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Papers by Philip Eyckens

International Journal of Plasticity, 2015

This paper presents a new hierarchical multi-scale framework that allows taking into account evol... more This paper presents a new hierarchical multi-scale framework that allows taking into account evolution of the plastic anisotropy during sheet forming processes. The evolution of crystallographic texture, which is identified as the main source of the plastic anisotropy, is predicted by the ALAMEL crystal plasticity model. An extension to the phenomenological anisotropic planestress yield criterion BBC2008 is proposed, which provides adaptive updates of the local anisotropy in the integration points of the macroscopic finite element model. To this end, the BBC2008 is systematically recalibrated to data provided by the crystal plasticity virtual experiment framework (VEF). An enhanced identification algorithm is proposed. The new algorithm exploits comprehensive material characterization delivered by the VEF.

The grain interaction ALAMEL model allows predicting the evolution of the crystallographic textur... more The grain interaction ALAMEL model allows predicting the evolution of the crystallographic texture and the accompanying evolution in plastic anisotropy. A FE constitutive law, based on this multilevel model, is presented and assessed for a cup deep drawing process followed by an ironing process. A Numisheet2011 benchmark (BM-1) is used for the application. The FE material model makes use of the Facet plastic potential [2] for a relatively fast evaluation of the yield locus. A multi-scale approach has been recently developed in order to adaptively update the constitutive law by accommodating it to the evolution of the crystallographic texture. The identification procedure of the Facet coefficients, which describe instantaneous plastic anisotropy, is accomplished through virtual testing by means of the ALAMEL model, as described in more detail in the accompanying conference paper . Texture evolution during deformation is included explicitly by re-identification of Facet coefficients in the course of the FE simulation. The focus of this paper lies on the texture-induced anisotropy and the resulting earing profile during both stages of the forming process. For the considered AKDQ steel material, it is seen that texture evolution during deep drawing is such that the anisotropic plastic flow evolves towards a more isotropic flow in the course of deformation. Texture evolution only slightly influences the obtained cup height for this material. The ironing step enlarges the earing height.

International Journal of Plasticity, 2011

Some metal sheet forming processes may induce an amount of plastic shear over the sheet thickness... more Some metal sheet forming processes may induce an amount of plastic shear over the sheet thickness. This paper investigates how formability of anisotropic sheet metal is affected by such Through-Thickness Shear (TTS). The Marciniak-Kuczynski (MK) model framework, a commonly used analytical tool to predict the limit of sheet formability due to the onset of localized necking, is extended in this paper in order to explicitly account for TTS in anisotropic metal sheets. It is a continuation of previous work by the present authors ), in which TTS is incorporated for isotropic sheet. This is achieved by the introduction of additional force equilibrium and geometric compatibility equations that govern the connection between matrix and groove in the MK model. Furthermore, in order to integrate plastic anisotropy, a material reference frame available in recent literature is incorporated, as well as a particular model for anisotropic yielding that relies on virtual testing of anisotropic properties (Facet plastic potential), since out-of-plane anisotropy related to TTS cannot be measured experimentally. *Manuscript Click here to view linked References * Corresponding author. Philip.eyckens@mtm.kuleuven.be; Tel.: +32 16321305; fax: +32 16321990.

International Journal of Plasticity, 2009

The Marciniak-Kuczynski (MK) forming limit model is extended in order to predict localized neckin... more The Marciniak-Kuczynski (MK) forming limit model is extended in order to predict localized necking in sheet metal forming operations in which Through-Thickness Shear (TTS), also known as out-of-plane shear, occurs. An example of such a forming operation is Single Point Incremental Forming. The Forming Limit Diagram (FLD) of a purely plastic, isotropic hardening material with von Mises yield locus is discussed, for monotonic deformation paths that include TTS. If TTS is present in the plane containing the critical groove direction in the MK model, it is seen that formability is increased for all in-plane strain modes, except equibiaxial stretching. The increase in formability due to TTS is explained through a detailed study of some selected deformation modes. The underlying mechanism is a change of the stress mode in the groove that results in a delay of the onset of localized necking.

The International Journal of Advanced Manufacturing Technology, 2010

The aim of the study was to establish practical formulae allowing to predict the forces occurring... more The aim of the study was to establish practical formulae allowing to predict the forces occurring during the single point incremental forming process. This study has been based on a large set of systematic experiments on the one hand and on results of finite elements modeling simulations on the other. This led to analytical formulae allowing to compute the three main components of the force for five selected materials in function of the working conditions (sheet thickness, wall angle, tool diameter, and step down) with a good precision. Moreover, a general model has been deduced, allowing to compute an approximate value for the force for any material, based on knowledge of the tensile strength only.

