Plastic Stage Research Articles

Additively manufactured truss-core sandwich cylinders: Materials, processes and performances

To explore a structure with a superior load-to-mass ratio applying in rockets, two types of lattice truss sandwich cylinders with corrugated-core and X-core were designed, printed, tested and analyzed. The powder bed fusion (PBF) technique of laser additive manufacturing (AM) was applied to make the structures. Uniaxial compression results reveal that these truss-core sandwich cylinders have excellent mechanical performances with long plastic deformation, and their ultimate failure modes were global or local plastic post-buckling. The finite element method (FEM) was used to explore the influences of geometrical parameters to the failure mode and the bearing capacity. A failure theory was proposed, which can consistently predict the plastic buckling loads consistent with the experimental results. The load-to-mass ratio of the X-core cylinder is slightly about five percent improved compared to the corrugated-core cylinder, but the plastic plateau stage is greatly increased. Truss-core sandwich cylindrical shell configurations with high load-to-mass ratio, long plastic stage, easy-manufacturing and controllable process were successfully proposed for aeronautical structures through this research.

The influence of palm fiber reinforcement on the cement content of vegetated concrete substrate under the condition of equal strength.

Vegetated concrete substrate (VCS) is a kind of ecological cemented soil, which has very wide application prospect in high and steep rock slope eco-protection. Cement is an important component of VCS, but it has high energy consumption and environmental pollution. Fiber reinforcement plays an positive role in improving the mechanical properties of soil, and its use as a substitute for cement content in VCS under the condition of equal strength is rarely investigated. In this study, the unconsolidated-undrained (UU) triaxial compression test of unreinforced substrate as blank control samples (BCS) and reinforced substrate as fiber reinforced samples (FRS) were carried out. The test results showed that the stress-strain curve of VCS can be divided into compaction stage, elastic stage, plastic stage and strain hardening stage. The average peak strength increased by 34.3kPa, 53.6kPa, 218kPa and 81.8kPa as cement content of VCS was 0%,4%, 6% and 8%, respectively. The relationship between the peak strength and cement content of VCS could be better fit by Boltzmann function. The mathematical model of fiber instead of cement in VCS under the condition of equal strength was established. It is found that there is a critical point of cement content according to the mathematical model. The cement of VCS can be completely replaced by plam fiber as the cement content is less than the critical point. While the cement content is higher than the critical point, the cement of VCS can be partially replaced by plam fiber. The decrease of average cement content was 17.23%, 19.00%, 24.27% and 25.34% with 0.2%, 0.4%, 0.6% and 0.8% fiber content in reinforced substrate, respectively. The theoretical method and interpolation method for fiber substitute cement content of VCS under equal strength condition were proposed, which can provide technical guidance for ecological slope protection engineering practice of vegetated concrete.

Amorphization of Gd2Zr2O7 induced by mechanical deformation in nanoindentation

Amorphization has become an important plastic deformation mechanism for covalently-bonded ceramics, which can disturb their crystalline order and affect their mechanical behaviors. Unlike the covalently-bonded ceramics, however, amorphization induced by mechanical deformation is rarely observed in ionically-bonded ceramics. In this work, we show direct experimental evidence that nano-scale amorphous zones in Gadolinium Zirconate ceramic are the ultimate stage of plasticity and relate to an accumulation of dislocations. In addition, the occurrence of amorphization also causes the unusual indentation size effect.

Граничний стан армованих двошарнірних арок двотаврового перерізу

The method of determining the limit load for double-hinged arches is considered. The calculation is performed using the limit equilibrium method. The cross-section of the arch is taken in the form of a reinforced I-beam. The strain-deformed state of the I-beam material is described by the Prandtl diagram. But unlike the classical diagram, it has different yield points under tension and compression. The reinforcement material is described by the classic Prandtl diagram with the same yield strength in tension and compression.In most cases, the ultimate equilibrium of the cross-section is based on the use of one factor ˗ the plastic moment, upon reaching which the cross-section enters a plastic state with the possibility of unlimited deformation. But such an approach cannot be adopted for an arch, as significant longitudinal forces arise in its sections. Ignoring longitudinal forces leads to errors in determining the ultimate load. Therefore, for arches, when determining the limit state of the cross section, it is important to take into account both the bending moment and the longitudinal force. That is, for the transition of the section to the limit state, it is necessary to apply a limit moment to it, which corresponds to a certain longitudinal force. This leads to the concept of the region of cross-sectional strength, constructed in the coordinates of the bending moment - longitudinal force. The boundary of this region indicates the limit state of the cross-section and is described by the flow conditions, which can be obtained by considering the plastic equilibrium of the cross-section. Using the flow conditions, the equilibrium equation of the arch and some constraints, it is possible to formulate an optimization problem for finding the limit load. Limit load (objective function) is the smallest load that satisfies the arch equilibrium equation, yield conditions and constraints. The project variable is the coordinate of the cross-section that enters the plastic stage. The solution of this problem for arches of constant and variable stiffness was performed using electronic spreadsheets. Arch calculations were also performed in PC Lira-CAD. A comparison of the calculation results showed a satisfactory convergence.

