The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Daniel Bonn - One of the best experts on this subject based on the ideXlab platform.
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quasi periodic and irregular motion of a solid sphere falling through a thixotropic yield stress fluid
Applied Physics Express, 2017Co-Authors: Mina Fazilati, Nahid Malekijirsaraei, S Rouhani, Daniel BonnAbstract:We report the observation of the oscillatory and irregular motion of solid spheres settling under the influence of gravity in a thixotropic yield-stress fluid, namely, a suspension of Laponite. The size of the ball and the aging time of the Laponite suspension are found to be two important parameters that determine whether oscillations occur. The irregular motion may be related to the existence of an unstable Flow region and shear banding as is concluded from comparisons with rheological measurements, namely, the Flow Curve and creep tests, using the same Laponite suspensions.
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s shaped Flow Curves of shear thickening suspensions direct observation of frictional rheology
Physical Review E, 2015Co-Authors: Zhongcheng Pan, Henri De Cagny, Bart Weber, Daniel BonnAbstract:We study the rheological behavior of concentrated granular suspensions of simple spherical particles. Under controlled stress, the system exhibits an S-shaped Flow Curve (stress vs shear rate) with a negative slope in between the low-viscosity Newtonian regime and the shear thickened regime. Under controlled shear rate, a discontinuous transition between the two states is observed. Stress visualization experiments with a fluorescent probe suggest that friction is at the origin of shear thickening. Stress visualization shows that the stress in the system remains homogeneous (no shear banding) if a stress is imposed that is intermediate between the high- and low-stress branches. The S-shaped shear thickening is then due to the discontinuous formation of a frictional force network between particles upon increasing the stress.
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viscosity bifurcation in thixotropic yielding fluids
Journal of Rheology, 2002Co-Authors: Philippe Coussot, Q D Nguyen, H T Huynh, Daniel BonnAbstract:Most concentrated colloidal suspensions such as cement, drilling fluids, paints, muds, etc., have been considered until now thixotropic fluids with a Flow Curve of an ideal yield stress fluid. We start by showing from inclined plane tests, intended to determine the yield stress, that these systems in fact exhibit peculiar properties. Unlike ideal yield stress fluids, they stop Flowing abruptly below a critical stress, and start Flowing at a high velocity beyond a critical stress, which in addition increases with the time of preliminary rest. In order to clarify these features we carried out a complete set of rheometrical tests with a model fluid, a bentonite suspension. Our results show that under controlled stress, in some cases after significant Flow, there is bifurcation of the behavior towards either stoppage or rapid shear, depending on the relative values of the imposed and critical stresses. As an immediate consequence, we find that no (homogeneous) steady state Flows at a shear rate below a critical value can be obtained. These results can be qualitatively predicted by a simple theoretical model that assumes that the viscosity of the material results from the competition between aging and shear rejuvenation, associated to, respectively, the organization or disorganization of the network of particle interactions. This shows that the Flow Curve in the steady state of concentrated colloidal suspensions and, more generally, of structured fluids, is strongly affected by their thixotropy.
Wolfgang Bleck - One of the best experts on this subject based on the ideXlab platform.
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quantification of the effect of transformation induced geometrically necessary dislocations on the Flow Curve modelling of dual phase steels
International Journal of Plasticity, 2013Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Alexander Schwedt, Peyman Goravanchi, Wolfgang BleckAbstract:Abstract The current work aims to predict the work-hardening behaviour of dual-phase (DP) steel, focusing on the effect of transformation-induced geometrically necessary dislocations (GNDs). Equiaxed and banded microstructures were produced through suitable heat treatment cycles in a laboratory. Electron backscatter diffraction measurements were performed to characterise GNDs. The Flow behaviour was modelled within the microscale finite element method, considering the effect of the microstructures using the representative volume element (RVE) approach. 2-D RVEs were created based on real microstructures. The Flow behaviour of single phases was modelled using the dislocation-based work-hardening approach. The volume change during the austenite-to-martensite transformation was also modelled, and the resulting prestrained areas in ferrite were considered to be the storage place of GNDs. The thickness of the GND layer around martensite islands was quantified experimentally and numerically. Subsequently, three criteria were developed to describe the strength, thickness, and amount of prestrain in the GND zone as a function of microstructural features in DP steel. Then, numerical uniaxial loading in the rolling direction was applied on the RVEs to study the effect of GNDs on the stress and strain distribution in the microstructures, Flow Curve, and hardening behaviour of DP steel. A computational first-order homogenisation strategy was employed to obtain the true stress–true strain Curves from the RVE calculations. The Flow Curves of simulations that took the GNDs into account were in better agreement with the experimental Flow Curves, compared with those of simulations that did not consider the GNDs.
