Showing papers on "Strain hardening exponent published in 1988"
TL;DR: In this article, the axisymmetric deformation behavior of 0.9999 Cu is investigated at strain rates from 10−4 to 104 s−1, and it is shown that the athermal dislocation accumulation rate, or Stage II hardening rate, becomes a strong function of strain rate.
1,145 citations
TL;DR: In this article, the structure and micromechanisms of the localization process of single crystals and polycrystals of an Al-3 wt% Cu alloy were studied.
278 citations
TL;DR: In this paper, the deformation mechanisms of a strengthened nickel base alloy (Inconel 718) have been studied and a new type of precipitate shearing mechanism has been shown.
190 citations
TL;DR: In this article, a procedure for delineating directional and isotropic hardening from uniaxial hardening data has been developed for the Bodner-Partom model and applied to a nickel-base superalloy, B1900 + Hf.
Abstract: Modeling of hardening and thermal recovery in metals is considered within the context of unified elastic-viscoplastic theories. Specifically, the choices of internal variables and hardening measures, and the resulting hardening response obtained by incorporating saturation-type evolution equations into two general forms of the flow law are examined. Based on the analytical considerations, a procedure for delineating directional and isotropic hardening from uniaxial hardening data has been developed for the Bodner-Partom model and applied to a nickel-base superalloy, B1900 + Hf. Predictions based on the directional hardening properties deduced from the monotonic loading data are shown to be in good agreement with results of cyclic tests.
155 citations
TL;DR: In this paper, the authors examined the behavior of tensile specimens containing random and regular arrays of holes and found that those with random arrays are less ductile than their regular-array counterparts, depending on the minimum spacing between holes, hole size and the strain hardening of the material.
100 citations
TL;DR: In this article, the brittle-ductile transition for a high nitrogen steel is investigated by numerical analyses of the Charpy impact test, where the material is described in terms of an elastic-viscoplastic constitutive model that accounts for the nucleation and growth of micro-voids, leading to ductile fracture, as well as for cleavage failure by micro-crack nucleation.
Abstract: The brittle-ductile transition for a high nitrogen steel is investigated by numerical analyses of the Charpy impact test. The material is described in terms of an elastic-viscoplastic constitutive model that accounts for the nucleation and growth of micro-voids, leading to ductile fracture, as well as for cleavage failure by micro-crack nucleation. The temperature dependence of flow strength and strain hardening is included in the model, and this leads to the prediction of a transition from cleavage fracture to predominantly ductile fracture as the temperature increases. For the particular steel considered it is found that the variation of strain hardening with temperature has a strong effect on the failure mode transition. Both slow loading and impact loading of the Charpy specimen are analyzed. Most of the computations are based on a quasi-static formulation since, even at the strain rates encountered in the Charpy impact test, material strain rate sensitivity is the main time effect. The influence of material inertia is investigated in a few transient analyses.
78 citations
TL;DR: In this article, the authors investigated the internal structure of macroscopic deformation bands after a change of deformation path and performed uniaxial tension tests on predeformed by rolling sheets of mild steel, parallel and perpendicular to the rolling direction.
76 citations
TL;DR: In this paper, the formation of localized shear bands in single crystals of internally nitrided alloys of Fe-Ti-Mn was studied experimentally and theoretically, including the dislocation substructures that are formed at and near shear band/matrix interfaces along with documentation of the crystallography of the localized shearing process.
70 citations
TL;DR: In this article, the authors presented flow visualization and flow birefringence results for polystyrene and low density polyethylene (LDPE) in planar entry flow for contraction ratios of 4 : 1 and 8 : 1.
Abstract: Both flow visualization and flow birefringence results are presented for polystyrene and low density polyethylene (LDPE) in planar entry flow for contraction ratios of 4 : 1 and 8 : 1. It was found that vortex growth was a different function of Weissenberg number ( We ) for each polymer and hence fluid elasticity could not be solely used to account for the onset and growth of vortices. Based on extensional stress measurements along the centerline,a normalized by means of the down-stream wall shear stress, it was proposed that vortex growth and intensity are a function of the extensional viscosity and to some degree the shear viscosity. In particular, LDPE, which exhibits a strain hardening extensional viscosity and an early onset of shear-thinning, exhibits large and intense vortices while polystyrene, which does not exhibit strain hardening, exhibits only small low intensity vortices. The purpose of the study was to emphasize the importance of a fluid's extensional properties, relative to its shear properties, in determining entry flow behavior.
