Plastic Instability Research Articles

Scale-dependent pop-ins in nanoindentation and scale-free plastic fluctuations in microcompression

Abstract

Integrating tumor hypoxic stress in novel and more adaptable strategies for cancer immunotherapy.

Immunotherapy is poised to become an increasingly utilized therapy in the treatment of cancer. However, several abnormalities in the tumor microenvironment (TME) that can thwart the efficacy of immunotherapies have been established. Microenvironmental hypoxia is a determining factor in shaping aggressiveness, metastatic potential and treatment resistance of solid tumors. The characterization of this phenomenon could prove beneficial for determining a patient's treatment path and for the introduction of novel targetable factors that can enhance therapeutic outcome. Indeed, the ablation of hypoxia has the potential to sensitize tumors to immunotherapy by metabolically remodeling their microenvironment. In this review, we discuss the intrinsic contributions of hypoxia to cellular plasticity, heterogeneity, stemness and genetic instability in the context of immune escape. In addition, we will shed light on how managing hypoxia can ameliorate response to immunotherapy and how integrating hypoxia gene signatures could play a role in this pursuit.

Joining aluminium profiles to composite sheets by additive manufacturing and forming

Abstract This paper explores the application of the ‘mortise-and-tenon’ concept for joining hollow section aluminium profiles to composite strips or sheets. Wire arc additive manufacturing is combined with joining by forming to fabricate the tenons and to obtain the mechanical interlocking with the mortises available in the strips (or sheets). The workability limits are established by means of an analytical model that combines plastic deformation, instability and fracture. Experimental and finite element modelling are utilized to develop the overall joining process and to validate the round ‘mortise-and-tenon’ design resulting from the analytical model. Pull-out and shear destructive tests are carried out to evaluate the overall strength of the joints and results allow concluding that the new joints can easily and effectively replace existing solutions based on welding, fastening or adhesive bonding. The proposed joining process also circumvents the need to design extra fixing and interlocking features in low cost hollow section aluminium profiles for easy assembling.

Recent progress and future research prospects on the plastic instability of medium-Mn steels: a review

Lightweight materials are desired for energy saving and emission reduction of automobiles. A promising material for automobile parts is advanced high strength steel (AHSS). A recently developed material called medium-Mn steel, with excellent mechanical properties, has attracted increasing attention as the third-generation AHSS for automotive processing. However, medium-Mn steel is disadvantaged by plastic instability during tensile tests. This plastic instability is usually associated with localized and propagative bands on the material surface, which cause an unexpected surface roughening effect and premature failure in the most unfavorable cases. Therefore, plastic instability has severely impeded the commercialization of medium-Mn steels. The phenomenon manifests as discontinuous yielding followed by a yielding plateau (the Luders strain), along with flow stress serrations (the Portevin-Le Chatelier (PLC) effect). Both effects are influenced by the composition, annealing process, and microstructure (phase morphology and constituents) of the steel. Both effects are also correlated with the austenite-to-martensite transformation during deformation to a greater or lesser extent, which is rarely observed in metallic materials. Consequently, the mechanisms of both effects are complicated and explainable by diverse theories. This paper reviewed the current research results on the influences of various factors on the Luders strain and PLC effect, and discussed their corresponding mechanisms. This paper particularly emphasized the limitations of the existing theoretical explanations and proposed future researches to elucidate the existing disputes. Based on the current research and our preliminary experiment, this paper finally suggested ways of eliminating the plastic instability of medium-Mn steel, while guaranteeing ultrahigh strength, and excellent ductility. These improvements will drive the future development of this field.

Effect of Inertial on the Adiabatic Shear Instability: Numerical Analysis

Adiabatic shear instability, as a common plastic instability model of metal materials, has attracted more and more attention. In the process of metal cutting, adiabatic shear instability is observed in the chip with different cutting speed. The plastic instability of the chip usually has two effects: one is the quality of the machined parts, and the other is the fatigue service life of the machined tool. These two factors are usually determined by the parameters of the tool (front corner, back corner), the processing parameters (cutting thickness and cutting speed) and other conditions. In order to improve the machining efficiency, the main way is to raise the cutting speed under the condition that the tool parameters are fixed. With the continuous increase of cutting speed, the inertial effect will become stronger and stronger. By means of commercial finite element software Abaqus/Explicit, this paper analyzes the variation of metal cutting morphology, the fluctuation law of cutting force, the change trend of average cutting force, and the role of inertia effect in different machining speed ranges.

