Accumulation Of Plastic Deformation Research Articles

Effect of primary carbides on rolling contact fatigue behaviors of M50 bearing steel

It is perceived that the fatigue performance of high-strength steel is preponderantly affected by internal defects. Inclusions may be efficaciously controlled and primary carbides have been proved to be the principal type of defects in M50 bearing steels. Primary carbides in service-status M50 were systematically characterized and the rolling contact fatigue (RCF) properties were evaluated under different friction conditions in this work. An innovative method of shakedown analysis was proposed for the first time, extending the shakedown analysis of materials as an integral based on traditional shakedown theory to that of weaker internal defects. By the response of primary carbides, the RCF process was separated into a plastic instability period and an elastic shakedown period. The accumulation of plastic deformation around carbides before shakedown was proposed as the RCF damage parameter, and the influence of primary carbides was studied based on the damage criterion. The competition between surface and internal fatigue initiation was revealed via different friction coefficients. A predictive model for RCF initiation was established, which could predict the critical sizes for disparate types of primary carbides not deleterious to RCF performance, and the critical sizes under typical industrial service condition were reckoned.

Cumulative buckling deformation of stiffened panel under cyclic loading

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

Subgrains of γ′ phase in a single-crystal superalloy induced by ultrahigh temperature creep

A new type of γ′ phase subgrain has been found in a single-crystal superalloy after exposure to 360 hours of creep at 1200 °C and 60 MPa. This signifies the capacity of dynamic recrystallization (DRX) of γ′ phase, overturning the widely accepted idea that it is incapable of undergoing DRX. The remarkable plasticity of the γ′-precipitate, activated at ultrahigh temperature, is likely due to the slip/rotation and cracking of the γ′-subgrains. Microstructural evidence of the subgrain boundary originating from γ′-shearing dislocations and their associated walls was obtained, which explains the subgrain formation dependence on plastic deformation accumulation. These findings help us understand the γ′-deformation mechanism and guide to improve creep resistance.

The effects of normal load on the dry-sliding, micro-scratch and nanoindentation behaviors of the Zr-based bulk glassy alloys

In this work, the macro-sliding, micro-scratch and nanoindentation behaviors of the Zr55Cu30Al10Ni5 bulk glassy alloy (BGA) were investigated. It was found that the frictional coefficient and wear rate of the BGA in the dry-sliding test increase with the increasing normal load, and the wear mechanism includes abrasive and oxidative wear. The micro-scratch test results show that the plastic deformation of the alloy and the density of shear bands at the edge of scratch scar increase with the increase in normal load. The friction mechanism between the indenter and the BGA shifts from stick-slip friction to ploughing friction and to micro-cutting as the applied normal load elevates. The nanoindentation result reveals that increasing the normal load leads to the plastic deformation accumulation, and the decreasing elastic modulus and hardness, which are related to the increase in the free volume and shear bands during the deformation process.

Fatigue failure of amorphous alloys under cyclic shear deformation

The accumulation of plastic deformation and flow localization in amorphous alloys under periodic shear are investigated using molecular dynamics simulations. We study a well-annealed binary mixture of one million atoms subjected to oscillatory shear deformation with strain amplitudes slightly above a critical value. We find that upon approaching a critical strain amplitude from above, the number of shear cycles until the yielding transition is well described by a power-law function. Remarkably, the potential energy at the end of each cycle as a function of the normalized number of cycles is nearly independent of the strain amplitude, which allows for estimation of the fatigue lifetime at a given strain amplitude. The analysis of nonaffine displacements of atoms elucidates the process of strain localization, including irreversible rearrangements of small clusters until the formation of a system-spanning shear band.

