaluation test of fibre concrete, an adjustable deflection test device is designed based on existing codes, which can effectively solve the drawback of the existing bracket for fixing the deflection test instrument in the span, which needs to know the type of instrument in advance and then reserve the through-holes that can be clamped to the instrument.

Role of grain boundaries and precipitates in small, fatigue crack-growth resistance of additively-manufactured, Ni-based superalloy 718 under torsional cyclic loading

In essence, additively-manufactured, Ni-based superalloy 718 (AM Alloy 718) possesses a strong crystallographic texture, δ-phases and Laves phases, whose effect on fatigue resistance under torsional cyclic loading is yet to be investigated. In order to elucidate the role of microstructural features in small-crack behavior, torsional fatigue testing was conducted on an AM Alloy 718. Test results revealed that crack-growth was controlled by the twist angle, but also by the tilt angle which had been reported to play a minor role in fatigue resistance under uniaxial cyclic loading. Furthermore, the impact of precipitates appeared to be insignificant, with no strong correlation made between their dispersion and the crack-growth rate.