Showing papers on "Strain hardening exponent published in 1989"

Effects of Deformation Induced Phase Transformation and Twinning on the Mechanical Properties of Austenitic Fe–Mn–Al Alloys

Abstract: Structure and mechanical properties of austenitic Fe–(20 and 30)Mn–(0 to 7) Al alloys in the temperature range between 77 and 295 K have been investigated in relation to the occurrences of phase transformation and deformation twinning. Additions of aluminum to the 20 wt% Mn alloys significantly decreased the γ→e transformation temperature. The yield stress of these alloys increased with increasing aluminum content, whereas the strain hardening of them decreased. This tendency is prominent at low temperatures. In the 30 wt% Mn alloys the yield stress and strain hardening were almost identical regardless of aluminum contents. Additions of aluminum strongly suppress the γ→e transformation and give birth to the occurrence of deformation twinning. Calculated stacking fault energy based on a regular solution approach shows that the austenitic Fe–Mn–Al alloys which have the stacking fault energy approximately larger than 20 erg/cm2 favor the deformation twinning leading to the increase in low temperature ductility.

The influence of strain hardening on cumulative plastic deformation in rolling and sliding line contact

Abstract: The influence of strain hardening on the cumulative plastic deformation (ratchetting) which takes place in repeated rolling and sliding contacts has been assessed by the use of a non-linear kinematic hardening law proposed and tested by B ower (J. Mech. Phys. Solids37,455, 1989). Both the sub-surface flow, which occurs at low traction coefficients ( 0.25), have been investigated. Two materials have been studied: hard-drawn copper and rail steel. Good correlation was found for copper between the theory and rolling contact experiments.

Martensite formation, strain rate sensitivity, and deformation behavior of type 304 stainless steel sheet

Abstract: The strain and strain rate dependence of the deformation behavior of Type 304 stainless steel sheet was evaluated by constant temperature tensile testing in the temperature range of −80 °C to 160 °C. The strain rate sensitivity, strain hardening rate, and ductility reflected the compctition of two strengthening mechanisms: strain-induced transformation of austenite to martensite and dislocation substructure formation. At low temperatures, the strain rate sensitivity and strain hardening rate correlated with the strain-induced transformation rate. A maximum in total ductility occurred between 0 °C and 25 °C, and the contributions of strain rate sensitivity and strain hardening to independent maxima with temperature of the uniform and post-uniform strains are discussed.

Cyclic hardening properties of hard-drawn copper and rail steel

Abstract: A cyclic hardening law due to Armstrong and Frederick (CEGB Report RD/B/N731, 1966) has been extended to describe plastic strain accumulation (ratchetting) in hard-drawn copper and rail steel. The four parameters of the theoretical model were determined from a single uniaxial test on each material, in which unequal tension and compression were applied. Using these parameters the model was found to give good predictions of the ratchetting rate measured in non-proportional cycles of tension-torsion-compression, which are representative of the stress cycles experienced by surface elements in rolling and sliding contact.

Dynamic Shear Band Development in Plane Strain

Abstract: : Plane strain compression of a rectangular block is used as a model problem to investigate the dynamics of shear band development from an internal inhomogeneity. The material is characterized as a von Mises elastic-viscoplastic solid, with a hardness function that exhibits a local maximum. Regardless of whether the material is hardening or softening, plastic strain development involves the evolution of finger-like contours emanating from the inhomogeneity at 45 degrees to the compression axis. Once a given strain contour crosses the specimen, it fans out about its initial direction of propagation. For a softening solid, this fanning out ceases for some strain level greater than the strain at the hardness maximum and further straining takes place in an ever narrowing band. Many of the qualitative features of shear band development under dynamic loading conditions are the same as under quasi-static loading conditions, but a significant retardation of shear band development due to inertial effects is found.

Strain hardening and substructural evolution in NiCo solid solutions at large strains

Abstract: Torsion deformation was used to investigate dislocation substructure evolution at large strains in high purity nickel and NiCo solid solutions. Observations of small strain dislocation structures formed in stage III revealed that the laminar dislocation structure observed after large strains in stage IV develops from short paired dislocation sheets within the tangled dislocations of an equiaxed cell wall. The development of these short paired dislocation sheets into long microbonds occurs gradually by a multiple-slip process in accordance with the principles of low energy dislocation structures and without the occurrence of a shear instability. The plane of these sheets and /or microbands does not correspond to a {111} slip plane. As these microbands form, a misorientation between the interior of the paired sheets and the surrounding matrix develops and increases with increasing strain.

