Showing papers on "Strain hardening exponent published in 2006"
TL;DR: In this paper, a focused ion beam microscope was used to study single crystal Au columns with diameters ranging from 180'nm to 8'µm. The columns were machined into the surface of a large-grained Au sheet and then mechanically tested using a nanoindenter outfitted with a flat diamond punch.
Abstract: Uniaxial compression tests have been performed on single crystal Au columns ranging in diameter from 180 nm to 8 µm. The columns were machined into the surface of a large-grained Au sheet using a focused ion beam microscope and then mechanically tested using a nanoindenter outfitted with a flat diamond punch. Images of the compressed columns show that deformation occurs by localized shear on the slip systems with the largest resolved shear stresses. After an elastic loading regime, the columns exhibit yielding in discrete strain bursts. The compressive yield stress scales roughly as the inverse square root of the column diameter. The apparent strain hardening rate also increases strongly with decreasing column diameter and stresses as large as 1 GPa are reached. Both of these size effects are attributed to dislocation source-limited behaviour in small volumes.
692 citations
TL;DR: In this article, the influence of texture and grain size on work hardening behavior and dynamic recovery of magnesium alloys was studied, in addition to the direct effect of texture through the change in the orientation factor for basal and prismatic slip, effects were found on dynamic recovery and the appearance of stage II of workhardening.
598 citations
TL;DR: In this article, the uniaxial compressive responses of silicone rubber (B452 and Sil8800) and pig skin have been measured over a wide range of strain rates (0.004 −4000 s−1).
413 citations
TL;DR: Strain softening caused by localized shearing was found to be effectively prevented by nanocrystallization that is in situ produced by plastic flow within the shear bands, leading to large plasticity and strain hardening.
Abstract: Shear bands generally initiate strain softening and result in low ductility of metallic glasses. In this Letter, we report high-resolution electron microscope observations of shear bands in a ductile metallic glass. Strain softening caused by localized shearing was found to be effectively prevented by nanocrystallization that is in situ produced by plastic flow within the shear bands, leading to large plasticity and strain hardening. These atomic-scale observations not only well explain the extraordinary plasticity that was recently observed in some bulk metallic glasses, but also reveal a novel deformation mechanism that can effectively improve the ductility of monolithic metallic glasses.
281 citations
TL;DR: It is reported that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed ‘multi-junctions’, which are responsible for the strong orientation dependence of strain hardening in body-centred cubic crystals.
Abstract: At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects--dislocations. First theorized in 1934 to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed only two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening: a common phenomenon in which continued deformation increases a crystal's strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions tying dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed hereafter multi-junctions. The existence of multi-junctions is first predicted by Dislocation Dynamics (DD) and atomistic simulations and then confirmed by the transmission electron microscopy (TEM) experiments in single crystal molybdenum. In large-scale Dislocation Dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in BCC crystals.
279 citations
TL;DR: In this article, a more accurate material model and a continuous measure of cross-section deformation capacity were proposed to provide more rational and efficient designs for structural carbon steel sections, and the proposed method offers average increases in member resistances of around 20% over the current Eurocode approach, and a reduction in scatter of the prediction.
274 citations
TL;DR: In this article, the effect of deformation twinning on the strain hardening behavior of polycrystalline AM30 Mg alloy was investigated, and the softening effect induced by double and contraction twinning was responsible for the abnormal strain-hardening behavior.
273 citations
TL;DR: In this paper, a new Pd-Si binary bulk metallic glass was developed, which exhibits a uniform plastic deformation and a large plastic engineering strain of 82% and a plastic true strain of 170%.
Abstract: Usually, monolithic bulk metallic glasses undergo inhomogeneous plastic deformation and exhibit poor ductility (<2%) at room temperature. We report a newly developed Pd–Si binary bulk metallic glass, which exhibits a uniform plastic deformation and a large plastic engineering strain of 82% and a plastic true strain of 170%, together with initial strain hardening, slight strain softening and final strain hardening characteristics. The uniform shear deformation and the ultrahigh plasticity are mainly attributed to strain hardening, which results from the nanoscale inhomogeneity due to liquid phase separation. The formed nanoscale inhomogeneity will hinder, deflect, and bifurcate the propagation of shear bands.
272 citations
TL;DR: In this paper, a model for the low temperature tensile response of copper polycrystals with grain sizes in the range of 2-50μm was proposed and the initial work hardening behavior was strongly grain size dependent and was considered to arise from a combination of kinematic and isotropic hardening due to dislocation-grain boundary interactions.
265 citations
TL;DR: In this paper, the authors present design criteria to achieve saturated PSH behavior in fiber rupture type ECCs, which are measures of energy exchange during steady state flat crack propagation and stress level to initiate micro-cracks.
