Stage Of Plastic Deformation Research Articles

Acoustic Emission Characteristics and Damage Evolution of Sandstone with Different Pores under External Load

The rock will be damaged and destroyed when the external load reaches the bearing limit, which will be accompanied by complex AE signals and damage evolution laws. Therefore, in order to obtain the relationship between AE signal and damage evolution characteristics of rocks, 4 kinds of sandstones of a mine are used for AE test. Firstly, the porosity of 4 kinds of sandstone is tested. Secondly, the AE signal parameter characteristics of sandstone with different porosity are analyzed. Finally, the AE parameters obtained are combined with cellular automata and damage theory to analyze the damage evolution law and critical damage value of different sandstones. The results show that the pore size of the four sandstones is QSYX > QSYZ > FSYX > FSYZ. The loading process is divided into compaction stage, elastic deformation stage, and plastic deformation stage, with peak strengths of 46.92 MPa, 43.32 MPa, 57.87 MPa, and 54.31 MPa, respectively. Or the AE event rate, the missing area, missing parts and missing number are different. The QSYX missing area is larger than QSYZ and FSYZ; the macrocrack growth speed is also faster; and the signs of fracture are obvious. The number of FSYX missing is more than QSYX, QSYZ, and FSYZ. The first two missing parts are caused by internal defects; the last two missing parts are signs of fracture; QSYX, QSYZ, and FSYZ are shear failure, and FSYX is tensile failure. The damage evolution process of the four sandstones corresponds to the loading process one by one. The calm stage of damage corresponds to the compaction stage, the damage expansion stage corresponds to the elastic deformation stage, and the damage acceleration stage corresponds to the plastic deformation stage. The critical damage values are 0.438, 0.499, 0.576, and 0.476, respectively, which are higher than the critical damage values of the sandstone cell model of 0.43, indicating that when the damage values reach the critical value, instability exists and instability failure will occur with continuous load.

Strain Rate-Dependent Anomalous Work Hardening of a Single-Crystalline CoNi-Base Superalloy

The deformation mechanisms operating in superalloys depend on different parameters such as composition, temperature and deformation rate. So far, the transition from shearing by APB-coupled dislocations to shearing under the formation of stacking faults has been studied exclusively as a function of temperature but not as a function of the strain rate. Therefore, interrupted compression tests with strain rates between 10–3 s−1 and 10–5 s−1 were performed on the single-crystalline CoNi-base superalloy ERBOCo-4 at a temperature of 850 °C. The evolution of the defect structures has been analyzed by conventional transmission electron microscopy. A change of the deformation mechanism from APB-coupled dislocation shearing to stacking fault shearing is found to depend on the strain rate. At lower strain rates, an increased stacking fault density is associated with a higher yield strength and higher work hardening rates at the early stages of plastic deformation due to a very high stacking fault density. After approximately 2.0 pct plastic strain, the stress reaches a plateau and decreases subsequently, which is associated with the formation and thickening of twins as shown by high-resolution scanning transmission electron microscopy. At higher strain rates, the work hardening rate is significantly lower in the early deformation stage. The role of segregation to planar defects and the influence of local phase transformations (LPT) at SESFs is further discussed in reference to the influence of the strain rate. The segregation of W as an η stabilizing element is found to be crucial for the formation of a local phase transformation in ERBOCo-4. At higher strain rates the phase transformation is hindered by insufficient W segregation, resulting in a higher twin density.

Investigations on the tensile deformation of pure Mg and Mg–15Gd alloy by in-situ X-ray synchrotron radiation and visco-plastic self-consistent modeling

In this study, the texture evolutions of two Mg materials during tension are explored. In-situ X-ray synchrotron and Visco-Plastic Self-Consistent (VPSC) modeling are employed to investigate the different deformation modes between pure Mg and Mg–15Gd (wt.%) alloy. These two materials with a strong extrusion texture show large different slip/twinning activity behaviors during tensile deformation. The basal 〈a〉 slip has the highest contribution to the initial stage of plastic deformation for pure Mg. During the subsequent plastic deformation, the prismatic slip is dominant due to the strong ED // 〈100〉 fiber texture. In contrast, the deformation behavior of Mg–15Gd alloy is more complex. Twinning and basal slip are dominant at the early stage of plastic deformation, but further deformation results in the increased activation of prismatic and pyramidal slips. In comparison to pure Mg, the ratios of the critical resolved shear stress (CRSS) between non-basal slip and basal slip of the Mg–15Gd alloy are much lower.