ANALYSIS OF THE STATE OF STRESS AND DEFORMATION OF THE MATERIAL OF THE BILLET WHEN PLANTING BY THE RESOURCE-SAVING METHOD OF STAMPING BY ROLLING

The processes of planting blanks by the rolling stamping method allow for the efficient production of a wide range of high-quality products, but the possibility of material destruction during deformation prevents the expansion of their technological capabilities. Further development of the processes is possible through the development of new technological schemes based on the analysis of deformation kinematics and the appointment of favorable technological parameters, taking into account their influence on the stress-strain state and deformability of the material of the workpieces. In order to widely use the method of assessing the deformability of workpieces, a reliable technique is needed, which provides for the presence of a mathematical model of the trajectory of deformation of material particles in the coordinates "stress state indicator - accumulated plastic deformation before failure." The work uses an approach to finding an analytical representation of the deformation trajectory based on the construction of a differential equation between the components of plastic deformation increments, followed by the solution of this equation and the identification of its parameters based on experimental data. According to the results of the research, the deformation trajectories of the material particles of the peripheral surface of the flange when planting by rolling stamping method were schematically constructed in the coordinates "intensity of deformations - stress state indicator". Based on the built model, damage accumulation can be simulated by changing the values of the model parameters for different materials and deformation paths. An analytical representation of the deformation trajectory in a parametric form was obtained. The advantages of representing the deformation trajectory in the form of parametric equations are the convenience of analyzing these trajectories. The advantage of the model of the trajectory of deformation of material particles in the coordinates "stress state indicator - plastic strain accumulated before failure" is the absence of a material constant in the analytical expression for the stress state, and the consequence is additional convenience of analyzing ratios and selecting the value of the material constant based on experimental data.

Formation mechanism and microstructure evolution of light etched region during rolling contact fatigue in M50 steel

The microstructure evolution and formation mechanism of lighting etched region (LER) during rolling contact fatigue (RCF) in M50 steel are systematically investigated. The results indicates that LER is a kind of microstructure degeneration due to the accumulation of plastic strain. Finite element simulation indicates that, under 5 GPa contact pressure, the equivalent stress at the subsurface exceeds the yield strength of M50 steel and, thus, induces plastic deformation. The dislocation density and fraction of grain boundaries increase in LER and thus the grain size is refined and the micro-hardness increases. The microstructure evolution in LER during RCF is divided into four stages. Stage 1: dislocations and GBs increase significantly, while the micro-hardness remains almost unchanged with the increase of RCF cycles; Stage 2: GBs and micro-hardness increase gradually with the increase of RCF cycles; Stage 3: LAGBs evolve to HAGBs and thus the total length of LAGBs decreases and the total length of HAGBs increases. Dislocation density and micro-hardness both increase with the increase of RCF cycles; Stage 4: dislocations and GBs continue to increase with the increase of RCF cycles due to accumulation of plastic deformation, and the micro-hardness also increases.

Accumulated plastic deformation behavior of granite residual soil under cyclic loading considering the influence of initial unloading

In engineering practice, excavation near existing transport infrastructures is inevitable due to space limitations. The effect of excavation-induced unloading on soil response to subsequent traffic loads is well worth studying, yet previous studies mainly focused on the effect of initial loading (rather than unloading). Undrained cyclic triaxial tests on undisturbed granite residual soil were conducted to study the effect of initial unloading on accumulated plastic deformation behavior. The unloading effect was achieved by reducing cell pressure before cyclic loading. The influences of amplitude of cyclic stress (σd) and degree of initial unloading (Di) on accumulated plastic deformation behavior were investigated. The results indicate that accumulating mode of axial plastic strain changes gradually from “stabilization” to “failure” as σd increases. Initial unloading has a significant promoting effect on the development of accumulated axial strain, and the promoting effect heightens with the increase in Di. Besides, a comparative study was conducted by employing another two stress paths, and it is found that initial shearing stress path makes a considerable difference to accumulated plastic deformation behavior. Under the same Di and σd, soil specimen experiencing initial unloading shows a poorer capacity for resisting cyclic deformation, compared to the other initial shearing stress paths. An empirical model was developed for predicting accumulated axial strain under the condition that initial unloading occurs prior to cyclic loading, considering the effects of amplitude of cyclic stress and degree of initial unloading.

