Strain Hardening Characteristics Research Articles

Influence of partial slip and direction of traction on wear rate in wheel-rail contact

Abstract The ‘dynarat’ computer model is used to simulate accumulation of plastic shear strain (ratcheting) in rails and to predict wear and crack initiation. Recent developments have focussed on models of microstructure and strain hardening characteristics, based on twin-disc tests with British normal grade (Grade 220) rail steel. In Europe, and increasingly in Britain, the harder Grade 260 rail steel is more common. A set of twin-disc tests using Grade 260 rail steel has been performed, providing wear rates, micro-hardness data and shear strain estimates in order to select a new strain-hardening model. Calibration has led to developments in the core ratcheting model, including the ratcheting equation, the criterion for material failure and the influence of surface micro-roughness. Having calibrated the model against twin-disc data, wear rates are predicted for a set of generic wheel-rail contacts. The ratcheting model is developed here to simulate transverse traction, and a new quasi-static partial slip model for wheel-rail contact is presented. For the range of contact conditions studied, wear rate is found to vary linearly with peak contact pressure, and an equation for estimating wear rate as a function of peak pressure and traction coefficient is fitted to the simulation data.

Basal slip localization in zinc single crystals. The Considère analyses

The tensile and compression tests were performed on zinc single crystals oriented for slip in the basal slip system. During the first stage of the stress–strain curve, the localized necking was typical of strain localization in the tensile specimens. Single or multiple necks were formed along the specimen length. The range of temperatures and the strain rates for single necking of the sample was determined. The formation of such necking depends on strain hardening characteristics and can be predicted by the Considère criterion. On the other hand, propagation of the necked area along the sample length was not predictable by this criterion. Localized sliding and specimen kinking was indicative of the strain localization observed for different specimens compressed under the same conditions, i.e. temperature and strain rate. A decrease in the compression force and in the cross-sectional area with anvil displacements produced localized sliding. On the other hand, a continuous increase in the compression force was representative of tests leading to specimen kinking.

Comparative experimental study of hot-rolled and cold-formed rectangular hollow sections

Square and rectangular hollow sections are generally produced either by hot-rolling or cold-forming. Cross-sections of nominally similar geometries, but from the two different production routes may vary significantly in terms of their general material properties, geometric imperfections, residual stresses, corner geometry and material response and general structural behaviour and load-carrying capacity. In this paper, an experimental programme comprising tensile coupon tests on flat and corner material, measurements of geometric imperfections and residual stresses, stub column tests and simple and continuous beam tests is described. The results of the tests have been combined with other available test data on square and rectangular hollow sections and analysed. Enhancements in yield and ultimate strengths, beyond those quoted in the respective mill certificates, were observed in the corner regions of the cold-formed sections—these are caused by cold-working of the material during production, and predictive models have been proposed. Initial geometric imperfections were generally low in both the hot-rolled and cold-formed sections, with larger imperfections emerging towards the ends of the cold-formed members—these were attributed largely to the release of through thickness residual stresses, which were themselves quantified. The results of the stub column and simple bending tests were used to assess the current slenderness limits given in Eurocode 3, including the possible dependency on production route, whilst the results of the continuous beam tests were evaluated with reference to the assumptions typically made in plastic analysis and design. Current slenderness limits, assessed on the basis of bending tests, appear appropriate, though the Class 3 slenderness limit, assessed on the basis of compression tests, seems optimistic. Of the features investigated, strain hardening characteristics of the material were identified as being primarily responsible for the differences in structural behaviour between hot-rolled and cold-formed sections.