Room Temperature Recovery of Cryogenically Deformed Aluminium Alloys

Production of complex lightweight parts for transport applications requires a high level of formability. This has been shown to be achievable via deep-drawing of aluminium alloys at cryogenic temperatures. Although the effect of an extended plastic deformation regime at low temperature is well known, the mechanisms behind it are unclear due to the limits of the characterisation methods applied. In previous studies, the microstructures of samples deformed at low temperatures were examined after reheating to room temperature (RT) and storage. However, after heating the dislocation structure and density of the deformed material change and do not reflect the cryogenic situation. In this work, we investigate the evolution of flow stress during recovery in Al-Mg and Al-Mg-Si alloys. We examine the RT recovery behaviour of samples pre-strained at 77 K to different strain levels, and evaluate the structural stability upon subsequent deformation. We also study microstructural evolution via in-situ synchrotron X-ray diffraction, starting from initial conditions at cryogenic temperatures to long-term RT-recovery. Recovery of cryogenically deformed samples at RT results in reduction of the flow stress, in dependence on RT storage. The recovery process can be divided into three distinct sections, each based on a different mechanism characterised by either the arranging or the annihilation of dislocations. Subsequent further straining at room temperature after cryogenic forming also generates plastic instabilities and premature fracture due to unfavourable hardening and recovery assisted softening interplay.

Влияние магнитного поля удлиненного соленоида на деформирование металлических кумулятивных струй

The inertial stretching of a metal shaped-charge jet in the magnetic field of a long solenoid is considered. The purpose of the magnetic field influence on the shaped-charge jet is to slow down the development of plastic instability of the jet with an increase in its ultimate elongation and penetration ability. Using a number of simplifying assumptions, an analytical description of electromagnetic processes in the jet section during its motion in the solenoid cavity and after exit from it is obtained. The induction heating of the jet section and the stress state in it under the action of electromagnetic forces are calculated in relation to the conditions of previous experiments with the influence of the magnetic field of solenoids on shaped-charge jets. The obtained results are used to substantiate the hypothesis about the possible cause of a large discrepancy in the experimental data of different authors on the increase in the penetrative action of shaped charges.

Experimental and numerical FEM-based determinations of forming limit diagrams of St14 mild steel based on Marciniak-Kuczynski model

Forming Limit Diagrams (FLD) are used to design metal sheet processes and to choose appropriate material according to their formability. Development of simple and robust methods to determine the formability domain is then a major industrial issue. Marciniak-Kuczynski (M-K) plastic instability criterion is a classical and common approach used to predict numerical FLD. A convergence of the experimental conditions and theoretical M-K hypotheses is investigated. This analysis leads in the proposition of a simple experimental procedure based on classical instrumented biaxial testing machines to reproduce M-K in-plane loading conditions. Experimental results are compared with numerical simulations based on a finite element method implantation of M-K model. This versatile implementation allows the coupling of the plastic instability criterion in ABAQUS with a large range of material behaviors. Application of these experimental and numerical procedures on commercial St14 mild steel leads to the relative differences between lower experimental points and numerical forming limits predictions that do not exceed 4% and 10% in the case of uniaxial and equibiaxial tension for initial prescribed defect ratio of 0.99 and 0.95, and 22% in the case of plane tension. These procedures constitute ways easily implementable in the industry to obtain FLD at reduced costs that show good correlations between experimental results and numerical M-K predictions.

