Uniaxial Compression Conditions Research Articles

Thermomechanical coupling in glassy polymers: An effective temperature theory

Glassy polymers exhibit a strong thermomechanical coupling when subjected to mechanical loading. A homogeneous strain distribution can be achieved in uniaxial compression conditions. However, a clear temperature increase is observed when the loading rate is relatively high, which further results in a decrease in stress due to thermal softening. In tensile tests, necking instability can easily occur. A temperature change is accompanied by the nucleation and propagation of necked regions. In this work, we apply the effective temperature theory, which can capture the nonequilibrium structure evolution of amorphous polymers, to investigate the thermomechanical behavior of glassy polymers. We demonstrate that a finite element model based on the effective temperature theory can well capture the stress response and the temperature increase of polycarbonate (PC) compressed at different loading rates. We further combine experiments and simulations to investigate the effects of the loading rate and aging treatment on the necking behavior in amorphous glassy polymer poly(ethylene terephthalate)-glycol (PETG) in uniaxial tension loading tests. Through employing digital image correlation and infrared thermometry, the strain distribution and the temperature field can be fully characterized during the formation and propagation of necked regions. The model captures all important features of localization behavior observed in experiments, including the force–displacement relationship, and the strain and temperature distribution with necking propagation. However, it underestimates the maximum strain and temperature when the displacement is large. Both experimental and simulative results indicate that increasing loading rate and aging time can induce an increase of the intrinsic strain softening due to a more active structural state caused by mechanical rejuvenation. This further leads to more pronounced localized behavior and a ductile–brittle transition. Our work proves that the effective temperature model is a powerful theoretical framework to predict the complex thermomechanical response of glassy polymers.

Transient processes in electric transport in the powder MoS2 samples

The transient processes in the electric conduction of fine disperse 2D-MoS2 powder samples, occurring at changing voltages applied to the sample under the conditions of uniaxial compression, have been studied. The long-term transient changes in conduction occurring both after switching on and off the applied voltage were observed. The current–voltage characteristics have a hysteresis of branches corresponding to increasing and decreasing voltages. The observed results are explained within the frame of the migration (interlayer) polarization model.

Effect of Strain Rate on Anisotropic Mechanical Behavior of the Shale Under Uniaxial Compression Conditions

Effect of Strain Rate on Anisotropic Mechanical Behavior of the Shale Under Uniaxial Compression Conditions

A quantitative model considering crack closure effect of rock materials

The crack closure effect of rock materials plays a key role in the evaluation of its compressive mechanical behavior. Based on the effective medium theory, an exponential model is proposed to describe the influence of the crack closure effect on the constitutive relation. The crack deformation is characterized by natural strain, and the crack propagation in the compaction phase and elastic phase can be captured by this quantitative model. Three model parameters can be determined from experimental data. The model was verified by our uniaxial compression tests on marble and other types of rocks from the published literature. The results show that the model can simulate the nonlinear phase of stress–strain curves of rocks under uniaxial and triaxial compression conditions. The crack closure behavior in the compression tests can be well reflected by this model. In addition, the microcrack volume ratio is positively correlated with the crack density parameters, and the material damage caused by microcracks can be identified and quantified by the microcrack volume ratio.

Three-dimensional finite element analysis for structural parameters of asphalt pavement: A combined laboratory and field accelerated testing approach

To solve the issues that material parameters measured under specific laboratory conditions are extremely different from the actual pavement, a method of structural parameters correction in semi-rigid base asphalt pavement combining numerical simulations, laboratory tests, and accelerated pavement testing (APT) was proposed. The dynamic modulus and the uniaxial static compression creep tests were conducted to explore the effect of temperature, size, and fatigue. Then, these factors were coupled to the modified Burgers model that can reflect the creep law of the whole process. A constitutive model reflecting the real parameters of the pavement was subsequently constructed with the ABAQUS user-defined material (UMAT) subroutine. Experimental results illustrated it was verified that the modified subroutine has good feasibility both in single-layer and multi-layer materials from the actual pavement. Furthermore, simulated results also showed excellent agreement with field-tested findings. Finally, the rutting deformation curves and the demarcation points of each stage were obtained, which could guide the rutting prediction and maintenance decision of semi-rigid asphalt pavement.