62 citations
TL;DR: In this article, a simple constitutive model for nonsmared cracking nonlinear finite element analysis of reinforced concrete structures is described, and the effect of biaxial stress conditions on peak strengths is represented by a variation of the Kupfer-Hilsdorf failure curve in stress space, where the compressive and tensile envelopes are separately specified.
Abstract: A simple constitutive model for ‘smeared cracking’ nonlinear finite element analysis of reinforced concrete structures is described. The model divides the uniaxial response curve for concrete into fivedamage regions, described as linear elastic; compressive strain hardening; compressive strain softening; tensile strain softening; and tensile stiffening regions. Recommendations for incorporating strain localization effects into homogenized material properties in the softening regions are given. The effect of biaxial stress conditions on peak strengths is represented by a variation of the Kupfer‐Hilsdorf failure curve in stress space, in which the compressive and tensile envelopes are separately specified. Recommendations for the effects of tensile cracking and confinement on the shear modulus and the compressivesoftening modulus, respectively, are included. The central thrust of the paper is to incorporate those attributes of the constitutive model which affect the prediction of structural failure modes as...
61 citations
TL;DR: In this article, a study of the plastic deformation of several fcc metals and alloys at large strains was conducted, where the assumption of steady-state flow at low and intermediate temperatures was examined.
Abstract: A study of the plastic deformation of several fcc metals and alloys at large strains was conducted. The purpose of this study was to take a critical look at the assumption of steady-state flow at low and intermediate temperatures. For this purpose, large strain data were obtainedvia torsion testing of thin-walled tubes. The stress-strain results from these tests followed two distinct trends: at low temperatures, strain hardening continued at shear strains of 8; at higher temperatures strain softening occurred. Continued strain hardening was observed in pure nickel, nickel-cobalt solid-solutions, pure aluminum, and two aluminum alloys. A laminar arrangement of closely spaced dislocation walls arises at large strains and low temperatures, which differs from the well-recovered equiaxed subgrain structure observed at high temperature. Thus, it appears that dynamic recovery processes are not sufficient to establish a steady-state dislocation structure at low temperatures. Strain softening in nickel and nickel-cobalt at higher temperatures was attributed to dynamic recrystallization. In none of the large strain tests conducted was a steady-state flow stress, independent of strain, observed. Torsion test data were found to match data from steady-state tensile creep and compression tests. In the case of the large strain torsion tests of nickel, recrystallization occurred. This suggests that recrystallization and boundary migration can be important processes in creep.
TL;DR: In this article, the effects of cyclic hardening, changes of specimen cross-section, slip traces and dislocation arrangements have been studied at 295 and 524 K and a strain rate of 1·6 x 10−3 s−1.
Abstract: Single crystals of iron and its alloys with 0·5, 0·9, 1·8 and 3 wt% Si oriented for single slip have been cyclically deformed in tension–compression at plastic strains between 10−4 and 10−2. The effects of cyclic hardening, changes of specimen cross-section, slip traces and dislocation arrangements have been studied at 295 and 524 K and a strain rate of 1·6 x 10−3 s−1. The cyclic deformation of b.c.c. Fe–Si alloy crystals at both temperatures and of f.c.c. metal crystals shows similarly shaped cyclic hardening and cyclic stress–strain curves, and similar strain localization in the persistent slip bands (PSBs) and development of the dislocation arrangement. Unlike in f.c.c. metals, in Fe–Si crystals there is appreciable secondary slip at the beginning of the saturation stage, leading to a cell structure in PSBs. The dislocation cell structure is typical of PSBs throughout their whole existence. The surface relief of emerging PSBs also differs from that in f.c.c. metals. Changes in specimen cross-s...