Effect of hydrogen on the flow behavior of a TiAl based alloy during plane strain compression at elevated temperature

Effect of hydrogen on the flow behavior of a TiAl based alloy during plane strain compression at temperatures of 1150 and 1200 °C was investigated in this paper. The results showed that hydrogen obviously softened the alloy and had significant effects on the texture evolution. Hydrogen decreased the number of sharp yield points and hindered the occurrence of Portevin-Le Chatelier effect, which decreased plastic instability of the alloy during plane strain compression. Hydrogen enhanced textures related to γ-phase recrystallization and twinning, as well as α2-phase pyramidal texture component. Additionally, weak α2-phase transverse texture component {11–20}<0001> was present in the hydrogenated samples, and the occurrence of such texture was mainly attributed to hydrogen-increased B2 phase.

Elementary plastic events in amorphous silica.

Plastic instabilities in amorphous materials are often studied using idealized models of binary mixtures that do not capture accurately molecular interactions and bonding present in real glasses. Here we study atomic-scale plastic instabilities in a three-dimensional molecular dynamics model of silica glass under quasistatic shear. We identify two distinct types of elementary plastic events, one is a standard quasilocalized atomic rearrangement while the second is a bond-breaking event that is absent in simplified models of fragile glass formers. Our results show that both plastic events can be predicted by a drop of the lowest nonzero eigenvalue of the Hessian matrix that vanishes at a critical strain. Remarkably, we find very high correlation between the associated eigenvectors and the nonaffine displacement fields accompanying the bond-breaking event, predicting the locus of structural failure. Both eigenvectors and nonaffine displacement fields display an Eshelby-like quadrupolar structure for both failure modes, rearrangement, and bond breaking. Our results thus clarify the nature of atomic-scale plastic instabilities in silica glasses, providing useful information for the development of mesoscale models of amorphous plasticity.

Effect of Hydrogen Charging on Pop-in Behavior of a Zr-Based Metallic Glass

In this work, structural and mechanical properties of hydrogen-charged metallic glass are studied to evaluate the effect of hydrogen on early plasticity. Hydrogen is introduced into samples of a Zr-based (Vit 105) metallic glass using electrochemical charging. Nanoindentation tests reveal a clear increase in modulus and hardness as well as in the load of the first pop-in with increasing hydrogen content. At the same time, the probability of a pop-in occurring decreases, indicating that hydrogen hinders the onset of plastic instabilities while allowing local homogeneous deformation. The hydrogen-induced stiffening and hardening is rationalized by hydrogen stabilization of shear transformation zones (STZs) in the amorphous structure, while the improved ductility is attributed to the change in the spatial correlation of the STZs.

Prediction of necking in HCP sheet metals using a two-surface plasticity model

In the present contribution, a two-surface plasticity model is coupled with several diffuse and localized necking criteria to predict the ductility limits of hexagonal closed packed sheet metals. The plastic strain is considered, in this two-surface constitutive framework, as the result of both slip and twinning deformation modes. This leads to a description of the plastic anisotropy by two separate yield functions: the Barlat yield function to model plastic anisotropy due to slip deformation modes, and the Cazacu yield function to model plastic anisotropy due to twinning deformation modes. Actually, the proposed two-surface model offers an accurate prediction of the plastic anisotropy as well as the tension–compression yield asymmetry for the material response. Furthermore, the current model allows incorporating the effect of distortional hardening resulting from the evolution of plastic anisotropy and tension–compression yield asymmetry. Diffuse necking is predicted by the general bifurcation criterion. As to localized necking, it is determined by the Rice bifurcation criterion as well as by the Marciniak & Kuczynski imperfection approach. To apply both bifurcation criteria, the expression of the continuum tangent modulus associated with this constitutive framework is analytically derived. The set of equations resulting from the coupling between the Marciniak & Kuczynski approach and the constitutive relations is solved by developing an efficient implicit algorithm. The numerical implementation of the two-surface model is assessed and validated through a comparative study between our numerical predictions and several experimental results from the literature. A sensitivity study is presented to analyze the effect of some mechanical parameters on the prediction of diffuse and localized necking in thin sheet metals made of HCP materials. The effect of distortional hardening on the onset of plastic instability is also investigated.

Analysis of Plastic Deformation Instabilities at Elevated Temperatures in Hot-Rolled Medium-Mn Steel.