Abstract: Engineered Cementitious Composites (ECCs) have recently demonstrated their high performance with pseudo strain hardening (PSH) behavior in civil engineering structures and buildings. These materials incorporate low cost fibers such as Polyvinyl Alcohol fibers, which often rupture in composites. Such fiber rupture type ECCs tend to have inferior and unsaturated PSH behavior compared with those incorporating properly designed pull out type fiber. The present study focuses on presenting practical design criteria to achieve saturated PSH behavior in fiber rupture type ECCs. These criteria are proposed based on two performance indices, which are measures of energy exchange during steady state flat crack propagation and stress level to initiate micro-cracks. The latter performance index necessitates a new cracking strength prediction theory, which is proposed in the current study. Finally the cracking strength theory is justified using tensile test data, and the criteria are proposed based on the data in terms of these two indices.
251 citations
TL;DR: In this paper, the potentialities of dislocation dynamics simulations for performing analyses of crystal plasticity and obtaining information that cannot be reached by experiment are emphasized, such as the mesoscopic coefficients of interaction between non-coplanar slip systems.
TL;DR: In this paper, the effect of deformation twinning on the mechanical response of high-purity α-titanium deformed at room temperature was investigated and it was shown that the newly formed deformation twins were harder than the matrix.
Abstract: Novel experiments were conducted to elucidate the effect of deformation twinning on the mechanical response of high-purity α-titanium deformed at room temperature. Orientation-imaging microscopy (OIM), microhardness, and nanohardness evaluations were employed in conjunction with optical microscopy and quasi-static compression testing to obtain insight into the deformation mechanisms. Hardness measurements revealed that the newly formed deformation twins were harder than the matrix. This observation is perhaps the first experimental evidence for the Basinski mechanism for hardening associated with twinning, arising from the transition of glissile dislocations to a sessile configuration upon the lattice reorientation by twinning shear. This work also provided direct evidence for two competing effects of deformation twinning on the overall stress-strain response: (1) hardening via both a reduction of the effective slip length (Hall-Petch effect) and an increase in the hardness of twinned regions (Basinski mechanism) and (2) softening due to the lattice reorientation of the twinned regions.
TL;DR: In this paper, the tensile behavior of ultra-fine grain (UFG) pure Ti fabricated by equal channel angular pressing was investigated at ambient temperature, and the dislocation-based model was used to discuss the properties of UFG pure Ti.
TL;DR: In this paper, deformation-induced phase transformation in a type 304 austenitic stainless steel has been studied in tension at room temperature and −50 °C. The evolution of transformation products was monitored using X-ray diffraction (XRD) line profile analysis of diffraction peaks from a single XRD scan employing the direct comparison method.
Abstract: Deformation-induced phase transformation in a type 304 austenitic stainless steel has been studied in tension at room temperature and −50 °C. The evolution of transformation products was monitored using X-ray diffraction (XRD) line profile analysis of diffraction peaks from a single XRD scan employing the direct comparison method. Crystallographic texture transitions due to deformation strain have been evaluated using (111)
γ
pole figures. The tensile stress-strain data have been analyzed to explain the influence of underlying deformation-induced microstructural changes and associated texture changes in the steel. It is found that the initial stage of rapidly decreasing strain hardening rate in type 304 steel is primarily influenced by hcp ɛ-martensite formation, and the second stage of increasing strain hardening rate is associated with an increase in the α′-martensite formation. The formation of ɛ-martensite is associated with a gradual strengthening of the copper-type texture components up to 15 pct strain and decreasing with further strain at −50 °C. Texture changes during low-temperature deformation not only change the mechanism of ɛ-martensite formation but also influence the strain rate sensitivity of the present steel.
TL;DR: In this article, the effects of loading rate, peak/valley strain or stress holds, ambient temperature and non-proportional loading path on the cyclic softening/hardening and ratchetting behaviors of the material were discussed.
TL;DR: In this paper, a constitutive material law with a damping controlled glide process was used to describe the material flow behavior of the tested alloys under high strain rates loading with the assumption of domination of damping control.
TL;DR: In this paper, the critical stress for initiation of dynamic recrystallization (DRX) can be identified from the inflection point on the strain hardening rate (qds/de) versus flow stress (s) curve.
Abstract: The critical stress for initiation of dynamic recrystallization (DRX) can be identified from the inflection point on the strain hardening rate (qds/de) versus flow stress (s) curve. This kind of curve can be described by an equation that fits the experimental q-s data from zero to the peak stress. Such a curve must have an in- flection point and the simplest relation that has such properties is a third order equation. Hot compression tests were carried out on a 304 H stainless steel over the temperature range 900-1 100°C and strain rate range 0.01-1 s � 1 to a strain of 1. An appropriate third order equation was fitted to the strain hardening data. The results show that the critical stress at initiation s c�� B/3A where A and B are coefficients of the third order equation. It is evident that this value depends on the deformation condi- tions. The stress-strain curve was then normalized with respect to the peak stress, leading to a normalized value of the critical stress (uc) equal to ucs c/s p�� B� /3A� . Here Aand Bare coefficients of the normal- ized third order equation. This value is constant and independent of the deformation conditions.