Mesoscopic damage mechanism and a constitutive model of shale using in-situ X-ray CT device

With the continuous development of shale gas engineering in China, an accurate and adequate understanding of shale damage mechanism has become a critical scientific problem addressed by rock mechanics. So far, many researchers have studied the fracturing process and mechanism of shale in multi-scales. The mesoscopic structural characteristics of shale during loading can be obtained using the in-situ X-ray computed tomography (CT) technology. Some scholars have observed the structure evolution of shale under in-situ uniaxial compression conditions, but the quantitative investigation of damage evolution and constitutive model of shale still requires further studies. Given this, the in-situ micro-CT device was adopted to study the macroscopic mechanical response and the mesoscopic structural evolution of Longmaxi shale under uniaxial compression conditions. The evolution law of the damage variable with the axial strain was analysed according to the characterisation of the damage variable based on the CT image grey variance. In combination with the macroscopic stress-strain curve shape, evolution stage of the mesoscopic void, evolution of the damage variable, mesoscopic mechanism of the macroscopic mechanical response was investigated, and a damage constitutive model of shale under the uniaxial compression test was proposed. The conclusion of this study can provide a foundation for further research into the multi-scale failure mechanism of shale and exert theoretical guidance significance on shale gas fracturing engineering practice.

Experimental Study on Uniaxial Compression Fracture Process of Marble Specimens with Circular Holes with Different Diameters

After a long period of geological tectonic action, the rock mass develops defects such as pores and fissures of different sizes and scales. The mechanical properties of the rock are largely affected by these discontinuous structures, and there will be obvious stress concentration at the edge of the hole inside the rock. Under the action of concentrated stress, cracks initiate, expand, and penetrate from the edge of the hole to form the final fracture surface and cause instability and failure of the rock mass. In practical engineering, due to the limitation of various factors, it is impossible to conduct uniaxial compressive strength test for each geotechnical engineering. Therefore, the rock in the weak part of the project can be selected as the test specimen before the indoor physical tests. In this paper, the static uniaxial compression test of specimens with circular holes with different diameters is carried out. Digital Image Correlation (DIC) monitoring system is used to observe the test process in real time; then, the data from uniaxial compression test and DIC monitoring test are obtained. At the same time, the macroscopic failure characteristics and crack propagation law of rock specimens are obtained, and the evolution law of strain field during the failure process of rock specimens is also obtained. Meanwhile, Realistic Failure Process Analysis (RFPA) is carried out to verify the accuracy of static uniaxial compression physical tests numerically. Physical tests show that the uniaxial compressive strength of marble specimens with circular holes and the dispersion of measured peak stress data decrease with the increase of hole diameter. The main failure type of marble specimens with circular holes is tension shear failure; the macroscopic failure zone of the sample, the strain field localization zone, and the maximum shear stress field of the numerical test are distributed in a semi-“X” shape with the hole as the center. The localization zone of the strain field on the specimen is usually the region of crack initiation, propagation, and penetration on the surface of the marble specimens. It also shows that specimen breaks in the radial direction along the y -axis of the circular hole, and cracks connecting both ends of the circular hole and the specimen are generated on the specimen. It is of great theoretical value and practical significance to the influence of the cavities in the rock specimens on the stress and strain characteristics and the initiation, propagation, and penetration modes.

Anisotropic Mechanical Behavior of Stratified Sandstone Subjected to Cyclic Loading

Abstract To explore the influence of bedding structure on mechanical properties and failure mechanism of anisotropic rocks, six stratified sandstone specimens with different bedding orientations were tested by uniaxial static and cyclic loading. Acoustic emission (AE) and digital image correlation (DIC) were exploited to analyze the mechanism of crack initiation and propagation. The results show that with the increase of the bedding orientation, the peak strength of the stratified sandstone decreases first and then increases, behaving an obvious anisotropic characteristic. The AE characteristic-stress-strain curve can be divided into four stages: crack closure stage, linear elastic deformation stage, stable crack growth stage, and unstable crack growth stage. Moreover, as the bedding orientation increases from 30° to 90°, the crack closure stage gradually lengthens, while the linear elastic deformation stage first shortens and then lengthens. Under uniaxial static compression, due to the presence of bedding surface, the rock surface has a relative slip trend as a result of the uneven distribution of strain on the left and right parts, and the strain field boundary is almost consistent with the bedding orientation. Furthermore, cyclic loading can increase the rock stiffness, and the stain field boundary of 60° stratified sandstone sample in each cycle is slightly different in the cyclic loading.

