Key Engineering Materials Vols. 504-506

Abstract: In this work, we discuss a finite strain material model for evolving elastic and plastic anisotropy combining nonlinear isotropic and kinematic hardening. The evolution of elastic anisotropy is described by representing the Helmholtz free energy as a function of a family of evolving structure tensors. In addition, plastic anisotropy is modelled via the dependence of the yield surface on the same family of structure tensors. Exploiting the dissipation inequality leads to the interesting result that all tensor-valued internal variables are symmetric. Thus, the integration of the evolution equations can be efficiently performed by means of an algorithm that automatically retains the symmetry of the internal variables in every time step. The material model has been implemented as a user material subroutine UMAT into the commercial finite element software ABAQUS/Standard and has been used for the simulation of the phenomenon of earing during cylindrical deep drawing.

Abstract: In this work, distinct algorithmic strategies for the implementation of complex anisotropic criteria in finite element codes are presented. Two different algorithm classes are presented: semi-explicit and semi-implicit procedures, trying to conjugate the best of conventional return-mapping techniques, and accounting for sub-incrementation and subdivision procedures to improve the quality of the obtained results. In the present study, two complex (non-quadratic) anisotropic yield criteria were implemented: Barlat et al. 1991 (Yld91) and Barlat et al. 2004 with 18 coefficients (Yld2004-18p). The performance of the developed algorithms is inferred in cup drawing simulations for aluminum alloys, with the convergence rate as well as the quality of the solutions being assessed, when compared to experimental results. As result, an algorithm and programming framework is provided, suitable for direct implementation in commercial finite element codes, such as Abaqus (Simulia) and Marc (MSC-Software) packages.

Abstract: The optimization of automotive security components requires good knowledge of the material state after fabrication, particularly with respect to damage that may have been done to the material by the manufacturing process. To achieve this, numerical simulation of the fabrication process is often undertaken. However, classical continuum damage models, like the Gurson [3] model, are not appropriate for the simulation of the blanking by punching operation because the material damage is primarily the result of shear stresses. This work is focused on the use and validation of a modified Gurson type damage model capable of modeling this process, which has recently been proposed by Nahshan [7]. After a brief description of the modification, the implementation and the validation of the modified Gurson model is detailed. A comparison between the original Gurson model and the modified model is presented in order to highlight the importance of the modification for a pure shear stress state and to show that the two models are equivalent for a purely hydrostatic stress state. It is also shown that the results from the modified model are dependent on the finite element mesh size.

Abstract: For numerical simulations of rapid forming processes the most used constitutive equations are based on the Johnson-Cook (JC) law and on the Zerilli-Armstrong (ZA) one. Starting from physical points of view, detailed in the first part, the present paper proposes a more general constitutive equation available for both static and dynamic loadings. It is then possible to describe correctly gradients of strain rate, plastic strain and temperature which occur during a complex deformation path of a metal forming process. From this new law, the JC formulation can be easily obtained from an asymptotic variation of the proposed law at high values of strain rates and, in similar way, the ZA formulation is obtained at small values of strain rates. Application to an aluminum alloy AA5083 will be presented to validate the proposed constitutive equation by a Finite Element Model (FEM) of a rapid upsetting test performed with the Split Hopkinson Pressure Bars.

Abstract: Nowadays a wide number of constitutive models are available to describe the plastic behaviour of metals at large deformations including anisotropy, strain rate effect, kinematic hardening etc. Often the main limitation to the practical applicability of a model resides in the difficulty of experimentally identifying the constitutive parameters. At large deformations the identification can become very complicated because of the occurrence of instabilities, e.g. localized necking or buckling, which create a complex three-dimensional stress and strain state inside the specimen. In these cases, the identification problem is usually tackled by Finite Element Model Updating where a numerical model of the test is built up and the parameters are iteratively varied to achieve the best agreement with the experiments. Recently it has been demonstrated that the Virtual Fields Method (VFM) can be successfully adopted to solve the identification problem using three dimensional displacement data [Rossi and Pierron, Identification of plastic constitutive parameters at large deformations from three dimensional displacement fields, Comp. Mech., 2011]. This paper presents the first experimental validation of this approach. It deals with the identification of plastic constitutive parameters at large strains looking at the post-necking behaviour of a notched specimen. The experiment consisted in a tensile test performed on a notched specimen obtained from a 3 mm thick sheet of 316L stainless steel. An experimental procedure has been developed to reconstruct the volume deformation history of the specimen during the test. A set of four cameras, two for each side of the specimen, has been used to measure the displacement field on the two faces at the same time using Digital Image Stereo-Correlation. The adopted spatial resolution is good enough to describe the out-of-plane movement which takes place at the surface after the occurrence of necking. Starting from the surface measurements, the volume displacement field inside the specimen has been reconstructed using a non-linear volume interpolation. The procedure as well as the reliability of the reconstruction process is described in the paper. Finally, the three dimensional displacement field is used to identify the parameters of a plastic constitutive model using the VFM. The anisotropy of the sheet metal is also taken into account. The main outcome of the paper is to illustrate that it is possible to retrieve the three dimensional deformation history of a specimen in the post-necking region using surface measurements and that these data can be quantitatively used to identify the constitutive model.