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Correlation between 2D and 3D Flow Curve modelling of DP steels using a microstructure-based RVE approach
Materials Science and Engineering: A, 2013Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Hendrik Quade, Wolfgang BleckAbstract:Abstract A microstructure-based approach by means of representative volume elements (RVEs) is employed to evaluate the Flow Curve of DP steels using virtual tensile tests. Microstructures with different martensite fractions and morphologies are studied in two- and three-dimensional approaches. Micro sections of DP microstructures with various amounts of martensite have been converted to 2D RVEs, while 3D RVEs were constructed statistically with randomly distributed phases. A dislocation-based model is used to describe the Flow Curve of each ferrite and martensite phase separately as a function of carbon partitioning and microstructural features. Numerical tensile tests of RVE were carried out using the ABAQUS/Standard code to predict the Flow behaviour of DP steels. It is observed that 2D plane strain modelling gives an underpredicted Flow Curve for DP steels, while the 3D modelling gives a quantitatively reasonable description of Flow Curve in comparison to the experimental data. In this work, a von Mises stress correlation factor σ3D/σ2D has been identified to compare the predicted Flow Curves of these two dimensionalities showing a third order polynomial relation with respect to martensite fraction and a second order polynomial relation with respect to equivalent plastic strain, respectively. The quantification of this polynomial correlation factor is performed based on laboratory-annealed DP600 chemistry with varying martensite content and it is validated for industrially produced DP qualities with various chemistry, strength level and martensite fraction.
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transformation induced geometrically necessary dislocation based Flow Curve modeling of dual phase steels effect of grain size
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2012Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Wolfgang BleckAbstract:The Flow behavior of dual-phase (DP) steels is modeled on the finite-element method (FEM) framework on the microscale, considering the effect of the microstructure through the representative volume element (RVE) approach. Two-dimensional RVEs were created from microstructures of experimentally obtained DP steels with various ferrite grain sizes. The Flow behavior of single phases was modeled through the dislocation-based work-hardening approach. The volume change during austenite-to-martensite transformation was modeled, and the resultant prestrained areas in the ferrite were considered to be the storage place of transformation-induced, geometrically necessary dislocations (GNDs). The Flow Curves of DP steels with varying ferrite grain sizes, but constant martensite fractions, were obtained from the literature. The Flow Curves of simulations that take into account the GND are in better agreement with those of experimental Flow Curves compared with those of predictions without consideration of the GND. The experimental results obeyed the Hall-Petch relationship between yield stress and Flow stress and the simulations predicted this as well.
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modelling the effect of microstructural banding on the Flow Curve behaviour of dual phase dp steels
Computational Materials Science, 2012Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Wolfgang BleckAbstract:Abstract Dual-phase steels (DP) are well suited for automotive application due to their attractive mechanical properties, such as high strength and good formability. These properties are achieved by the dispersion of hard martensite particles in the soft and ductile ferrite matrix. The current work aims to predict the mechanical properties of dual-phase steels. A microstructure based approach by means of representative volume element (RVE) was employed for this purpose. Available and novel routines were used to create the 2D RVEs from the real microstructures. Periodic and homogeneous boundary conditions were imposed. Dislocation based model was implemented to predict the Flow behaviour of the single phases. Computational first order homogenization strategy was employed to obtain the true stress–true strain Curves from the RVE calculations. The implementation of the periodic boundary condition results in a better agreement with the converged effective value compared to the displacement boundary condition. Equiaxed microstructures show higher strength and work hardening compared to that of the banded microstructures. In the same fraction of martensite, the yield stress of DP steels decreases by increasing the aspect ratio of martensite bands.
Ali Ramazani - One of the best experts on this subject based on the ideXlab platform.