Study on the Dynamic Properties of Concrete under Freeze-Thaw Cycles and CT Fine View Damage

In order to study the dynamic mechanical properties of concrete under freeze-thaw cycles, uniaxial impact compression tests were conducted on concrete specimens with different number of freeze-thaw cycles (0, 25, 50, 75, and 100) using a 50 mm diameter split Hopkinson pressure bar (SHPB) test device at an impact air pressure of 0.4 MPa and combined with a CT scanning system to analyze the internal fissures of concrete specimens before and after freeze-thaw cycles. The results showed the following. (1) The concrete specimens were divided into linear elastic stage, plastic stage, and damage stage under the freeze-thaw cycle, and the macroscopic mechanical parameters of the specimens were weakened with the increase of the number of freeze-thaw cycles. (2) Freeze-thaw intensified the expansion and penetration of the internal fracture of the specimens, and the CT scan results showed that the internal fracture parameters of the concrete specimens were increased with the increase of the number of freeze-thaw cycles, and the structural characteristics of the internal fracture show gradually complex expansion and evolution characteristics. (3) The degree of damage was defined at the fine level, which increased with the number of freeze-thaw cycles, establishing an intrinsic link between the fine deterioration of concrete specimens and the loss of macroscopic parameters under freeze-thaw cycles. The results are of great significance to further elucidate the internal structural characteristics of concrete material damage under freeze-thaw conditions and to reveal the damage deterioration mechanism.

The investigation on shale mechanical characteristics and brittleness evaluation

Rock mechanical property is significant for shale gas development and exploitation. Shale compressive strength, tensile strength, elastic deformation and so on, are necessary parameters for drilling, completion and fracturing work in shale formation. Among all these shale mechanical parameters, brittleness is a tricky and significant rock property, which has been widely used to hydraulic fracturing design. Currently, although so many works have been conducted to investigate shale brittleness, there is no precise definition of brittleness. In particular, there is no consensus on which method is the most reliable for shale brittleness evaluation. It is vital to figure out how to evaluate shale brittleness in a reliable method. Thus, this paper presents an experimental study on shale mechanical properties, analyzing mechanical features in stress strain curve, relation between mineral content and strength, mechanical parameters at varying confined stress. Based on shale mechanical characteristics and its brittle exhibition, stress strain curve from triaxial compression test is divided into 3 stages, namely, elastic stage, plastic stage and post peak stage. In combined with brittle characteristics in 3 stages of axial and radial stress–strain curves, a new brittleness index has been established for assessing shale brittleness. In order to prove the applicability of new brittleness index, its result is compared with shale failure sample after triaxial test and existing brittleness indexes based on mineral content, elastic deformation, energy, stress and strain, showing a good consistency and proving its practicability. Based on this brittleness index, influence factors of shale brittleness have been discussed. It is shown that elastic module is the most important factor of shale brittleness. Bedding plane makes shale brittleness have strong anisotropy. Brittleness is not only relied on its structure and mineral (like bedding plane, silicate and clay mineral content), but is also highly affected by external stress. Large confined pressure is able to impair shale brittleness. Outcome in this study can offer theoretical guidance for shale exploitation.

Investigation on crack resistance of full aeolian sand high-performance concrete synergistically modified by supplementary cementitious materials and superabsorbent polymer

