Evolution Of Plastic Strain Research Articles

Effect of atomized hybrid nanofluid on machined subsurface layer of nickel alloy

Nickel-based alloys have encountered microstructural changes during machining, which forces machining industries to use cutting fluid excessively. The progress of sustainable methods for machining operations is necessary due to their adverse effect on the environment, operator health, and manufacturing economics. In the present research, the application of an Ultrasonically Atomized Hybrid Nanofluid (UAHNF), a near-dry lubrication technique, has been utilized for performing machining of nickel alloy, and its impact on the machined subsurface layer has been examined. The machining-affected subsurface has been analyzed for plastic strain and microstructural analysis using XRD and EBSD techniques. It has been found that the UAHNF method provides lesser plastic strain and better microstructural evolution than dry cutting. Moreover, it uses a little cutting fluid and is an environmentally benign and economical process.

Permeability properties of natural gas hydrate-bearing sediments considering dynamic stress coupling: A comprehensive experimental investigation

As a key exploitative parameter, permeability affects the extraction efficiency and program for natural gas hydrate (NGH). However, few studies have considered the effect of dynamic stress (continuously changing stress) on permeability, let alone permeability evolution during reservoir instability. Given the dynamic stress-seepage coupling process of actual NGH exploitation, this paper upgrades the triaxial apparatus to new equipment that can realize the unified tests of dynamic stress and water permeability. Subsequently, the seabed clay recovered from the South China Sea and quartz sand were used to remold fine-grained hydrate-bearing clayey-silty sediments (HBCSS). Finally, a series of stress-seepage coupling tests were carried out to reveal the permeability characteristics during sediment destabilization and to explain the coupling mechanism. The results show that seepage can reduce the strength and stiffness, and enhance the softening or weaken the hardening of the stress-strain curve. The dynamic stress-influenced permeability exhibits a non-monotonic evolution, which is essentially an expansion or contraction of seepage channels. Stress-strain relationship, volume deformation, and permeability are coupled and have good correspondence. The shear-seepage coupling process can be divided into four stages: initial compression, plastic strain development, plastic expansion and fracture development, plastic failure, and rapid expansion. This paper enriches the existing research from three aspects: permeability properties affected by dynamic stress, mechanical behavior affected by seepage, and stress-seepage coupling mechanism. It has played a significant reference for the safe and efficient exploitation of fine-grained NGH.

Load bearing behavior of concrete encased composite columns with high‐performance materials

AbstractThe use of steel‐concrete composite columns, such as concrete‐encased steel columns have increased in the construction of high‐rise buildings and longspan structures. In particular, the use of high‐performance materials in composite columns is expanding, enabling optimisation of the column dimensions, theultimate load capacity, improve economic design and resource efficiency. However, high‐strength concrete shows a more brittle behaviour compared to normal concrete compression strength classes that may limit the development of plastic strains in the encased steel section. In addition concrete spalling is a serious issue that may become decisive for the determination of the ultimate resistance. Therefore, to extend the design methods for composite columns using high‐performance materials, the ideitified knowledge gaps need to be filled. For this reason an experimental campaign has been performed consisting of nine concrete‐encased steel composite column test specimens; five specimens with circular and four with rectangular cross‐sections. The testing campaign was carried out in a 20 MN servo‐hydraulic universal testing machine at the Ruhr‐Universität Bochum.Abaqus® FEM software has been used to develop numerical models able to reproduce the behavior of the tested specimens. The comparisons of the experimental and numerical results show a very good correlation. The numerical analysis enable more detailed understanding of the tests results and behavior of the columns.

