Journal: IOP Conference Series: Materials Science and Engineering

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IOP Publishing

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1757-8981
1757-899X

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2015 - 2019222020 - 20228
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Publications 1 - 10 of 30
  • Strain Rate and Temperature Dependent Plastic Response of AA7075 during Hot Forming
    Item type: Conference Paper
    Pandya, Kedar Sanjay; Roth, Christian C.; Mohr, Dirk (2019)
    IOP Conference Series: Materials Science and Engineering
    High strength 7xxx series aluminum alloys are being increasingly considered for automotive applications due to their superior specific strength compared to 6xxx series alloys. Complex structural components made from 7xxx series alloys need to be manufactured through warm forming or hot stamping due to their low ductility at room temperature. For hot stamping, the AA7075 blanks are heat treated to above 480°C, yielding a supersaturated solid solution, corresponding to a W temper state. The preheated blanks are then simultaneously formed and rapidly quenched using cooled dies. The entire forming process takes place in the W temper condition. The stamped parts are further subject to an artificial aging heat treatment to achieve the desired temper through precipitation hardening. Therefore, understanding the strain rate and temperature dependent plasticity behavior of AA7075-W is critical to establish optimum design parameters for the hot stamping process. The present work is concerned with the strain rate and temperature dependent plastic response of AA7075 in the W temper state. In particular, the results from a comprehensive experimental program on uniaxial tension specimens are presented. High temperature experiments are carried out at strain rates ranging from 0.001-2/s in a universal testing machine. An induction heating system with infrared high speed imaging is used to span the entire temperature range from 180°C to 480°C, over the full range of strain rates considered. Based on the experimental results, a candidate plasticity model with an empirical mixed Swift-Voce hardening law and a Johnson-Cook type strain rate and temperature dependence is calibrated.
  • Virtual tryout planning in automotive industry based on simulation metamodels
    Item type: Conference Paper
    Harsch, David; Heingärtner, Jörg; Hortig, Dirk; et al. (2016)
    IOP Conference Series: Materials Science and Engineering
  • Development of membrane cryostats for large liquid argon neutrino detectors
    Item type: Conference Paper
    Montanari, David; Bremer, Johan; Gendotti, Adamo; et al. (2015)
    IOP Conference Series: Materials Science and Engineering ~ Advances in Cryogenic Engineering: Proceedings of the Cryogenic Engineering Conference (CEC)
    A new collaboration is being formed to develop a multi-kiloton Long-Baseline neutrino experiment that will be located at the Surf Underground Research Facility (SURF) in Lead, SD. In the present design, the detector will be located inside cryostats filled with 68,400 ton of ultrapure liquid argon (less than 100 parts per trillion of oxygen equivalent contamination). To qualify the membrane technology for future very large-scale and underground implementations, a strong prototyping effort is ongoing: several smaller detectors of growing size with associated cryostats and cryogenic systems will be designed and built at Fermilab and CERN. They will take physics data and test different detector elements, filtration systems, design options and installation procedures. In addition, a 35 ton prototype is already operational at Fermilab and will take data with single-phase detector in early 2016. After the prototyping phase, the multi-kton detector will be constructed. After commissioning, it will detect and study neutrinos from a new beam from Fermilab. These cryostats will be engineered, constructed, commissioned, and qualified by an international engineering team. This contribution presents the on-going effort on the development of the cryostats and details the requirements and the current status of the design.
  • Reduction in mechanical anisotropy through high temperature heat treatment of Hastelloy X processed by Selective Laser Melting (SLM)
    Item type: Conference Paper
    Etter, Thomas; Kunze, Karsten; Geiger, Fabian; et al. (2015)
    IOP Conference Series: Materials Science and Engineering
    Selective Laser Melting (SLM) is an additive manufacturing technology used to directly produce metallic parts from thin powder layers. To evaluate the anisotropic mechanical properties, tensile test specimens of the Ni-base alloy Hastelloy X were built with the loading direction oriented either parallel (z-specimens) or perpendicular to the build-up direction (xy- specimens). Specimens were investigated in the "as-built" condition and after high temperature heat treatment. Tensile tests at room temperature and at 850°C of "as-built" material have shown different mechanical properties for z- and xy-specimens. The anisotropy is reflected in the Young's modulus, with lower values measured parallel to the build-up direction. It is shown that the anisotropy is significantly reduced by a subsequent recrystallization heat treatment. The characterization of microstructural and textural anisotropy was done by Electron Back Scatter Diffraction (EBSD) analysis. Predictions of Young's modulus calculated from the measured textures compare well with the data from tensile tests.