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quantification of the effect of transformation induced geometrically necessary dislocations on the Flow Curve modelling of dual phase steels
International Journal of Plasticity, 2013Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Alexander Schwedt, Peyman Goravanchi, Wolfgang BleckAbstract:Abstract The current work aims to predict the work-hardening behaviour of dual-phase (DP) steel, focusing on the effect of transformation-induced geometrically necessary dislocations (GNDs). Equiaxed and banded microstructures were produced through suitable heat treatment cycles in a laboratory. Electron backscatter diffraction measurements were performed to characterise GNDs. The Flow behaviour was modelled within the microscale finite element method, considering the effect of the microstructures using the representative volume element (RVE) approach. 2-D RVEs were created based on real microstructures. The Flow behaviour of single phases was modelled using the dislocation-based work-hardening approach. The volume change during the austenite-to-martensite transformation was also modelled, and the resulting prestrained areas in ferrite were considered to be the storage place of GNDs. The thickness of the GND layer around martensite islands was quantified experimentally and numerically. Subsequently, three criteria were developed to describe the strength, thickness, and amount of prestrain in the GND zone as a function of microstructural features in DP steel. Then, numerical uniaxial loading in the rolling direction was applied on the RVEs to study the effect of GNDs on the stress and strain distribution in the microstructures, Flow Curve, and hardening behaviour of DP steel. A computational first-order homogenisation strategy was employed to obtain the true stress–true strain Curves from the RVE calculations. The Flow Curves of simulations that took the GNDs into account were in better agreement with the experimental Flow Curves, compared with those of simulations that did not consider the GNDs.
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Correlation between 2D and 3D Flow Curve modelling of DP steels using a microstructure-based RVE approach
Materials Science and Engineering: A, 2013Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Hendrik Quade, Wolfgang BleckAbstract:Abstract A microstructure-based approach by means of representative volume elements (RVEs) is employed to evaluate the Flow Curve of DP steels using virtual tensile tests. Microstructures with different martensite fractions and morphologies are studied in two- and three-dimensional approaches. Micro sections of DP microstructures with various amounts of martensite have been converted to 2D RVEs, while 3D RVEs were constructed statistically with randomly distributed phases. A dislocation-based model is used to describe the Flow Curve of each ferrite and martensite phase separately as a function of carbon partitioning and microstructural features. Numerical tensile tests of RVE were carried out using the ABAQUS/Standard code to predict the Flow behaviour of DP steels. It is observed that 2D plane strain modelling gives an underpredicted Flow Curve for DP steels, while the 3D modelling gives a quantitatively reasonable description of Flow Curve in comparison to the experimental data. In this work, a von Mises stress correlation factor σ3D/σ2D has been identified to compare the predicted Flow Curves of these two dimensionalities showing a third order polynomial relation with respect to martensite fraction and a second order polynomial relation with respect to equivalent plastic strain, respectively. The quantification of this polynomial correlation factor is performed based on laboratory-annealed DP600 chemistry with varying martensite content and it is validated for industrially produced DP qualities with various chemistry, strength level and martensite fraction.
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transformation induced geometrically necessary dislocation based Flow Curve modeling of dual phase steels effect of grain size
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2012Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Wolfgang BleckAbstract:The Flow behavior of dual-phase (DP) steels is modeled on the finite-element method (FEM) framework on the microscale, considering the effect of the microstructure through the representative volume element (RVE) approach. Two-dimensional RVEs were created from microstructures of experimentally obtained DP steels with various ferrite grain sizes. The Flow behavior of single phases was modeled through the dislocation-based work-hardening approach. The volume change during austenite-to-martensite transformation was modeled, and the resultant prestrained areas in the ferrite were considered to be the storage place of transformation-induced, geometrically necessary dislocations (GNDs). The Flow Curves of DP steels with varying ferrite grain sizes, but constant martensite fractions, were obtained from the literature. The Flow Curves of simulations that take into account the GND are in better agreement with those of experimental Flow Curves compared with those of predictions without consideration of the GND. The experimental results obeyed the Hall-Petch relationship between yield stress and Flow stress and the simulations predicted this as well.
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modelling the effect of microstructural banding on the Flow Curve behaviour of dual phase dp steels
Computational Materials Science, 2012Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Wolfgang BleckAbstract:Abstract Dual-phase steels (DP) are well suited for automotive application due to their attractive mechanical properties, such as high strength and good formability. These properties are achieved by the dispersion of hard martensite particles in the soft and ductile ferrite matrix. The current work aims to predict the mechanical properties of dual-phase steels. A microstructure based approach by means of representative volume element (RVE) was employed for this purpose. Available and novel routines were used to create the 2D RVEs from the real microstructures. Periodic and homogeneous boundary conditions were imposed. Dislocation based model was implemented to predict the Flow behaviour of the single phases. Computational first order homogenization strategy was employed to obtain the true stress–true strain Curves from the RVE calculations. The implementation of the periodic boundary condition results in a better agreement with the converged effective value compared to the displacement boundary condition. Equiaxed microstructures show higher strength and work hardening compared to that of the banded microstructures. In the same fraction of martensite, the yield stress of DP steels decreases by increasing the aspect ratio of martensite bands.