The study addressed the microstructure and mechanical properties of hot-rolled advanced high-strength medium manganese steel. Some of the curves that were obtained in static tensile tests at deformation temperatures of 20–200 °C showed the occurrence of the heterogeneous plastic deformation phenomenon, called the Portevin-Le Chatelier (PLC) effect. The deformation temperature significantly influenced a serration character. The correlations between the deformation temperature, serration range, microstructural features, and fracture behavior were investigated. The curves showed no Lüders elongation as a result of the thermomechanical processing applied. The serrated flow phenomenon was observed at 60 and 140 °C. The serration type was different and the most enhanced at 140 °C, where the PLC effect was present in both uniform and post-uniform elongation ranges. The disappearance of serrations at 200 °C was related to the increased diffusion intensity.

Explanation of the PLC Effect in Advanced High-Strength Medium-Mn Steels. A Review.

The paper reviews the recent works concerning the Portevin–Le Chatelier (PLC) effect in Advanced High-Strength Steels (AHSSs) with a special attention to high-strength medium-manganese steels. Theories explaining the mechanism of the plastic instability phenomenon in steels with medium- and high-Mn contents were discussed. The relationships between microstructural effects such as TRIP (Transformation-Induced Plasticity), TWIP (Twinning-Induced Plasticity) and the PLC effect were characterized. The effects of processing conditions including a deformation state (hot-rolled and cold-rolled) and strain parameters (deformation temperature, strain rate) were addressed. Factors affecting the value of critical strain for the activation of serrated flow behavior in particular in medium-manganese steels were described.

Effects of transformation-induced plasticity (TRIP) on tensile property improvement of Fe45Co30Cr10V10Ni5-xMnx high-entropy alloys

A new metastable high-entropy alloy (HEA) system was suggested by thermodynamic calculations based on the Gibbs free energies of FCC and HCP and the associated stacking fault energy (SFE). The Fe45Co30Cr10V10Ni5-xMnx (x = 0, 2.5, and 5 at.%) alloys were fabricated, and their tensile properties were evaluated at room and cryogenic temperatures. The relationship between the deformation mechanism and strain hardening behavior was investigated to reveal the role of deformation-induced martensitic transformation on tensile properties. The difference in Gibbs energy decreases with increasing Mn content, leading to the decreased SFE in sequence. At room temperature, ~60% of BCC martensite in the 5Mn HEA contributes effectively to the steady strain hardening, suppressing the plastic instability. This TRIP effect achieves much eminence in the cryogenic deformation, enabling the tensile strength to reach over 1.6 GPa due to 100% of BCC and HCP martensite. In addition to the fraction of martensite, the increased Mn content reduces a critical strain required to trigger the martensitic transformation and then raises the transformation rate. The present findings may provide a guide for the design of metastable HEAs to enhance tensile properties for cryogenic applications through adjusting SFE and TRIP effect.

Instabilities of time-averaged configurations in thermal glasses.

In amorphous solids at finite temperatures the particles follow chaotic trajectories which, at temperatures sufficiently lower than the glass transition, are trapped in "cages." Averaging their positions for times shorter than the diffusion time, one can define a time-averaged configuration. Under strain or stress, these average configurations undergo sharp plastic instabilities. In athermal glasses the understanding of plastic instabilities is furnished by the Hessian matrix and its eigenvalues and eigenfunctions. Here we propose an uplifting of Hessian methods to thermal glasses, with the aim of understanding the plastic responses in the time-averaged configuration. We discuss a number of potential definitions of Hessians and identify which of these can provide eigenvalues and eigenfunctions which can explain and predict the instabilities of the time-averaged configurations. The conclusion is that the nonaffine changes in the average configurations during an instability is accurately predicted by the eigenfunctions of the low-lying eigenvalues of the chosen Hessian.