Investigation of dynamic characteristics and cumulative plastic strain prediction model of clay-fouled round gravel under cyclic subway loading

This study conducts a series of saturated undrained dynamic triaxial tests using the DYNTTS large triaxial cyclic test system to investigate the dynamic characteristics and cumulative plastic strain evolution of clay-fouled round gravel under cyclic subway loading. The effects of clay fouling level (VCI), confining pressure, and dynamic stress amplitude on round gravel's dynamic properties and cumulative plastic strain are analyzed. In addition, a cumulative plastic strain prediction model is proposed by combining the time hardening theory with clay fouling level, confining pressure, and dynamic stress amplitude. The study's results indicated that with the increase in VCI, the accumulated plastic strain of the specimen tends to increase and then decrease, and the fouled clay changes from lubrication to filling, with the cut-off VCI generally between 20% and 30%. VCI significantly affects the cumulative plastic strain of specimens with small confining pressures and slightly impacts specimens with large confining pressures. With the increase in VCI, the specimen's elastic strain tends to increase and then decrease, while its resilient modulus shows the opposite trend, but both trends are more pronounced with the decrease in the confining pressure and the increase in the dynamic stress amplitude. Generally, the research results can be used as a reference for designing subway roadbed structures and their post-construction settlement predictions.

Controlled radial deformation of AZ31B magnesium alloy bar during cyclic rotating-bending process

Extruded magnesium alloy bars are susceptible to uneven microstructure after the heat treatment process, which is characterised by coarse grain size in the surface, thus affecting the further processing properties of the overall bars. In this paper a new cyclic rotating-bending (CRB) process is proposed to improve the microstructure by achieving a gradient plastic deformation of the bars from the surface to the core. The paper is based on finite element simulations to qualitatively and quantitatively investigate the plastic strain distribution and evolution in the middle radial section of the bar under different CRB processes. In addition, the influence on the microstructure distribution under the processes is also further clarified. The results show that during CRB deformation, the plastic strain in the middle radial section of AZ31B magnesium alloy bars gradually decreases from the surface to the core radially, and the plastic strain in the surface is significantly positively correlated with the bending angle and the period of rotation. When the bending angle reaches 150°–170° and the period of rotation reaches 30 r, the section of the bars is fully plastic deformed. Simultaneously, compared to the initial microstructure, significant twinning deformation occurs in the surface of the bars after different CRB processes, which further promotes grain refinement.

Experimental study on thermal effect of deep-sea cohesive sediment under monotonic and cyclic loads

In this study, a series of monotonic and cyclic triaxial tests were conducted to investigate the thermal effect on the mechanical behaviors of sediment from the South China Sea (SCS). The excess pore water pressure (including temperature-induced and shear-induced), shear strength, accumulative plastic strain, and stiffness degradation index were measured and analyzed. Two modified empirical models were proposed to predict the evolution of the accumulative plastic strain and stiffness degradation index with the cyclic number (N) at different temperatures. A unified index of normalized cyclic shear ratio (CSR‾) taking the cyclic stress and temperature into account was proved to be suitable to describe the model parameters . Compared with the constant temperature condition (4 °C), both the monotonic and cyclic shear strength of the samples decreased as the temperature increased. Moreover, a temperature-independent correlation between cyclic and monotonic shear strengths was derived. A more markedly cyclic softening behavior could be observed under high-temperature conditions in which the undrained shear behavior showeds somewhat quasi-overconsolidated (QOC) characteristics.

Extension of Process Limits in High‐Strength Aluminum Forming by Local Contact Heating

The aluminum alloy EN AW‐7075 T6 is used in the automotive sector for its favorable strength‐to‐weight ratio. However, the limited cold formability is currently addressed by energy‐ and time‐consuming temperature‐assisted processes. In order to limit the effort to critical forming areas only, the state‐of‐the‐art shows promising results for increasing the blank temperature in the range of warm forming. The design of new processes in an industrial context requires appropriate numerical simulation with inherent complexity due to time‐ and temperature‐dependent effects. Herein, the potential of a newly developed tool setup and process chain with integrated local contact heating of the EN AW‐7075 T6 blank is investigated on the basis of a curved hat profile. A thermomechanically coupled FE model of the process is developed and validated. The influence of the local heating layout is analyzed in experimental forming tests and a corresponding process window is derived. The influence of local heating on the occurring failure mechanisms is discussed based on simulation results. The equivalent plastic strain evolution is successfully used to evaluate the local heating dependent failure behavior. A significant increase in the overall formability of the part is achieved by the proposed process chain.

