Bekim Berisha

Forming Limit Prediction of Metastable Materials with Temperature and Strain Induced Martensite Transformation

Stainless steels as well as TRIP and TWIP steels show a hardening behavior, which can be described only in dependency on the deformation and temperature history during the real forming process. Because the hardening behavior is the determinate factor for the necking phenomenon, the prediction of rupture becomes also deformation path and temperature dependent. As a consequence, the common FLC‐method, using a single curve for the prediction of the failure state is not accurate enough. In this paper, a temperature dependent Forming Limit Surface (FLS) is presented.

Modeling of friction phenomena in extrusion processes by using a new torsion-friction test

During the extrusion processes very complex adhesion and friction effects between the die surface and the extruded material occur. If they can not be controlled, the process can not be controlled either. In the framework of this study, a recently developed experiment for the investigation of the frictional phenomena under extrusion conditions will be presented. Based on the so called Torsion-Tribo-Test, low and high pressure load cases are considered. The experimental layout, the evaluation method as well as some measurements will be presented.

Stress Limit Model With Deformation Dependent Damage For Failure Prediction In Bulk Forming Processes

Based on the experimental results, a model for the description of damage in materials during bulk forming processes is presented. Unlike the most of the existing models which suppose no damage occurs under compressive stresses, the presented model considers all deformations as the cause for damages. The damages are strongly oriented according to the deformations. Therefore, it cannot be described using just a scalar value. However, failure appears only when the tensile stress exceeds the limit value in the material. In order to perform the failure prediction in FE‐simulation of bulk forming processes, an ellipsoid in the stress space is introduced to describe the limit stress. The ellipsoid, or a sphere for an isotropic material, changes its shape according to the deformation applied on the material. The stress state works as the criterion for the rupture in the material.

FE Simulation of Cross Roll Straightening: a Strain Tensor Field Approach

Cross roll straightening is an operation aiming at reducing the undesirable curvature of drawn round bars. Previous attempts to describe the process are mainly based on analytical considerations. This paper presents a new approach to the problem. The deformation of the bar—the strain tensor field—between the rolls is obtained through a FE simulation. The path of a material point within the bar during the straightening process is computed analytically. Assuming that the chosen material model for the bending simulation does not influence significantly the strains obtained, the strain tensors associated with the elements along the deformation path of a material point are considered as successive deformation states. Following this strain history, the corresponding stresses are obtained using a mixed isotropic‐kinematic constitutive model. The validity of the method is tested by comparing the predicted internal stresses appearing at the surface of a bar with experimental measurements.

Modelling of Instability and Fracture Effects in the Sheet Metal Forming Based on an Extended X-FLC Concept

The industrial necking prediction in sheet metal forming is still based on the Forming Limit Diagram (FLD) as initially proposed by Keeler. The FLD is commonly specified by the Nakajima tests and evaluated with the so called cross section method. Although widely used, the FLC concept has numerous serious limitations. In the paper the influences of bending on the FLC as well as postponed crack limits will be discussed. Both criteria will be combined to an extended FLC concept (X-FLC). The new concept demonstrates that the Nakajima tests are not only appropriate for the evaluation of the necking instability, but also for the detection of the real crack strains. For the evaluation of the crack strains, a new local thinning method is proposed and tested for special 6xxx Al-alloys.

Numerical models for the prediction of failure for multilayer fusion Al-alloy sheets

Initiation and propagation of cracks in monolithic and multi-layer aluminum alloys, called “Fusion”, is investigated. 2D plane strain finite element simulations are performed to model deformation due to bending and to predict failure. For this purpose, fracture strains are measured based on microscopic pictures of Nakajima specimens. In addition to, micro-structure of materials is taken into account by introducing a random grain distribution over the sheet thickness as well as a random distribution of the measured yield curve. It is shown that the performed experiments and the introduced FE-Model are appropriate methods to highlight the advantages of the Fusion material, especially for bending processes.

Constitutive Modelling of Dynamic Strain Aging Effect under Various Loeading Conditions

Various analytical rules of mixture are commonly used to take into account heterogeneous features of a material and to derive global properties. But, with such models, one may not be able to fulfil the requirements for separating appropriately the different lengthscales. This might be the case for some issues such as strain localisation, surface effect, or topological distributions. At an intermediate lengthscale, which we refer to as the mesoscopic scale, one can still apply continuum mechanics. So why not perform calculations using the finite element method on volumes of material to obtain the response of Representative Elementary Volumes (R.E.V.). The construction of digital microstructures for such calculations is performed in two steps. First, a series of R.E.V.s with statistics of features of real materials should be defined. Then, finite element meshes should be produced for these R.E.V.s and updated when calculations involve large strains. Powerful automatic three-dimensional mesh generators and remeshing techniques prove necessary for this latter task. This strategy is applied to create digital R.E.V.s which match statistical features of forgings. Measurements provide micromechanical parameters of each subvolume. As an example of calculations, numerical simulations provide the anisotropic fatigue properties of forgings.