Morton M Denn - One of the best experts on this subject based on the ideXlab platform.
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nonmonotonic Flow Curves of shear thickening suspensions
Physical Review E, 2015Co-Authors: Romain Mari, Ryohei Seto, Jeffrey F Morris, Morton M DennAbstract:The discontinuous shear thickening (DST) of dense suspensions is a remarkable phenomenon in which the viscosity can increase by several orders of magnitude at a critical shear rate. It has the appearance of a first-order phase transition between two hypothetical ``states'' that we have recently identified as Stokes Flows with lubricated or frictional contacts, respectively. Here we extend the analogy further by means of stress-controlled simulations and show the existence of a nonmonotonic steady-state Flow Curve analogous to a nonmonotonic equation of state. While we associate DST with an $\mathsf{S}$-shaped Flow Curve, at volume fractions above the shear jamming transition the frictional state loses Flowability and the Flow Curve reduces to an arch, permitting the system to Flow only at small stresses. Whereas a thermodynamic transition leads to phase separation in the coexistence region, we observe a uniform shear Flow all along the thickening transition. A stability analysis suggests that uniform shear may be mechanically stable for the small Reynolds numbers and system sizes in a rheometer.
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disappearance of extrusion instabilities in brass capillary dies
Journal of Rheology, 1999Co-Authors: Venu G Ghanta, Brian L Riise, Morton M DennAbstract:We have extruded a linear low-density polyethylene through capillary dies fabricated from stainless steel and brass. We confirm a result first reported by Ramamurthy [Ramamurthy, A. V., “Wall Slip in Viscous Fluids and Influence of Materials of Construction,” J. Rheol. 30, 337–357 (1986)]: sharkskin can be eliminated by the use of a brass die. We also find a substantially enhanced throughput from the brass die relative to the stainless-steel die at stresses in the range where sharkskin is observed with the latter. Finally, the large pressure oscillations and periodic extrudate distortions observed in the “slip–stick” region with stainless steel are absent with brass, where the transition to the “upper branch” of the Flow Curve is more gradual.
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the effect of die materials and pressure dependent slip on the extrusion of linear low density polyethylene
Journal of Rheology, 1997Co-Authors: Morton M DennAbstract:The Flow of linear low-density polyethylene through stainless-steel slit dies occurred at shear rates approximately 12% higher than in identical α-brass dies at the same wall shear stresses, indicating near-wall slip. The Flow Curves were independent of gap spacing. We show through the slip theory of Hill and co-workers [J. Rheol. 34, 891–918 (1990)] that a measurable gap dependence of the Flow Curve is not a necessary consequence of wall slip; the Flow Curves for both stainless steel and α-brass dies can be fit with the same rheological parameters, with a difference in the work of adhesion accounting for the differences in the Flow Curves. X-ray photoelectron spectroscopy revealed differences in the chemistry of brass surfaces with different pretreating, corresponding to small differences in Flow Curves. Fluorocarbon-coated die surfaces showed no more slip than stainless steel, while the Flow Curve with gold-coated surfaces followed stainless steel at intermediate stress and brass at high stress.
K. Mukherjee - One of the best experts on this subject based on the ideXlab platform.
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quantification of the effect of transformation induced geometrically necessary dislocations on the Flow Curve modelling of dual phase steels
International Journal of Plasticity, 2013Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Alexander Schwedt, Peyman Goravanchi, Wolfgang BleckAbstract:Abstract The current work aims to predict the work-hardening behaviour of dual-phase (DP) steel, focusing on the effect of transformation-induced geometrically necessary dislocations (GNDs). Equiaxed and banded microstructures were produced through suitable heat treatment cycles in a laboratory. Electron backscatter diffraction measurements were performed to characterise GNDs. The Flow behaviour was modelled within the microscale finite element method, considering the effect of the microstructures using the representative volume element (RVE) approach. 2-D RVEs were created based on real microstructures. The Flow behaviour of single phases was modelled using the dislocation-based work-hardening approach. The volume change during the austenite-to-martensite transformation was also modelled, and the resulting prestrained areas in ferrite were considered to be the storage place of GNDs. The thickness of the GND layer around martensite islands was quantified experimentally and numerically. Subsequently, three criteria were developed to describe the strength, thickness, and amount of prestrain in the GND zone as a function of microstructural features in DP steel. Then, numerical uniaxial loading in the rolling direction was applied on the RVEs to study the effect of GNDs on the stress and strain distribution in the microstructures, Flow Curve, and hardening behaviour of DP steel. A computational first-order homogenisation strategy was employed to obtain the true stress–true strain Curves from the RVE calculations. The Flow Curves of simulations that took the GNDs into account were in better agreement with the experimental Flow Curves, compared with those of simulations that did not consider the GNDs.