Investigation of slope deterioration mechanism under freeze–thaw cycles: centrifuge modelling

Although deterioration of soil due to freeze–thaw cycles has been investigated based on element tests and small-scale laboratory model tests, there still lacks fundamental understanding of slope response under cyclic freeze–thaw action. In this study, a novel in-flight freeze–thaw system was developed to investigate the thermal, hydraulic and mechanical response of a typical unsaturated slope under seven freeze–thaw cycles in the centrifuge. Temperature, suction and deformation were measured during the test. The test results show that the suction at the freezing front can be up to about 1 MPa. The comparison between measured and theoretically predicted suction elucidates that using the temperature-dependent Clausius–Clapeyron equation is inadequate to model suction evolution precisely when considering water migration. The subsequent thawing reduces the suction to 9 kPa, which is about 60% lower than during the initial unfrozen state. The suction destruction softens the surface soil layer, leading to downslope movement during thawing. During the seasonal downward ratcheting movement, the lower elevation of the slope can flatten from 59° to 50°. The repeated freeze–thaw action generates significant fractures at the soil surface while deep cracks can form due to tensile rupture. Freeze–thaw-induced seasonal variation of suction softens the post-thawed slope, leading to accumulated plastic deformation and shallow slope failure.

Evaluation of Plastic Deformation Considering the Phase-Mismatching Phenomenon of Nonlinear Lamb Wave Mixing

Nonlinear guided elastic waves have attracted extensive attention owing to their high sensitivity to microstructural changes. However, based on the widely used second harmonics, third harmonics and static components, it is still difficult to locate the micro-defects. Perhaps the nonlinear mixing of guided waves can solve these problems since their modes, frequencies and propagation direction can be flexibly selected. Note that the phenomena of phase mismatching usually occur due to the lack of precise acoustic properties for the measured samples, and they may affect the energy transmission from the fundamental waves to second-order harmonics as well as reduce the sensitivity to micro-damage. Therefore, these phenomena are systematically investigated to more accurately assessing the microstructural changes. It is theoretically, numerically, and experimentally found that the cumulative effect of difference- or sum-frequency components will be broken by the phase mismatching, accompanied by the appearance of the beat effect. Meanwhile, their spatial periodicity is inversely proportional to the wavenumber difference between fundamental waves and difference- or sum-frequency components. The sensitivity to micro-damage is compared between two typical mode triplets that approximately and exactly meet the resonance conditions, and the better one is utilized for assessing the accumulated plastic deformations in the thin plates.

Numerical Investigation of Presliding in Viscoplastic Spherical Contacts

AbstractPresliding contacts play an important role in stiffness, damping, and thus dynamic response of assembled structures. Load-dependent nonlinearities in presliding contacts still hinder predictive modeling. Classical models apply only to smooth elastic contacts and a small subset of materials. Recently, the authors tested high density polyethylene (HDPE) inside a scanning electron microscope (SEM) and observed that nonlinearity trends in tangential stiffness and damping deviate from the predictions of the classical models. This discrepancy was attributed to HDPE’s nonlinear viscoplastic response. The aim of this study is to model aforementioned experiments numerically and investigate the influence of nonlinear material response on the presliding response of spherical contacts. A finite element model of a rigid spherical indenter pressed and sheared on a nonlinear viscoplastic half-space is constructed. The indenter geometry and boundary conditions are set in accordance with the experiments, and the constitutive model is tuned to the measured indentation responses. The tuned model delivers a shear response in agreement with the experiments. Accumulated plastic deformations are also found to correlate well with the wear profiles. The model further reveals that nonlinear viscoplasticity dominates tangential stiffness and dissipation at high normal preloads. Our results confirm further that nonlinear material response contributes significantly to the load-dependent nonlinearities in viscoplastic presliding contacts.

Implementation of the Lemaitre damage model with kinematic hardening in the ANSYS finite element complex

Currently, one of the most popular in fracture mechanics is taking into account damage and its effect on the stress-strain state of structural elements. In this work, the Lemaitre damage model was integrated taking, into account the combined hardening law, which combines the Armstrong - Frederick kinematic hardening law and the Voce isotropic hardening law, into the ANSYS finite element software. The model is implemented in the form of a dynamically linked library of user material for three-dimensional objects, which is tested on a cylindrical specimen with an external annular notch, both in an axisymmetric setting and in a three-dimensional one. The article presents model representations of the above-listed standard systems. This work demonstrates only one of the two stages of verification of the created program - comparison of damage fields under monotonic loading with data known in the literature - and doesn’t take into account the verification of cycle-by-cycle kinetics of plastic deformation accumulation with experimental data for low-cycle fatigue. The result of verification, consistent with similar experiments known in the literature, is confirmed in accordance with similar experiments. In addition, an analogy was found using the TSL law of the cohesive fracture mechanics approach. Despite the fact that two different types of constitutive equations are used in the cohesive model and Lemaitre, the physical meaning of these equations consists in one thing - visualization and identification of mechanisms and coordinates of damage.

