Papers by Keyword: Strain Path Change

Abstract: The effect of strain path change on formability of TRIP590 and TRIP780 was investigated experimentally. Two-step uniaxial tension tests, which consist of the first loading in the rolling direction (RD) and the second loading in the directions varied from RD to transverse direction (TD) in every 15º, were conducted. The evolution of strain rates inside and outside the localized necking zone were inspected by using DIC measuring technique. When the angle between the two loading directions was increased from 0º to 90 º, the subsequent hardening behavior in second step was transited from cross-loading type to Bauschinger type. The total elongation was increased when the two loading directions are close to each other and then it was decreased with the increase of angle. When the angle further increased to 90 º, the total elongation is increased again. It is believed that both of the martensite transformation and Bauschinger type transient has a positive impact on the formability of TRIP steels.

Abstract: In this paper the effect of the strain path change was studied in aluminum alloy containing 0,25% Mg. In metals alloys different metal forming processes can create strong crystallographic and morphologic texture. Both of them can cause direction dependency of the mechanical and other properties. The aim of this paper is to analyze how the effect of the increasing reduction of cold drawn wire appears as a result of the uniaxial compression performed in the direction opposite of drawing. Compression test were performed. Through the results of these tests the changes of the direction of deformations were analyzed qualitatively and numerically. These results provide the possibility to use uniaxial compression test to evaluate the mechanical behavior of cold drawn aluminum.

Abstract: In the current work, the recently proposed homogeneous anisotropic hardening (HAH) model, featuring a distorted yield surface, is applied to commercially pure aluminium. A dislocation-based hardening rule is incorporated into the HAH model to describe the transient stagnation of the hardening rate during strain reversal. A cast and homogenized material with random texture previously investigated by Mánik et al. [1] is selected. The material is prestrained either by compression or rolling, and then tested in uniaxial tension to acquire either reverse softening or orthogonal hardening. The Bauschinger effect, the permanent softening during reverse loading and the hardening in the course of orthogonal loading are captured by the model. However, the permanent softening during orthogonal loading cannot be predicted, and the transient variations of the R-value predicted by the HAH model are neither in qualitative nor quantitative agreement with the experimental data.

Abstract: The present paper aims at a theoretical study of the forming limits of a sheet metal subjected to double strain path changes by using as reference material the AA6016-T4 aluminum alloy sheet. The simulation of plastic instability is carried out through the Marciniak-Kuczynski analysis. The initial shape of the yield locus is given by the Yld2000-2d plane stress yield function. The strain hardening of the material is described by the Voce type saturation law. Linear and several complex strain paths involving single and double strain path changes are taken into account. The validity of the model is assessed by comparing the predicted and experimental forming limits under linear and selected one strain path change. A good accuracy of the developed software on predicting the forming limits is found. A sensitive analysis of the influence of the type and value of the double prestain in the occurrence of the plastic flow localization is performed. A remarkable effect of the double strain path change on the sheet metal forming limits is observed.

Abstract: This paper presents two crystal plasticity based computational constitutive models for the intrinsic formation of plastic microstructure during monotonic loading and its altered evolution under strain path changes in metal forming operations. The formation step is modeled via a non-convex strain gradient crystal plasticity framework which could simulate the intrinsic development of the plastic microstructures. The evolution under strain path changes is modeled via phenomenologically based constitutive equations incorporated into crystal plasticity framework. The latter is capable of simulating the transient anisotropy effects (e.g. cross hardening, Bauschinger effect) depending on the change in the strain path. The paper discusses the unification of such models for the continuous modeling of microstructure formation and evolution processes.