An analysis of the meso-structural damage evolution of coal using X-ray CT and a gray-scale level co-occurrence matrix method

To study the meso-damage evolution in coal under loading conditions, the industrial CT scanning tests under the uniaxial compression condition were carried out using an industrial CT scanner equipped with a loading control system. The gray level co-occurrence matrix (GLCM) theory was applied to quantitatively analyze the meso-damage evolution and the fracturing characteristics using the acquired CT images at each scanning stage. Four statistical features (i.e., contrast, energy, correlation, and homogeneity) of the CT images were extracted to evaluate the internal damage evolution in coal. The results show that the contrast generally took a changing trend of first decreasing and then increasing with the closure, initiation, and expansion of the fractures, that is, it decreased slowly at the compaction stage, slightly increased at the elasticity stage, and drastically increased at the post-peak stage. On the contrary, the energy, correlation, and homogeneity showed a changing trend opposite to that of the contrast. The changing tendency of mean values of the four statistical features could be depicted by the Boltzmann function, and in addition, the statistical features followed Gaussian distribution during the complete stress-strain process. The maximum damage cross-section of the coal sample could be transferred with the increase of deformation in the coal sample, indicating that the final failure location may not be the maximum initial damage location. The fracture ratio measured by the binarization and K-Means clustering algorithm methods typical showed three changing stages (i.e., a slow reduction process, a slight increase process, and a dramatic increase process) and could be used to reflect the meso-damage development process in coal at different deformation stages.

Creep characterization and modelling of ordinary refractory ceramics under combined compression and shear loading conditions

Creep behaviour of ordinary refractory ceramics is evidently asymmetric under uniaxial tension and compression. In service, they are often exposed to multiaxial stress states. In the present paper, the modified shear test specimens were applied for a creep study in the shear-compression zone of the p-q diagram, and the pure shear creep parameters following the Norton-Bailey strain hardening equation were inversely identified in combination with a weighting function between pure shear and uniaxial compressive conditions. The weighting function was implemented in an in-house asymmetric creep constitutive model. The experimental curves can be well predicted with identified parameters of the asymmetric creep constitutive model. It shows that the shear creep of ordinary refractory ceramics is evidently different to uniaxial compressive/tensile creep. Consideration of shear creep in the thermomechanical modelling of industrial vessels increases the accuracy of simulation results and supports the lining concept optimization investigation.

Study on Mechanical Properties of Modified Polyurethane Concrete at Different Temperatures

The objective of the present research was to study the effect of temperature on the mechanical properties, failure mode and uniaxial compression constitutive relationship of a modified polyurethane concrete. A total of 24 cube and 27 prism specimens were fabricated, and the uniformity of the polyurethane concrete was checked. The compressive test, splitting tensile test and static uniaxial compression test were carried out at 0, 15, 40 and 60 °C. The failure mode, cube compressive strength, splitting tensile strength, axial compressive strength, elastic modulus and the compressive stress–strain curves of the modified polyurethane concrete were obtained. Based on the experimental results, a uniaxial compression constitutive model of the modified polyurethane concrete considering temperature characteristics was proposed. The results show that the elastic modulus, cubic compressive strength, splitting tensile strength and axial compressive strength of the modified polyurethane concrete decrease with the increase of temperature, and the peak strain and ultimate strain increase significantly. When the temperature rises from 0 to 60 °C, the cubic compressive strength, splitting tensile strength and axial compressive strength are decreased by 67.1%, 66.4% and 73.3%, respectively. The calculation results of the proposed constitutive model are in good agreement with the test results. The results are expected to guide the application of the modified polyurethane concrete in bridge deck pavement.