Quantitative analysis of the influence of strain hardening on equal channel angular pressing process

Equal channel angular pressing is a promising severe plastic deformation method. The objective is to achieve high and homogeneous deformation in the workpiece. However, corner gap formed during the process has a pronounced effect on strain homogeneity and should be taken into account for proper die design. Although there exits several publications reporting the effect of strain hardening behavior of the workpiece on corner gap formation, the literature lacks a quantitative study on the subject. In this study, the effects of strain hardening characteristics of the material, strain hardening coefficient (K) and exponent (n) of Hollomon’s law, on corner gap formation and strain homogeneity in equal channel angular pressing process were investigated quantitatively by finite element method. The results were compared with the upper bound analysis for the verification of the finite element findings. A series of numerical simulations showed that the process performance can be improved by modifying the die corner curvature accordingly, without running time consuming simulations.

Strain hardening and plasticity of low-carbon steels with a heterophase structure: I. Strain hardening characteristics of steels during uniaxial tension

The strain hardening characteristics of low-carbon 10kp, 05G2S2, and 05G2R steels with a heterophase structure (ferritic-martensitic, ferritic-bainitic, ferritic-pearlitic) formed upon quenching from the intercritical temperature range and upon normalization are studied during uniaxial tension. The dislocation structures of these steels are examined by electron microscopy and internal friction, and the results obtained are used to explain the factors that cause the high strain-hardening rate of steels with a two-phase ferritic-martensitic structure. Stress-strain diagrams are constructed for steel samples that have various heterophase structures and are subjected to a small preliminary deformation (up to 10%); these diagrams can be used to compare the strain-hardening characteristics of the steels.

The role of additives on dope rheology and membrane formation of defect-free Torlon ® hollow fibers for gas separation

The high demands on high performance membranes for energy, water and life science usages provide the impetus for membrane scientists to search for a comprehensive understanding of membrane formation from molecular level to design membranes with desirable configuration and separation performance. This pioneering work is to elaborate the importance of polymer rheology on hollow fiber formation and reveal the integrated science bridging polymer fundamentals such as polymer cluster size, shear and elongational viscosities, molecular orientation, stress relaxation to membrane microstructure and separation performance for gas separation. Torlon ® poly(amide imide) was employed in this study with various solvent/nonsolvent additives. The effects of additives on polymeric cluster size, hydrogen bonding and dope rheology during the phase inversion have been examined. It is found that hydrogen bonding and strain-hardening characteristics play very important roles in dope rheology and membrane separation performance. Torlon ® possesses strong hydrogen bonds with NMP/water mixtures, the addition of a small amount of water enlarges polymer cluster size, strengthen molecular network (i.e., strain hardening) and facilitate macrovoid-free morphology. However, strong hydrogen bonding may retard chain unfolding during spinning, induce faster relaxation for highly oriented dense-selective skin, and thus reduce gas-pair selectivity. By adjusting dope chemistry and spinning conditions with balanced solubility parameters and dope rheology, we have developed defect-free Torlon ® hollow fiber membranes with an O 2/N 2 selectivity of 8.55 and an ultra-thin layer of 488 Å simply using water as the additive. Fibers spun from dopes containing other additives have the optimal O 2/N 2 selectivity varying from 7.69 to 9.97 at 25 ± 2 °C, and the dense layer thickness varying from 500 Å to 2000 Å. Their corresponding mixed-gas O 2/N 2 selectivity for compressed air varies from 7.12 to 9.00.

Strain hardening behaviour in sintered Fe—0.8%C—1.0%Si—0.8%Cu powder metallurgy preform during cold upsetting

Cold upsetting experiments were performed on sintered Fe—0.8%C—1.0%Si—0.8%Cu steel preforms in order to evaluate the strain hardening characteristics. Powder preforms of 86 per cent theoretical density and an initial aspect ratio of 0.4 were prepared using a suitable die and a 1 MN capacity hydraulic press. Sintering was carried out in an electric muffle furnace for a period of 90 min at 1150 °C. Each sintered compact was subjected to an incremental compressive loading of 0.04 MN until fractures appeared on the free surface. Experiments were performed with no lubricant and using graphite as a lubricant. The behaviour of the applied stress as a function of both strain and densification level exhibits a continuous enhancement over three different response modes. The first and third stage responses offer a high resistance to deformation, whereas the second stage shows virtually steady-state behaviour. The instantaneous strain hardening exponent ni and strength coefficient Ki of the steel preforms were calculated and found to continuously increase with an increase in the deformation and densification levels.