Complex Network Methods for Plastic Deformation Dynamics in Metals

Plastic deformation of DC04 steel is regarded as a nonlinear, complex, irreversible, and self-organized process. The stress–strain time series analysis provided the possibility to identify areas of (quasi-)elastic deformation, plastic deformation, and necking. The latter two regions are the most informative. The area of inelastic deformation is reflected by collective, self-organized processes that lead to the formation of pores, and finally, the development of microcracks and a general crack as the cause of sample failure. Network measures for the quantitative assessment of the structural deformations in metals are proposed. Both spectral and topological measures of network complexity were found to be especially informative. According to our results, they can be used not only to classify the stages of plastic deformation, but also, they can be applied as a precursor of the material destruction process.

Dislocation behavior in initial stage of plastic deformation for CoCrNi medium entropy alloy

The effects of crystal orientation on the plastic deformation behavior for typical CoCrNi medium entropy alloy under tensile and compressive loading were investigated by molecular dynamics simulations. The Lomer-Cottrell (L-C) structure and parallel hexagonal close packed (HCP) bands are produced in the [111] and [11̅0] orientations at the initial stage of tensile deformation, respectively. However, perfect dislocations and two types of non-slip dislocations (Hirth and Stair-rod) are generated in the [111] and [11̅0] orientations during compressive deformation, respectively. The L-C structure produced by tensile loading in the [111] orientation and Hirth/Stair-rod dislocations produced in the [11̅0] orientation under compression act as a dislocation pinning and limit the continued glide of the dislocations. The dislocation reactions under various conditions were analyzed, which is helpful to understand the plastic deformation mechanism of CoCrNi alloy.

Experimental study of corrosion behavior on 7005 aluminum alloy under acidic 3.5 wt% NaCl environment

AbstractThe corrosion failure behavior of 7005 aluminum alloy is a critical issue in the durability studies of vessels operating. In this study, the corrosion behavior of aluminum alloy 7005‐T4 in 3.5 wt% NaCl solution with pH 2, pH 4, and pH 6 was investigated by surface observation, electrochemical, and mechanical testing. The experiment results that the corrosion susceptibility is more at a solution of pH 2 compared with that at pH 4 and pH 6. The maximum depth and width of corrosion pits increase linearly with aggravation of corrosion. In addition, the probability distribution of the corrosion pit depth in each corrosion environment, which generally follows a Lognormal distribution, is statistically analyzed. The tendency of stress−strain curves at the elastic−plastic transition stage indicated that corrosion has an effect on mechanical performance, and revealed that corrosion promotes the advance of the plastic deformation stage. Fractography indicated the presence of a quasi‐cleavage zone beneath the corrosion layer. The results provide theoretical guidance for safe service and design guidelines for 7005 aluminum alloy.

Formation condition and effect on the early stages of plastic deformation of chemical short-range order in Cr-Co-Ni medium-entropy alloy

The variation in the chemical short-range order (CSRO) of Cr-Co-Ni medium entropy alloy is investigated by measuring the electrical resistivity at various temperatures. According to the measurements at various cooling rates, the thermodynamic equilibrium at temperatures higher than 1075 K cannot be quenched in most conventional sample preparations. The resistivity quenched at various temperatures increases with decrease in annealing temperature, especially below 873 K. The activation energy of the reaction is determined 240–270 kJ mol−1. The activation energy is almost the same as that of atomic diffusion in the Cr-Mn-Fe-Co-Ni high-entropy alloy, indicating that the electrical resistivity change is associated with the change in the atomic arrangement. Using the results of electrical resistivity measurements, the effect of the degree of CSRO on the early stage of plastic deformation was investigated by compression tests on single crystals with varying degrees of CSRO. The critical resolved shear stress and activation volume of the slip are found to be virtually independent of the degree of CSRO. Repetitive strain-rate jump tests indicates that the degree of CSRO does not significantly affect the dislocation motion process in the early stages of the plastic deformation in Cr-Co-Ni medium entoropy alloy.