  • Design of a simple shear test for large strains with sequential re-machining of the specimen edges
    Item type: Conference Paper
    Colon, Xavier; Adlafi, Morwan; Galpin, Bertrand; et al. (2022)
    IOP Conference Series: Materials Science and Engineering
    The simple shear test is a standard experiment used for the calibration of plasticity models. For thin sheets, the specimen can be as simple as a rectangular strip, or feature optimized geometries of the free edges. These enhancements are mainly motivated by the undesirable initiation of fracture from the free edges, which limit the usable strain range of the test. Previous studies have shown that fracture from the edges occurs due to a local stress state close to uniaxial tension in these areas. In an attempt to increase the maximum strain, a sequential simple shear test is proposed. The specimen is a sheet metal strip with two opposing cut-outs with rounded concave edges. The specimen is mounted in a shear testing device, composed of two jaws with a prismatic joint connection. The shear test protocol includes multiple two-steps sequences. First, an interrupted shear test is performed up to a given value of displacement. Second, the shear testing device, along with the clamped specimen, is positioned in a milling machine and the rounded free edges of the specimen are re-machined. The application of the proposed testing protocol is presented for three engineering materials. It is found that the valid range of this experiment (i.e. the maximum strains attained before specimen failure) can be substantially extended through repeated re-machining of the specimen boundaries.
  • Analysis of yield locus description on springback behaviour of CR700Y980T-DP steel
    Item type: Conference Paper
    Grubenmann, Michael; Barth, Konrad; Heingärtner, Jörg; et al. (2018)
    IOP Conference Series: Materials Science and Engineering
    To precisely predict springback, the stress and strain state has to be described correctly during the forming process and after load removal. In this work, different yield locus descriptions are considered for springback prediction of a CR700Y980T-DP steel with a thickness of 1.5 mm. Experiments and Finite Element Analyses (FEA) with LS-DYNA of a U-bending validation tool were performed and springback predictions compared. In the FEA Hill48, Barlat89 and Barlat's YLD2000 for isotropic hardening, Yoshida-Uemori as a kinematic hardening model and the homogeneous anisotropic hardening (HAH) model are chosen. Monotonic and two-step tensile tests with a directional change of stress are performed and cyclic tension-compression tests are used to determine the material parameters of the investigated hardening models. It is seen that both Yoshida-Uemori and HAH models provide a significant improvement in the springback prediction. The results, however greatly depend on the identification procedure as well as on the modelling of cross-loading contraction effects.
  • A failure model for non-proportional loading under plane stress condition based on GFLC in comparison to eMMFC and PEPS
    Item type: Conference Paper
    Hippke, Holger; Berisha, Bekim; Hora, Pavel (2018)
    IOP Conference Series: Materials Science and Engineering
    A mathematical model is presented for a forming limit for non-proportional loading under plane stress condition. The model results in an approach to reduce the number of experiments needed for the Generalized Forming Limit Concept (GFLC) and presents a numerical approach to calculate the linearized FLC from real Nakajima measurements. The mathematical model has been analyzed in comparison to Polar Effective Plastic Strain (PEPS) diagram and enhanced Modified Maximum Force Criterion (eMMFC), discussing both consistency with plasticity modeling and industrial applicability. An experimental setup based on Nakajima specimens is presented and DIC measurements are used to capture loading paths. The measured loading paths are used to validate predictions made by PEPS, eMMFC and the presented mathematical approach. The latter model shows promising results for prediction of failure for non-proportional loading.
