Plastic Flow Stress Research Articles

Analisys of hot deformation behavior of super duplex stainless steel UNS S32760 through processing maps

The need for materials with higher strength and corrosion resistance in corrosive environments, such as in the oil extraction in saline media, has led to the use of super duplex stainless steels in projects such as the Pre-sal. The manufacture of these materials involves the step of thermomechanical processing, whose performance depends on the workability of the material. Processing conditions in which the super duplex stainless steel UNS S32760 can be worked safely and in which the material can fail were investigated in this presentation. The physical simulation was performed by means of hot torsion testing. The tests were performed at temperatures ranging from 900°C to 1200°C and strain rates of 0.01s-1 to 10s-1. The evolution of strain rate sensitivity of flow stress (m) for deformation of 0.5 at all temperatures investigated here was determined. After attaining the values of m for each deformation condition, the values of the power dissipation efficiency (η) were calculated, an instability criterion (ξ) was applied, and processing maps were constructed. Using these maps, the effects of deformation conditions on the power dissipation efficiency and the material plastic instability were discussed. The domains of processing maps, the observed microstructures and the shape of plastic flow stress curves were associated.

On compressive deformation behavior of hollow-strut cellular materials

We investigated the compressive deformation behavior of hollow-strut cellular materials. The present cellular structure consists of pentagon and/or hexagonal shaped cellular network, where the individual strut cross-section is hollow triangular prism. The porosity of entire cellular material network is 96.5%. Uni-axial compressive test was applied, and both macroscopic (network-level) and microscopic (strut-level) deformation behaviors were investigated. The macroscopic nominal stress–strain curve showed a linear relationship during elastic deformation, and then a stress plateau region was observed, followed by the gradual increase in plastic flow stress. Next, by using X-ray micro-CT technique, the strut geometry was quantitatively identified, and based on which finite element method (FEM) was carried out to elucidate the relationship between the strut geometry and the microscopic/macroscopic elastoplastic deformation. A three dimensional spatial structure unit model was established to mimic the present open-cell structure, where the employed strut material properties were obtained from micro-indentation experiments. The FEM computational result agrees reasonably with experimental one of the macroscopic Young's modulus and yield stress. It also suggests that the stress concentration occurs in the minimum cross-section of strut, and then plastic deformation starts at this local point. Such a local yield phenomenon becomes a trigger of buckling for strut, leading to macroscopic plastic deformation characteristics. Furthermore, strain rate effect of the present cellular material was investigated numerically. The obtained result showed reasonable deformation behavior, but this may be improved in the future.

Static and Dynamic Properties of Semi-Crystalline Polyethylene.

Properties of extruded polymers are strongly affected by molecular structure. For two different semi-crystalline polymers, low-density polyethylene (LDPE) and ultra-high molecular weight polyethylene (UHMWPE), this investigation measures the elastic modulus, plastic flow stress and strain-rate dependence of yield stress. Also, it examines the effect of molecular structure on post-necking tensile fracture. The static and dynamic material tests reveal that extruded UHMWPE has a somewhat larger yield stress and much larger strain to failure than LDPE. For both types of polyethylene, the strain at tensile failure decreases with increasing strain-rate. For strain-rates 0.001–3400 s−1, the yield stress variation is accurately represented by the Cowper–Symonds equation. These results indicate that, at high strain rates, UHMWPE is more energy absorbent than LDPE as a result of its long chain molecular structure with few branches.

Model for charge/discharge-rate-dependent plastic flow in amorphous battery materials

Plastic flow is an important mechanism for relaxing stresses that develop due to swelling/shrinkage during charging/discharging of battery materials. Amorphous high-storage-capacity Li–Si has lower flow stresses than crystalline materials but there is evidence that the plastic flow stress depends on the conditions of charging and discharging, indicating important non-equilibrium aspects to the flow behavior. Here, a mechanistically-based constitutive model for rate-dependent plastic flow in amorphous materials, such as LixSi alloys, during charging and discharging is developed based on two physical concepts: (i) excess energy is stored in the material during electrochemical charging and discharging due to the inability of the amorphous material to fully relax during the charging/discharging process and (ii) this excess energy reduces the barriers for plastic flow processes and thus reduces the applied stresses necessary to cause plastic flow. The plastic flow stress is thus a competition between the time scales of charging/discharging and the time scales of glassy relaxation. The two concepts, as well as other aspects of the model, are validated using molecular simulations on a model Li–Si system. The model is applied to examine the plastic flow behavior of typical specimen geometries due to combined charging/discharging and stress history, and the results generally rationalize experimental observations.