Development of elastic and plastic strains in concrete damaged by alkali–silica reaction during various compression loading tests

In this study, the fracture process of concrete in the presence of alkali silica reaction (ASR) cracks under compressive stress was evaluated. Concrete specimens damaged by ASR expansion at various expansion levels were tested under monotonic, stepwise cyclic, and sustained compressive loadings to evaluate the change in mechanical properties. The evolution of expansion cracks under these loading conditions was assessed using digital image correlation (DIC). In stepwise cyclic and sustained loadings, the elastic and plastic strains generated under different stress levels were separately investigated to elucidate the stress resistance mechanism and the impact of expansion cracks on mechanical properties. An increase in the expansion level was found to remarkably decrease the elastic modulus of concrete but only slightly reduce the compressive strength. Using DIC, expansion cracks were observed with the accumulation of principal strain development even at low stress level. Elastic strains linearly developed in specimens with and without ASR damage, whereas plastic strains increased non-linearly with expansion. Despite the high plastic strains observed before reaching the peak load under stepwise cyclic and sustained loadings, the compressive strength and elastic modulus of concrete remained unchanged regardless of the loading pattern. Based on this evidence, the stress resistance mechanism in the cross-section of ASR-damaged concrete was explained.

Damage to fracture in offshore engineering materials under several stress states: Blowout preventer valve application

This contribution proposes a study of the mechanical behavior from damage to fracture of normalized and annealed samples of AISI 4340 steel. Besides, a numerical study in a security device of offshore industry is conducted. Experimental tests were initially performed on smooth and notched cylindrical specimens subjected to monotonic tension as well as on rectangular specimens subjected to pure shear and a combination of shear and tension loads. Such experimental tests were selected to observe different combinations of the stress triaxiality and the normalized third invariant concerning the stress state. The two types of heat treatments were considered to achieve different levels of ductility and verify their influence on the fracture mode of the alloy. The approach based on the second invariant of the deviatoric stress tensor ( J 2), with nonlinear isotropic hardening, is assumed to describe the mechanical behavior of the material in axisymmetric and three-dimensional numerical simulations. Finite elements simulations were carried out to analyze the performance of a simplified version of a mechanism of a blow-out preventer-BOP valve. The experimental and numerical curves of force versus displacement were compared, highlighting the effect of the stress triaxiality, the third invariant and calibration condition on the behavior of the material. The evolution of the equivalent plastic strain was plotted, and the fracture onset compared with its maximum value. The results show that the AISI 4340 alloy is highly dependent on the stress triaxiality and third invariant. Moreover, it can be concluded that different calibration conditions for the isotropic hardening curve can result in different levels of forces for the correct performance of the BOP in a critical situation.

A nonlinear constitutive model for whole stress–strain behaviors of compressed rocks incorporating crack closure effect

AbstractThis paper is devoted to establishing a nonlinear constitutive model incorporating crack closure effect for capturing the whole process of rock deformation and failure behavior under compression. The model formulated in the framework of irreversible thermodynamics is based on the additive decomposition of the total strain into crack closure strain, elastic, and plastic parts. New specific criteria are proposed for the description of crack closure strain and plastic strain evolution. Notably, analytical solutions of the model under conventional triaxial compression loading conditions are derived. For application, a robust semi‐implicit return mapping (SRM) algorithm with a crack closure strain correction and a plastic strain correction is developed. The analytical solutions, herein, are subtly used to calibrate model parameters and as reference results for assessing the accuracy and robustness of the SRM algorithm. Validation of the model conducted by comparing with experimental data from the literature shows that the model can properly describe the nonlinear mechanical behaviors of rocks, including crack closure effect, strain hardening/softening, peak and residual strength, as well as volumetric compaction/dilation transition. In addition, a nonlocal formulation is introduced as an extension of the model to remedy the mesh dependency problem.