Lateral and vertical size effects on nanoindented microstructures

In this paper we study numerically the size effect of different constitutive relationships based on bulk silicon subjected to nanoindentation. The constitutive relationships of multilinear kinematic hardening rules are considered in order to capture elastic-perfectly plastic to strain-hardening characteristics. The size effect is estimated from the hardness of the varied contact depths normalized by that of the standard size sample that has been previously validated. Both lateral and vertical sample size effects of the microstructure are evaluated by varying respective size with an identical indentation depth. The contact depths equaling to 1/10th of the sample thickness bring around ±10% of the vertical size effect, while those equaling to one quarter lead to approximately ±20%. A softer material with an elastic-perfectly plastic characteristic generates a smaller vertical size effect because of the continuum mechanism occurs only at the near field. Narrowing down the lateral width, leads to lower loads due to no lateral boundary could conduct. Hard samples are less affected by the lateral size effect than soft ones.

Strain-hardening behaviour of fibre-reinforced sand in view of filament geometry

The results from a series of laboratory drained standard triaxial tests conducted on sand specimens reinforced with randomly oriented discrete fibres are presented. The purpose of the testing programme was to quantify the stress–strain–strength behaviour of polypropylene fibre reinforced sand specimens considering different fibre lengths (up to 50 mm) and diameters (from 0.023 mm to 0.1 mm fibre), resulting in a large spectrum of aspect ratios (up to approximately 2200). Fibre reinforced specimens exhibited either strain-softening or strain-hardening characteristics, depending on the specific ranges of aspect ratio. Generally, longer fibres (high aspect ratios) produce higher strengths and strain-hardening behaviour in sand, allowing the mixture to be used as a geomaterial for embankments over soft soil, in which the material must withstand large deformations without losing its strength. However, very thin fibres (such as 0.023 mm diameter), longer than 24 mm, tangle during mixture, significantly reducing the effects of the fibres on the soil behaviour.

Sparse Long-chain Branching's Effect on the Film-casting Behavior of PE

Abstract The degree of film-width reduction or necking during film-casting is analyzed for three metallocene-catalyzed linear low density polyethylenes, LLDPE, with varying degrees of sparse long-chain branching, LCB. The resins included three sparsely LCB polyethylene, PE, materials with varying LCB content ranging from linear to 0.57 LCB/10000 CH2 along with a Ziegler-Natta polymerized LLDPE and a tubular free-radical polymerized low density polyethylene, LDPE. The last two resins are used as reference materials. The primary objective of this analysis is to evaluate whether uniaxial extensional rheological characteristics, in particular strain-hardening, that are a result of LCB influence the film-necking properties. At the lowest drawdown ratio, necking is observed to be reduced with increasing LCB, and thus strain-hardening characteristics. At the higher drawdown ratios it is observed that LCB no longer reduces necking and the curves merge to the results found for linear PE, except in the case of LDPE, which shows reduced necking at all drawdown ratios. Furthermore, comparisons of film necking are also made to separate the effects of molecular weight distribution, MWD, and LCB. The results indicate that both broadening the MWD and the addition of sparse LCB reduce the degree of necking, but to a lesser degree in the case of broadening the MWD. Analysis of the uniaxial extensional and dynamic shear rheology with the pom-pom constitutive model reveals that a distribution of branches along shorter relaxation time modes is important in reducing necking at higher drawdown ratios. Factors such as shear viscosity effects, extrudate swell, and non-isothermal behavior were eliminated as contributing factors.