The Dynamic Mechanical Properties of Oxygen Free Copper under the Impact Load

The dynamic mechanical properties of oxygen free copper has been tested under the different strain rate (4700s-1~21000s-1) at the room temperature by split Hopkinson pressure bar (SHPB), the true stress-true strain curves has been obtained. Power-Law constitutive model and Johnson-Cook constitutive model have been built to fit the experimental result from SHPB test of oxygen free copper, meanwhile, the constitutive model can be applied to the simulation analysis of cutting process. The results show that the oxygen free copper is sensitive to the strain rate. In addition, the Johnson-Cook constitutive model predicts the plastic flow stress of the oxygen free copper more accurately than the Power-Law constitutive model at the high strain rate.

Mechanical response and microstructural evolution of Ti-13Zr-13Nb biomedical alloy under high strain rate load.

This study uses a split-Hopkinson bar to investigate the mechanical response and microstructural evolution of Ti-13Zr-13Nb biomedical alloy under dynamic loading. Dynamic compression tests are performed at room temperature (25°C) and 500°C under strain rates of 900 s - 1, 1900 s - 1 and 2800 s - 1, respectively. It indicates that the mechanical response of Ti-13Zr-13Nb biomedical alloy is very influenced by strain rate and temperature. The plastic flow stress and strain rate sensitivity increase with the increasing strain rate but decrease with increasing temperature. It reveals that the Ti-13Zr-13Nb alloy fail due to the intensive localized shearing by fracture observation. Furthermore, TEM observations reveal that the dislocation density increases with an increasing strain rate, but decrease with increasing temperature.

Effects of Strain Rate and Temperature on Mechanical Property of Nickel-based Superalloy GH3230

Abstract High temperature tensile tests of nickel-based superalloy GH3230 were carried out at various strain rates from 10−3 s−1 to 10−1 s−1 over temperature range from 1144 K to 1273 K. The emphasis has been put on the effect of strain rate and temperature on the plastic flow stress. The results show that flow stress increases and the hardening exponent n decreases with the strain rate increasing or temperature decreasing. The strain rate sensitivity exponent m is a constant independent of temperature according to the interdependencies of flow stress, strain rate and temperature. The thermal deformation activation energy of GH3230 was calculated as 441 kJ/mol. The full recrystallization will occur when the temperature is about 1273 K and the microstructures with uniform and fine grains can be obtained. The SEM result shows that the fracture mode of all specimens is micro-porous aggregation ductile fracture caused by damage, the variation of temperature has not much effect on fracture morphology at various temperatures, but fracture dimple becomes deeper and larger while the strain rate decreasing.

The Dynamic Mechanical Properties of Oxygen Free Copper under the Impact Load

The dynamic mechanical properties of oxygen free copper has been tested under the different strain rate( 2 500s- 1~ 15 500 s- 1) at the room temperature by split Hopkinson pressure bar( SHPB),the true stress-true strain curves has been obtained. Power-Law constitutive model and Johnson-Cook constitutive model have been built to fit the experimental result from SHPB test of oxygen free copper. The results show that the plastic flow stress of oxygen free copper is not sensitive to the strain rate,however,the yield stress increases obviously and the strain strengthening effect decrease when the the strain rate is higher than 14 000 s- 1. In addition,the Power-Law constitutive model can predict the plastic flow stress of the oxygen free copper more accurately than the Johnson-Cook constitutive model at the high strain rate.

