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J W Coenen - One of the best experts on this subject based on the ideXlab platform.
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plastic deformation of recrystallized tungsten potassium wires constitutive deformation law in the temperature range 22 600 c
International Journal of Refractory Metals & Hard Materials, 2018Co-Authors: Dmitry Terentyev, J Riesch, S Lebediev, T Khvan, A Zinovev, M Rasinski, A Dubinko, J W CoenenAbstract:Abstract Recent efforts dedicated to the mitigation of tungsten (W) brittleness have demonstrated that tungsten fiber-reinforced composites acquire extrinsic toughening even at room temperature, which is due to the outstanding strength of W wires. However, high temperature operation/fabrication of the fiber-reinforced composite might result in the degradation of the mechanical properties of W wires. To address this, we investigate mechanical and microstructural properties of potassium-doped tungsten wires, being heat treated at 2300 °C and tested in temperature range 22–600 °C. Based on the microscopic analysis, the engineering deformation curves are converted into actual stress - strain dataset, accounting for the Local Necking. The analysis demonstrates that Local strain in the Necking region can reach up to 50% and the total elongation monotonically increases with temperature, while the ultimate tensile strength goes down. Preliminary transmission electron microscopy analysis using FIB-cut lamella from the Necking region revealed the presence of curved dislocation lines in the sample tested at 300 °C, proving that plastic deformation occurred by dislocation glide.
Thilo F. Morgeneyer - One of the best experts on this subject based on the ideXlab platform.
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On deformation and damage micromechanisms in strong work hardening 2198 T3 aluminium alloy
Acta Materialia, 2018Co-Authors: Ante Buljac, François Hild, Lukas Helfen, Thilo F. MorgeneyerAbstract:Abstract The deformation and damage micromechanisms ahead of a notch in a large flat specimen have been assessed using in situ synchrotron laminography combined with digital volume correlation for the strain evaluation in the material bulk. Despite the enhanced work hardening of the naturally aged Al-Cu-Li alloy several slanted strained bands were found from very early loading onward in the slanted fracture region. The final slanted crack followed one of the strained bands without significant ductile damage growth. In the high stress triaxiality region, close to the notch that underwent substantial Local Necking, two damage micromechanisms were observed, namely, i) limited void nucleation and growth from intermetallic particles, and ii) slanted shear cracks, even starting from the specimen surface and also located in single grains as shown by post mortem EBSD analyses. In the intermediate region a new deformation mechanism of flip-flopping strain bands and resulting flip-flopping cracks have been revealed using “projection DIC” measurements.
Dmitry Terentyev - One of the best experts on this subject based on the ideXlab platform.
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plastic deformation of recrystallized tungsten potassium wires constitutive deformation law in the temperature range 22 600 c
International Journal of Refractory Metals & Hard Materials, 2018Co-Authors: Dmitry Terentyev, J Riesch, S Lebediev, T Khvan, A Zinovev, M Rasinski, A Dubinko, J W CoenenAbstract:Abstract Recent efforts dedicated to the mitigation of tungsten (W) brittleness have demonstrated that tungsten fiber-reinforced composites acquire extrinsic toughening even at room temperature, which is due to the outstanding strength of W wires. However, high temperature operation/fabrication of the fiber-reinforced composite might result in the degradation of the mechanical properties of W wires. To address this, we investigate mechanical and microstructural properties of potassium-doped tungsten wires, being heat treated at 2300 °C and tested in temperature range 22–600 °C. Based on the microscopic analysis, the engineering deformation curves are converted into actual stress - strain dataset, accounting for the Local Necking. The analysis demonstrates that Local strain in the Necking region can reach up to 50% and the total elongation monotonically increases with temperature, while the ultimate tensile strength goes down. Preliminary transmission electron microscopy analysis using FIB-cut lamella from the Necking region revealed the presence of curved dislocation lines in the sample tested at 300 °C, proving that plastic deformation occurred by dislocation glide.
Odd Sture Hopperstad - One of the best experts on this subject based on the ideXlab platform.
