Thomas Henke

Bio: Thomas Henke is an academic researcher from RWTH Aachen University. The author has contributed to research in topics: Recrystallization (metallurgy) & Flow stress. The author has an hindex of 6, co-authored 21 publications receiving 99 citations.

Derivation of anisotropic flow curves of ferrite–pearlite pipeline steel via a two-level homogenisation scheme

Abstract: In order to calculate the effective macroscopic stress–strain curves of a pearlitic–ferritic pipeline steel, a multi-scale approach based on the homogenisation method is presented. Starting from an experimental material characterisation after the cooling process, material models involving microstructural features (e.g. grain size, carbon content) are derived for each phase. Since pearlite is a eutectoid phase mixture, embedded in a ferrite matrix, three length scales are introduced: the nano-scale of a ferrite–cementite bi-lamella of pearlite, the mesoscopic pearlite/ferrite microstructure and the macro-scale of the component. Firstly homogenisation techniques are applied to a bi-lamella Representative Volume Element (RVE) of pearlite. Initial predictions based on the assumption of a hard, elastic cementite phase are further improved by considering its yield behaviour. The present study outlines that cementite is not only anisotropic in elasticity but also in plasticity. Due to uncertainties about the yield behaviour of cementite, a sensitivity analysis has been performed. Secondly pearlite is treated at the micro-scale as an effective phase in an elasto-plastic ferrite matrix. Virtual tensile and shear tests are performed in order to derive the effective flow curves of the pearlite/ferrite microstructures. Comparisons with experimental stress–strain curves in rolling and transverse directions illustrate the accuracy and entitlement of the two-level homogenisation scheme.

Modelling of static recrystallization kinetics by coupling crystal plasticity FEM and multiphase field calculations

Abstract: In multi-step hot forming processes, static recrystallization (SRX), which occurs in interpass times, influences the microstructure evolution, the flow stress and the final product properties. Static recrystallization is often simply modeled based on Johnson-Mehl-Avrami-Kolmogorov (JMAK) equations which are linked to the visco-plastic flow behavior of the material. Such semi-empirical models are not able to predict the SRX grain microstructure. In this paper, an approach for the simulation of static recrystallization of austenitic grains is presented which is based on the coupling of a crystal plasticity method with a multiphase field approach. The microstructure is modeled by a representative volume element (RVE) of a homogeneous austenitic grain structure with periodic boundary conditions. The grain microstructure is gener-ated via a Voronoi tessellation. The deformation of the RVE, considering the evolution of grain orientations and disloca-tion density, is calculated using a crystal plasticity finite element (CP-FEM) formulation, whose material parameters have been calibrated using experimental flow curves of the considered 25MoCrS4 steel. The deformed grain structure (disloca-tion density, orientation) is transferred to the FDM grid used in the multiphase field approach by a dedicated interpolation scheme. In the phase field calculation, driving forces for static recrystallization are calculated based on the mean energy per grain and the curvature of the grain boundaries. A simplified nucleation model at the grain level is used to initiate the recrystallization process. Under these assumptions, it is possible to approximate the SRX kinetics obtained from the stress relaxation test, but the grain morphology predicted by the 2d model still differs from experimental findings.

Towards integrative computational materials engineering of steel components

Abstract: This article outlines on-going activities at the RWTH Aachen University aiming at a standardized, modular, extendable and open simulation platform for materials processing. This platform on the one hand facilitates the information exchange between different simulation tools and thus strongly reduces the effort to design/re-design production processes. On the other hand, tracking of simulation results along the entire production chain provides new insights into mechanisms, which cannot be explained on the basis of individual simulations. Respective simulation chains provide e.g. the basis for the determination of materials and component properties, like e.g. distortions, for an improved product quality, for more efficient and more reliable production processes and many further aspects. After a short introduction to the platform concept, actual examples for different test case scenarios will be presented and discussed.

