Abstract: This work investigates the relation between shear stress and plastic yield considering that a crystal can only deform in a limited set of directions. The shear stress in arbitrary directions is mapped for some cases showing relevant differences. Yield loci based on mean shear stress are constructed. The Tresca yield criterion can be improved by averaging the shear stress over directions near the direction of maximum shear stress. Yield criteria based on averaging over crystallographic direction show a clear influence of the actual orientation of these direction, notably in case of few crystallographic directions. The general finding is that the higher the isotropy of a material, the lower the plane strain factor. The shape of the yield loci is comparable to those derived by the Hershey criterion with exponents lower than 3.
Abstract: High tensile strength steel sheets have large springback after being formend at room temperature. Warm forming can be a solution to reduce springback of high tensile strength steel parts. NANOHITENTM is a high strength ferritic steel precipitation-strengthened by nanometer-sized carbides developed by JFE Steel Corporation. Tensile strength of the steel at room temperature does not change before and after deformation at elevated temperatures up to 873K since the carbides in the steel are stable at high temperatures less than 973K. Therefore, the steel is suitable for warm forming. Springback of 980MPa NANOHITENTM parts warm formed at 873K is the same level of that of cold formed conventional 590MPa steel parts. In this study, two kinds of material testing at room temperature and at elevated temperatures between 573K and 937K were performed to understand the mechanical behavior of 980MPa NANOHITENTM: uniaxial tensile tests and bending tests. The steels flow stress depends on not only material temperature but also strain rate in uniaxial tensile tests. After a bending test, the specimen shows springback measured by the change of an angle between the two sides. Stress relaxation happens while a test specimen is held at the bottom dead point after bending. And the stress relaxation could be used to reduce springback of warm formed parts.
Abstract: A constitutive model that accounts for mixed isotropic-nonlinear kinematic hardening, suitable for any non-quadratic yield criteria, is proposed. The finite strain model is derived from a thermodynamically consistent framework and relies on the multiplicative split of the deformation gradient in the context of hyperelasticity. The nonlinear kinematic hardening approach is introduced in the constitutive model by means of the multiplicative split of the plastic deformation gradient. The constitutive equations are consistently derived by exploiting the dissipation inequality, and expressed by symmetric tensor-valued internal variables only. The exponential map algorithm was employed in the integration of the evolution equations. This algorithmic strategy has the advantage of preserving both the plastic incompressibility and the symmetry of the internal variables. The model was implemented into a material user-subroutine of a commercial finite element code (ABAQUS), and some numerical results are presented to assess the performance of the present model.
Abstract: In the present study, the deformation behavior of a cast Mg alloy sheet that had random crystallographic orientations was studied both experimentally and numerically. Although the crystallographic orientations were random, the stress-strain curve was asymmetric between tension and compression: the flow stress under tension was higher than that of compression. Moreover, the stress-strain curve exhibited a strain path dependency: a slightly sigmoidal curve occurred under tension following compression, while it did not occur under compression following tension. Clearly, such tendencies were similar to those observed in rolled Mg alloy sheets although the tendencies were less pronounced in the cast Mg alloy sheet. A crystal plasticity finite-element method was used to understand the mechanism of these results. Simulation results showed that the asymmetry and the strain path dependency in the stress-strain curves occurred in the cast Mg alloy sheet because of the asymmetry in the activity of twinning between tension and compression as in the case of rolled Mg alloy sheets.
Abstract: In-plane tension/compression tests of a dual phase steel sheet with a tensile strength of 780 MPa were carried out using in-plane stress reversal testing machine. Remarkable tension/ compression asymmetry of flow stress (TCA) has been observed. Moreover, biaxial tensile tests using cruciform specimens were performed to measure contours of plastic work. The test material exhibited differential work hardening (DWH). In order to reproduce the TCA, an asymmetric quadratic yield function proposed by Verma et al. (2011) was used. The parameters of the yield function were changed as a function of reference plastic strain to reproduce the DWH. Furthermore, to assess the springback prediction accuracy of the developed model, a 3-point bending experiment and finite element analyses (FEA) were performed. It is concluded that the use of the material model that is capable of reproducing DWH and TCA is a must for a highly accurate FEA of springback.