TL;DR: In this article, a formal analogy between the equations of pure plastic and viscoplastic flow theory for void-containing metals and those of standard non-linear elasticity is presented, particularized for the analysis of axisymmetric sheet metal forming problems using simple two node linear finite elements.
Abstract: A formal analogy between the equations of pure plastic and viscoplastic flow theory for void-containing metals and those of standard non-linear elasticity is presented. The formulation is particularized for the analysis of axisymmetric sheet metal forming problems using simple two node linear finite elements. Details of the treatment of friction and strain hardening phenomena, time increment computation and elastic effects are also given. Examples of the effect of void porosity on the hemispherical stretching of a circular sheet are presented.
TL;DR: In this paper, a mathematical model was developed to describe the high-speed melt-spinning behavior of polymers, including the effects of acceleration, gravity, and air friction on the kinematics of the process; temperature and molecular orientation on the crystallization kinetics of the polymer; and temperature, molecular weight, and crystallinity on the elongational viscosity of the material.
Abstract: A mathematical model was developed to describe the high-speed melt-spinning behavior crystallizable polymers. This model included the effects of acceleration, gravity, and air friction on the kinematics of the process; temperature and molecular orientation on the crystallization kinetics of the polymer; and temperature, molecular weight, and crystallinity on the elongational viscosity of the material. Experimental on-line diameter, birefringence, and temperature profiles were obtained for a 12,000 Mn nylon-66 at 2.5 g/min spun at take-up speeds ranging from 2800 to 6600 m/min. These profiles were qualitatively and reasonably quantitatively in agreement with the predicted profiles. They indicated that orientation induced crystallization occurs at spinning speeds greater than 4000 m/min. The experimental diameter and birefringence profiles were compared to those predicted by the model using Avrami indices of 3, 2, and 1. There was a small increase in the crystalline index at the lower speeds with decreasing index. The effect of the strain hardening was more significant at the higher speeds, this being shown by decreasing the exponent in the relationship for the crystallinity on the elongational viscosity. The model developed in this study indicates that high spinning speeds provide the high stress environment that increases the molecular orientation within the fiber. It is this higher molecular orientation that is the driving force for rapid crystallization on the spinline. This rapid crystallization causes a strain hardening, preventing any further drawdown in the fiber diameter and an abrupt rise in the birefringence. This behavior closely corresponds to the observed spinline profiles.
01 Dec 1988
TL;DR: In this paper, a viscoplastic model is developed which is applicable to anisothermal, cyclic, and multiaxial loading conditions, and three internal state variables are used in the model; one to account for kinematic effects, and the other two are isotropic effects.
Abstract: A viscoplastic model is developed which is applicable to anisothermal, cyclic, and multiaxial loading conditions. Three internal state variables are used in the model; one to account for kinematic effects, and the other two to account for isotropic effects. One of the isotropic variables is a measure of yield strength, while the other is a measure of limit strength. Each internal state variable evolves through a process of competition between strain hardening and recovery. There is no explicit coupling between dynamic and thermal recovery in any evolutionary equation, which is a useful simplification in the development of the model. The thermodynamic condition of intrinsic dissipation constrains the thermal recovery function of the model. Application of the model is made to copper, and cyclic experiments under isothermal, thermomechanical, and nonproportional loading conditions are considered. Correlations and predictions of the model are representative of observed material behavior.
TL;DR: In this paper, the processing behaviour of similar and different samples of polyethylene film grades were compared with laboratory data, and the experimental results confirmed the pulsations to be a kind of draw resonance.
Abstract: In assisting product development the processing behaviour of similar and different samples of polyethylene film grades were compared with laboratory data. Bubble stability of the samples in film blowing on a commercial production line is related to rheological properties in elongation. Strain hardening behaviour enhances stability as is well known. The influence of even small differences can be shown, but in these cases the linear viscoelastic stress growth, the absolute value of the viscosity, and the activation energy of flow become important as well. The experimental results are discussed confirming the pulsations to be a kind of draw resonance.
01 Mar 1988
TL;DR: In this paper, the mechanical behavior of a composite of Si3N4 (with 3 % MgO) containing 0-20 % SiC whiskers has been investigated by compressive creep experiments (100-300 MPa) at temperatures between 1 250 °C and 1 500 °C.