Differential Energy Criterion of Brittle Fracture as a Criterion for Wood’s Transition to the Plastic Deformation Stage

An experimental study and modeling of the behavior of wood during compression along the fibers was carried out. The nonlinear analytical dependence of the load on the strain was considered. Attention was focused on the post-peak stage of deformation in order to determine the load and displacement at which the transition to the stage of plastic deformation occurs. The work was aimed at substantiating the application of the energy criterion of brittle fracture as a criterion for the transition to the stage of plastic deformation. To achieve this goal, methods of mathematical modeling and analysis of test results were used. As an upshot, a simple and practical procedure was developed to predict the transition point to the above stage of plastic deformation. The simulation results were consistent with laboratory tests of samples and fragments of structures. The practical significance of this criterion lies in its possible use as an additional tool for analyzing the condition of some wooden structures. Energy criteria, including the one mentioned above, belong to fairly universal criteria. Accordingly, the research methodology can be adapted to analyze the behavior of, for example, composites under other types of loads in further studies.

Molecular simulation of the plastic deformation and crack formation in single grit grinding of 4H-SiC single crystal

Silicon carbide (SiC) is a promising semiconductor material for high-performance power electronics devices, but difficult to machine. The development of cost-effective machining technology for SiC wafers requires a complete understanding of the interaction between diamond grit and workpiece material. This work systematically investigated the deformation and crack formation of monocrystalline 4H-SiC involved in single grit grinding using molecular dynamics (MD) simulation. The mechanism for crack initiation and propagation in monocrystalline 4H-SiC was revealed for the first time. Our simulation results showed that the subsurface damage at the earlier stage of plastic deformation was caused by amorphization and dislocation initiation. With the progress of penetration, slip bands were formed. The extension of slip bands in the later stage of plastic deformation was the cause of crack initiation. Stress analysis demonstrated that the tensile stresses parallel to the scratch direction and perpendicular to the scratch plane were the driving force for the crack initiation ahead of a diamond abrasive grit, while the tensile stress perpendicular to the scratch plane was responsible for the crack initiation underneath the grit. The propagation of the two cracks was driven by the maximum principal stress at the crack tip, and the propagation direction depends on the direction of the maximum principal stress.

Role of slip and twinning on strain hardening, and correlation with geometric hardening, latent hardening, and grain boundary strengthening in Titanium

This work investigates the role of slip & twinning on the strain hardening in Titanium (Ti) that develops profound {101¯2}<101¯1¯> extension twins (ET) during compression. The microstructure and texture evolution studies were carried out by ex-situ electron backscattered diffraction (EBSD) at intermittent strains. The strain hardening could be catagorized into multiple plastic deformation stages governed by twin-induced geometrical hardening & softening, intrinsic latent hardening, and grain boundary strengthening. Crystal plasticity was applied to simulate the flow curve and texture evolution, thereby obtaining slip and twin activities. The initial axisymmetric {101¯0} - {112¯0} fiber texture was favourable for prismatic <a> slip and ET evolution on compression. ET nucleate till ε=0.10 and broadens to consume almost the entire microstructure up to ε=0.22. The ET reorient the microstructure to a geometrically hard orientation with c-axis ∼∥ to compression direction (CD) escalating the Taylor factor (M). The ET leads to sigmoidal flow behavior from ε=0.06 to 0.17, increasing SHR in stage II. The ET domains majorly deform by pyramidal <c+a> and basal slips, though having high shear strength, and produce {112¯7}−{101¯7} fiber texture from ε=0.1 to 0.69. Beyond ε=0.11, {112¯2}<112¯3¯> contraction twins (CT) also evolve inside ET-domains, consuming ∼0.29 of its fraction. The CT reorient the ET domains to a softer {112¯1}−{112¯2} fiber texture with a low M. The geometrically soft CT domains, latent hardening and grain fragmentation contribute toward flattening the strain hardening curve and decreasing SHR in stage III after ε=0.17.