Tensile Properties of Low Shrinkage Engineered Cementitious Composite

This paper reports the tensile properties of a new class of engineered cementitious composite with characteristic of low drying shrinkage. Experimental results show that drying shrinkage of the composite is greatly reduced as using the low shrinkage cementitious material in matrix, while the composite remains strain-hardening and multiple cracking characteristics. The measured drying shrinkage strain at 28 days is only 10910-6 to 24210-6 for low shrinkage ECCs. For traditional ECC, the shrinkage strain at 28 days is nearly 120010-6. The average tensile strain capacity after 28 days curing is 2.5% of the low shrinkage ECC with tensile strength of 4-5MPa.

Material flow stress and failure in multiscale machining titanium alloy Ti-6Al-4V

Titanium Ti-6Al-4V alloy is a typical difficult-to-machine material due to its unique physical and mechanical properties. The material properties of Ti-6Al-4V play an important role in process design and optimization. However, the dynamic mechanical behavior is poorly understood and accurate predictive models have yet to be developed. This work focuses on the dynamic mechanical behavior of machining Ti-6Al-4V beyond the range of strains, strain rates, and temperatures in conventional materials testing. The flow stress characteristics of strain hardening and thermal softening can be predicted by the Johnson–Cook model coupled with the adiabatic condition. The predicted flow stresses at small strains agree very well with those from the split Hopkinson pressure bar (SHPB) tests, while the predicted flow stresses at large strains also agree with the calculated flow stresses based on the cutting tests with a suitable depth of cut. Heat fraction and temperature parameter control the range of thermal softening and the decrease rate of flow stress. The material may exhibit super plasticity at a small depth of cut with a large radius of the cutting edge in micromachining. Strain rate is one important factor for material fracture close to the cutting edge. The failure strain increases linearly with the increase of homologous temperature, while it only increases slightly with the strain rate.

EFFECT OF MICROSTRUCTURES ON THE DYNAMIC DEFORMATION BEHAVIOR OF Ti-6Al-4V ALLOY

The effects of microstructures of Ti -6 Al -4 V alloy on the flow stresses and fracture behaviors at quasi-static and dynamic deformation conditions were investigated. Specimens of different sizes and fractions of α globules in equiaxed and bimodal structures were compressed at the strain rate of 2×10−3/ s and 3×103/ s using hydraulic testing machine and split Hopkinson pressure bar, respectively. The a globule size in equiaxed structure changed the level of flow stresses, but did not affect the strain hardening characteristics. Meanwhile, the volume fraction of α globule (or lamellar phase) in bimodal structures influenced both the flow stress and strain hardening exponent at quasi-static and dynamic deformation conditions. Bimodal structure of 50% lamellar fraction is considered to be more favorable in dynamic deformation condition at strain rate regime of 3×103/ s than equiaxed or bimodal one having higher lamellar fraction.

Predicting the strain hardening characteristics of ageing alloys under combined loading

The paper examines the effect of the initial structure and prestrain on the yield stress of a number of ageing steels and alloys under combined loading in the cases of like and opposite directions of preloading and reloading. The parameters of yield loci are calculated for the case of combined two-axial tension. Their predicted dependence on temperature, ageing time, and prestrain is experimentally checked. As they increase, strain hardening changes from isotropic to kinematic and then to mixed

Twinning and texture development in two Mg alloys subjected to loading along three different strain paths

The evolution of twinning and texture in two Mg-based (+Al, Mn, Zn) alloys was investigated using uniaxial tension, uniaxial compression and ring hoop tension testing at temperatures from ambient to 250 °C and a strain rate of 0.1 s −1. The results indicate that the initial extrusion texture plays an important role in the formation of different types of twins and that the twinning behavior also depends on the strain path. Contraction and double twinning are the dominant twinning mechanisms in uniaxial tension, while extension twinning prevails in uniaxial compression and ring hoop tension testing. Schmid factor analysis indicates that only components that are favorably oriented (i.e., with the highest SF values) can undergo rapid and complete twinning. The different twinning behaviors are shown to be responsible for the sharply contrasting strain hardening characteristics of the experimental flow curves and dramatic texture changes.