Numerical Simulation of the Taylor Impact Test for Laser Powder Bed Fusion Parts Based on Microstructural Internal State Variables

The response of any engineering design components to stresses should be predictable, While the response of a material to complex loading, such as high strain rates experienced during service, is difficult to represent with simple tests, the Taylor impact test is one of a number of tests devised for high strain rate complex loading. To expedite the acceptance of LPBF Ti6Al4V (ELI) for use in demanding structural applications, there is a need to develop numerical models based on the internal microstructural state variables to predict the performance of the alloy over a wide range of high strain rates using such complex tests. This paper documents the numerical simulation of Taylor impact tests for direct metal laser-sintered and post-processed Ti6Al4V (ELI—Extra Low Interstitial) alloy. A microstructural variable-based constitutive model was used to predict the mechanical properties (stresses and evolution of plastic strains) of the material. The corresponding material parameters of the model were based on the specific microstructure obtained upon post-process heat treatment. The model was first implemented as a user material subroutine in the explicit finite element program ABAQUS using the VUHARD subroutine. Subsequently, the symmetrical Taylor impact tests of Laser Powder Bed Fusion (LPBF) Ti6Al4V (ELI) parts were numerically simulated using the VUHARD subroutine at different impact velocities. The equivalent von Mises stress and plastic strain obtained from numerical simulations were compared with the analytical solutions based on the strain rates obtained. It was shown that the instantaneous and average absolute errors between the numerical and analytical values of the model were generally less than 5%. The mushroom end, commonly observed in a Taylor test specimen, was also seen in the numerical model.

Stress-bearing Mechanism of Concrete Damaged by Delayed Ettringite Formation under Compressive Stress with Various Loading Patterns

The involvement of expansion cracks in reducing compressive properties was experimentally evaluated. Concrete specimens deteriorated by delayed ettringite formation were subjected to three loading patterns (monotonic, stepwise cyclic and sustained loadings) and digital image correlation was performed to observe the behavior of expansion cracks during compressive loading. As a result, while significantly large plastic deformation was generated in the pre-peak, the reduction in compressive properties was hardly influenced by the loading patterns. The elastic strain, obtained from the loading hysteresis, increased linearly until a maximum load was reached. Consequently, two possible stress-bearing mechanism of concrete damaged by delayed ettringite formation under compressive stress was proposed to explain the development of elastic and plastic strains and the reduction in the compressive property.

Crack tip enhanced phase-field model for crack evolution in crystalline Ti6Al from concurrent crystal plasticity FE-molecular dynamics simulations

Dislocations nucleating from a crack-tip contribute to the plasticity evolution in the vicinity of crack, as well as to the crack propagation process. This paper systematically develops a method for enhancing the defect energy, accounting for crack-tip nucleated dislocations in the Helmholtz free energy density functional, associated with the phase-field formulation of crack evolution, in a coupled crystal plasticity finite element phase field (CPFE-PF) model. A concurrent crystal plasticity FE- hyper-dynamics accelerated molecular dynamics (CPFE-MD) model is created. The model that identifies, transfers, and propagates dislocations from the atomistic to continuum domain, is used to generate a dataset of crack tip dislocation density evolution along with different state variables. The paper focuses on crack tip plasticity mechanisms for the crystalline alloy Ti6Al. Using a Bayesian inference approach the critical state variables that affect the evolution of crack tip nucleated dislocation density are inferred. A functional form of the evolution of dislocation density in terms of the state variables is derived by employing a genetic programming based symbolic regression (GPSR) approach. The contribution of nucleated dislocation densities to effective plastic strain evolution at the crack tip is validated using the CPFE-MD model simulations. A comparison of the crack path and other state variables in the vicinity of the crack with and without contributions from the nucleated dislocations shows the importance of this augmentation on crack evolution.

Constitutive modeling of a laumontite-rich tight rock and the application to poromechanical analysis of deeply drilled wells

Geological formations containing laumontite-rich rock are usually treated as problematic for geo-energy production projects because the presence of laumontite mineral can promote complex mechanical behaviors. Previous laboratory results indicate that rock formations with a higher laumontite content display severe stress sensitivity in poromechanical responses. With an increase in confining pressure, there is a transition from dilation to compression regime and the resulting localization styles range from shear dilation to compaction bands. In this study, we conduct finite element modeling of constitutive behaviors of rocks retrieved from the tight glutenite reservoir formation using a thermodynamic-consistent plasticity model. The shear dilation to compaction transition is well characterized. Poromechanical analysis is also conducted to analyze the plastic zone development around a borehole drilled in an over pressured reservoir. The simulated stress-paths of key points around the borehole are used to demonstrate the plastic strain development processes. The impact of in-situ stress on the wellbore stability is highlighted, and a comparison with the results from using the traditional plastic constitutive model is conducted.