Modelling the constitutive behaviour of aluminium alloy B206 in the as-cast and artificially aged states

The constitutive behaviour of the aluminium-copper casting alloy B206 has been investigated in both the as-cast and artificially aged states. For the as-cast material, a unified plastic flow stress model has been developed using experimental data acquired from compression tests performed using a Gleeble 3500 thermo-mechanical simulator. The unified model considers different constitutive models, each for a specific temperature interval, and the transition between these models at an intermediate temperature based on the material's strain rate sensitivity. For the artificially aged material, a linear-fit yield strength evolution model as a function of the heat treatment parameters has been developed using experimental data from tensile tests and hardness measurements. The yield strength model takes advantage of an independently developed microstructure model that specifically describes the precipitation kinetics of the material based on differential scanning calorimetry measurements. Evaluations of the developed models show good fits between the predicted strengths and data from experiments as well as the literature.

Cast aluminium matrix composites modified with using FSP process – Changing of the structure and mechanical properties

The aim of this study was to explain the influence of the plastic deformation generated in friction stir processing on the changing concentration and distribution of SiC reinforcement particles in the cast composite A339/SiC/p, as well as determine its mechanical properties. Aluminum matrix reinforced with 10% vol. SiC particles was modified using friction stir processing method. Changing of distribution of the reinforcement particles was calculated and analysed using new mathematical RVE theory with Mityushev–Eisenstein–Rayleigh sums (M-sum) and PointSel software. The mechanical properties were determined on the basis of compression and hardness tests.The significant changes in the concentration and distribution of SiC particles were observed. Mechanical testing of selected deformed areas showed significant differences in the values of the plastic flow stress and hardness in thermomechanical hardening areas, approximately from about 30–40%.

Plastic Flow Stress and Constitutive Model for H96 Brass Alloy

To explore the thermo-mechanical response of H96 brass alloy, the quasi-static (universal-testing machine) and dynamic (the split Hopkinson pressure bar apparatus) uniaxial compression experiments have been performed under the temperatures from 293 K to 873 K and the strain rates from 0.001 s-1 to 6000 s-1, and the strains over 60% are obtained. Results show that, H96 brass alloy has strong strain hardening behavior, and it becomes weaker with the increasing temperature. In addition, this alloy is sensitive to strain rates; and, it has temperature sensitivity, the dynamic strain aging occurs at the temperature of 473 K and a quasi-static strain rate of 0.001 s-1. Based on the thermal activation dislocation mechanism, paralleled with the experimental results, a plastic flow constitutive model with the physical conception is developed. The model is suitable to predict the plastic flow stress at different temperatures and strain rates. According to comparing results, the model predictions are in good agreement with the experimental results.

Dynamic Recrystallization of Spheroidal Graphite Iron during Hot Cutting Operations

The objective of the present paper is to analyse the dynamic recrystallization process occurring during the hot cutting of spheroidal graphite iron. In the first instance, hot cutting tests are performed with an orthogonal cutting test bench. The emergence of dynamic recrystallization is highlighted thanks to microstructural observations. Then, a finite element model of the hot cutting operation is introduced. The material behaviour is modelled with a specific constitutive model considering the influence of dynamic recrystallization on the plastic flow stress. Finally, the predictive response of this model is compared to the experimental observations.

Plastic flow modeling of Ti-5Al-2Sn-2Zr-4Mo-4Cr alloy at elevated temperatures and high strain rates

The true stress-strain relationships of Ti-5Al-2Sn-2Zr-4Mo-4Cr(TC17) alloy with a wide range of strain rates were investigated by uniaxial quasi-static and dynamic compression tests, respectively. Quasistatic compression tests were carried out with Instron 8874 test machine, while dynamic compression tests were performed with the split Hopkinson pressure bar (SHPB) which was installed with heating device and synchroassembly system. The dynamic mechanical behaviors tests of TC17 were carried out from room temperature to 800 °C at intervals of 200 °C and at high strain rates (5 500–1 9200 s−1). The stress-strain curves considering temperature-strain rate coupling actions were obtained. The Johnson-Cook constitutive model was developed through data fitting of the stress-strain curves. The material constants in the developed constitutive model can be determined using isothermal and adiabatic stress-strain curves at different strain rates. The Johnson-Cook constitutive model provided satisfied prediction of the plastic flow stress for TC17 alloy.