CIRP Annals - Manufacturing Technology, 2010

The aim of this article is to study the accuracy of finite element simulations in predicting the ... more The aim of this article is to study the accuracy of finite element simulations in predicting the tool force occurring during the single point incremental forming (SPIF) process. The forming of two cones in soft aluminum was studied with two finite element (FE) codes and several constitutive laws (an elastic-plastic law coupled with various hardening models). The parameters of these laws were identified using several combinations of a tensile test, shear tests, and an inverse modeling approach taking into account a test similar to the incremental forming process. Comparisons between measured and predicted force values are performed. This article shows that three factors have an influence on force prediction: the type of finite element, the constitutive law and the identification procedure for the material parameters. In addition, it confirms that a detailed description of the behavior occurring across the thickness of the metal sheet is crucial for an accurate force prediction by FE simulations, even though a simple analytical formula could provide an otherwise acceptable answer.

the case t =0rev/s ('NO_ToolRot') is considered in a separate experiment.

Single Point Incremental Forming is a sheet forming process that uses a smooth-ended tool followi... more Single Point Incremental Forming is a sheet forming process that uses a smooth-ended tool following a specific tool path and thus eliminates the need for dedicated die sets. Using this method, the material can reach a very high deformation level. A wide variety of shapes can be obtained without specific and costly equipment. To be able to optimize the process, a model and its material parameters are required. The inverse method has been used to provide material data by modeling experiments directly performed on a SPIF set-up and comparing them to the experimental measurements. The tests chosen for this study can generate heterogeneous stress and strain fields. They are performed with the production machine itself and are appropriate for the inverse method since their simulation times are not too high.

Key Engineering Materials, 2011

ABSTRACT The Incremental Sheet Forming (ISF) process offers a large variety in tool path strategi... more ABSTRACT The Incremental Sheet Forming (ISF) process offers a large variety in tool path strategies to obtain a particular final product shape. As fundamental understanding of the relevant deformation modes in ISF is growing, the selection of the tool path strategy may be shifted from trial-and-error towards more fundamentally based knowledge of the process characteristics. Truncated cones and pyramids have been fabricated by both unidirectional (UD) and bidirectional (BD) contour-based tool path strategies, considering different wall angles and materials (Mn-Fe alloyed aluminum sheet and low carbon steel sheet). It is shown that the induced through-thickness shear along the tool movement direction is clearly non-zero for UD, in which case the sense of tool movement is the same for all contours, while it is close to 0 for BD, due to the alternating tool sense during consecutive contours. Furthermore, the heterogeneity in product thickness, as observed for the UD strategy in [1,2], is avoided by using the BD strategy. It is verified that this difference in deformation may affect the mechanical properties in the walls of pyramids by means of tensile testing, but the results are material-dependent. For the aluminum alloy, the re-yield stress along the tool movement direction is smaller for BD in comparison to UD, and the fracture strain in large wall angle products is higher. For the steel, no statistically significant differences in mechanical properties between UD- and BD-processed parts are observed. Finally, for both materials a (slightly) higher limiting wall angle has been repeatedly measured using the BD tool strategy. In light of these results, the bidirectional tool path strategy is to be preferred over the unidirectional one, as thickness distribution and formability are more favorable, while both strategies require similar resources and processing time.

on the process parameters. The identification of the most determining forming parameter that cont... more on the process parameters. The identification of the most determining forming parameter that controls the relative importance of either mechanism is an interesting topic for future research.

International Journal of Material Forming, 2008

In this paper, the Finite Element submodelling technique is applied to model the small plastic zo... more In this paper, the Finite Element submodelling technique is applied to model the small plastic zone in the Incremental Forming process with an adequately fine (sub-millimetre) mesh at a computationally acceptable cost. The focus lies on the distribution of the contact pressure between sheet and forming tool. Different forming conditions of truncated cones are considered. Results show that the contact consists of two distinguishable parts. The obtained insights can be applied in the physical modelling of forming forces, which allows improving part accuracy through compensation for the mechanical stiffness of the forming machine.