Cyclic deformation and dynamically induced short-range ordering in small particles reinforced Al composite

Cyclic deformation of an Al-Cu-Mg composite strengthened by small non-shareable particles was firstly investigated through strain controlled low-cycle fatigue (LCF) tests. In comparison with the reported Al-Cu-Mg composites, the in-situ TiB2/Al-Cu-Mg composite presented higher fatigue life at all the tested strain amplitudes. The composite exhibited cyclic hardening response, which was contributed by non-shareable particles, dislocations and dynamically deformation-induced short-range orderings in the Al matrix. Considering plastic strain evolution, the cyclic hardening model was established and well fitted with experimental results. When cyclic loading was controlled at higher strain amplitudes, the serrated flow occurred on stress-strain hysteresis loops and then it disappeared after the first few cycles related to the occurrence of dynamic strain aging. Onset of the stress serration on stress-strain hysteresis loop was greatly dependent on the strain amplitude rather than cumulative plastic strain. Finally, the correlative cyclic hardening model of small particles reinforced Al composite was analytically discussed based on microstructure evolution and cyclic stress-strain responses.

Study on Damage Evolution of Tunnel Lining under Strike-Slip Fault Movements

For the purpose of investigating the damage mechanism and failure characteristics of tunnel lining under the strike-slip fault movements, a three-dimensional elastoplastic finite element model was established in the present study including a railway tunnel across an active strike-slip fault. With the assistance of this numerical model, the tensile and compressive damage, plastic strain development process, and shear failure of the secondary lining at different fault plane positions were analyzed, and further, the damage laws of the secondary lining at different fault displacement and tunnel-fault intersection angles were summarized. The simulation results showed that when the maximum imposed fault displacement is 30 cm, the most unfavorable fault plane appears at the junction between the moving block and the fracture zone, and serious tensile cracks and shear failures occur on each fault rupture plane. Besides, the maximum plastic strain and compressive damage are distributed in the vault and invert. In addition, with the increase of the fault displacement, the axial strain of the secondary lining increases as well, of which the main part shows the tensile strain. Interestingly, with the decrease of the crossing angle, the axial strain gradually changes from the tensile strain to the compressive strain, which is consistent with the direction of the fault angle. Furthermore, the tensile and compressive damage of the secondary lining increases with the increase of movement distance. At the same time, the tensile damage along the tunnel ring develops, and the compressive damage propagates mainly in tunnel vault and invert. With the decrease of the crossing angle, the degree of the tensile damage in the lining reduces, as well as its range. The compressive damage area of the lining develops from the vault and inverted arch to the tunnel wall on both sides, which has a trend of penetrating the side walls on both sides. The distribution area and maximum value of the overall lining damage indices in tensile (OLDT) and overall lining damage indices in compressive (OLDC) increase with the increase of fault movement distance. With the decrease of the crossing angle, the distribution range and maximum value of OLDT decrease, and the distribution range of OLDC increases, while the maximum value basically remains the same. The research results of the present work can provide reference for the antidislocation design of railway tunnels crossing strike-slip faults.