Quasi in-situ observations of microstructure evolution during low cycle fatigue in carbide-free bainitic steel

Microstructure evolution during fatigue is crucial to the initiation and propagation of fatigue cracks in carbide-free bainitic steels. Utilizing quasi in-situ electron back-scattered diffraction method, the morphology of retained austenite and the accumulated plastic deformation within the bainitic laths at different fatigue stages were characterized. The observations disclosed that the retained austenite in various locations has different mechanical stability, and the dislocations in the bainitic laths will evolve to low-energy substructures continually throughout the fatigue process. Furthermore, combined with hysteresis curves analysis, a microstructural evolution model that comprehensively considered the retained austenite stability and the evolution of dislocations was proposed.

A novel elastic–plastic damage model for rock materials considering micro-structural degradation due to cyclic fatigue

In this work, a novel constitutive model is developed to describe the mechanical behavior of rock like materials subjected to cyclic loading. The fatigue damage is physically related to the progressive micro-structural degradation which is described by a convolutional law. The number of cycles is considered as an equivalent time in the description of creep deformation. The fatigue damage evolution is further coupled with the plastic deformation. The evolution of fatigue damage is driven by the applied stress amplitude and number of cycles through the coupling with the accumulated plastic deformation. The proposed model is able to describe the mechanical responses of materials under both monotonic and cyclic loading conditions. The mechanical responses of two representative rocks, argillaceous quartz siltstone and salt rock, are investigated by the proposed model. All main behavior features of these materials are well captured by the proposed model.

Gradient damage model for ductile fracture introducing degradation of damage hardening modulus: implementation and experimental investigations

This study presents a gradient damage model for ductile fracture, in which the damage hardening modulus is degraded by the accumulation of plastic deformation and the volume expansion caused by negative hydrostatic pressure. The proposed model fulfills the thermodynamic requirements, and the governing equations are derived from energy minimization principles. Two parameter studies are carried out to confirm the basic performance of the proposed model, in which some typical ductile fracture responses are demonstrated by changing parameters for degrading the damage hardening modulus. Also, a series of numerical experiments are presented to reveal the ability of the proposed model to successfully simulate the fracture tests of advanced high strength steel sheets with different tensile strengths. It is indeed confirmed by the close agreement with experimental results that the proposed model is capable of realizing the breaking elongation, the transitional behavior from unstable to stable crack propagations, and the corresponding load–displacement curves. Also, the model successfully reproduces and predicts the crack initiation positions in notched specimens with different notch radii.

Evaluation of elastic-plastic deformation in HSS work roll under coupling of residual stress thermal stress and contact stress during hot rolling

Fatigue damage such as crack and spalling on the work roll surface in hot rolling is caused by comprehensive action of residual stress, thermal stress and contact stress. In this work, a HSS work roll was studied based on the actual hot rolling condition, and high temperature performances of the work roll materials are considered. Finite element method was used to obtain the residual stress distribution of the work roll after quenching and tempering. Then, the coupling mechanism of residual stress, thermal stress and contact stress of the work roll in the bite region are analyzed by a transient thermomechanical model. The results showed that, the maximum equivalent strain and equivalent plastic strain appear at the exit of the bite region. And residual stress and thermal stress account for about 90% of the coupling stress. The accumulation of plastic deformation only occurs in the initial rolling, and the initial temperature influences the plastic deformation accumulation. Furthermore, abnormal rolling often leads to large plastic deformation on the roll surface.

Stress wave propagation characteristics and impact resistance of laminated composites under impact loading

SHPB simulation of TC4/2024AL/TC4, TC4/TA2/TC4, and TC4/Al3Ti/TC4 laminated composites under different impact velocities of strike bar was carried out by ANSYS/LS-DYNA. Then stress distribution of the laminated composites under impact loading was theoretically analyzed according to the one-dimensional stress wave theory. Influencing factors of stress distribution and energy attenuation were emphatically analyzed, including impact velocities of the strike bar, wave impedance of interlayer, and plastic deformation. The results show that, when the impact velocity is determined, impact resistance is negatively correlated with the impedance of the interlayer. Under certain conditions, impact resistance is negatively correlated with impedance of interlayer, and positively correlated with impact velocity. In addition, the accumulation of plastic deformation could make residual stress distribution along the position coordinate axis shows a trend of gradual increase from both sides to the middle position.