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Correlation between 2D and 3D Flow Curve modelling of DP steels using a microstructure-based RVE approach
Materials Science and Engineering: A, 2013Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Hendrik Quade, Wolfgang BleckAbstract:Abstract A microstructure-based approach by means of representative volume elements (RVEs) is employed to evaluate the Flow Curve of DP steels using virtual tensile tests. Microstructures with different martensite fractions and morphologies are studied in two- and three-dimensional approaches. Micro sections of DP microstructures with various amounts of martensite have been converted to 2D RVEs, while 3D RVEs were constructed statistically with randomly distributed phases. A dislocation-based model is used to describe the Flow Curve of each ferrite and martensite phase separately as a function of carbon partitioning and microstructural features. Numerical tensile tests of RVE were carried out using the ABAQUS/Standard code to predict the Flow behaviour of DP steels. It is observed that 2D plane strain modelling gives an underpredicted Flow Curve for DP steels, while the 3D modelling gives a quantitatively reasonable description of Flow Curve in comparison to the experimental data. In this work, a von Mises stress correlation factor σ3D/σ2D has been identified to compare the predicted Flow Curves of these two dimensionalities showing a third order polynomial relation with respect to martensite fraction and a second order polynomial relation with respect to equivalent plastic strain, respectively. The quantification of this polynomial correlation factor is performed based on laboratory-annealed DP600 chemistry with varying martensite content and it is validated for industrially produced DP qualities with various chemistry, strength level and martensite fraction.
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transformation induced geometrically necessary dislocation based Flow Curve modeling of dual phase steels effect of grain size
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2012Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Wolfgang BleckAbstract:The Flow behavior of dual-phase (DP) steels is modeled on the finite-element method (FEM) framework on the microscale, considering the effect of the microstructure through the representative volume element (RVE) approach. Two-dimensional RVEs were created from microstructures of experimentally obtained DP steels with various ferrite grain sizes. The Flow behavior of single phases was modeled through the dislocation-based work-hardening approach. The volume change during austenite-to-martensite transformation was modeled, and the resultant prestrained areas in the ferrite were considered to be the storage place of transformation-induced, geometrically necessary dislocations (GNDs). The Flow Curves of DP steels with varying ferrite grain sizes, but constant martensite fractions, were obtained from the literature. The Flow Curves of simulations that take into account the GND are in better agreement with those of experimental Flow Curves compared with those of predictions without consideration of the GND. The experimental results obeyed the Hall-Petch relationship between yield stress and Flow stress and the simulations predicted this as well.
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modelling the effect of microstructural banding on the Flow Curve behaviour of dual phase dp steels
Computational Materials Science, 2012Co-Authors: Ali Ramazani, Ulrich Prahl, K. Mukherjee, Wolfgang BleckAbstract:Abstract Dual-phase steels (DP) are well suited for automotive application due to their attractive mechanical properties, such as high strength and good formability. These properties are achieved by the dispersion of hard martensite particles in the soft and ductile ferrite matrix. The current work aims to predict the mechanical properties of dual-phase steels. A microstructure based approach by means of representative volume element (RVE) was employed for this purpose. Available and novel routines were used to create the 2D RVEs from the real microstructures. Periodic and homogeneous boundary conditions were imposed. Dislocation based model was implemented to predict the Flow behaviour of the single phases. Computational first order homogenization strategy was employed to obtain the true stress–true strain Curves from the RVE calculations. The implementation of the periodic boundary condition results in a better agreement with the converged effective value compared to the displacement boundary condition. Equiaxed microstructures show higher strength and work hardening compared to that of the banded microstructures. In the same fraction of martensite, the yield stress of DP steels decreases by increasing the aspect ratio of martensite bands.