Abstract: The mechanical behaviour of a composite of Si3N4 (with 3 % MgO) containing 0-20 % SiC whiskers has been investigated by compressive creep experiments (100-300 MPa) at temperatures between 1 250 °C and 1 500 °C. No significant influence of the SiC whiskers on the creep behaviour could be detected up to about 5 % of strain. Two temperature regions could be identified : a) at high temperature the creep deformation occurs at a quasiconstant strain rate. Extended cavitation in the triple junctions occurs. b) At low temperatures, creep rates rapidly decrease due to a strain hardening coefficient much more important than for higher temperatures. In the microstructure, cavities and microcracks along grain boundaries and fiber-matrix interfaces were observed. Dislocations play an insignificant role for the deformation in both temperatures regions.
TL;DR: In this paper, the various tensile properties, such as yield strength, tensile strength, strength coefficient, uniform elongation, strain hardening exponent and strain rate sensitivities, of stainless steel-clad aluminium sandwich sheet metals have been analyzed on the basis of the fact that the flow stresses of the sandwich sheets follow the rule of mixtures.
Abstract: The various tensile properties, such as yield strength, tensile strength, strength coefficient, uniform elongation, strain hardening exponent and strain rate sensitivities, of stainless steel-clad aluminium sandwich sheet metals have been analysed on the basis of the fact that the flow stresses of the sandwich sheets follow the rule of mixtures, an average of component properties weighted by the volume fractions. The rule of mixtures can be applied to the tensile strengths and strength coefficients of the sandwich sheets, whereas the yield strengths do not follow the mixture rule. The force weighted average rule, an average of component properties weighted by volume fractions and forces, can be applied to uniform elongations, strain hardening exponents and strain rate sensitivities of the sandwich sheets.
TL;DR: In this paper, the authors used finite element methods (FEM) to simulate the deformation behavior of a polycarbonate plate with a circular hole as a special case of two-phase polymer alloys and found that the maximum octahedrai shear stress shifts from the equatorial region toward the 45 ° region at the interface between the hole and the ductile matrix phase.
TL;DR: Asymptotic crack tip fields, for dynamic crack propagation in an elastic-plastic material, have been calculated in this paper, where the possibility of plastic reloading on the crack flank is taken into account.
TL;DR: In this article, the post-yield deformation characteristics of Al-Li-Cu-Mg-Zr alloys in the solution treated and naturally aged conditions have been investigated and two essential features were observed: the existence of a region with no work hardening and pronounced serrations superimposed on the flow curve.
Abstract: The post-yield deformation characteristics of Al–Li–Cu–Mg–Zr alloys in the solution treated and naturally aged conditions have been investigated. Two essential features were observed: the existence of a region with no work hardening and pronounced serrations superimposed on the flow curve. The no work hardening regime is associated with the activation of a single slip system in unrecrystallised and textured material; activation of further slip systems and consequent hardening is assisted by natural aging. Serrated post-yield flow is attributed to the production of free dislocations from those pinned by lithium atom atmospheres; this form of serrated yielding is suppressed as the concentration of lithium solute falls during natural aging.MST/816
TL;DR: In this paper, a non-linear elastic strain hardening/softening plastic model was used to analyse two embankment failures and two distinct mechanisms of progressive failure generation were uncovered.
TL;DR: In this paper, the importance of strain hardening and strain-rate hardening in sheet tensile tests has been investigated using finite-element modeling (FEM), and the results of these simulations showed that the uniform elongation(eu) depends mainly onn, with a small dependence onm.
Abstract: The importance of strain hardening and strain-rate hardening in sheet tensile tests has been investigated using finite-element modeling (FEM). A Hollomon-type material law was examined with n-values (logarithmic work hardening rate) between 0.0 and 0.5 andm- values (logarithmic strainrate sensitivity) between -0.002 and 0.3. A von Mises yield condition with isotropic hardening rule completed the material model. The results of these simulations showed that, as expected, the uniform elongation(eu) depends mainly onn, with a small dependence onm. The post-uniform elongation (ef—eu;ef = total elongation) depends on bothn andm, and is highly nonlinear in either variable for large parametric values. That is, combined increases ofn andm extend ductility more than the additive increase expected from separate effects. Earlier presumptions thatn is unimportant in the post-uniform region appear incorrect. Approximate relations between (ef,eu) and (n,m) were developed quantitatively. These relations have been compared with superplastic data in the literature and appear to agree well, given uncertainties in specimen geometry and test procedure.