The development of full aeolian sand high-performance concrete (FA-HPC) can alleviate the dilemma of river sand shortage. However, the high cracking potential appears due to the characteristic of low water-to-binder ratio and high cementitious materials content. Herein, supplementary cementitious materials (SCMs), i.e., fly ash and silica fume, and superabsorbent polymer (SAP) are used to optimize the crack resistance synergistically. The crack resistance in plasticity and hardening stages are investigated based on flat plate cracking and ring restrained cracking combined with chloride ions penetration, respectively. Meanwhile, capillary pressure, mechanical strength, and internal temperature field were also involved. Additionally, the modification mechanism of SCMs and SAP is analyzed from a microscopic perspective via hydration heat, phase analysis, and microstructure. The results showed that whether single use or binary mixing, fly ash and silica fume significantly improved the crack resistance of FA-HPC. Moreover, it was further enhanced by the SAP, presenting a 63.39% and 96.96% increase in crack resistance at plasticity stage and hardening stage respectively as well as a 78.31% reduction in chloride ions penetration. Contrary to silica fume, fly ash hindered early hydration leading to a decrease in capillary pressure, strength, and internal temperature field with content dependence. Besides, the most intense volcanic ash reaction was observed in binary mixing. Although SAP delayed the early hydration, the later hydration degree was improved obtaining more hydration products at 28d since the internal curing effect. Thereby, the more refined microstructure was achieved for more optimized crack resistance. This study provides an available strategy for reducing the cracking risk of FA-HPC to promote wide application in engineering.

Early-age properties of cement paste with mechanically ground Yellow River sediment

Yellow River sediment(YRS) is a rich, low-activity silica-aluminum dredged sediment resource, which can provide a new source of supplementary cementitious materials(SCMS). This study investigated the influence of mechanically ground Yellow River sediments (MGYRS) on the hydration kinetics and early-age shrinkage of cement paste. Hydration kinetics revealed that MGYRS could significantly accelerate the hydration process, proceeding directly from the nucleation and crystal growth stage (NG) to the diffusion stage (D). In the plastic stage of cement paste, the early-age shrinkage of MGYRS at 40 min and 70 min increased by 250 µm/m and 99 µm/m, respectively. Microscopic analysis showed that MGYRS could promote the generation of Ca(OH)2, and the nucleation, micro-aggregate filling effect of MGYRS could refine the pore structure. This study can provide theoretical support for realizing the gelatinization of YRS.

The Mechanism of Deformation Compatibility of TA2/Q345 Laminated Metal in Dynamic Testing with Split-Hopkinson Pressure Bar.

The laminated metal materials are widely used in military, automobile and aerospace industries, but their dynamic response mechanical behavior needs to be further clarified, especially for materials with joint interface paralleling to the loading direction. The mechanical properties of TA2/Q345 (Titanium/Steel) laminated metal of this structure were studied by using the split Hopkinson pressure bar (SHPB). To shed light on the stress-state of a laminated metal with parallel structure, the relative non-uniformity of internal stress R(t) was analyzed. The mechanism of deformation compatibility of welding interface was discussed in detail. The current experiments demonstrate that in the strain rate range of 931-2250 s-1, the discrepancies of the internal stress in specimens are less than 5%, so the stress-state equilibrium hypothesis is satisfied during the effective loading time. Therefore, it is reasonable to believe that all stress-strain responses of the material are valid and reliable. Furthermore, the four deformation stages, i.e., the elastic stage, the plastic modulus compatible deformation stage, uniform plastic deformation stage and non-uniform plastic deformation stage, of the laminated metal with parallel structure were firstly proposed under the modulating action of the welding interface. The deformation stages are helpful for better utilization of laminated materials.

Elastoplastic mechanical behavior of high-field magnet of REBCO high temperature superconducting composite tape under extreme environment

Rare Earth-Barium-Copper-Oxide (REBCO) high temperature superconducting (HTS) coated conductors (CCs) are one of the best candidate materials for high-field magnets. However, due to its laminated structure, each component materials of the CCs are inevitably subjected to residual thermal stress and electromagnetic force during heat treatment, processing, cooling, coil winding, and operation of the magnet structure. It causes changes in its superconducting properties, and even leads to irreversible degradation of superconducting critical characteristics, brittle layer fracture and peeling delamination within the coil. To reveal the synthesis of the tape and the stress state of the high-field coil under multi-physical fields. The residual thermal stress/strain accumulation of each component material of the tape is investigated under two thermal processing and manufacturing methods by combining analytical theory with numerical calculation. In which, the two thermal processing methods are metal-organic chemical vapor deposition (MOCVD) process and one-step cooling simplified process respectively. The results show that the residual thermal stresses accumulated by five materials, Hastelloy, Buffer, REBCO, Ag and Cu, under the step-by-step synthesis process are about 14.7%, 97.3%, 96.3%, 80% and 49.2% of the simplified model, respectively. And the residual thermal strains are about 15.1%, 96.8%, 95.2%, 85.2% and 27.5% of the simplified model. In addition, after the five materials are manufactured according to the step-by-step thermal processing process, the accumulated residual thermal stresses are about 1.88%, 86.3%, 61%, 114.3%, and 34% of their respective yield strengths, respectively. The elastoplastic mechanical behavior of the magnet under the background of high magnetic field and strong current is also studied. The results indicate that the maximum values of effective and hoop stresses in each component material of the coil are distributed in the innermost side of the magnet, and decrease gradually along the radial direction of the magnet. In the innermost coil of the magnet, the effective stress of each component material increases with the increase of the transmission current. When the transmission current Iope = 183 A, the material Cu reaches the yield strength and enters the plastic deformation stage. Whereas the material Hastelloy reaches the yield strength and enters into the plastic deformation stage when the transmission current Iope = 222 A.