Ultrahigh plastic flow in Au nanotubes enabled by surface stress facilitated reconstruction

Metal nanowires are usually strong but are not able to maintain a high plastic flow due to a lack of strain hardening. In this study, we investigated the tensile deformation of a new category of metal nanowires with hollow interiors by atomistic simulations – the so-called metal nanotubes – and revealed that they possess a combination of ultrahigh strength and plastic flow. In particular, it was found that by controlling the wall thickness and axial orientation, ultrahigh plastic flow stress of more than 2GPa could be maintained at up to ∼60% tensile strain in Au nanotubes, whereas the solid Au nanowires of similar size yielded at tensile strain of less than 5%, after which the stress dropped immediately below 1GPa. Furthermore, a universal trend of surface reconstruction to the energetically favorable close-packed {111} orientation was found in Au nanotubes regardless of the initial orientation, which may be responsible for the unique plasticity in Au nanotubes with extremely thin walls.

Multi-physics Modelling in Machining OFHC Copper – Coupling of Microstructure-based Flow Stress and Grain Refinement Models

In metal machining, the workmaterial undergoes severe thermomechanical loading, which has a consequence on the microstructure change at different zones in the machined workpiece (chip, tool tip zone, machined surface). In this paper, a multi-physics modelling in machining OFHC copper was proposed. The plastic flow stress of the workmaterial is described by the so-called Mechanical Threshold Stress (MTS) model. For comparison purpose the classical Johnson–Cook (JC) thermo-viscoplastic flow stress model is also introduced. In order to predict the microstructure change, precisely the grain size evolution in the workmaterial during machining, a physical-based Dislocation Density (DD) model was coupled with the MTS model in the framework of an Arbitrary Lagrangian Eulerian (ALE) Finite Elements (FE) approach. The ALE-FE model is developed for the orthogonal cutting process simulation in 2D case. Coupled MTS–DD material models were implemented in Abaqus/Explicit software via a user-material program. The first part of the multi-physics model is validated by comparison of predicted cutting force components with experimental ones and those predicted by the JC model. In the second part, the grain refinement during the cutting process is predicted, revealing zones where the microstructure is highly affected, particularly in the depth of the newly formed surface. This allows estimating the thickness of the effected subsurface by the cutting process.

Hall–Petch k dependencies in nanopolycrystals

Measurements at nanopolycrystal grain sizes of the Hall–Petch plastic flow stress dependence on the reciprocal square root of polycrystal grain diameter occasionally fall short of expectations. The present report reviews experimental and theoretical aspects of such behaviours for a number of metal systems. Comparison with the results on copper had led to prediction on a dislocation pile-up mechanics basis of an even stronger grain size dependence that is now shown to carry over to explanation of similar results reported recently for cold-drawn eutectoid steel wires. In the current review, emphasis is given on additional important results achieved on the production of nanoscale zinc and tungsten carbide materials by Professor Jay Narayan and colleagues at North Carolina State University.

A new approach to the determination of plastic flow stress and failure initiation strain for aluminum alloys cutting process

Abstracts Material flow stress in cutting is difficult to measure experimentally due to the complex deformation behavior involved. In this work, a finite element (FE) simulation of cutting and a theoretical flow stress model including plastic and failure criteria were developed to determine the flow stress during aluminum alloy (7075-T6) cutting process. In a stable plastic stage, the Johnson–Cook model considering the adiabatic temperature rise (ATR) was proposed to calculate the plastic flow stress, and the relative errors of the calculated and FE simulated stresses were less than 4%. In the ductile failure stage, an improved failure initiation criterion in terms of temperature, pressure and strain rate was proposed to determine the failure initiation strain in cutting. In addition, an energy-density based ductile failure criterion was used to predict the flow stress in the failure stage. By combining the proposed theoretical model results and the FE simulation results, the entire flow stress during cutting was obtained. The influence of the failure initiation strain of the chip layer on cutting performance was also discussed. In addition, a set of aluminum alloy (A2024-T351) orthogonal cutting simulations were performed to validate the effect of the failure initiation strain on the cutting process. The predicted variation trends of the failure initiation strain for the two types of aluminum alloys were similar, and the predicted forces and tool–chip contact lengths were compared with the measured values in Atlati et al. [9]. Finally, the prediction of the failure initiation strain and its influence on tool–chip contact behavior were validated indirectly.