Abstract: Commercial purity Ti was subjected to equal channel angular pressing (ECAP) for up to three passes at 400oC using a die with die angle of 120o. Compression testing of the ECAP specimens was carried out to determine the subsequent flow behavior. Two types of compression test specimen orientations, one parallel to the axis of ECAP and the other at 45o to the axis of the ECAP, were prepared from the specimens subjected to ECAP. Anisotropy in flow behaviour (as indicated by values of strength co-efficient, K and strain hardening exponent, n) was observed. The strain hardening rates were also calculated from the experimentally determined flow curves for the specimens tested in compression in the two orientations. The results have been interpreted in terms of the strain path change parameter between the two deformation steps (ECAP and compression). Strain hardening behaviour and microstructure evolution is discussed in terms of strain path change parameter. Specimens compressed in the direction parallel to the ECAP direction had lower strain hardening exponents while exhibiting higher initial flow stresses. The strain hardening rates were lower for specimens compressed at 45o to the ECAP direction compared to specimens compressed parallel to the ECAP direction.

Abstract: The artificial aging response of Al-Mg-Si 6082 aluminum alloy is investigated over a wide temperature range. Samples aged to under aged, peak aged and over aged conditions are further subjected to plastic deformation by simple compression, plane strain compression and simple shear. The flow behavior and the corresponding hardening rates are documented. Equivalent stress – strain curves are generated for the three stress states for an aging temperature of 160oC. Strain reversal experiments in simple shear were carried out in order to characterize the Bauschinger effect. Strain path change experiments were also conducted, in which the gage section that was first deformed by simple shear was further deformed by simple compression.

Abstract: This work studies the deformation mechanisms active in two pure hexagonal close packed metals, beryllium (Be) and zirconium (Zr), during equal channel angular extrusion processing. An experimental-theoretical approach is employed to assess their relative contributions through measurement and calculation of texture evolution. A new multi-scale constitutive model, incorporating thermally activated dislocation density based hardening, is shown to effectively predict texture evolution as a function of processing route, number of passes (up to four), initial texture, pressing rate, and processing temperature. Texture predictions are shown to be in very good agreement with experimental measurements. Also, it is found that the two most active deformation modes in Be are basal slip and prismatic slip, where the predominant one is interestingly found to depend on die angle. Deformation in Zr during the first pass is predicted to be accommodated not only by its easiest mode, prismatic slip, but by basal slip and tensile twinning.

Abstract: Specimens of commercial purity aluminum were subjected to a strain path change test during high temperature deformation. Specimens were deformed at 4000 C and strain rate of 0.1 s-1 up to various strains of 0.2, 0.5, and 1. Then in a strain path change test, specimens were first deformed to a strain of 0.5, and subsequently deformed to strains of 0.2 and 0. In order to further the understanding of the deformation mechanisms in aluminum, the subgrain sizes and misorientations were characterized in detail by comparative studies using optical microscopy in polarized light (POM), orientation imaging microscopy (OIM/SEM) and transmission electron microscopy (TEM). The analysis revealed that while subgrain size is relatively insensitive to strain, overall misorientations increased with increasing strain. These analyses confirmed a strong bimodal distribution of boundaries during deformation coupled with a low fraction of medium angle boundaries. The results contribute to the understanding that dynamic recovery in aluminum maintains subboundaries with low misorientation but as grains elongate and more subgrain become adjacent to grain boundaries the fraction of high angle boundaries rises.

Abstract: Sequences of fatigue-tension tests were performed on copper polycrystal sheet, with 32µm mean grain size. The effect of strain path change on subsequent reloading yield stress as well as work hardening rate has been investigated. Dislocation microstructure was observed by transmission electron microscopy after mechanical tests. Under present conditions, it was found that fatigue prestraining caused the increase of reloading yield stress, larger amplitude of strain path change resulted in higher reloading yield stress and lower work hardening rate. Reloading tensile curves are independent of predeformation plastic strain amplitudes in both cases. Many isolated dislocation lines between cell walls have been detected for Φ=0° case when the subsequent tension strain amount is 5%, this can be well understood from the microscopic dislocation slip mechanisms. When the reloading tension tests have been done until rupture, dislocation structures become typical of monotonic tension without preloading. The correlation of mechanical properties and microstructural observations was discussed in this paper.