Effects of dynamic velocity on the dynamic compressive mechanical properties and energy evolution of coal specimens

ABSTRACT Dynamic compressive parameters of coal specimens were driven not only by impact velocities, but also by preloaded confining pressure, and impact loading methods. This paper investigates the influences of dynamic parameters based on the split Hopkinson pressure bar (SHPB) experimental data obtained from the dynamic compressive impact tests on 12 coal specimens under four different impact velocity. The analysis of the experimental results confirmed that with an increase of dynamic velocity, and then the dynamic compressive strength is constantly increases. When the dynamic velocity is 8.10 m/s, and then reacheed the maximum dynamic strength of 25.11 MPa of the coal specimen. Furthermore, the dynamic velocity of 8.10 m/s was defined as the threshold value of the dynamic strength increased. The greater of dynamic velocity, the higher degree of fragmentation of the coal specimens. The transmitted energy, reflected energy, and absorbed energy of coal specimens shows a directly proportional relationship with the incident energy. The results highlight the first and second dynamic deformation moduli can clearly explained the deformation laws of elastic and plastic stages of the coal specimens. It is also found that the maximum of strain rate is 177.28 s−1 and the maximum ultimate strain is 2.47 × 10−2of the coal specimens. The research results were beneficial to supplement the coal-rock dynamics theory, thus also providing a direction for safety and effective mining and crushing in coal mines.

Atomic study on deformation behavior and anisotropy effect of 3C-SiC under nanoindentation

3C silicon carbide (3C-SiC) has great potential and value for innovated high-precision devices. However, the high hardness make it a daunting task to understand its deformation behavior and anisotropy effect, which impedes its processing and application. In this study, we performed molecular dynamic simulations of nanoindentation to study the deformation behavior and anisotropy effect of 3C-SiC. It is found that the elastic deformation and incipient plastic deformation are orientation-dependent, exhibiting fourfold symmetry, central symmetry and threefold symmetry while indenting on 3C-SiC (010), (110) and (111) surfaces. The dislocations nucleate and propagate due to the high shear stress induced by indentation, which in turn leads to the dissipation of shear stress in the sample. In the plastic stage, the dislocations evolve from embryonic dislocations to ribbon dislocations, and finally to prismatic dislocations. The prismatic dislocation loops are surrounded by stacking faults in {111} planes, most of which are enclosed by four stair-rod dislocations in <110> orientations. Moreover, we find that Thompson tetrahedron is suitable for understanding the anisotropy effect of ceramics with cubic diamond structure, and it is applied to investigate the formation and evolution of dislocations systematically in 3C-SiC. This research is meaningful for understanding the deformation behavior and anisotropy effect of 3C-SiC. The results may provide theoretical support for the processing and application of 3C-SiC.

Study on correlation and scaling protocol between indentation stress-strain and uniaxial stress-strain

Most important mechanical structures produce complex local material changes during the moulding and servicing process. Obtaining the specific local mechanical properties of materials and conducting field tests are problems in structural integrity evaluation. The instrumented indentation technique (IIT) is a solution to these problems. However, the core challenge with the IIT is the quantitative correlation between indentation response and uniaxial stress-strain (USS) response. In this study, we investigate the scaling relationship between indentation stress-strain (ISS) and uniaxial stress-strain (USS). A new protocol for obtaining the mechanical properties of alloy steels using ISS curves is proposed, which can directly obtain the plastic stage of the USS curves from the ISS curves. We used a finite element inversion method based on existing theories. Models with different mechanical properties (USS curves) were established using the ABAQUS software to obtain the corresponding ISS curves. We then explored the relationship between the two; the strain-scaling factor and stress-scaling function were specifically identified. Our work proposes an ISS protocol for power-law-hardening materials. Two alloy steels (SA508 and 42CrMo) were experimentally verifieds. The predicted results are approximately consistent with the practical curves and the error was within ± 10%. Because of the universality and reliability of the protocols, it is expected to provide a theoretical reference and technical approach for the development of IITs and structural integrity evaluation of important mechanical structures.

