Enhanced material models for the process design of the temperature dependent forming behavior of metastable steels

In this paper, the main parameters for the process design of thermo active deep drawing processes with metastable steels are specified. As a consequence of the strain- and temperature-dependent martensitic phase transformation, the CrNi-steels have a very complex forming and failure behavior. Particularly, the strong heating of the blank, induced by the hardening increase, influences the yield properties, the failure and the tribological system. Hence, a robust process design with FEM is subordinated to the correct virtual description of these process parameters. Both, experimental methods for an accurate measurement of the forming, friction and failure properties, as well as numerical models were introduced in this study. Using these models, temperature dependent parameters like phase transformation from γ-austenite to α′-martensite, yield properties, failure or friction can be easily included into the virtual process modeling. The model was successfully validated by biaxial stretching tests and rectangular cups, drawn at different tool temperature

New algorithm for a robust user-independent evaluation of beginning instability for the experimental FLC determination

The failure prediction in sheet metal forming is typically realized by evaluating the so called forming limit curves (FLC). The standard experimental method is the Nakajima test, where sometimes also the Marciniak test setup is used. Up to now, the FLC determination was performed with failed specimens and an one-directional intersection line method or by manually analyzing the estimated strains before cracking. Both methods determine the failure by considering the occurrence of cracking and do not consider the possibility of time continuous recording of the Nakajima test. Consequently forming limit curves which have been evaluated in such way are often "laboratory dependent” and deviate for identical materials significantly. This paper presents an algorithm for a fully automatic and time-dependent determination of the beginning plastic instability based on physical effects. The algorithm is based on the evaluation of the strain distribution based on the displacement field which is evaluated by optical measurement and treated as a mesh of a finite element calculation. The critical deformation states are then defined by 2D-consideration of the strain distribution and their time derivates using a numerical evaluation procedure for detecting the beginning of the localization. The effectiveness of the proposed algorithm will be presented for different materials used for the Numisheet'08 Benchmark-1 with Nakajima tes

A dislocation based material model for warm forming simulation

Based on experimental results, a dislocation material model describing the influence of dynamic strain aging on the deformation behavior at elevated temperatures is presented. One and two stage loading tests at different temperatures were performed in order to describe the hardening behavior during the deformation at elevated temperatures as well as the hardening behavior after the dynamic strain aging process. Bergström's theory of work hardening was used as a basis for the model development. In the proposed model a relationship between material coefficients of the classical Bergström model and temperature was investigated. The aim of the new material model was to introduce the least possible amount of new parameters as well as to facilitate the mathematical determination of parameters during the fitting of the model with the experimental data. The developed model was implemented in an in-house FE-Code in order to simulate the material behavior due to the dynamic strain aging. Representative simulation results were compared with the experimental data in order to validate the efficiency and the application range of the model. Simulation of the forming process provided data for optimizing strength properties and enabled process contro

Damage dependent stress limit model for failure prediction in bulk forming processes

Stress limit, combined with deformation and orientation dependent damages, is proposed as the criterion for failure prediction in bulk forming processes. Dedicatedly designed experiments are carried out to investigate this model. The results verify the existence of the stress limits. The value of stress limit can vary in different directions due to the material processing histories. The experiments revealed also strong kinematical hardening behaviours for the investigated material. The constitutive law has to include this effect to evaluate the stresses with sufficient precisio

A strain rate dependent anisotropic hardening model and its validation through deep drawing experiments

In the present work, a modified version of the widely used Yld2000-2d yield function and its implementation into the commercial FE-code LS-Dyna is presented. The difference to the standard formulation lies in the dependency of the function parameters on the equivalent plastic strain. Furthermore, strain rate dependency is incorporated. After a detailed description of the model and the identification of the parameters, the numerical implementation i.e., the stress-update algorithm used for the implementation is explained. In order to validate the model, two different materials, namely Formalex™5x, a 5182-based aluminum alloy and a DC05 mild steel were characterized. The results of the tensile and hydraulic bulge tests are presented and used for the parameter identification. The experimental curves are reproduced by means of one element tests using the standard and modified model to demonstrate the benefit of the modifications. For validation purposes, cross die geometries were drawn with both materials. The outer surface strains were measured with an optical measurement system. The measured major and minor strains were compared to the results of simulations using the standard and the modified Yld2000-2d model. A significant improvement in prediction accuracy has been demonstrated

Thermal Emission and Tidal Heating of the Heavy and Eccentric Planet XO-3b

We determined the flux ratios of the heavy and eccentric planet XO-3b to its parent star in the four IRAC bands of the Spitzer Space Telescope: 0.101% +- 0.004% at 3.6 micron; 0.143% +- 0.006% at 4.5 micron; 0.134% +- 0.049% at 5.8 micron and 0.150% +- 0.036% at 8.0 micron. The flux ratios are within [-2.2,0.3, -0.8, -1.7]-sigma of the model of XO-3b with a thermally inverted stratosphere in the 3.6 micron, 4.5 micron, 5.8 micron and 8.0 micron channels, respectively. XO-3b has a high illumination from its parent star (Fp ~(1.9 - 4.2) x 10^9 ergs cm^-2 s^-1) and is thus expected to have a thermal inversion, which we indeed observe. When combined with existing data for other planets, the correlation between the presence of an atmospheric temperature inversion and the substellar flux is insufficient to explain why some high insolation planets like TrES-3 do not have stratospheric inversions and some low insolation planets like XO-1b do have inversions. Secondary factors such as sulfur chemistry, atmospheric metallicity, amounts of macroscopic mixing in the stratosphere or even dynamical weather effects likely play a role. Using the secondary eclipse timing centroids we determined the orbital eccentricity of XO-3b as e = 0.277 +- 0.009. The model radius-age trajectories for XO-3b imply that at least some amount of tidal-heating is required to inflate the radius of XO-3b, and the tidal heating parameter of the planet is constrained to Qp < 10^6 .Comment: Accepted for publications in The Astrophysical Journa

Nanosized TiO2: a promising catalyst for the aldol condensation of furfural with acetone in biomass upgrading

Nanosized TiO2catalyst was successfully prepared by a simple green procedure and used in liquid phasealdol condensation of furfural with acetone, a key step in bio-fuel processing. In order to determinethe effect of calcination temperature on catalytic properties of TiO2, the as-prepared TiO2and calcinedTiO2(150–900◦C) were studied by XRD, BET, TPD-CO2/NH3, TGA/DTG and FTIR evaluation. The catalyticperformance of TiO2samples in aldol condensation of furfural with acetone was evaluated and comparedwith that of Mg–Al hydrotalcites and a BEA zeolite. These experiments showed that uncalcined TiO2possessed reasonable activity in aldol condensation of furfural to acetone and resulted in commonlyproduced condensation products. The observed catalytic behavior of TiO2could be competitive withthat reported for other inorganic solids. The calcination of TiO2resulted, however, in a decrease in itscatalytic activity due to extensive dehydration and surface dehydroxylation as well as due to changes oftextural properties resulting in a decrease in the amount of accessible active sites. Thanks to its advancedproperties, nanosized TiO2is a promising catalyst for aldol condensation of furfural with acetone andcould broaden possibilities for optimizing conditions for bio-fuel production