Analysis of large-strain shear in rate dependent face-centred cubic polycrystals: correlation of micro- and macromechanics

Abstract: Micro- and macroscopic aspects of large-strain deformation are examined through analyses of shear by using physical and phenomenological models. Past experiments and analyses are first reviewed to reveal current issues and put the present work in perspective. These issues are addressed by a complete set of simulations of large-strain shear with a finite-strain, rate-dependent polycrystal model. The model is based on a rigorous constitutive theory for crystallographic slip that accounts for the development of crystallographic texture and the effects of texture on constitutive response. The influences of strain hardening, latent hardening, strain-rate sensitivity, boundary constraints, and initial textures on texture evolution and constitutive response are studied. Coupled stress and strain effects such as axial elongation during unconstrained shear and the development of normal stresses during constrained shear are related to material properties, boundary constraint and texture. The formation of ideal textures and their role in determining polycrystalline behaviour is discussed in quantitative terms. Large-strain shear is also studied by using several phenomenological constitutive theories including J 2 -flow theory, J 2 -corner theory, and two versions of finite-strain kinematic hardening theory. The behaviours predicted by these phenomenological theories and the physically based polycrystal model are directly compared. A noteworthy outcome is the close correspondence found between the predictions of J 2 -corner theory and those of the micromechanically based physical model.

On the creep crack-growth prediction by a non local damage formulation

Abstract: Using the general framework of the thermodynamics of irreversible processes, a class of elastoviscoplastic constitutive equations accounting for positive strain hardening and continuum damage softening is developed as a local approach to creep fracture prediction

The study of work hardening in Fe-Mn-Al-C alloys

Abstract: Three austenitic Fe-Mn-Al-C alloys with different aluminium content from 0, 5.1 and to 8.5wt% are chosen for the present work hardening study. Serrated stress-strain curves with pronounced work hardening were observed during tensile testing, also the serration of stress-strain curve is found to be decreasing as the increase of aluminium content. The serration can, however, still be observed even if the aluminium content is increased to as much as 8.5wt%. According to morphology studies and electron microscopic investigations, it is found that strain-induced deformation twins are closely related to the work hardening in the present alloys. Therefore, deformation twinning is strongly suggested as a major cause of work hardening in Fe-Mn-Al-C alloys, and also plays an important role on the serration of stress-strain curve.

Three-dimensional void growth before a blunting crack tip

Abstract: L arge deformation finite element analysis has been used to study the growth of initially spherical voids directly ahead of a blunting mode I plane strain crack tip. Conditions of small scale yielding were assumed, and the three-dimensional numerical method accounted fully for the interaction between the voids and the crack tip. The near tip stress and deformation fields were obtained for different void-size-to-spacing ratios, and for perfectly plastic and strain hardening materials. The calculations show that the holes spread towards the crack tip and towards each other at a faster rate than they elongate in the tensile direction. The computed void growth rates are compared with previous models for void growth. Finally, several hole coalescence criteria were used to predict fracture initiation from the numerical data. The estimates agree quite well with experimental data for materials containing loosely bonded spherical inclusions.

Dependence of strength–ductility characteristics on thermal history in lowcarbon, 5 wt-%Mn steels

Abstract: As a result of investigations into the improvement of the strength–ductility characteristics of some C–Mn steels, it has been found that the strength–ductility combination (expressed numerically as tensile strength × uniform elongation) of as hot rolled 0·1C–5Mn steels, with or without 2%Si, is greatly improved if an optimum reheating treatment in the range 650–675°C is applied to the steel. Since there is a direct connection with a large increase in the work hardening rate, it is very likely that this improvement is the result of the transformation induced plasticity of well stabilised austenite which has been formed by the reheating treatment.MST/798

Strong Extensional and Shearing Flows of a Branched Polyethylene

Abstract: Simultaneous fits of the Kaye‐BKZ and Wagner equations to shear and uniaxial extensional flow data are not the most critical tests of these equations, because their memory functions depend on two strain invariants, and this dependence can be varied independently for shear and for uniaxial extension to obtain a fit for each. In planar deformations, however, the memory function depends on only one invariant; this dependence can be measured in single‐step shear experiments, as is reported here for a branched polyethylene, IUPAC X, using a sliding plate rheometer. Predictions are then made for three different planar deformation histories: start‐up of steady simple shear and steady planar extension, and exponentially growing shear, all tests in which the memory function depends on only one invariant. The predictions in steady shear and exponential shear are in rough agreement with the data. The theory for planar extension, however, greatly underpredicts the experimental strain hardening of IUPAC X, which has been reported to be similar to the strain hardening usually seen in uniaxial extension for LDPE. Thus, the Kaye‐BKZ and Wagner single‐integral equations cannot simultaneously describe both strain softening in shear and extreme strain hardening in planar extension using a damping function obtained from one of these flows.