Study on the Characteristics of Coal Ultrasonic Response during Loading and Its Influence Mechanism

The prediction and prevention· of mine disasters are crucial to identifying the stress and strain state of coal using ultrasonic response characteristics. In this study, ultrasonic testing experiments of primary structure coal samples under uniaxial loading were conducted using a low-frequency rock physics measuring device. Based on the experimental results, the study focused on analyzing the relationship of the stress–strain state of coal samples with the ultrasonic velocity and quality factor of coal samples during stress loading, and exploring the influence mechanism of ultrasonic propagation in coal during stress loading. The results demonstrated that the stress-loading process of coal samples falls into the elastic deformation stage and the plastic deformation stage. In the elastic deformation stage, the ultrasonic velocity and the quality factor of the coal sample increased with the increase in the coal axial strain. In the plastic deformation stage, the ultrasonic velocity and quality factor of coal samples decreased as the axial strain of coal samples increased. Coal porosity was the fundamental factor affecting the coal wave velocity variation and the coal quality factor variation. In the elastic deformation stage, increased coal axial stress was accompanied by decreased coal porosity, contributing to the increase in coal wave velocity and coal quality factor. In the plastic deformation stage, the increase in the coal axial strain increased coal porosity and thus curtailed the wave velocity and quality factor of coal. Significant differences were observed in ultrasonic response characteristics of coal under various stress and strain states. The research results can lay a theoretical foundation for the safe and efficient development of coal resources and the prevention and control of mine disasters.

Simultaneously improved strength and ductility in aluminum matrix composite with heterogeneous structures under impact loadings

Heterostructured composites with the coexistence of hard and soft phases can achieve a superior strength-ductility synergy. However, the deformation and fracture mechanisms of these composites under impact loading has not been comprehensively understood. In this work, an in-situ TiB2/2024 Al composite was adopted to investigate the role of heterogeneous structures on the tensile mechanical responses at different strain rates and temperatures. Surprisingly, a simultaneously enhancement of strength and ductility was found in the composite under impact loading. To explore the underlying mechanisms responsible for the observed mechanical behavior, microstructural analyses were performed on the samples before and after deformation. Results reveal that the composite has obvious heterogeneous structure consist of particle-rich regions (regard as hard phase) and particle-lean regions (soft phase). The interaction between the two phases can affect the strain hardening behavior of the composites at different plastic deformation stages. Compared with the strain rate insensitive matrix, the particle-induced constraint makes the flow stress of the composite increases substantially with an increase in strain rate. Meanwhile, the tortuous crack propagation paths induced by the cooperation of hard and soft phases contribute to the improved ductility at high strain rates.

Acoustic characteristics of deformable metals

The stages of plastic flow in Fe-18 wt.% Cr-10 wt.% Ni structural steel were studied by performing non-destructive testing. In particular, the acoustic parameters of the steel were measured via uniaxial tensile tests. According to the changes in the velocity of propagation and attenuation of ultrasound in a wide temperature range, the stages of plastic deformation and destruction were clearly distinguished. Based on the results of this study, a non-destructive testing parameter characterizing the transition to the pre-destruction stage of the material was proposed.