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low velocity impact on high strength steel sheets an experimental and numerical study
International Journal of Impact Engineering, 2016Co-Authors: Gaute Gruben, M Langseth, Egil Fagerholt, Odd Sture HopperstadAbstract:Abstract Low-velocity impact tests were performed on dual-phase and martensitic steel sheets and compared with corresponding quasi-static tests. The geometry and loading condition of the specimens were similar to formability tests, and the average strain rates before failure were in the range 80–210 s − 1 for the low-velocity tests and 0.002-0.005 s − 1 for the quasi-static tests. For both loading rates, the sheets failed under pre-dominant membrane loading, and by varying the specimen geometry, the stress states prior to failure ranged from uniaxial tension to equi-biaxial tension. Thus, the most important stress states occurring during an impact event in a thin-walled structure are covered. The experiments were complemented by nonlinear finite element simulations, where higher-order solid elements and a refined mesh were applied to capture the failure of the sheets. The materials were modelled using the Hershey high-exponent yield function combined with the associated flow rule and isotropic hardening. Quasi-static tensile and shear tests and tensile tests at elevated strain rates were performed to calibrate the constitutive relation. The results in terms of force-displacement curves and strain histories at critical positions in the specimens were similar for low-velocity and quasi-static loading, independent of material and specimen geometry. This indicates that the quasi-static test gives a good description of the sheet behaviour under low-velocity impact loading. The numerical simulations were found to be in good agreement with the experimental results, and strengthened the experimental finding that all the sheet-impact tests, except the martensitic steel sheet in a state close to equi-biaxial tension, displayed Local Necking before final fracture.
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Experimental detection of forming limit strains on samples with multiple Local necks
Journal of Materials Processing Technology, 2016Co-Authors: Dmitry Vysochinskiy, Odd Sture Hopperstad, O.-g. Lademo, Térence Coudert, Aase Gavina Roberg ReyesAbstract:Abstract Sheet metal formability is traditionally described by the forming limit curve (FLC). Experimental FLCs are obtained by performing formability tests and determining failure strains. The strains are usually measured either by etching a grid on the sheet surface or by digital image correlation (DIC). Ductile metal sheets fail primarily by Local Necking which introduces a severe strain gradient in the failure region. This makes accurate detection of the failure strains challenging. An international standard (ISO12004-2:2008) was introduced in 2008 to unify the procedure of FLC detection; prior to this large discrepancies were observed between the results reported by different laboratories. The main limitation of the standard method for detection of forming limits is that its application is limited to cases where a single Local neck is formed in the metal sheet prior to fracture. In the case of multiple Local necks, the samples are simply discarded. Furthermore, the standard method does not include any guidelines to distinguish the failure by Local Necking and direct failure by fracture. One of the advantages of DIC over the traditional etched-grid technique is that the former allows us to obtain not only the strain distribution but also its history. This allows for alternative methods for detection of forming limit strains. This paper introduces a DIC-based method which was specially developed to handle the case of multiple Local necks and to distinguish failure by Local Necking from direct fracture automatically. The method is not confined to a single test type and can be used in combination with different formability tests as long as DIC is used to measure strains.
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Experimental detection of the onset of Local Necking in an aluminium sheet
Materials Science Forum, 2014Co-Authors: Dmitry Vysochinskiy, Odd Sture Hopperstad, O.-g. Lademo, Térence Coudert, Aase Gavina Roberg ReyesAbstract:Forming limit diagrams (FLDs) are widely used to assess metal sheet formability. Experimental FLDs are obtained by performing formability tests and determining failure strains. The standard method for detection of forming limits is based on the spatial distribution of the strains and requires formation of a single Local neck. Some aluminium alloys, such as AA6016, have a tendency to form multiple strain Localizations in formability tests, which can be interpreted as multiple Local necks. Thus, use of the standard method is questionable for these aluminium alloys. The present paper presents an alternative, digital-image-correlation-based method for experimental detection of the onset of Local Necking in an aluminium sheet. The method is based on monitoring the sheet-thickness evolution, and is developed to be user independent and resistant to noise in the measurements. The method can be used in combination with different types of formability tests. The main requirement is that digital image correlation is used for strain measurements. Here, the method is initially tested on uniaxial tension tests of AA6016 aluminium alloy sheets and then extended to formability tests.