TL;DR: A temperature-dependent anisotropic material model for use in a coupled thermo-mechanical finite element analysis of the forming of aluminum sheets was developed in this article, where the anisotropy properties of the aluminum alloy sheet AA3003-H111 were characterized for a range of temperatures 25 −260 −C (77 −500 −F) and for different strain rates.
TL;DR: The dependence of strain hardening on strain and entanglement density is also consistent with these models, but thetemperaturedependence is not consistent with the trend as discussed by the authors.
Abstract: The strain hardening behavior of model polymer glasses is studied with sim- ulations over a wide range of entanglement densities, temperatures, strain rates, and chain lengths. Entangled polymers deform affinely at scales larger than the entangle- mentlengthasassumedinentropicnetworkmodelsofstrainhardening.Thedependence of strain hardening on strain and entanglement density is also consistent with these models,butthetemperaturedependencehastheoppositetrend.Thedependenceontem- perature,rate,and interaction strength can instead be understood as reflecting changes in the flow stress. Microscopic analysis of local rearrangements and the primitive paths betweenentanglementsisusedtotestmodelsofstrainhardening. ©2006WileyPeriodicals,
TL;DR: In this paper, it was shown that the high work hardening rate at true strains above about 0.15 is attributed mainly to mechanical twinning, which contributes about twice the effect of that from dislocation accumulation.
TL;DR: In this paper, a closed-cell aluminum was investigated at high strain rates, utilizing AZ31 magnesium alloy bars as compared to maraging steel bars, to estimate the validity of the mechanical response of the foam.
TL;DR: In this article, the authors studied the mechanics of compactant failure in four sandstones associated with a broad range of failure modes in the brittle-ductile transition and found that the initial yield stresses were identified as the critical stresses at the onset of shear-enhanced compaction and subsequent yield stresses depended on hardening given by plastic volumetric strain.
Abstract: [1] We studied the mechanics of compactant failure in four sandstones associated with a broad range of failure modes in the brittle-ductile transition. While Berea and Bentheim sandstones can fail by compaction localization, homogeneous cataclastic flow dominates failure modes in Adamswiller and Darley Dale sandstones at high effective pressures. We acquired new experimental data to complement previous studies, focusing on the strain hardening behavior in samples under drained conditions. The initial yield stresses were identified as the critical stresses at the onset of shear-enhanced compaction, subsequent yield stresses were considered to depend on hardening given by plastic volumetric strain. The yield stresses were described by elliptical yield caps in the stress space, and we compared the cap evolution with two constitutive models: the critical state model and the cap model. Bentheim sandstone showed the best agreement with both models to relatively large strains. Darley Dale sandstone showed the best agreement with the associated flow rule as prescribed by the normality condition, which is implicitly assumed in both constitutive models. Shear-enhanced compaction in Bentheim and Berea sandstones was appreciably more than that predicted for an associative flow rule, with the implication that a nonassociative model is necessary for capturing the inelastic and failure behavior of these sandstones over a broad range of effective pressures. With reference to the nonassociative model formulated by Rudnicki and Rice, bifurcation analysis would predict the transition of failure mode from shear band to compaction band and ultimately to cataclastic flow, in qualitative agreement with the experimental observations.
TL;DR: In this paper, an elastic plastic damage formulation is proposed to circumvent the disadvantages of pure plastic and pure damage approaches, based on an isotropic damage model combined with a hardening yield plastic surface.
25 May 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the authors used the electron back scattering diffraction technique to reveal changes of bulk microstructure due to cyclic loading and minor changes of dislocation microstructures were detected by transmission electron microscopy, which was attributed to high stability of the grain structure and lower purity of the examined ultrafine-grained copper.
Abstract: Fatigue lifetime under stress control of ultrafine-grained Cu of 99.9% purity prepared by equal channel angular pressing is shown to exceed that of conventionally grained cold worked counterparts by a factor of 1.7 in the low-, high- and very-high-cycle region. The electron back scattering diffraction technique did not reveal changes of bulk microstructure due to cyclic loading. Minor changes of dislocation microstructure were detected by transmission electron microscopy. Qualitative change from moderate cyclic hardening to cyclic softening was observed with increasing stress amplitude. Comparison of S–N data with those available in literature shows substantially higher lifetime of the material studied in this work in the high- and very-high-cycle region. This effect is attributed to the high stability of the grain structure and lower purity of the examined ultrafine-grained copper.
