Philip Eyckens

Forming Limit Predictions for the Serrated Strain Paths in Single Point Incremental Sheet Forming

The forming limits of sheets subjected to the Single Point Incremental Forming process (SPIF) is generally several times higher than those found in the Forming Limit Curve (FLC). In this paper it is shown that the non‐monotonic, serrated strain paths to which the material is subjected to during the SPIF process, play a role in the high formability, compared to the monotonic loading in the traditional FLC. The deformation history of an aluminium alloy truncated cone formed with the SPIF process is retrieved through a finite element (FE) model, and discussed. Subsequently, the strain paths at three different depths in the sheet are used as input into a Marciniak‐Kuczynski (MK) forming limit model. The usage of different constitutive models in this analysis shows that anisotropic hardening contributes to the delay of the onset of necking in the SPIF process. The large difference in the predicted forming limits that were obtained from the different layers indicates that an interaction between these layers sho...

Forming Limit Predictions for Single-Point Incremental Sheet Metal Forming

A characteristic of incremental sheet metal forming is that much higher deformations can be achieved than conventional forming limits. In this paper it is investigated to which extent the highly non‐monotonic strain paths during such a process may be responsible for this high formability. A Marciniak‐Kuczynski (MK) model is used to predict the onset of necking of a sheet subjected to the strain paths obtained by finite‐element simulations. The predicted forming limits are considerably higher than for monotonic loading, but still lower than the experimental ones. This discrepancy is attributed to the strain gradient over the sheet thickness, which is not taken into account in the currently used MK model.

Evolving texture-informed anisotropic yield criterion for sheet forming

In this work we compare the predictions of the phenomenological anisotropic plane-stress plasticity model BBC2008, calibrated either classically by means of mechanical tests, or by crystal plasticity virtual experiments, to those of a HMS type model with continuous calibration of the same phenomenological model BBC2008. An industrial-grade aluminum alloy AA-6016 is chosen for the test case. Experimental part of the study includes tensile tests and deep drawing of cylindrical cups, preceded by measurements of crystallographic texture. It was found that the material exhibits a noticeable through-thickness gradient in terms of both the texture and plastic anisotropy. The classical calibration of the 16 parameters of BBC2008 was done from tensile experiments (yield stresses and Lankford coefficients) in directions every 15° from the rolling direction and the biaxial yield stress and anisotropy coefficient. The initial texture for the HMS-BBC2008 model was determined from as received samples. The ALAMEL model ...

Polycrystalline Model Predictions of Flow Stress and Textural Hardening during Monotonic Deformation

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.

Validation of the Texture-Based ALAMEL and VPSC Models by Measured Anisotropy of Plastic Yielding

Several multilevel plasticity models that make use of the crystallographic texture have been developed in the past for the prediction of deformation textures. State-of-the-art models that consider grain interaction, such as Alamel and VPSC, are known to give superior deformation texture predictions compared to the well-known (full constraint) Taylor model. In this paper, these models are assessed on a different basis, namely their ability to predict plastic anisotropy in single-phase steel sheet. A wide range of mechanical tests is considered: uniaxial tension, plane strain tension, simple shear and sheet normal compression. Furthermore, the sensitivity of the anisotropy predictions is analyzed, considering the variability in textures measured by routine XRD. The considered grain interaction models clearly produce improved predictions of plastic anisotropy over the Taylor model.

An Efficient Strategy to Take Texture-Induced Anisotropy Point-by-Point into Account during FE Simulations of Metal Forming Processes

Cup drawing of sheet material (carbon steel DC06 and aluminium alloy AA3103-O) is simulated using a Finite Element (FE) method configured as a hierarchical multi-scale model. It performs a two-way simulation of the interactions between the metal flow and the crystallographic textures of the polycrystalline material. In this, the evolution of the deformation textures is simulated by the Taylor and ALAMEL models, and this in every integration point of the FE mesh. The resulting textures have been compared with experimentally measured ones at different positions within the work-piece. An anisotropic constitutive model is used based on the Facet model identified from the current texture in every location by means of the Taylor and/or ALAMEL model. The updating procedure has been highly optimized. Simulated and experimental results (cup profiles, deformation textures) are compared. The effect of texture updating is assessed.

Tool Directionality in Contour-Based Incremental Sheet Forming: an Experimental Study on Product Properties and Formability

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.

Numerical biaxial tensile test for sheet metal forming simulation of aluminium alloy sheets based on the homogenized crystal plasticity finite element method

The simulation of the stretch forming of A5182-O aluminum alloy sheet with a spherical punch is performed using the crystal plasticity (CP) finite element method based on the mathematical homogenization theory. In the simulation, the CP constitutive equations and their parameters calibrated by the numerical and experimental biaxial tensile tests with a cruciform specimen are used. The results demonstrate that the variation of the sheet thickness distribution simulated show a relatively good agreement with the experimental results.

Anisotropic sheet forming simulations based on the ALAMEL model: Application on cup deep drawing and ironing

The grain interaction ALAMEL model [1] 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 [3] 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 [4]. Texture evolution during deformation is included explicitly by re‐identification of Facet coefficients in the course...

Identification of constitutive equation in hierarchical multiscale modelling of cup drawing process

In this paper we discuss extensions to a hierarchical multi‐scale model (HMS) of cold sheet forming processes. The HMS model is capable of predicting changes in plastic anisotropy due to the evolution of crystallographic textures. The ALAMEL polycrystal plasticity model is employed to predict the texture evolution during the plastic deformation. The same model acts as a multilevel model and provides “virtual experiments” for calibration of an analytical constitutive law. Plastic anisotropy is described by means of the Facet method, which is able to reproduce the plastic potential in the entire strain rate space. The paper presents new strategies for identification of the Facet expression that are focused on improving its accuracy in the parts of the plastic potential surface that are more extensively used by the macroscopic FE model and therefore need to be reproduced more accurately. In this work we also evaluate the applicability of identification methods that (1) rely exclusively on the plastic potenti...