Abstract: Especially for bending/hemming operations, aluminum alloys lack sufficient formability. The aim is to use them in the same way as other structural materials such as conventional steel. In this study, a combined laser-assisted roller hemming process is set up. For this, a 4000 W Nd:YAG-laser with a wave-length of 1096 nm is used. Several parameters are defined and the effects of heat treatment on the hemming ability of AA6014 were investigated. Taking into account the kinds of components that are expected to be formed, the experiment is set up with two flexible robots that can rotate on six axes. One moves the roller for the forming process and the other guides the laser system. Radiation tests by the laser were conducted before the forming processes. Sheets were irradiated with a laser energy level between 10 J/mm and 40 J/mm. The heat-treat condition was confirmed by micro-hardness tests. Roll (in/out) for straight contours after final hemming were measured and the effect from the heat treatment was investigated. Furthermore, the influence of the applied heat on the final hem geometry was investigated. Limitations of the conventional roller hemming process were highlighted and the transition to laser-assisted roller hemming defined.

Abstract: Superplastic forming and diffusion bonding (SPF/DB) is a near-net-shape forming and joining process used with alloys having superplastic properties in order to make manufact which should have light weight and high stiffness. The aerospace is one of those sectors in which such technology is mainly used. This process allows to reduce the buy-to-fly ratio and consequently the production costs thanks to the possibility to produce complex shape components in a single shot. The material widely used for this application is the Ti-6Al-4V alloy for its high strength vs weight ratio, excellent mechanical proprieties, corrosion resistance and galvanic compatibility with carbon fiber reinforced composite materials. In this study, finite element analysis of the SPF/DB process has been carried out in order to investigate the thickness prediction, the optimization of the tooling’s geometry and the definition of the sheets initial thickness in the blow forming process of a multi-sheets configuration.

Abstract: The parts of smart components consisting of structural and smart materials are conventionally produced separately and assembled in additional processes afterwards. An alternative approach to combine the forming of metallic parts and the assembly of components in one process step is proposed in this paper. Numerical and experimental investigations are carried out to investigate the influence of the axial clamping of the tube during the integration of a ring part through rotary swaging. The experiments also demonstrate the producibility of smart components by incremental forming processes without damaging the sensitive functional parts, which is proven by a functioning test.

Abstract: Wavy interface is a characteristic of an impact weld. Electromagnetic pulse welding is a solid state welding process, which produces a weld between two mating surfaces at high velocity of impact. In this paper studies on metallurgical characterization of electromagnetic welds to find the presence or absence of wavy weld interface, by using optical microscope and scanning electron microscope, are reported. EM welds of Al to Al do not show wavy interface due the low energy of impact. EM welds of Al to Steel show wavy weld interface due to instability at the weld interface. Studies have revealed that discharge energy and critical velocity of impact are the important parameters for creating stronger electromagnetic welds and the wavy weld interface.

Abstract: Light weight construction is a major task within automotive and aircraft industry due to lower fuel consumption or increase the possible payload. Structural or exterior shell components are more and more manufactured out of aluminum alloys for this reason. A further weight reduction could be achieved by the substitution of aluminum alloys by magnesium alloys. Also the application of blanks with a varying thickness is a possibility to realize light weight design. To combine the advantages of weight reduction by the use of magnesium alloys and tailored welded blanks (TWBs), an effective joining technique is required. Friction Stir Welding can be used for difficult to be welded magnesium alloys to manufacture magnesium TWBs. During this process the different thickness of the two sheets can cause an unequal heat distribution below the tool, affecting material flow and therefore components strength. In the paper the main results of a numerical and experimental campaign on Friction Stir Welding of AZ31 magnesium alloy tailored blanks are presented. The numerical simulation was validated by experimental observations and the occurring bonding conditions have been analyzed in both approaches.