ОСОБЕННОСТИ ДЕФОРМИРОВАНИЯ СЕЧЕНИЙ ИЗГИБАЕМЫХ ЖЕЛЕЗОБЕТОННЫХ ЭЛЕМЕНТОВ ПРИ ДЕЙСТВИИ НАГРУЗОК БОЛЬШОЙ ИНТЕНСИВНОСТИ

Many buildings are exposed to special effects of high intensity. When calculating reinforced concrete constructions both for special emergency loads and for seismic loads, the development of plastic strains in reinforcement is supposed just at the design stage. The experimental investigation of plastic deformations influence of the stress-strain state of normal sections of bending reinforced concrete elements has been made. According to the results of experiments, it has been found, that the hypothesis of plane sections is not observed in all cases. The approximation of deformations graph along the cross-section height by the bilinear function, variable during the load process, i.e hypothesis of bilinear sections, is more accurate. The dependence of coefficient A of the hypothesis of bilinear sections on the coefficient of plasticity for reinforcement deformations has been determined based on the results of numerical calculations of beams with symmetric reinforcement and beams without compressed reinforcement in a finite element software package Abaqus. The comparison of obtained numerical results with experimental data has been made, which has shown their satisfactory convergence. The maximum deviation does not exceed 13 %. The assessment of influence of coefficient of hypothesis of bilinear sections on the limit value of coefficient of plasticity for curvature, corresponding to the beginning of the destruction of compressed area of concrete is given.

The limit value of plasticity coefficient of reinforcement under seismic loads

Plastic deformations develop in bending reinforced elements under seismic loads. In calculation of reinforced concrete constructions by spectral method they are taken into account only at the stage of seismic loads determination by introduction of reducing coefficients. But the development of plastic strains in bending elements leads to appearance of cracks, which don’t close after changing the internal forces sign. Due to it the work height of a section decreases, and the strength of elements reduces during subsequent loading cycles. The experimental evaluation of the influence of plastic strains on the stress-strain state of beams under low-cycle alternating effects of high intensity has been carried out. The first group of beams, considered as reference, was loaded with a monotonously increasing load until complete destruction. The other groups of beams were loaded by the monotonically increasing load up to the specified value of plastic deformations was reached. Then they were unloaded and loaded by the force of the opposite sign up to destruction. The stress-strain state of normal sections at all stages of loading was studied. It has been discovered that when changing the sign of load the through cracks are formed which close later if the plastic deformations of reinforcement are not large. The limit value of plasticity coefficient in the first semi cycle of loading preventing the formation of non-closing through cracks when changing the sign of internal forces has been determined.

Composite Beams with Web Openings Employing Alternative Layout of Shear Connectors

It is generally accepted that bearing systems consisting of composite elements, combining steel and concrete, are highly efficient. Nevertheless, the current economic situation generates continuous demand for more effective solutions. Hence, by the finite element model, this paper investigates an impact of alternative layout of shear connectors adopting a combination of continuous and discontinuous distribution. On the basis of numerical investigation, the study seeks an optimal layout of shear studs for simply supported composite beams under uniform static loading utilizing the same number of connectors for each span. The paper is divided into three parts. At first, investigated specimens are designed against European Standards and limitations according to the literature. This is followed by a parametric study using finite element models examining shear flow magnitudes at the steel-concrete interface and plastic strain evolution at the level of the bottom flange. Afterwards, the study reveals relations regarding the modified parameters and overall structural performance of composite girders. Despite that, the paper is unable to encompass the entire matter of issue but serves as a preliminary attempt to define a novel structural connection arrangement presenting more unit utilization of composite beams' resistance.

AN ANISOTROPIC EXTENSION FOR A THERMOVISCOPLASTIC GTN DUCTILE DAMAGE MODEL

In this work, a thermoviscoplastic GTN ductile damage model is extended to account for anisotropic plastic behavior. The novel feature of the proposed model is the enrichment of a GTN model, with the Hill's yield criterion accounting for hardening, softening, strain-rate effect, plastic work heating, thermal diffusion, and void shearing mechanism. The anisotropy is introduced by changing the equivalent stress definition, which provides greater flexibility to the model. The resultant modifications in the yield criterion and in the damage evolution equations are described. Two numerical problems involving plates subjected to tension loading that leads to ductile damage and eventual fracture are presented and discussed in order to validate the present approach. A mixed finite element formulation is adopted together with the fully damaged zone technique to represent the ductile fracture. Results obtained through computational simulations highlight the effect of the anisotropic parameters on mechanical behavior in terms of the global stress-strain curve and the local evolution of plastic strains, porosity and temperature. We conclude that the proposed anisotropic model is simple to implement yet provides more flexibility in capturing complex ductile fracture processes.