Effects of Anisotropic Yield Functions on Prediction of Forming Limit Diagram for AHS Steel

In this study, experimental and numerical analyses of Forming Limit Diagram (FLD) for Advanced High Strength (AHS) steel grade 980 were performed. Forming limit curve was first determined by means of the Nakazima stretch-forming test. Then, analytical calculations of the FLD based on the Marciniak-Kuczynski (M-K) model were carried out. Different yield criteria, namely, Hill’48 (r-value and stress-based), Yld89 (r-value and stress-based) and Barlat2000 (Yld2000-2d) were investigated. The strain hardening law according to Swift was applied. To identify parameters of each model, uniaxial tension, balanced bi-axial bulge test and in-plane biaxial tension test were performed. As a result, predicted plastic flow stresses and plastic anisotropies of the AHS steel by various directions were evaluated. In addition, effects of the anisotropic yield functions, strain rate sensitivities, imperfection values and work hardening coefficient on the predicted FLD were studied and discussed. It was found that the FLD based on the Yld2000-2d yield criterion was in better agreement with the experimental curve. Accuracy of the FLD predictions based on the M-K theory, especially in the biaxial state of stress, significantly depended on the applied yield criteria, for which yield stresses and r-values of different loading directions were required.

Finite hyperelastic–plastic constitutive equations for atomistic simulation of dynamic ductile fracture

The atomistic-based consistent finite hyperelastic–plastic constitutive (LH) model for multiscale simulation is proposed. The system Helmholtz energy is constructed from an embedded-atom method (EAM) potential with uniformly deformation assumption, which consists two parts: the volumetric part and the deviatoric part. A lattice structure related upscaling strategy is introduced to split the total energy. The volumetric strain energy is used to determine the elastic responses, while the deviatoric strain energy governs the plastic evolution. Based on the maximum plastic dissipation principle and the deviatoric strain energy, we derive the general form of atomistic-based plastic flow rule, which lays the theoretical foundation to build LH model. The evolution equation can be explicitly expressed by the kinematic variables of multiplicative decomposition, which directly relates the flow rule to basic physical processes that induce plasticity such as dislocation multiplications. The isotropic hardening parameters in von Mises yield function are fitted by plastic flow stresses. The integration algorithm of standard finite strain plasticity is developed for LH model. In case of zero deviatoric strain energy, LH model reduces to the classical elastic Cauchy-born (CB) model. Full atomistic simulation of dynamic crack propagation is carried out to validate this model. Plasticity captured by LH model, instead of the elasticity obtained from CB model, is observed after lattice instability, which implies that the ductile fracture can be governed by the collective behaviors of dislocations.

Microstructural aspects of superplasticity in Ti–6Al–4V alloy

It is generally accepted that fine-grained and equiaxed microstructure enables superplastic forming of two-phase titanium alloys. Appropriate microstructure is usually developed in the thermomechanical process with careful selection of the parameters of plastic deformation and heat treatment. The results of previous studies proved that β solution treatment preceding plastic deformation in thermomechanical process distinctly increased superplasticity of Ti–6Al–4V alloy despite the fact that obtained microstructure considerably differed from equiaxed one. In current studies fine-grained microstructure of the Ti–6Al–4V alloy was obtained by several variants of thermomechanical process. It was found that the alloy after thermomechanical process comprising β solution treatment followed by forging in α+β temperature range exhibited high superplastic deformability. Metallographic examination of the alloy was carried out at various stages of superplastic tensile test in order to analyse microstructure evolution during superplastic deformation. Tensile tests were interrupted at the strain values corresponding to characteristic points of stress–strain curve, i.e. maximum and steady-state plastic flow stress. Digital image analysis was employed to quantify the changes of strereological parameters of the microstructure of Ti–6Al–4V alloy. The effect of spheroidization and growth of α grains on superplastic behaviour of the alloy was identified.