Monotonic and cyclic load of pine wood under uniaxial compression: experiments and modeling

To analyze the behavior of wood under uniaxial monotonic deformation, we applied an analytical model to separately control of the pre-peak and post-peak branches of the load-displacement curve. Two criteria for the transition of wood into the plastic stage of deformation are considered. To analyze the behavior of wood under cyclic uniaxial deformation, we used the load-displacement curve obtained for monotonic deformation as an envelope. Equations for the plotted of load-displacement curves under cyclic loading with incomplete unloading are proposed. As an experimental and model illustration, the behavior of a pine specimen under cyclic uniaxial compression with incomplete unloading in the range from 8 to 15 kN is considered. Because the highest cyclic load exceeds the load at the point of transition of wood into the plastic stage, the deformation process has already after 54 cycles moved to the downward branch of the load-displacement curve for monotonic deformation. The simulation results are in agreement with experimental and literature data, which suggests the suitability of this method for the study of some wood-based composites.

Exploration of electrodeposited process of Ni-Co-Cr coating and investigation of Cr doping strengthening mechanism by indentation simulation

Ni-Co-Cr coating with excellent mechanical properties was successfully prepared by the electrodeposition process. Compared with the binary Ni-Co alloy coating, the hardness of the ternary coating is increased to 1247HV, an increase of 61%. The wear rate decreased by 40.7%. The exploration of the deposition process found that sodium formate and urea can effectively dissociate [Cr(H 2 O) 6 ] +3 complexes to realize the co-deposition of Cr 3+ . The strengthening mechanism of Cr doping was then investigated by molecular dynamics indentation simulation. It is found that grain refinement and solid solution strengthening contribute 20% (0.33 GPa) and 80% (1.33 GPa) to the hardness, respectively. The enhanced covalent character between Cr and solvent atoms improves the elastic modulus and promotes the dislocation nucleation, which originates from body-centered cubic-like (BCC-like) defect clusters. In the plastic deformation stage, Cr doping reduces the stacking fault energy of the alloy and makes the release of the plastic strain more uniform and stable. The denser 3D stacking fault network structures hinder the further propagation of dislocation loops and increase the dislocation reaction probability. More Lomer-Cottrell and Hirth Locks are generated and improve the strength of the coating. • An electrodeposited Ni-Co-Cr coating with excellent mechanical properties was developed. • The contribution of grain refinement and solid solution strengthening to hardness was quantitatively investigated. • The strengthening in the elastic stages comes from the enhanced covalent character between Cr and solvent atoms. • The strengthening in the plastic stages comes from the increase in the number of immobile dislocations.

Modeling of trabecular bone transition into plastic deformation stage under uniaxial compression

Modeling of trabecular bone transition into plastic deformation stage under uniaxial compression

KINETIC INSTABILITY IN AN ENSEMBLE OF DISLOCATIONS UNDER PLASTIC DEFORMATION OF METALS

Based on a system of evolutionary equations for an ensemble of edge dislocations, the problem associated with the development of instability of a homogeneous distribution of dislocations is considered. Despite the large number of theoretical studies on this topic, most of them are considered within the framework of reaction-diffusion phenomenological models, which contradicts the mathematical structure of the initial evolutionary equations, which are hyperbolic equations. For the initial system of hyperbolic equations, the case is considered when the process of plastic deformation develops along a given axis. The elastic interaction of dislocations is neglected in order to reveal the instability features of the homogeneous state of the system due to the local kinetics of dislocations. The stability analysis of the homogeneous state was carried out for a system of dislocations of two types differing in mobility. As a result of linearization of the initial system, a system of homogeneous linear algebraic equations is obtained. From the condition of the existence of its nontrivial solutions, a dispersion equation is found, using which, on the basis of the Raut – Hurwitz criterion, the range of parameters of the initial system at which kinetic instability is realized is established. In the instability region, an expression is obtained for the increment of the unstable mode, which determines the behavior of the system beyond the bifurcation point, and an expression for the characteristic scale of the inhomogeneous structure. The region of system parameters where the diffusion dynamics of the system is realized is found. It is shown that the Turing instability for the total dislocation density is realized in this parameter range. Within the framework of the initial system of hyperbolic equations, the dynamics of both total and excess dislocation densities is significant. It is the excess density that leads to disorientation in the formation of heterogeneous dislocation structures. This is a fundamental point, since at the advanced stage of plastic deformation, the dislocation structure is basically a system of disoriented cells and ragged sub-boundaries.