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analytical and numerical analysis of sheet metal instability using a stress based criterion
International Journal of Solids and Structures, 2008Co-Authors: Hagbart S Alsos, Odd Sture Hopperstad, Rikard Tornqvist, Jorgen AmdahlAbstract:Abstract Strain based Keeler–Goodwin diagrams are widely used in forming processes to predict onset of Local Necking. Plastic instability is determined once the forming limit strain is exceeded. Use of these diagrams requires proportional strain paths, which is not necessarily the case in sheet metal forming operations. In many forming processes, the strain path changes during deformation. This may change the forming limit curve significantly. In the paper, a stress based forming limit criterion is adopted to deal with strain path non-linearities. Comparisons with earlier published work on forming limits are made through analytical considerations. Furthermore, the criterion is implemented into the finite element code LS-DYNA and verified numerically against results from large scale bulge tests.
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Modelling of Local Necking and Fracture in Aluminium Alloys
AIP Conference Proceedings, 2007Co-Authors: D. Achani, Odd Sture Hopperstad, Magnus Eriksson, O.-g. LademoAbstract:Non‐linear Finite Element simulations are extensively used in forming and crashworthiness studies of automotive components and structures in which fracture need to be controlled. For thin‐walled ductile materials, the fracture‐related phenomena that must be properly represented are thinning instability, ductile fracture and through‐thickness shear instability. Proper representation of the fracture process relies on the accuracy of constitutive and fracture models and their parameters that need to be calibrated through well defined experiments. The present study focuses on Local Necking and fracture which is of high industrial importance, and uses a phenomenological criterion for modelling fracture in aluminium alloys. As an accurate description of plastic anisotropy is important, advanced phenomenological constitutive equations based on the yield criterion YLD2000/YLD2003 are used. Uniaxial tensile tests and disc compression tests are performed for identification of the constitutive model parameters. Ductile fracture is described by the Cockcroft‐Latham fracture criterion and an in‐plane shear tests is performed to identify the fracture parameter. The reason is that in a well designed in‐plane shear test no thinning instability should occur and it thus gives more direct information about the phenomenon of ductile fracture. Numerical simulations have been performed using a user‐defined material model implemented in the general‐purpose non‐linear FE code LS‐DYNA. The applicability of the model is demonstrated by correlating the predicted and experimental response in the in‐plane shear tests and additional plane strain tension tests.
M Rasinski - One of the best experts on this subject based on the ideXlab platform.
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plastic deformation of recrystallized tungsten potassium wires constitutive deformation law in the temperature range 22 600 c
International Journal of Refractory Metals & Hard Materials, 2018Co-Authors: Dmitry Terentyev, J Riesch, S Lebediev, T Khvan, A Zinovev, M Rasinski, A Dubinko, J W CoenenAbstract:Abstract Recent efforts dedicated to the mitigation of tungsten (W) brittleness have demonstrated that tungsten fiber-reinforced composites acquire extrinsic toughening even at room temperature, which is due to the outstanding strength of W wires. However, high temperature operation/fabrication of the fiber-reinforced composite might result in the degradation of the mechanical properties of W wires. To address this, we investigate mechanical and microstructural properties of potassium-doped tungsten wires, being heat treated at 2300 °C and tested in temperature range 22–600 °C. Based on the microscopic analysis, the engineering deformation curves are converted into actual stress - strain dataset, accounting for the Local Necking. The analysis demonstrates that Local strain in the Necking region can reach up to 50% and the total elongation monotonically increases with temperature, while the ultimate tensile strength goes down. Preliminary transmission electron microscopy analysis using FIB-cut lamella from the Necking region revealed the presence of curved dislocation lines in the sample tested at 300 °C, proving that plastic deformation occurred by dislocation glide.