Key Engineering Materials Vols. 611-612

Abstract: The present work deals with the calibration strategy of yield functions used to describe the plastic anisotropic behavior of metallic sheets. In this paper, Bron and Besson yield criterion is used to model the plastic anisotropic behavior of AA5086 sheets. This yield model is flexible enough since the anisotropy is represented by 12 parameters (4 isotropic parameters and 8 anisotropic parameters in plane stress condition) in the form of two linear fourth order transformation tensors. The parameters of this anisotropic yield model have been identified from a single dedicated cross biaxial tensile test. It is shown, from finite element simulations, that the strain distribution in the center of the cruciform specimen is significantly dependent on the yield criterion. Moreover, this cross biaxial test involves a large range of strain paths in the center of the specimen. The calibration stage is performed by means of an optimization procedure minimizing the gap between experimental and numerical values of the principal strains along a specified path in the gauge area of the cruciform specimen. It is shown that the material parameters of Bron and Besson anisotropic yield model can be determined accurately by a unique biaxial tensile test.

Abstract: The main objective of the present work is to investigate the effect of the residual stresses originated by the friction stir welding (FSW) process in the compressive strength of aluminium alloy plates. The finite element method (FEM) is used to simulate the welding process and calculate the distribution of the residual stresses. The model is validated using a residual stress map obtained by means of the contour method from a friction stir welded AA2024-24 plate. The results from the welding simulation were then used to numerically assess the influence of the residual stresses on the collapse load of the plate.

Abstract: The subject discussed in this article concerns the determination of optimal sensor (pressure & temperature) configurations for polymer injection moulds. A sensor configuration is considered optimal when it is able to predict the product quality (dimension, warpage, etc.) with a good accuracy (from experimental data provided by these sensors). Initially, plastic engineers integrated sensors in moulds to acquire knowledge about their processes and to have better understanding of physical phenomenon. This article presents a numerical methodology to identify optimal combinations of sensors. The methodology is firstly based on polymer injection molding simulation to collect virtual sensor data. In a second step, virtual sensor data are analyzed by modern data-driven modeling techniques to identify optimal sensor configurations.

Abstract: Presently, the need to characterize the constitutive parameters of materials has increased due to the manufacture of new materials and development of computational analysis software intending to reproduce the real behavior which depends on the quality of the models implemented and their material parameters. However, in order to identify all constitutive parameters of materials a large number of mechanical tests is required. Thus, only one mechanical test that could allow to characterize all the mechanical properties could be desired. Hence, the aim of this work is to propose a methodology that find the most informative loading path in the sense of display normal and shear strains as clear aspossible to warrantee that the solution is the most unique and distinguishable for the parameter identification process. To achieve this objective the proposed methodology uses Finite Element Analysis (FEA) and Singular Value Decomposition (SVD) coupled together with optimization strategies. Thismethodology is presented for elastoplasticity behavior.

Abstract: The mechanical behavior of thin sheets of aluminium alloy 6016, of thickness 1.14 mm, was investigated in unconstrained bending, with or without a tensile pre-strain. In the case of bending without pre-strain, no rupture was observed. Therefore, a tensile pre-strain, ranging from 0.19 up to 0.45 (longitudinal strain), was applied prior to bending. The highest pre-strain values were reached within the necking area. Tensile tests on rectangular samples were performed, then reduced samples were cut out and submitted to bending. A Digital Image Correlation (DIC) system was used to measure the maximum local strains reached during both tests. The evolution of the applied load on the bending tool versus its displacement showed that a rupture in bending was obtained for tensile pre-strains higher than 0.25, as evidenced by a load drop. These results showed that very high strains can be reached in bending, which is consistent with previously obtained numerical results.

Abstract: Roll levelling is a forming process used to remove the residual stresses and imperfections of metal strips by means of plastic deformations. During the process the metal fibres are subjected to cyclic tension-compression deformations leading to achieve flat product. The process is especially important to avoid final geometrical errors when coils are cold formed or when thick plates are cut by laser. In the last years, and due to the appearance of high strength materials such as Ultra High Strength Steels, machine design engineers are demanding a reliable tool for the dimensioning of the levelling facilities. In response to this demand, Finite Element Analysis and Analytical methods are becoming an important technique able to lead engineers towards facilities optimization through a deeper understanding of the process. Aiming to this study two different models have been developed to analyze the roll levelling operations: an analytical model and a finite element model. The FE-analysis was done using 2D-modelling assuming plane strain conditions. Differing settings, leveller configuration and materials were investigated. The one-dimensional analytical levelling model is based on classical beam theory to calculate the induced strain distribution through the strip, and hence the evolving elastic/plastic stress distribution. Both models provide a useful guide to process-sensitivities and are able to identify causes of poor leveller performance. The theoretical models have been verified by a levelling experimental prototype with 13 rolls at laboratory.

Abstract: In the present work, pultrusion of a composite rod is simulated for various part thicknesses using the finite element method. The pultrusion process set-up is taken from literature in which the temperature and the degree of cure evolutions inside the rod were measured. The predicted temperature and degree of cure profiles in the three dimensional (3D) thermo-chemical analysis are found to agree well with the measured data. The contact pressure between the part and the heating die is calculated using a mechanical contact formulation in the 2D mechanical process model for 9 different part thickness values. Using the contact pressure distribution along the die, the process induced pulling force is predicted. For the simulated cases, a non-linear relation is found between the total force and the product size.

Abstract: A latent hardening model based on binary junction-induced hardening can effectively describe the anisotropy measured in multiaxial tests. However, this approach still has some descriptive and predictive limitations. Recent findings show that binary junctions generated by interactions of pairs of dislocations can only induce short-term hardening effect due to the unzipping process of binary junctions. By contrast, multi-junctions, which are formed via multiple interactions of dislocations, can exert a strong and enduring influence on the hardening of polycrystals. In this study, we extend the modeling of dislocation junctions from the binary to multi-junctions, and implement this evolution into a self-consistent visco-plastic model. An application of this model for predicting the yield surface and texture evolution of AA5754 during uniaxial and plane strain loadings is given as a demonstration of the capabilities of the evolutionary binary-multi junction approach.

Abstract: Plastic deformation induces various types of dislocation microstructures at different length scales, which eventually results in a heterogeneous deformation field in metallic materials. Development of such structures manifests themselves as macroscopic hardening/softening response and plastic anisotropy during strain path changes, which is often observed during forming processes. In this paper we present two different non-local plasticity models based on non-convex potentials to simulate the intrinsic rate-dependent and rate-independent development of plastic slip patterns, which is the simplified mechanism for the intrinsic microstructure development. For the sake of mechanistic understanding, the formulation and the simulations will be conducted in one-dimension which does not exclude its extension to multi-dimensions resulting in a crystal plasticity framework.

Abstract: In the last decades, manufacturing of layered composite materials has become an interesting topic in industrial development. Joining properties of adhesively bonded materials are characterized by a complex interaction of plastic deformation, thermo-mechano-chemical coupling effects, adhesion and diffusion. Additionally, the interactions between the microstructures involved in the process have to be taken into account. In this paper the microstructure of materials (as e.g. Al1050, Al2024 and Al5754), which have a wide range of applications in engineering structures, is numerically and experimentally investigated. The results are compared with experimental data.