Mechanical properties of Pd–Cu–Si bulk metallic glass

In present work, Pd 77.5Cu 6Si 16.5 bulk metallic glass balls with diameter up to 6 mm have been prepared by fluxing and water quenching method. It has been found that the Pd–Cu–Si glassy alloy exhibits a compressive plastic strain of about 11.4%, together with strain hardening characteristics. A large localized shear band, accompanied by a shear step of about 220 μm in size, has been clearly observed on the deformed specimen. The related shear plane is estimated to have an angle of 42 degrees with respect to the loading axis. The good ductility of the glassy alloy is believed to be partially attributed to strain hardening and the higher resistance of the glassy alloy to crack nucleation and propagation due to its large Poisson's ratio.

Fe-based bulk metallic glass with high plasticity

Fe-based bulk metallic glasses usually exhibit very poor ductility (<0.5%), which has limited their applications. Here the authors report an Fe-based bulk metallic glass which shows a plastic strain of ∼5.2%, together with high strength and distinct strain-hardening characteristics. Its yield strength is ∼2.32GPa, while the ultimate strength is ∼2.80GPa due to the strain-hardening effect. Multiple shear bands and related shear ledges are observed on the deformed specimen. The high plasticity and strain hardening are attributed to the nanoscale inhomogeneity that resulted from liquid phase separation, which can hinder the propagation of shear bands and promote multiple shearing.

Strain rate dependence of deformation behavior of high-nitrogen austenitic steels

Abstract Tensile deformation of high-nitrogen austenitic steel under various strain rate conditions was characterized to investigate the deformation mechanism of the steel at different strain rates. Room-temperature tensile tests of the steel with nitrogen contents of 0.51 wt.% and 0.88 wt.% were performed at different strain rates ranging from 5 × 10 −5 s −1 to 5 × 10 −1 s −1 . Tensile curves of the steel were analyzed and compared, to obtain strain hardening characteristics, yield and tensile strengths as well as the elongation. Microstructures of the tested specimens were examined to understand the deformation behavior of the steel. The high-nitrogen steel with a complete austenite microstructure did not exhibit noticeable flow–stress dependence on strain rates. However, the steel with secondary phases such as nitrides in an austenite matrix showed strong strain rate dependency. The flow stress level increased significantly and the elongation decreased at strain rates higher than 5 × 10 −2 s −1 . The change of the mechanical property became more significant as the nitrogen content in the steel increased. The deformation behavior of the nitrogen steel was elucidated by planar slip and twinning, which operated differently at different strain rates.

Crystal Plasticity Analysis of Non-uniform Deformation in Symmetric Type Bicrystals under Tensile Load and Formation of Geometrically Necessary Dislocation Bands

Slip deformation in symmetric type bicrystal models subjected to tensile load is analyzed by a finite element crystal plasticity analysis code and accumulation of geometrically necessary dislocations (GNDs) is studied in detail. Uniform deformation was expected to take place because mutual constraint of crystal grains through the grain boundary plane does not occur in symmetric type bicrystals, but, some results of the analysis show non-uniform deformation and the high density of GNDs accumulated in the form of band. Such kind of non-uniform deformation is observed regardless of the model size and the strain-hardening characteristics. Mechanism of non-uniform deformation and accumulation of GNDs in the form of band in the symmetric type bicrystals is discussed from the viewpoint of the boundary condition and shape change of grains after slip deformation

Deformation Modes for Axial Crushing of Cylindrical Tubes Considered of the Edge Effect

In this paper, the elastoplastic deformation behaviors of cylindrical tube subjected to statically axial compression are studied by using finite element method (FEM). The effects of tube geometries and strain hardening characteristics of the material on the deformation behaviors are investigated. Generally, although it is recognized that the deformation situation along the circumferential direction is dependent on the thickness ratio (R/t) , it is found that the deformation is also greatly dependent on the edge constraint. The deformation mode along the circumferential direction also affects the deformation mode along the axis-direction. A method to control the deformation mode, such as adding a disk in tube center, is proposed to maintain the deformation keeping in an axi-symmetric mode.