Papers by Keyword: Rheometry

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Authors: Michael Modigell, Torsten Volkmann, Christoph Zang
Abstract: Rheometrical analysis of steel slurries in rotational rheometers requires a range of operating temperatures of about 1300 to 1600 °C, leading to a delicate device-related challenge with respect to the applied materials on the one hand and a suitable constructional set-up gaining unbiased measurements on the other hand. Accordingly, a new high-precision rotational rheometer for temperatures up to 1700 °C has been developed as an improvement on an existing rheometer previously developed at the Chair of Mechanical Process Engineering (AVT) of RWTH Aachen University. First measurements on the tool steel X210CrW12 in the mushy state with solid fractions of 20% to 40% demonstrate a remarkable improvement on the preciseness of torque gain which broadens the area of possible experimental operations on steel slurries in order to reliably characterize transient behavior of the material by e.g. creep, relaxation and oscillation experiments.
Authors: Michael Modigell, Lars Pape, Ksenija Vasilic
Abstract: The direct analysis and the inverse parameter fitting are methods for determination of unknown material parameters in rheometry. In this paper the efficiency of the both methods is examined on the case of a semi-solid alloy. The difference between the obtained material parameters is studied and the necessity of inverse parameter fitting is discussed.
Authors: Da Hu Yao, Kyung Bok Sun, Peng Li, Joong Hee Lee
Abstract: The curing reaction of the system bisphenol-A glycidol ether epoxy resin modified by poly (ethylene glycol) (PEO) and flexible amine (D-230) as curing agent has been studied by means of differential scanning calorimetry (DSC) and thermal scanning rheometry. The curing kinetic parameters have been calculated from the non-thermal DSC curve. The kinetic analysis suggests that the two-parameter autocatalytic model is more appropriate to describe the kinetics of the curing reaction of the system. Increasing the PU content leads to an increase in the heat of curing and has a little effect on the kinetic parameters apparent activation energy (Ea), pre-exponential factor (A), and order of the reaction (m and n). The rheological properties were measured by isothermal curing evolution. Introduction of PEO flexible chains delayed the polymerization. It has been confirmed that the introduction of PEO chains in the structure of the epoxy resin increases the mobility of the molecular segment of the epoxy networks and results in the decrease in glass transition temperature.
Authors: Andreas N. Alexandrou, Georgios C. Georgiou, Eva Athena Economides, Michael Modigell
Abstract: In this work we revisit the issue of obtaining true material constants for semisolid slurries. Therefore, we consider the circular Couette flow of Herschel-Bulkley fluids. We first show how true constants can be obtained using an iterative procedure from experimental data to theory and vice versa. The validity of the assumption that the rate-of-strain distributions across the gap share a common point is also investigated. It is demonstrated that this is true only for fully-yielded Bingham plastics. In other cases, e.g., for partially-yielded Bingham plastics or fully-yielded Herschel-Bulkley materials, the common point for the fully-yielded Bingham case provides a good approximation for determining the material constants only if the gap is sufficiently small. It can thus be used to simplify the iterative procedure in determining the material constants.
Authors: H.V. Atkinson
Abstract: The numerical modeling of semi-solid processing requires data on the rheological properties of materials. This data is often obtained by rheometry but there are difficulties with characterizing high solid fractions, where the torque which can be exerted with the rheometer is insufficient. A number of other methods for measuring the flow parameters, including compression between platens, have been utilized. The various methods will be reviewed in this paper. Computational fluid dynamics modelers have often used data from steady state experiments but it is the behaviour during rapid transients which is more relevant to the actual semi-solid processing route.
Authors: Boris Ouriev
Abstract: Understanding of flow properties and flow effects of liquid and semisolid aluminum became a key solution for know-how of casting process. Therefore such properties must be characterized a priory to layout of flow process parameters in order to predict structure formation of aluminum in flow. In order to reach target of material characterization it becomes essential to analyze materials under as close to real process conditions as possible. This task was solved by strong modification of commercial rotational rheometer and application of high-resolution temperature control. Besides understanding the flow properties it is essential to find the way of interactive structure control during flow process. Therefore controllable effects were generated and studied with the help of structure related rheological flow properties. For triggering structure formation an influence of mechanical vibration on flow properties of highly concentrated semisolid alloy is explored in this work. For that experimental set-up was designed and adapted to conventional rotational rheometer with precise rheological characterization capability. Priory to fundamental experiments with highly concentrated aluminum suspension a number of calibration tests were performed. Also prediction of wall slippage in shear flow under vibration was evaluated. Analysis of boundary conditions shows that no considerable side effects were present during shear experiment under vibration. The research reveals precise detection of transition temperatures with the help of steady and transient shear viscosity measurement besides selective measurement of full rheological curves within liquid and semisolid state temperature range. Rheological characterization was performed under shear flow conditions with and without presence of orthogonal to flow direction mechanical vibration. It was found that superposition of mechanical vibration and shear flow radically decreases shear viscosity but only in semisolid state. Liquid state rheological properties shows structural behavior but kept insensitive to application of mechanical vibration. For semisolid alloys, comparison between reference shear viscosities at specified shear rates and those measured under vibration shows considerable differences in flow properties. Conversion of concentrated suspension from strongly shear-thinning to almost Newtonian flow behavior is reported here. It is suggested to relate such phenomenon to non-equilibrium between structure formation and disintegration under vibration and hydrodynamic forces of shear flow. Influence of vibration on structure formation was also well observed during measurement of solidification process. Comparison to reference data shows how sensitive structure of concentrated suspension is to vibration in general and especially during solidification phase. The reveled effects and observations provide a solid bases for further fundamental investigations of structure formation regularities in flow of any highly concentrated systems.
Authors: Michael Modigell, Lars Pape, Horst R. Maier
Abstract: The investigation of the flow behavior of semi-solid steel alloys poses a great challenge by reason of high operation temperatures and the need of ceramic components for the measuring system. A high-temperature Couette rheometer has been developed to analyze flow properties of semi-solid alloys up to temperatures of 1500°C. In the present work the alloy under investigation is X210CrW12. Differently performed experiments clearly show its shear-thinning and time-dependent flow properties with a yield stress. This flow behavior is modeled using a Herschel-Bulkley approach where the experimental results achieved serve to identify model parameters.
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