  • Towards neural network models for describing the large deformation behavior of sheet metal
    Item type: Conference Paper
    Gorji, Maysam B.; Mohr, Dirk (2019)
    IOP Conference Series: Materials Science and Engineering
    Neural networks provide a potentially viable alternative to a differential equation based constitutive models. Here, a neural network model is developed to describe the large deformation response of a Levy-von Mises sheet material with isotropic strain hardening. Using a conventional return-mapping scheme, virtual experiments are performed to generate stress-strain data for random monotonic biaxial loading paths (up to strains of 0.2). Subsequently, a basic feedforward neural network model is trained and validated using the results from virtual experiments. The results for a shallow network with only two hidden layers show remarkably good agreement with all experimental data. The identified neural network model is implemented into a user material subroutine and used in basic structural simulations such as uniaxial tensile and notched tension experiments. In addition to demonstrating the potential of neural networks for modeling the rate-independent plasticity of metals, their application to more complex problems involving strain-rate and temperature effects is discussed.
  • Crystal plasticity as complementary modelling technique for improved simulations results of anisotropic sheet metal behaviour in forming processes
    Item type: Conference Paper
    Hirsiger, Sebastian; Berisha, Bekim; Hippke, Holger; et al. (2020)
    IOP Conference Series: Materials Science and Engineering
    The accuracy of the simulation results in terms of metal sheet forming strongly depends on the capability of modelling the anisotropic material behaviour. In addition, predictive capabilities of the models are strongly influenced by the way how the constitutive model parameters are calibrated. Macroscopic models lean towards to become more complex in order to map the material behaviour more precisely. As consequence the amount and complexity of the experiments is increasing as well. In addition, it is well known that, some of the experiments, for example the equibiaxial compression test, are difficult to perform and therefore, a reasonable coupling of crystal plasticity (CP) modelling and macroscopic models is proposed. It is worth to mention that, in the domain of CP, arbitrary load cases are possible and therefore, any stress ratio of the yield criterion can be used for calibration. Prediction of anisotropic material behaviour of AA6016-T4 and DC05 sheets based on CP simulations were previously presented and compared with the macroscopic Yld2000-2d model. Their data set is now used for the calibration of the parameters of the macroscopic model, where in contrast to the classical procedure, the exponent of the yield locus is defined as a fitting parameter. The strain distributions predicted by the models have been compared with DIC-measurements of Nakajima samples. The predictive capabilities of the CP-based fitting procedure, compared to the classical fitting, are highlighted. Additionally, a comparison of the strain distribution prediction between all model variants is performed on a cruciform shaped deep drawing part. It underlines the importance of the correct prediction of the yield normal, as it is given by the crystal plasticity computation.
  • Rheological characterization of a polyethylene melt, filled with different concentrations of fumed silica at 453 K
    Item type: Conference Paper
    Hadjistamov, Dimiter; Schweizer, Thomas (2018)
    IOP Conference Series: Materials Science and Engineering
    Fumed silica is widely used as filler / thickening agent to increase the viscosity or to change the flow behavior type of a suspension. Strong hydrogen bonds between silica-silica and silica-matrix can lead to a suspension with a yield stress. We wanted to know if fumed silica can also form a three-dimensional network in a polyethylene melt at 453 K. The systems with the polyethylene LDPE 1810H (LDPE) filled with different concentration of the fumed silica Cab-o-sil TS720 (silica) were investigated with the MCR300 rheometer from Anton Paar at 453 K. The shear stress τ and the 1st normal stress difference were measured simultaneously in stress growth experiments. The points of the flow curves represent the steady state values from the start-up experiments with a constant shear rate. Just after the steady state values are reached, the stress deformation is stopped, and the stress relaxation begins. The relative residual shear stress is an important value. The absence of residual shear stress means that the structure of the system is destroyed in the stress growth experiment and the system has shear-thinning flow behavior. The systems with high residual shear stress form a strong network that leads to a system with plastic flow behaviour with a yield stress. The shear stress changes insignificantly in the yield stress region with the shear rate. The yield stress can be determined also from the relative residual shear stress curve.
Publications 1 - 10 of 30