Manufacturing of high-strength multilayered composite by accumulative roll bonding

In this study, high strength Al/Mg composite was produced using Al 5052 and Mg AZ31B by applying three passes of the accumulative roll bonding (ARB) process at room temperature. For this purpose, microstructure, plastic instability, and mechanical properties were assessed by optical microscopy, x-ray diffraction (XRD), uniaxial tensile test, and Vickers microhardness. The microstructure showed that by rising the exerted strain, grains were drawn in the rolling direction and grain refinement in Mg layers occurred. The crystallite size for Al after first and second ARB passes reached 192 nm and 173 nm, respectively. The lamellar structure was also maintained after the second pass; however, necking, fracture, and waveform formation in Mg layers were quite visible. The results of mechanical properties showed that by increasing the number of ARB passes, the values of UTS and microhardness for both Al and Mg layers increased sharply at the zero pass and gradually at first and second ARB passes, respectively. Also, the maximum UTS reached 475 MPa, which was at least a 100% improvement compared to previous researches. Variation of microstructure and mechanical properties during ARB process are related to many factors such as severe plastic deformation, locking of dislocations, continuous recrystallization, lamellar structure and two mechanisms of work hardening and grain refinement at initial and last ARB cycles play a central role.

Shock velocity-dependent elastic-plastic collapse of pre-existing stacking fault tetrahedron in single crystal Cu

Abstract Stacking fault tetrahedron (SFT) is a characteristic vacancy clustered defect which can cause deterioration of metals through plastic instability. Here, we have studied the effect of shock loading on the structural transformation of the SFT and subsequent damage production using molecular dynamics simulation. According to the investigation, at low shock intensity (0.2 km/s), the SFT collapses partially through inverse Silcox-Hirsch mechanism and re-evolve during the post shock recovery. With increasing the intensity of the shock (0.5 km/s), complete disruption of the tetrahedron structure occurs through dissociation of unfaulted Frank loop into perfect and partial dislocations causing plastic deformation. A threshold pressure value has been identified where the transformation transits from elastic to plastic collapse of SFT. At very high shock velocities (above 1.1 km/s), it is observed that amorphization and martensitic phase transition of the specimen occurs. However, the rarefaction wave generated from the presence of SFT has contributed towards the stabilization of the martensitic phase during shock wave propagation.

Plastic leaf morphology in three species of Quercus: The more exposed leaves are smaller, more lobated and denser

AbstractPhenotypic plasticity and developmental instability in leaf traits are common in oak species but the role of environmental factors is not well understood. To decipher possible correlations between different leaf traits and effects of the position of leaves within the tree canopy, we quantified the plasticity of three leaf traits of 30 trees of Quercus alba L., Quercus palustris Muench and Quercus velutina Lam. We hypothesized that trees could modify the shape of their leaves for better adaptation to the variable microclimate within the canopy. Our results demonstrated that the south and north outer leaves were significantly smaller, more lobed and denser than those situated in the inner canopy. The order of leaves on the branch accounted for the plasticity of leaf traits in Q. alba only. Plasticity of lobing in Q. alba and Q. velutina depended on the height of the trees. We detected fluctuating asymmetry (FA) in all three species, but the source of variation depended on branch position in Q. velutina only. FA was more pronounced in north‐facing leaves. Plasticity of the leaf traits ranged from small to medium. Plasticity of leaf area and leaf mass per area (LMA) depended on the branch position. However, the plasticity of lobation was not affected by the location of a branch within the tree canopy. Quercus alba and Q. palustris had similar plastic responses but the plasticity of Q. velutina was significantly smaller. We concluded that individual plants detect and cope with environmental stress through vegetative organ modification.

Analytical prediction of failure pressure for pipeline with long corrosion defect

Corrosion assessment analysis and failure pressure prediction of pipelines with corrosion defects are essential in the integrity assessment of transmission pipelines. A series of semi-empirical corrosion criteria have been proposed to predict the failure pressure. However, the influence of defect width on the failure pressure is not considered in theory. In this paper, an improved model for the end-capped pipe with infinite length defect is developed based on plastic instability theory and finite strain theory, by considering the corrosion depth and width. The constitutive behavior of the pipe steel material is characterized by a power-law hardening stress-strain curve. Thus, two theoretical solutions are developed based on von Mises and Tresca criteria, by considering the strain hardening effect. Finite element analysis and sixty-one full-scale burst tests for each pipe with a single longitudinal defect are performed to validate and define the application scope of these new solutions. The results show that the proposed model based on von Mises criterion makes the best prediction and is suitable for the cases of defect length longer than 20Dt.