Features of animal babbling in the vocal ontogeny of the gray mouse lemur (Microcebus murinus)

In human infants babbling is an important developmental stage of vocal plasticity to acquire maternal language. To investigate parallels in the vocal development of human infants and non-human mammals, seven key features of human babbling were defined, which are up to date only shown in bats and marmosets. This study will explore whether these features can also be found in gray mouse lemurs by investigating how infant vocal streams gradually resemble the structure of the adult trill call, which is not present at birth. Using unsupervised clustering, we distinguished six syllable types, whose sequential order gradually reflected the adult trill. A subset of adult syllable types was produced by several infants, with the syllable production being rhythmic, repetitive, and independent of the social context. The temporal structure of the calling bouts and the tempo-spectral features of syllable types became adult-like at the age of weaning. The age-dependent changes in the acoustic parameters differed between syllable types, suggesting that they cannot solely be explained by physical maturation of the vocal apparatus. Since gray mouse lemurs exhibit five features of animal babbling, they show parallels to the vocal development of human infants, bats, and marmosets.

Macro-micro behaviors and failure mechanism of frozen weakly cemented mudstone

Understanding the mechanical properties and multiscale failure mechanism of frozen soft rock is an important prerequisite for the construction safety of tunnels, artificially frozen ground and other infrastructure in cold regions. In this study, the triaxial compression test are performed on mudstone in the weakly cemented soft rock strata in the mining area of western China, and the mechanical characteristics and failure mechanism of weakly cemented mudstone are systematically investigated under the combined action of freezing and loading. Furthermore, the quantitative relationship between the microstructural parameters and the macroscopic strength and deformation parameters is established based on fractal theory. Thus, the failure mechanism of frozen weakly cemented mudstone is revealed on both micro- and macro-scales. The results show that temperature and confining pressure significantly affects the elastic modulus and peak strength of weakly cemented mudstone. With decreasing temperature, the compressive strength increases, while the corresponding peak strain decreases gradually. On the deformation curve, the plastic deformation stage is shortened, and the brittle fracture feature at the post-peak stage is more prominent, and the elastic modulus correspondingly increases with decreasing temperature. Under low-temperature conditions, most of the weakly cemented mudstone undergoes microscopic shear failure along the main fracture surface. The micro-fracture morphology characteristics of weakly cemented mudstone under different temperatures are quantified via the fractal dimension, and an approximately exponential relationship can be obtained among the fractal dimension and the temperature, compressive strength and elastic modulus.

Revealing the deformation mechanisms of 〈110〉 symmetric tilt grain boundaries in CoCrNi medium-entropy alloy

The synergy of multiple deformation mechanisms responsible for the excellent mechanical properties attracts an increasing interest in CoCrNi medium entropy alloy (MEA). In this study, we employed molecular dynamics simulations to investigate the chemical properties of grain boundaries (GBs) and their influence on deformation mechanisms in CrCoNi MEA, with particular emphasis on role of lattice distortion and short-range order (SRO) in dislocation plasticity. After aging, we found Ni element segregate at GB through short-range diffusion driven by a more negative segregation enthalpy. The degree of SRO within the GB region is significantly correlated with the Ni concentration, and it markedly influences the structure and local stress distribution of the GB. Compared to the lattice distortion, SRO can effectively suppress the GB dislocations nucleation and slip, thereby increasing yield strength of the material. During the plastic stage, although the deformation microstructure is intimately linked to the active slip systems and grain orientation, the presence of SRO, as well as the resultant rugged dislocation pathways and increased slip resistance lowers the propensity for stacking faults and phase transformation. The current findings can enhance a fundamental comprehension of diverse deformation mechanisms in CoCrNi MEA and suggest that the chemical characteristics of GB could serve as a pivotal approach for modifying the mechanical properties of MEAs.

