Papers by Keyword: Work Softening

Abstract: The deformation behaviours and microstructure transformations during the cold rolling process of Al-1.4Fe-0.2Mn alloy sheets prepared from 99.7% pure aluminium were investigated by means of hardness-testing, transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS). The phenomena of work hardening and work softening were observed. The hardness of Al-1.4Fe-0.2Mn alloy sheets increased with the increasing of cold rolling reduction firstly, and reached to a peak at 80% cold rolling reduction, meaning work hardening. However, with further increasing of cold rolling reduction, the hardness decreased, which indicates work softening. During the initial deformation stage, the dislocation density and the number of sub-grain structures increased gradually, and many dislocations formed tangles, resulting in work hardening. When the cold rolling reduction exceeded 80%, the dislocation density decreased and sub-grain structures polygonized, leading to work softening. The forming of Mn, Fe and Si bearing compounds is an important reason for the work softening due to lowering solid solution content.

Abstract: Low cycle fatigue (LCF) of a molybdenum alloy was investigated under force-controlled cyclic tension with a zero minimum stress within the 850 – 1300 °C temperature range using Gleeble-3800 physical simulator. Also, the work softening (hardening) behavior was examined through a monotonic tension up to fracture after the force-controlled cyclic deformation. The investigated material under a force-controlled cyclic loading tends to work softening and its tensile strength decreases until the applied stress. The LCF fracture in such conditions occurs through the unrestricted plastic (creep) strain accumulation and exhaustion of the plasticity. The Morrow-type fatigue curves were obtained from the tests. The results of work softening tests were also used to predict the fatigue life.

Abstract: Mechanical behavior of a molybdenum alloy for high-temperature application was investigated at monotonic loading up to fracture, stress-and strain-controlled cyclic loading and short-term creep (less than 9 hours) under the temperatures from 293 to 1773 K using Gleeble-3800 physical simulator. The tests show that plastic strain corresponding to the tensile strength of the material under monotonic loading is small enough (<1%) whereas residual plastic strain after fracture exceeds by 50%. Repeated loading decreases the tensile strength and yield stress, but increases stable (rising) part of stress-strain curve. Increase in the test temperature leads to the change in fracture type from ductile to quasi-brittle distributed at a temperature above 1673 K. Under relatively low temperatures the rheological properties of the material depend strongly on the material processing history. Obtained creep data allows putting up a thermo-activational type equation used to calculate the steady creep rate. Coupling with the known Hoff's model for the creep prefracture stage, this equations allow not only strain rate but also adequate estimation of fracture time.

Abstract: Metal powders behave high strain rate, viscous effect and first hardening then softening deformation characteristics during the forming process of high velocity compaction. The characteristics of high strain rate and viscous effect are described by composite nonlinear viscoelastic body which consists of non-linear spring, linear spring and high strain rate Maxwell element. The deformation characteristics of first hardening then softening can be described by changing the degree of the term of nonlinear spring from greater than 1 to less than 1. Constitutive relation of metal powder in high velocity compaction is established. The degree of the term of nonlinear spring is considered as a function of strain. The function is approximated by linear, quadratic and cubic polynomial and the stress-strain curves are analyzed respectively. Analysis results indicate that the constitutive equation can describe the deformation characteristics of metal powder in high velocity compaction.

Abstract: The present work examines the reversal response of a face-centered cubic (fcc) polycrystalline metal after large pre-strains. While reversal responses among different fcc metals are similar after small pre-strains, they can vary widely after large pre-strains depending on material and microstructure. In this article, these characteristics are considered to be governed by three distinct mechanisms: (1) reverse glide of dislocations previously held by backstresses, (2) reverse glide of dislocations previously held by barriers, and (3) ‘reverse hardening’ by reverse glide over stable dislocation barriers formed in pre-straining. These small-scale mechanisms are incorporated into a polycrystal code to investigate their influence on the macroscopic reversal response and to interpret large strain reversal tests in the literature. It is shown that mechanism (2) is responsible for worksoftening and reductions in hardening rate and mechanism (3) for the overshoot seen in α- brass and other low stacking fault energy alloys. Mechanism (1) is responsible for the Bauschinger effect and occurs in all metals. A large fraction of second phases leads to a strong Bauschinger effect that can either reduce or postpone the effects of mechanisms (2) and (3).

Abstract: This paper deals with the results of three dimensional compression tests carried out for high stiffness urethane foams (Penguin-foam, Sunstar Engineering Ltd.), and also deals with the constitutive modelling base on Shima-Oyane’s consolidation condition for the tested foamed urethane. Three kinds of urethane foams, relative densities of which were 0.1, 0.2 and 0.33, were employed in the experiments. Like metallic porous materials, the tested urethane foams show the strong plastic-compressibility. On the other hand, in modelling, unlike metallic porous materials, the identified material constants for different density foams do not take the same (or unified) values but take the different values when Shima-Oyane’s constitutive model is assumed. Furthermore, the experimentally derived stress-relative density curves could not be satisfactorily described by Shima-Oyane’s original constitutive model; the experimental stress-relative density curves show stronger work hardening as compared with the simulated ones especially in the large deformation stage. To avoid those inconvenience, in this paper, a modified Shima-Oyane type constitutive equation was also proposed, and it was shown that the proposed model could well express both the low work hardening area of the stress-relative density curves at the initial deformation stage and the strong work hardening area at the final deformation stage by supposing the stress restriction at initial deformation stage due to the buckling of cell walls of each foam, and the rapid stress increase at the large deformation stage caused by the successive contact and the friction between the bent cellular walls, respectively.

Abstract: A constitutive model is applied to predict the flow stress of an fcc material up to 30% straining after rolling to reductions of 19%, 39%, and 50%. The model makes use of a single crystal hardening law which appreciates the directional anisotropy produced by planar dislocation boundaries, Bauschinger effects, and dissolution of substructure by new slip activity invoked by changes in strain path. Anisotropy between axial testing in the rolling (RD) versus the transverse direction (TD) and a tensioncompression stress- differential in RD are predicted. These and other characteristics of the flow curves are linked to changes in slip activity when deformation transitions from rolling to axial testing.