Lithium orthosilicate as nuclear fusion breeder material: Optimization of the drip casting production technology

The production of nuclear fusion energy, in the future DEMOnstration power plant (DEMO), is based on tritium self-sufficiency, achievable exploiting neutron-lithium reaction occurring in the breeding blanket modules. To fulfil the plant design conditions ceramic materials containing lithium are suggested. A lot of research efforts focused on the past years on the investigation and selection of the most suitable breeding material.In this study, the lithium orthosilicate, in pebble form, produced at room temperature at the University of Pisa was investigated and characterized from a thermo-mechanical point of view. The pebbles were produced by a drip casting forming technique, starting from an aqueous suspension of a Li4SiO4 precursor obtained by hydrolytic sol–gel method, and dropping it into a calcium chloride/lithium acetate solution. A high temperature thermal treatment was applied to green pebbles to synthesize Li4SiO4 and densify the structure. Thermal analysis and static uniaxial compression tests, without radial constraints, were carried out on different types of pebbles produced varying the composition of the precursor suspension by means of cellulose fiber additions. Results show the influence of the cellulose content on morphological and physical properties of the obtained pebbles. Li4SiO4 pebbles with a density of 88 % and a crush load of about 30 N were obtained when the suspension was modified by adding 15 wt% of cellulose fibers to the sol–gel precursor powders.

An improved yield criterion characterizing the anisotropic and tension-compression asymmetric behavior of magnesium alloy

A novel yield criterion based on CPB06 considering anisotropic and tension-compression asymmetric behaviors of magnesium alloys was derived and proposed (called M_CPB06). This yield criterion can simultaneously predict the yield stresses and the Lankford ratios at different angles (if any) under uniaxial tension, compression, equal-biaxial and equal-compression conditions. Then, in order to further describe the anisotropic strain-hardening characteristics of magnesium alloy, the proposed M_CPB06 criterion was further evolved to the M_CPB06ev model by expressing the parameters of the M_CPB06 model as functions of the plastic strain. As the model was developed, the stresses and Lankford ratios of AZ31B and ZK61M magnesium alloys at different angles under tensile, compressive and through-thickness compressive conditions were used to calibrate the M_CPB06/M_CPB06ev and the existing CPB06ex2 model. Calibration results reveal that compared with the CPB06ex2 yield criterion with equal quantity of coefficients, the M_CPB06 criterion exhibits certain advancement, and meanwhile the M_CPB06ev model can relatively accurately predict the change of the yield locus with increase of the plastic strain. Finally, the M_CPB06ev model was developed through UMAT in LS-DYNA. Finite element simulations using the subroutine were conducted on the specimens of different angles to the rolling direction under tension and compression. Simulation results were highly consistent with the experimental results, demonstrating a good reliability and accuracy of the developed subroutine.

Дослідження деформаційних характеристик ціликів вугілля як охоронних споруд підготовчих виробок

Purpose. Investigate the deformation characteristics of over-track coal aggregates as a way of protecting the preparatory workings and evaluate their impact on the stability of side rocks in the coal massif to ensure safe working conditions for miners in the mining areas. Method. To achieve the goal, laboratory studies of the deformation characteristics of protective structures in the form of whole coal were performed on models of equivalent materials. The models, with the ratio of sample length to width a/b=(1-2), were subjected to uniaxial compression. Results. It has been proven that the potential energy of coal aggregates is a function of their state, which takes into account the change in volume and shape under the action of an external load. It was experimentally established that under conditions of uniaxial compression in the range of changes in the relative deformation of coal aggregates 0.1≤λ≤0.25 depending on the size of the structures used, with an increase in the cross-sectional area of protective structures, the growth of the load continues without loss of their bearing capacity. Within the limits of the established safe deformation resource, the potential energy of overstretched coal units reaches a critical level, at which the stable state of the deformable structures is realised for a short time. At the transition of the critical level of the safe deformation resource, when λ>0.25, there is a loss of stability of the wholes, intensification of the convergence of the side rocks and their breaking, with subsequent collapse. Overstretched cells, within the limits of a safe deformation resource, are able to concentrate the potential energy of compression. The main element in the mechanism of destruction of over-stretched coal units is the critical density of elastic energy, the amount of which depends on their size. Scientific novelty. It was experimentally established that under the conditions of uniaxial compression of superstriped coal units with the ratio of their length to width a/b=(1-2) until the moment they lose stability, between the magnitude of the relative deformation and the change in the cross-sectional area of the used protective structures in contact with the side rocks in the produced space of the coal massif, there is a logarithmic functional dependence that allows determining their safe deformation resource. Practical significance. Coal targets, as protective structures of limited sizes, also have a limited safe deformation resource, which depends on the size of the structure and restrains the use of this method of protection of rollback stretches in the development of steep formations.