The strain hardening behaviour of dual-phase steel

Abstract: The strain hardening behaviour of a dual-phase steel was investigated using uniaxial tension load-elongation data, recorded by a computer in very small intervals. The double-logarithmic true stress-strain curve derived from the data is interpreted in terms of three separate stages of deformation behaviour. Within each stage the curve is approximated by a parabola and the differential (instantaneous) n values are derived analytically without making any prior assumptions about an arbitrary strain hardening law. It is shown that for each stage the n value is not constant and that n values changing linearly with log ϵ provide a better approximation to the experimental results.

Analysis of the temperature dependence of strain-hardening behavior in high- strength steel

Abstract: The stress-strain data of a high-strength low-alloy (HSLA) steel were measured at different temperatures and analyzed in terms of strain-hardening laws proposed by Hollomon,[1] Ludwik,[2] Swift,[3] and Voce.[4] Four methods of analysis, as suggested by Kleemola and Nieminen (K-N),[7] Crussard and Jaoul (C-J),[16] Ramani and Rodriguez (R-R),[11] and Guimaraes (G),[15] have been employed. The C-J analysis has been extended to the Voce equation for the first time. The results have been discussed in terms of the linear correlation coefficient and error in the estimation of uniform strain. The Voce equation has been found to describe the flow be-havior most accurately. The observed increase in flow stress in the dynamic strain aging (DSA) range has been explained in terms of temperature-dependent strain-hardening parameters. It has been established that with increase in the value of the Voce strain component, nv, the magnitude of the saturation stress approaches that of the ultimate tensile stress. A linear relationship has been established between ultimate tensile stress and saturation stress.

The relationship between the strain-hardening exponent n and the microstructure of metals

Abstract: To investigate the relationship between the strain-hardening exponent n and the microstructure of metals, two kinds of metal are chosen: pure aluminium specimens of different grain sizes, and plain carbon steel specimens with different sizes and amounts of spheroidized cementite. The results show that in the plain carbon steels the n value depends only on the interparticle spacing of cementite, which is related to the two parameters, the volume fraction f and the particle size d. If the interparticle spacing of cementite is considered as the mean free path (MFP) of dislocation motion, the n value of metals is proportional to the MFP. It is the MFP that controls the strain-hardening behaviour of materials.

1200 to 1400 K slow strain rate compressive behavior of small grain size NiAl/Ni 2 AlTi alloys and NiAl/Ni 2 AlTi–TiB 2 composites

Abstract: Since 1976 NiAl–Ni2AlTi alloys have been known to possess elevated temperature mechanical properties approaching those of Ni-base superalloys; however, due to their apparent brittleness, little additional work has been undertaken to exploit this strength. In an attempt to instill ductility in these materials, small grain size single (Ni–45Al–5Ti) and two (Ni–40Al–10Ti) phase intermetallics were fabricated by XDTM technology and tested (XDTM is a trademark of Martin Marietta Corporation). As these compositions have the potential for being the matrix material in high temperature composites, Ni–40Al–10Ti and Ni–45Al–5Ti with 20 vol.% TiB2 in the form of ∼1 μm diameter particles were also investigated. The as-fabricated materials were fully dense and polycrystalline. The grain sizes measured ∼8 μm for Ti-poor and about 15 μm for the Ti-rich unreinforced materials but could not be determined for either TiB2 containing composite. Elevated temperature compression testing was conducted to about 8% deformation between 1200 and 1400 K with strain rates varying from ∼10−4 to ∼10−7 s−1. The majority of the tests exhibited diffuse yielding over approximately 1% strain followed by negative strain hardening. However, a few experiments resulted in steady state behavior where deformation continued under a constant stress. The flow strengths on yielding of both forms of Ni–40Al–10Ti were higher than those for the Ni–45Al–5Ti versions. For each matrix composition the addition of 20 vol.% TiB2 decreased the strength at the higher strain rates in comparison to the TiB2-free forms. During slow deformation conditions, however, the particles do provide reinforcement. Light optical microscopy of tested specimens revealed that these materials are generally quite brittle as numerous longitudinal and transverse cracks were found irrespective of the type of stress-strain behavior.