TL;DR: In this paper, two expanding cavity models (ECMs) are developed for describing indentation deformations of elastic power-law hardening and elastic linear-hardening materials, which explicitly show that for a given indenter geometry indentation hardness depends on Young's modulus, yield stress and strainhardening index of the indented material.
15 Jan 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: The low-cycle fatigue behavior of SUS304-HP austenitic stainless steel was investigated systematically using tension-compression cycling under fully reversed total strain amplitude control conditions at room temperature in laboratory air.
Abstract: The low-cycle fatigue (LCF) behaviour of SUS304-HP austenitic stainless steel was investigated systematically using tension-compression cycling under fully reversed total strain amplitude control conditions at room temperature in laboratory air. In addition to tests at constant strain amplitudes, incremental step tests (IST) were also carried out. Cyclic stress response, during companion specimen tests (CST), revealed combinations of a variable cyclic hardening, stable behaviour and softening, depending on the applied cyclic strain amplitude, while during incremental step tests it exhibited cyclic hardening character at all strain levels. Microstructure observations using optical and transmission electron microscopy (TEM) revealed that with increasing total strain amplitudes the slip band density increased and the dislocation structure changed from a planar array to a more cellular-like structure. Cyclic deformation-induced austenite/martensite transformation was observed at higher cyclic strain amplitudes. The change in microstructures during cycling is responsible for the fatigue hardening/softening behaviour of the material. The SEM micrographs revealed that at low-strain amplitudes the inclusion-type nucleation occurred near the surface, while at the higher strain amplitudes crack initiation characterized by cleavage cracking occurred not only near the surface but also in the interior of the specimen. Linear or single-slope behaviour was seen both in cyclic stress–strain and Coffin-Mason plots. Masing cyclic stress–strain behaviour was presented only in the IST method but not in the CST method.
TL;DR: Canning et al. as mentioned in this paper investigated the quasi-static and dynamic compressive behavior of pyramidal truss cores made of 304 stainless steel using a combination of experimental techniques and provided a quantification of load-deformation response and associated failure modes across the sample.
TL;DR: In this article, the responses of nanocrystalline aluminum powder of different grain sizes, prepared by ball milling and consolidated into bulk specimens by hot pressing, were determined under quasi-static and dynamic compression.
TL;DR: In this paper, the authors analysed the stress and fracture conditions of a coated surface, that are the origin to wear, by three-dimensional finite element method (FEM) modelling on micro-level, by stress and strain computer simulations and by experimental studies with a scratch tester.
Abstract: The stress and fracture conditions of a coated surface, that are the origin to wear, were analysed by three-dimensional finite element method (FEM) modelling on microlevel, by stress and strain computer simulations and by experimental studies with a scratch tester The studied tribological contact was a 02 mm radius diamond ball sliding with increasing load on a thin, 2 Am thick titanium nitride (TiN) coating on a flat high speed steel substrate The ball was modelled as rigid, the coating linearly elastic and the steel substrate elastic–plastic taking into account strain hardening effects The stresses and strains generated in the surface during sliding are the result of four different mechanisms: the pulling and pushing by the friction force; the geometrical indent, groove, and torus shaped deformations of the flat surface; the bulk plasticity concentration and curvature minimum effects; and the residual stresses in the coating In a sliding contact the first crack is initiated at the top of the coating from bending and pulling actions and it grows down through the coating In the modelled scratch tester system a complex stress field is formed at the surface including remaining residual stresses in the coating behind the sliding contact The stress fields are very different in a scratched uncoated steel sample Some residual tensile stresses are formed in the groove behind the tip but they are very much lower than for the TiN coated case A displacement controlled FEM model was found to better represent the real situation and correspond to experimental results than a force controlled model D 2005 Elsevier BV All rights reserved
25 Jun 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the authors investigated the relationship among the stages of mechanical alloying, the powder characteristics, and the mechanical strength and hardness of the consolidated materials, and showed that the brittle particles accelerate the milling process by increasing the matrix deformation and enhancing the welding and the fracture of particles.
Abstract: This work investigates the use of mechanical alloying to produce aluminium AA6061 matrix composite powders reinforced with AlN, and the relationship among the stages of mechanical alloying, the powder characteristics, and the mechanical strength and hardness of the consolidated materials. The results showed that the brittle particles accelerate the milling process by increasing the matrix deformation and enhancing the welding and the fracture of particles. Moreover, the morphology and the structure of the powder, which changes with mechanical alloying, influence the powder compressibility and extrudability. The elimination of pre-existent cracks of the reinforcement particles, the smaller particle size and its better distribution throughout the matrix, associated with the strain hardening and oxide dispersion produced by milling, brought an about 100% increase in the ultimate tensile strength and hardness, as compared with conventionally mixed composites of the same composition.