Damage evaluation of rock salt under multilevel cyclic loading with constant stress intervals using AE monitoring and CT scanning

When salt caverns are used for the medium of underground energy storage, the surrounding rock experiences cyclic loading due to periodical injection-production. The cyclic load contains constant stress intervals and varied stress amplitudes, which threatens the stability of the salt caverns. In this study, multilevel cyclic loading with constant stress intervals on rock salt were conducted under uniaxial compression conditions. Acoustic emission (AE) monitoring and post-test computed tomography (CT) scanning were used to evaluate the damage evolution during the loading process. Results show that the density of the hysteresis loops in overall stress-strain curves present two-stage alternating sparse and dense loops during the loading. The cumulative AE count/energy increases with increasing the number of cycles and loading stage, and the growth rate for a single stage gradually becomes constant. The AE signals were divided into six kinds according to frequency-amplitude distribution. The presence of stress intervals increases the number of cracks and the median scale cracks account for about 90% of the total number of cracks. By analyzing the CT images, the crack morphology of the salt sample without hold time shows several inclined main cracks with numerous branch microcracks while that with the hold time of 20 s shows multiple branch cracks and numerous microcracks. To quantitatively evaluate damage evolution during the loading process, a damage variable is defined based on the relative variation of AE parameters. The damage accumulation for a single loading stage presents a two-phase pattern, and the damage degree during the accelerated deformation accounts for approximately half of the total damage. This study can provide guidance for designing the operation parameters when salt caverns are used for the storage of natural gas, air and CO 2 . • Multilevel cyclic loading tests with constant stress intervals are conducted on rock salt. • The AE parameters characteristics are analyzed. • The crack distribution and fracture behavior are visualized by CT images. • Damage evolution behavior is quantitatively evaluated.

ДОСЛІДЖЕННЯ ДЕФОРМАЦІЙНИХ ХАРАКТЕРИСТИК ОХОРОННИХ СПОРУД ПІДГОТОВЧИХ ВИРОБОК ДЛЯ ПРОГНОЗУВАННЯ СТІЙКОСТІ БІЧНИХ ПОРІД У ВУГЛЕПОРОДНОМУ МАСИВІ

Purpose. To study deformation characteristics of embedded massifs from crushed rock under different compression conditions to ensure the stability of side rocks in the coal massif. Methodology. To achieve the goal, the laboratory studies of deformation characteristics of embedded massifs during compression of crushed rock and during its uniaxial compression, with the possibility of lateral expansion of the source material, have been performed. Results. As a result of the performed experimental studies, the characteristic features of the deformation of embedded massifs made of crushed rock have been revealed. It has been established that the deposit array ambiguously determines the ability to respond to the influence of external factors, which are manifested when the coal array is unloaded with the workings. Such ambiguity is due to different compression conditions. The effect of laying the produced space as a method of protection of workings is ensured in conditions where the compression modulus of crushed rock, as a system of fractions of the embedded massif, is in the range of values △p ≥ (350...600) mm/m. In this case, the characteristics of the rigidity of the embedded massifs allow limiting the convergence of the side rocks in the produced space of the excavation site. Scientific novelty. Under conditions of compressive compression or uniaxial compression, with the possibility of lateral expansion of the crushed rock, when the rigidity of embedded massifs increases, when comparing the relative deformation of protective structures at different stages of deformation, the discrepancy between the values of this magnitude changes as a function of hyperbolic cotangent. Practical significance. It is necessary to take into account the deformation properties of the laying massifs in conditions of lateral expansion of crushed rock when applying the laying of the produced space as a method of protecting the preparatory workings in the excavation areas of coal mines.