Acoustic Emission and Deep Learning for the Classification of the Mechanical Behavior of AlSi10Mg AM-SLM Specimens

In this research paper, the acoustic emission technique and a deep learning framework based on two types of pre-trained CNN models (alexNet and squeezeNet) and a new model are proposed to characterize and classify the mechanical behavior of AlSi10Mg components. Specimens are built in a Selective Laser Melting machine with different bed orientations along X, Y, Z, and 45 degrees. Tensile tests are performed, and AE signals are recorded from these tests. To characterize the elastic and plastic deformation stages, a time-frequency domain analysis was performed using CWT-based spectrograms. Three different categories of damage classification strategies were implemented, and CNN models were trained for each strategy. CNN models including AlexNet, SqueezeNet, and the new model were used. Several training modes were performed to determine the CNN model that can accurately classify AE data. Understanding the minimum set of AE signals needed to train the CNN while having 100% accuracy and understanding the parameters affecting the accuracy of a CNN and the training time for the efficient classification of AE signals are the main objectives of this work. The results obtained demonstrated that the new simplified CNN model proposed can accurately classify the AE signals in a short time compared to AlexNet and SqueezeNet.

Dynamic shear fracture behavior of rocks: insights from three-dimensional digital image correlation technique

The influence of the loading rate on the mode I fracture property of rocks has been extensively investigated through experiments. However, the rate-effect of the dynamic mode II fracture has rarely been reported. This study employed the modified split Hopkinson pressure bar (SHPB) system to conduct a series of dynamic mode II fracture tests under a wide range of loading rates using the modified short-core-in-compression (SCC) method. The complete dynamic fracture processes of rock were observed using the three-dimensional high-speed digital image correlation (3D HS DIC) technique. The results reveal that the dynamic mode II fracture toughness increases with the loading rate at a power relationship. By virtue of the evolution of the surface principal strain and displacement, the dynamic fracture and damage process and the deformation characteristic of the SCC specimen were analyzed in detail. The dynamic fracture processes are divided into the elastic stage, plastic deformation stage, unstable crack growth stage, and post fracture stage. The crack propagation path of the SCC specimen was obtained under impact loading. It was found that new cracks first initiate from two notch tips and then propagate toward the central area in the expected shear fracture band (ESFB); the path is nearly independent of the loading rate. Additionally, the macro–micro fracture surface morphology of the SCC specimen was measured using a 3D optical scanner as well as scanning electron microscope (SEM) respectively. It was found that the roughness and curvature in the fracture surface created by shearing generally decrease as the loading rate increases, in accordance with fractal theory. The SEM result indicates intergranular fracture occurs more frequently on the shear fracture surface under static loading, while transgranular fracture is more common on the shear fracture surface at high loading rates.

Designing Mg alloys with high strength and ductility by reducing the strength difference between the basal and non-basal slips

The additions of alloying elements can significantly improve the mechanical properties of magnesium (Mg) alloys, mainly due to the fact that their additions change the critical shear stresses (CRSS) for dislocation slips. In this work, experimental and computational methods were used to explore the mechanisms responsible for the roles of Sm element addition in affecting the mechanical properties of Mg. The results showed that the addition of Sm obviously improves the microstructure and mechanical properties. It promotes the formation of twins and beneficially activated the non-basal <a> slip at the initial stage of plastic deformation, resulting in a high ductility. The Visco-Plastic Self-Consistent (VPSC) and the two-beam diffraction results confirmed that the pyramidal <c + a> slip and prismatic <a> slip were activated during tensile testing. The quantitative analysis of slip traces verified that the volume of non-basal slips reached 35 % after Sm addition. The additions of Sm with solid solution increased the activities of pyramidal <c + a> dislocation during deformation, which was beneficial to accommodate the c-axis strain, and finally improved the room temperature ductility of Mg. First-principle calculations demonstrate that the solute Sm atoms would reduce the stacking fault energy for basal and prismatic slips.