Abstract: The formability of sheet metal is highly related to the materials resistance to strain localisation and fracture. The Forming Limit Curve (FLC) is one way to evaluate the tendency to instabilities during the forming operation of a material for different strain states. The Nakazima test is a common technique used to experimentally determine the limiting strains. In this paper a slightly modified version of the proposed ISO-standard used at Outokumpu/Avesta Research Centre (ARC) is presented. The method considers the limiting principal strains before and after failure has occurred. The obtained results from the present approach are compared with previous internal methods used at the company. The previous internal methods consisted of evaluating the limiting principal strains at the maximum punch force before fracture or at a set punch distance before fracture. An austenitic stainless steel grade (254 SMO®) is used in the study. The method will in this work be called the Interpolation Method (IM). A Finite Element (FE) model of the Nakazima test is modelled in LS-DYNA® with the goal to be able to simulate the experimental test. In order to compare the FLC between the experimental and the numerical results, one instability indicator is proposed based on the onset of fracture in the FE model.The Barlat YLD2000 model  using 6 parameters based on proof stresses and anisotropic values for different material directions is applied as a constitutive model. The Interpolation Method is promising and will be used during a trial period at Outokumpu ARC in the future testing. Further development is needed for the simulation model.
Abstract: Intermediate annealing is a widely used process to recover formability in multi-stage deep drawing of austenitic stainless steel sheets. A special ability of metastable austenitic stainless steel is the so called strain-induced martensite formation, which causes the TRIP-effect (TRansformation Induced Plasticity). Major issues of intermediate annealing in serial production processes are long annealing times and high costs for annealing equipment and consumed heating energy. One suggestion for optimization of this process made in this paper is to anneal austenitic material only in regions of the part where it is required and to use suitable annealing parameters. This annealing parameters need to be optimized to requirements of follow-up forming processes. Therefor, basic intermediate annealing experiments were accomplished for austenitic grade EN 1.4301 within temperature range of 100-1100°C. Measurements of mechanical properties by uniaxial tensile tests, martensite contents measured by Feritscope and micrographs are showing the development of martensite reversion as well as reconstitution of austenitic phase. Especially 400°C, 600°C and 1000°C were identified as most interesting annealing temperature levels due to formation of carbides and recrystallization. This knowledge can be used as base for further local heat treatment concepts and may enhance, simplify and save costs of intermediate annealing process of deep draw components made of stainless steel.
Abstract: The multiaxial plastic deformation behavior of a cold rolled interstitial-free steel sheet with a thickness of 0.65 mm was measured using a servo-controlled multiaxial tube expansion testing machine for the range of strain from initial yield to fracture. Tubular specimens were fabricated from the sheet sample by roller bending and laser welding. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the contours of plastic work in stress space up to an equivalent plastic strain of 0.289 along with the directions of plastic strain rates. The test material exhibited differential hardening (DWH). A material modeling method for reproducing the DWH in a finite element simulation has been developed. Hydraulic bulge forming simulation results based on the DWH model had a closer agreement with the experimental results than those calculated using the isotropic hardening models with selected yield functions.
Abstract: In the last ten years, the automotive sector presents large interest for light alloys tubes for structural and body car parts to reduce CO2 emissions. Tubes hydroforming is one of the most popular processes to obtain complex parts by using liquids as active part of the dies (i.e. water-or oil-based emulsions) with reduced costs of equipment and machines. However, when elevated temperatures should be used to increase the material formability, hydroforming processes are strongly limited due to the boiling point of liquids. The use of gas at elevated temperature in the so-called Hot Metal Gas Forming process (HMGF) has shown promising capabilities thanks to the increased formability and the possibility to form parts with lower pressures. The paper focuses on a novel experimental set-up to evaluate the tubes formability at high temperatures. Tubes are heated by electric current and air in pressure is used to form the material. Aluminium alloy AA6060 tubes specimens were used to test the experimental equipment and evaluate temperature and pressure ranges able to shape the material.