Nonlinear dynamic response analysis of a hybrid concrete‐filled steel tubular truss lightweight bridge under strong earthquakes

AbstractConcrete‐filled steel tubular (CFST) trusses have seen recent development and application in bridges found in mountainous regions with deep valleys, and especially in seismic zones. In this study, a hybrid CFST truss lightweight bridge was selected as the research object. This bridge is composited of CFST composite truss girders (superstructure) and CFST lattice piers (substructure). Two different finite element models (FEM) for nonlinear seismic response analysis, one is detailed model and the other is simplified model, were established. By comparing with the detailed model, the results of the shaking table test as well as the real bridge test, the simplified model was found to be accurate. Considering the CFST columns' edge strain, the most unfavorable input direction of horizontal ground motion was perpendicular to the connection line of the bearings at both ends of the main girder. Also, the simultaneous influence of vertical ground motion and horizontal ground motion needs to be taken into consideration. By increasing the intensity of input ground motion, the yielding order of CFST lattice piers and its effect on the seismic response were also discussed. Results indicated that the bridge was in the elastic stage for the designed seismic ground motion. After the CFST lattice piers reached the plastic stage, the displacement and in‐plane bending moment had larger amplification coefficients compared to the increase coefficient of the input ground motion, while had a much smaller axial force amplification coefficient. Moreover, the in‐plane bending moment had a larger amplification coefficient in high piers compared to short piers.

Cyclic tests and finite element analysis on hybrid fiber–cement-based composite encased CFST columns

Six hybrid steel–polyvinyl alcohol (PVA) hybrid fiber–cement (HFC)-based composite encased concrete-filled steel tube (CFST) columns were tested for their seismic performance. The impacts of core concrete strength, steel tube thickness, and axial ratio on the seismic performance of the specimens were studied. According to the experimental findings, strengthening the core concrete can enhance the column's lateral bearing capacity and energy dissipation capabilities while also slowing the rate at which the steel tube's strain increases after yielding. Furthermore, HFC-encased CFST columns with a lower axial ratio and a thicker steel tube demonstrated superior seismic performance. An's equation of computation could better forecast the lateral bearing capacity of HFC-encased CFST columns when compared to other calculation methods. Finally, an experimentally validated finite element model of HFC-encased CFST columns was developed. Numerical studies showed that the seismic performance of composite columns can be significantly improved by increasing the strength of the HFC and the volume concentration of the steel fiber, and that PVA primarily acts in the plastic stage.

Dynamic deformation behavior and constitutive model of a Zr–W alloy

In this paper, a Zr–5W alloy was fabricated via casting. In order to obtain the mechanical properties of the material, quasi-static compression tests at room temperature and split Hopkinson pressure bar tests at various temperatures were carried out. The x-ray diffraction result showed that the main components of the alloy were αZr and W2Zr, where αZr is the matrix and W2Zr is the reinforcement. The metallographic characterization results showed that there were two main forms of W2Zr in the material, namely, large particle boundary and small diffuse submicrometer precipitates. The reinforcements of both distributions have the effect of increasing the strength of the material, but the small submicrometer W2Zr precipitates would cause microcrack nucleation during the late plastic deformation stage, resulting in damage softening. In order to make theoretical calculations of the mechanical properties of materials, the Johnson–Cook (JC) constitutive model and Zerilli–Armstrong (ZAM) constitutive model of the material were obtained. It was found that the JC constitutive model had poor consistency in describing material properties. Although the consistency of the ZAM constitutive model was higher than that of the JC constitutive model, it still had obvious shortcomings. Combined with the deformation mechanism of the alloy, a modified constitutive relation was established by adding damage softening terms based on the hexagonal close-packed metal constitutive model inferred by the kinetics of heat-activated dislocations. The relative error results of all working conditions show that the correlation consistency of the improved constitutive model in this paper is significantly better than that of JC constitutive and ZAM constitutive.

Study on mechanical and energy characteristics of coal samples under different unloading states

There are many types of coal seams in China, and the mining of protective layers will cause different rates of stress reduction in protected coal seams at different intervals. Therefore, experiments were conducted at different unloading rates to explore the strength, deformation, and energy characteristics of coal. Research findings: the AE (acoustic emission) signal of the coal body before unloading has a small range of changes and similar characteristics. After unloading begins, because of the different development rates of internal crack in the coal body under different unloading states, the AE signal of the coal body varies at different unloading rates. The maximum stress increases exponentially with the increase of unloading rate. It was found that the higher the unloading rate, the easier and earlier the coal sample is to be damaged. And it was discovered that the dissipated energy of the coal sample in the elastic stage is extremely low, and a large amount of total energy is converted into elastic energy and stored inside the coal sample. The dissipation energy increases during the plastic stage, while the trend of increasing elastic energy slows down. After the peak stage, the dissipated energy rapidly increases and the elastic energy decreases.