Experimental Uncertainties affecting the Accuracy of Stress‐Strain Equations by the Example of a Hensel‐Spittel Approach

Abstract: The accuracy of numerical simulations in metal forming highly depends on the description of the plastic flow behavior. Due to experimental uncertainties flow curves recorded at equal testing conditions (combinations of temperature and strain rate) show scatter. This scatter influences the fit of material models and the resulting fit parameters. In this paper, factors causing uncertainties and systematic errors as well as ways to statistically describe uncertainties in the flow stress are analyzed by means of finite element simulations and experimental analyses of compression tests. To this end, compression tests were conducted for a 25MoCr4 steel to record flow curves for various temperatures and strain rates. To grasp experimental uncertainties, each experiment was repeated five times. The well known Bootstrap method was applied to characterize the uncertainties in fitting a Hensel‐Spittel flow curve model to the experimental data. This method is compared with an alternative strategy of resampling the experimental data regarding the confidence intervals of the predictions made with the model.

Prediction of Microstructure and Resulting Rolling Forces by Application of a Material Model in a Hot Ring Rolling Process

Abstract: Ring rolling is a versatile incremental bulk forming process. Due to the incremental character of the process, it consists of a large number of deformation and dwell steps. Finite element (FE) simulations of bulk forming processes are capable of predicting loads, stresses and material flow. In recent years, the finite element analysis of ring rolling processes has become feasible both in terms of calculation time as well as regarding the closed loop control of the kinematic degrees of freedom [1]. Accordingly, the focus of interest now includes the prediction of the microstructure evolution. The accuracy of such numerical simulations strongly depends on the models characterizing the material behavior and boundary conditions. In this paper, a finite element based simulation study was conducted, in order to evaluate the impact of boundary conditions such as transfer time, radiation, heat transfer and friction on the target values of the ring rolling process. The results of the simulation study were compared to ring rolling experiments on an industrial size ring rolling device. A good accordance regarding the evolution of the outer diameter and radial force was observed. Strong contingencies of transfer time on the forces throughout the process were detected and considered in the simulation study. In a post processing step, the evolution of the microstructure considering the dynamic and static recrystallization as well as the grain growth was calculated using the FE results. The calculated grain sizes show good accordance with the experimentally observed microstructure of the ring before and after the rolling. Furthermore, the impact of process parameters on the evolution of the grain size was investigated.

The Design and Analysis of Experiments

Abstract: THE DESIGN AND ANALYSIS OF EXPERIMENTS. By Oscar Kempthorne. New York, John Wiley and Sons, Inc., 1952. 631 pp. $8.50. This book by a teacher of statistics (as well as a consultant for \"experimenters\") is a comprehensive study of the philosophical background for the statistical design of experiment. It is necessary to have some facility with algebraic notation and manipulation to be able to use the volume intelligently. The problems are presented from the theoretical point of view, without such practical examples as would be helpful for those not acquainted with mathematics. The mathematical justification for the techniques is given. As a somewhat advanced treatment of the design and analysis of experiments, this volume will be interesting and helpful for many who approach statistics theoretically as well as practically. With emphasis on the \"why,\" and with description given broadly, the author relates the subject matter to the general theory of statistics and to the general problem of experimental inference. MARGARET J. ROBERTSON

Recent development of ring rolling theory and technique

Abstract: Rings are basic mechanical components. Ring rolling is a well-known advanced plastic forming technique to manufacture high-performance seamless rings. Since the 21st century, the fast development of global manufacturing has raised an urgent request to high-performance rings, thus, comprehensive and in-depth studies on ring rolling technique were carried out. Based on long-term investigations and attentions on ring rolling, important developments of ring rolling theory and technique in recent 15 years were simply summarised, including the traditional radial ring rolling and radial-axial ring rolling, as well as the novel combined ring rolling. It is expected to give a valuable reference to the research and application of ring rolling.

A Cosserat crystal plasticity and phase field theory for grain boundary migration

Abstract: The microstructure evolution due to thermomechanical treatment of metals can largely be described by viscoplastic deformation, nucleation and grain growth. These processes take place over different length and time scales which present significant challenges when formulating simulation models. In particular, no overall unified field framework exists to model concurrent viscoplastic deformation and recrystal-lization and grain growth in metal polycrystals. In this work a thermodynamically consistent diffuse interface framework incorporating crystal viscoplasticity and grain boundary migration is elaborated. The Kobayashi–Warren–Carter (KWC) phase field model is extended to incorporate the full mechanical coupling with material and lattice rotations and evolution of dislocation densities. The Cosserat crystal plasticity theory is shown to be the appropriate framework to formulate the coupling between phase field and mechanics with proper distinction between bulk and grain boundary behaviour.