Investigation of local internal stresses under conditions of uniaxial compression of a quartz single crystal

Based on the analysis of the time intervals between acoustic emission signals obtained in the process of uniaxial compression of a cylindrical sample of a quartz single crystal, local stresses were estimated. It is shown that a decrease in the ability of a material to monotonic stress relaxation with time can serve as a signal for the imminent breakdown of the material. Keywords: acoustic emission, kinetic concept of strength, local mechanical stresses.

PHOTOGRAMMETRY MEASUREMENTS OF INITIAL IMPERFECTIONS FOR THE ULTIMATE STRENGTH ASSESSMENT OF PLATES

The objective of the present study is to develop a new approach to model the initial geometrical imperfections of ship plates by using Photogrammetry. Based on images, Photogrammetry is able to take measurements of the distortions of plates and to catch the dominant surface shape, including the deformations of the edges. Having this data, it is possible to generate faithful models of plate surface based on third order polynomial functions. Finally, the maximum load- carrying capacity of the plates is analysed by performing a nonlinear finite element analysis using a commercial finite element code. Three un-stiffened and four stiffened plates have been modelled and analysed. For each plate, two initial imperfection models have been generated one, based on photogrammetric measurements and the other, based on the trigonometric Fourier functions. Both models are subjected to the same uniaxial compressive load and boundary conditions in order to study the ultimate strength.

Experimental study of fracture geometry characteristics on rock mass strength and crack propagation evolution law

To explore the comprehensive effects of fracture geometric characteristics (length, penetration, quantity and dip angle) on rock mass strength, the uniaxial compression experiments were conducted on rock samples containing prefabricated fractures, the influence of fracture geometric characteristics on the deformation and failure of rock mass and the evolution law of crack propagation are systematically studied from macroscopic and microscopic perspectives. The results show that the conditions of uniaxial compression, the initial damage caused by prefabricated fracture further degrades the strength of rock samples. The rock sample mechanics process can be divided into the consolidation, secondary microcracks and crack extension and sample destruction of four stages, the peak strength, elastic modulus and peak strain of rock samples increase with the decrease of relative geometric characteristics. The failure form of intact rock sample was mainly tensile failure, the fracture (length and quantity) rock samples were subject to shear failure, the failure mode of rock sample with fracture penetration was mainly shear and supplemented by tension, the 0°, 30° and 90° fracture rock samples are X-shaped conjugate inclined plane shear failure, while the 45° and 60° fracture rock samples are shear failure.

Prediction of the stress state of the shotcreting support under repeated seismic load

The article assesses the impact of repeated blasts on the stress-strain state of the shotcreting support, which negatively affects the bearing capacity of the support and can lead to the formation of local rock falls in places of significant degradation of the shotcreting strength. Despite the fact that a single seismic load usually does not have a significant impact on the technical condition of the shotcreting support, repeated dynamic loading can lead to the development of negative processes and affect the safety. The article considers unreinforced and dispersed-reinforced shotcreting concrete as a shotcreting support. Models of deformation of rock and shotcreting support have been studied. To describe the deformation model of a rock mass, an elastic–plastic model based on the Hook-Brown plasticity condition has been accepted, which accurately describes the elastic-plastic behavior of a fractured medium. When performing the prediction of the stress-strain state of the shotcreting support, a model of plastic deformation of concrete with the accumulation of Concrete Damage Plasticity (CDP) was adopted, which allows to comprehensively consider the process of concrete deformation both under conditions of uniaxial compression and stress, and with minor edging draft. At the first calculation stage, a forecast of the seismic waves propagation in the immediate vicinity of the explosive initiation site was made. At the second stage, forecasts of the seismic waves propagation to the mine working and the stress-strain state of the support were made. On the basis of the performed studies, a methodology for assessing the impact of repeated blasts on the stress-strain state of the shotcreting support of the mine working is proposed.