Microstructure of Joints Processed by Ultrasonic Consolidation of Nickel Sheets

Ultrasonic consolidation is an advanced process of sequential solid-state joining of metal foils or sheets by ultrasonic welding. This process was used for joining six sheets of nickel with a thickness of 0.2 mm. Ultrasonic consolidation was accompanied by the formation of wear particles between the sheets. The appearance of microbonds between the sheet surface and the wear particles led to the formation of parallel rows of voids and swirl-like patterns near the interfaces. It was shown that ultrasonic consolidation of nickel sheets led to the formation of fine recrystallized grains near contact surfaces and a subgrain structure in the bulk of the consolidated layers. The microstructural changes were accompanied by an increase in the microhardness of nickel from 1567 MPa in the initial sheet to 2065 and 2400 MPa, respectively, in the bulk and joint zones of the consolidated sample. However, significant differences in the microstructure and microhardness of the layers were not revealed, despite the fact that the accumulated plastic deformation and thermal effects were different from layer to layer. This unexpected result was explained by an inhomogeneity of the microstructure of the nickel samples obtained by ultrasonic consolidation and by a possible interplay between ultrasonic residual hardening and softening.

Development of Artificial-Neural-Network-Based Permanent Deformation Prediction Model of Unbound Granular Materials Subjected to Moving Wheel Loading.

The majority of existing regression models for unbound granular materials (UGMs) consider only the effects of the number of loading cycles and stress levels on the permanent deformation characteristics and are thus unable to account for the complexity of plastic deformation accumulation behavior influenced by other factors, such as dry density, moisture content and gradation. In this study, research efforts were made to develop artificial-neural-network (ANN)-based prediction models for the permanent deformation of UGMs. A series of laboratory repeated load triaxial tests were conducted on UGM specimens with varying gradations to simulate realistic stress paths exerted by moving wheel loads and study permanent deformation characteristics. On the basis of the laboratory testing database, the ANN prediction models were established. Parametric sensitivity analyses were then performed to evaluate and rank the relative importance of each factor on permanent deformation behavior. The results indicated that the developed ANN prediction model is more accurate and reliable as compared to previously published regression models. The two major factors influencing the magnitude of accumulated plastic deformation of UGMs are the shear stress ratio (SSR) and the number of loading cycles, of which the calculated influence coefficients are 38% and 41%, respectively, while the degree of influence of gradation is twice that of the confining pressure.

Kinematics of die filling during upsetting of the head of a steel self-tapping screw

The aim of the work is to determine the kinematics of filling the die when upsetting the head of a steel self-tapping screw. It is noted that a significant part of the wire from ferrous metals is used for the manufacture of hardware products in the form of screws. For the analysis of the form change, a self-tapping screw made of 15Gps steel was selected. The process modeling was performed by the finite element method using the DEFORM software package. The distribution of the degree of deformation for the final position of the tool during the formation of the workpiece is presented. The contour of the center of forming and the accumulation of plastic deformations along the upsetting of the workpiece are given. The maximum strain up to a value of 2.70 is localized in the central part of the head with the spread of the zone of increased deformation in the transverse direction. A zone of increased deformations is identified in the area of the free surface and adjacent to it to the contact surface of the lower half-stamp. The distribution of the mean normal stress is shown. The highest values up to +504 MPa are observed in the zone of the free surface, not constrained by the action of the tool. Under real production conditions, the screw head profile was measured and recorded in coordinate form. For comparison, the calculated data obtained in solving the problem are given. Differences at small values of the current axial coordinate indicate a shift in the area of increased metal broadening in the calculated version closer to the upper half-stamp. The localization of tool wear in the real process is fixed. It is noted that the location of this zone corresponds to the position of the region where the highest level of plastic deformation is established by calculations, which corresponds to one of the variants of wear theories.