Maysam Gorji

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.

3D Plate-Lattices: An Emerging Class of Low-Density Metamaterial Exhibiting Optimal Isotropic Stiffness

In lightweight engineering, there is a constant quest for low-density materials featuring high mass-specific stiffness and strength. Additively-manufactured metamaterials are particularly promising candidates as the controlled introduction of porosity allows for tailoring their density while activating strengthening size-effects at the nano- and microstructural level. Here, plate-lattices are conceived by placing plates along the closest-packed planes of crystal structures. Based on theoretical analysis, a general design map is developed for elastically isotropic plate-lattices of cubic symmetry. In addition to validating the design map, detailed computational analysis reveals that there even exist plate-lattice compositions that provide nearly isotropic yield strength together with elastic isotropy. The most striking feature of plate-lattices is that their stiffness and yield strength are within a few percent of the theoretical limits for isotropic porous solids. This implies that the stiffness of isotropic plate-lattices is up to three times higher than that of the stiffest truss-lattices of equal mass. This stiffness advantage is also confirmed by experiments on truss- and plate-lattice specimens fabricated through direct laser writing. Due to their porous internal structure, the potential impact of the new metamaterials reported here goes beyond lightweight engineering, including applications for heat-exchange, thermal insulation, acoustics, and biomedical engineering.

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.

Modelling of fracture effects in the sheet metal forming based on an extended FLC evaluation method in combination with fracture criterions

The industrial based prediction in sheet metal forming bases still on the Forming Limit Diagrams (FLD) as formally proposed by Goodwin [1]. The FLD are 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 possibilities for a specific prediction of crack limits based on an extended FLC concept (X-FLC) will be discussed. The new concept demonstrates that the Nakajima tests are not only appropriate for the evaluation of the necking instability but for the detection of the real crack strains too. For the evaluation of the crack strains a local thinning method as proposed by Gorji et al. [3] is applied and tested for special 6xxx and 5xxx Al-alloys as well as for the corresponding multilayer FUSION material.

Prediction of localized necking for nonlinear strain paths using the modified maximum force criterion (MMFC) and the homogeneous anisotropic hardening model (HAH)

The reliable prediction of strain localization is a challenging task. It is generally known that in case of nonlinear strain paths the state of the art FLC cannot accurately predict the localization limit. The present paper aims to approach this problem using the MMFC criterion together with the novel HAH yield locus in order to define an adaptive FLC able to predict localization also in case of nonlinear deformation paths. Furthermore a reformulation of the MMFC is proposed which natively accommodates anisotropy as well as anisotropic hardening effects and is solely based on the yield locus function and its first and second derivatives with respect to the stress tensor. The proposed method will be compared against especially devised experiments featuring optical measurements. An implementation into the finite element framework predicting necking in form of fringe plots is also introduced.

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.