The purpose of this article is to develop an inverse method for adjusting the material parameters... more The purpose of this article is to develop an inverse method for adjusting the material parameters for single point incremental forming (SPIF). The main idea consists in FEM simulations of simple tests involving the SPIF specificities (the "line test") performed on the machine used for the process itself. This approach decreases the equipment cost. It has the advantage that the material parameters are fitted for heterogeneous stress and strain fields close to the ones occurring during the actual process. A first set of material parameters, adjusted for the aluminum alloy AA3103 with classical tests (tensile and cyclic shear tests), is compared with parameters adjusted by the line test. It is shown that the chosen tests and the strain state level have an important impact on the adjusted material data and on the accuracy of the tool force prediction reached during the SPIF process.

International Journal of Material Forming, 2009

%675$&7 This paper presents experimental measurements of the Through-Thickness Shear (TTS), (also... more %675$&7 This paper presents experimental measurements of the Through-Thickness Shear (TTS), (also known as out-of-plane shear) that occurs during Single Point Incremental Forming (SPIF) of low carbon steel into cone-shaped geometries. The measurements show the dependence of TTS on the cone wall angle. Also formability predictions are presented, using a Marciniak-Kuczy ski (MK) type of forming limit model which can take TTS explicitly into account [1]. It is seen that the presence of TTS in the process delays the onset of localized necking and thus can be a contributing factor to the very high formability during SPIF that is observed. .(<:25'6 Single Point Incremental Forming, Marciniak-.XF]\ VNL forming limit model, through-thickness shear.

Procedia Engineering, 2014

Multi-scale modelling offers physical insights in the relationship between microstructure and pro... more Multi-scale modelling offers physical insights in the relationship between microstructure and properties of a material. The macroscopic anisotropic plastic flow may be accounted for by consideration of (a) the polycrystalline nature and (b) the anisotropic grain substructure. The latter contribution to anisotropy manifests itself most clearly in the event of a change in the strain path, as occurs frequently in multi-step forming processes. Under monotonic loading, both the crystallographic texture and the loading-dependent strength contribution from substructure influence the macroscopically observed strength. The presented multi-scale plasticity model for BCC polycrystals combines a crystal plasticity model featuring grain interaction with a substructure model for anisotropic hardening of the individual slip systems. Special attention is given to how plastic deformation is accommodated: either by slip of edge dislocation segments, or alternatively by dislocation loop expansion. Results of this multi-scale modelling approach are shown for a batch-annealed IF steel. Whereas both model variants are seen to capture the transient hardening after different types of strain path changes, the dislocation loop model offers more realistic predictions under a variety of monotonic loading conditions.

Key Engineering Materials, 2013

ABSTRACT A series of mechanical tests in different specimen orientations was performed to study t... more ABSTRACT A series of mechanical tests in different specimen orientations was performed to study the anisotropic behavior of an IF steel (DC06). State-of-the-art polycrystalline models Alamel [1], VPSC [2], as well as the classical FC Taylor model were employed to predict flow stress curves. A two-stage Voce law was used to describe the single crystal shear stress-accumulated shear strain relationship. In this approach, the textural hardening and the dislocation hardening are effectively modeled separately. Results demonstrate that both the Alamel and VPSC models could reproduce the flow stress curves adequately. Also, the quantitative agreement of texture prediction is used to validate the model predictions. It is concluded that the better performance of grain interaction models compared to the FC Taylor model is mainly due to an improved prediction of the slip inside the constituting grains, and not in particular due to an improved prediction of texture evolution.

Materials Science and Engineering: A, 2013

ABSTRACT Two micro-macro scale transition models, namely the ALAMEL and visco-plastic self-consis... more ABSTRACT Two micro-macro scale transition models, namely the ALAMEL and visco-plastic self-consistent (VPSC) models (both predict the evolution of grain shape and texture), were used to study the effect of the initial grain shape. The evolution of plastic anisotropy was characterized by the q-value and was simulated in three directions for three low carbon steels assuming either equiaxed (hypothetical one) or a pancake (measured one) grain shape. In addition, texture evolution during rolling was investigated for one of the materials. Comparisons between the model predictions and the measured values demonstrate that the grain shape effect can be predicted well by both models. In most cases, the initial q-value can be nicely estimated by both models. VPSC is better than ALAMEL on q-value evolution prediction for simulations which consider the measured aspect ratios. For texture evolution of rolling, the effect of initial grain shape is only significant at large deformation.