TL;DR: In this article, the authors present a theoretical analysis of the ring test which takes into account the influence from strain hardening and instead of adopting Coulomb's law or the concept of a constant friction factor m for describing the frictional stresses, the friction model τ = fαk (f = friction factor, α = ratio between real and apparent area of contact, k = yield stress in pure shear) has been adopted.
TL;DR: In this paper, the cyclic stress-strain behavior of the single crystal nickel-base superalloy Mar-M200 has been investigated at ambient temperature and 843°C under constant strain rate cycling at 10−2 and 10−3/s.
TL;DR: In this article, the shape of the plastic zone for a particular case of plastic orthotropy agreed remarkably well with the zone of intense straining induced by an appropriately orientated crack within a graphite/epoxy laminate.
Abstract: Plane stress mode I near-tip fields in orthotropic materials are examined. Plastic orthotropy is described by Hill's quadratic yield function and the strain hardening behavior is given by an appropriate generalization of a uniaxial tensile power-law stress-strain relation. Pronounced changes in the pattern of the angular variations of crack-tip fields have been observed with the degree of plastic orthotropy and the amount of strain hardening. Possible shapes and sizes of plastic zones (as inferred from effective stress contours) are presented for high- and low-hardening materials and a wide range of plastic orthotropy. The shape of the plastic zone for a particular case of plastic orthotropy agreed remarkably well with the zone of intense straining induced by an appropriately orientated crack within a graphite/epoxy laminate.
TL;DR: In this article, the effect of alloy composition on both the grain growth kinetics and on the rate of oxidation was studied. But the results were limited to five FeAl alloys whose compositions span the B2 phase field (34-51 at. % Al).
Abstract: Rods of five FeAl alloys whose compositions span the B2 phase field (34–51 at. % Al) were produced by multiple hot extrusions of ingots. Samples of each alloy were annealed in air at 1173K and 1473K in order to study the effect of alloy composition on both the grain growth kinetics and on the rate of oxidation. Little oxidation occurred at 1173K but at 1473K the oxidation rate was found to decrease with increasing aluminum concentration. The predominant oxide was found to be α-A1 2 O 3 . The grain size was found to increase rapidly during the first hour of annealing at both temperatures after which it increased more slowly. The grain growth rate was found to decrease with increasing aluminum content. Hardness measurements were made on both as-extruded and annealed samples of FeAl. Hardness increased with increasing aluminum content. The hardness was higher in annealed and air-cooled samples due to vacancy retention. Compression tests of air-cooled, annealed samples also showed higher yield strengths than as-extruded samples. Yield strength and strain hardening rate were found to increase and the ductility was found to decrease with increasing aluminum content.
01 Apr 1988
TL;DR: In this paper, a continuum mechanics frame-vork for analyzing necking and shear band instabilities is discussed, and the predicted onset of instability depends sensitively on the material's constitutive characterization, not only through properties, such as strain hardening and strain rate sensitivity, that can be measured in proportional loading tests, but also through the material response to a change in loading path.
Abstract: The continuum mechanics framevork for analyzing necking and shear band instabilities is discussed. The predicted onset of instability depends sensitively on the material’s constitutive characterization, not only through properties, such as strain hardening and strain rate sensitivity, that can be measured in proportional loading tests, but also through the material’s response to a change in loading path. A specific problem is discussed that illustrates the influence of the curvature of flow potential surfaces even when a softening mechanism plays a major role in precipitating localization.
TL;DR: In this paper, the properties of sintered preforms of different initial densities and of greater length-to-diameter ratio were investigated for different compacting pressures and the optimal conditions of the billet geometry for the least non-uniform densification along the length of the compact were established.