Key Engineering Materials Vols. 554-557

Abstract: Cold forming, especially for steels of increased mechanical properties, encounters problem with a springback. Most of the tools require compensation of that effect, however it is not always feasible in a conventional way. In some special cases such as twisting springback, compensation of a tool remains an ineffective method of solving the problem. This paper aims to investigate the behaviour of springback deviation which may be reduced using the change of forming conception such as: crash forming, holding and stamping or stamping with pad. The results lead to an in-depth understanding of the design process and parameters for advanced high strength components. It is immensely important for tool shops which usually have only few weeks to make a tool. The essence of designing and numerical simulations must be emphasized. Without them some problems with twisting springback may be encountered during trial test what would involve labour absorbing corrections and some extra costs. Therefore, it is crucial to pay attention to forming conception, as using it we can obtain entirely different

Abstract: The manufacturing process of body parts starts with the step of sheet metal forming. The single parts, produced at the press shop, are put into clamping devices in order to align and to fix them. The fixation takes part before further operations like joining can be carried out. In order to simulate the process chain of add-on body parts realistically, the clamping process (closing the clamping device) has to be taken into account. The stationary surfaces of a clamping device are called passive and the moveable surfaces are called active surfaces. If the clamping process is calculated by means of active surfaces, their positions need to be measured in the state of a closed clamping device. While the passive surfaces of a body construction device can be measured with high reproducibility, the measurement of active surfaces in the state of a closed device is impracticable because of the loss of accessibility. Furthermore, if the parts to be clamped or the position of the clamping device differ from their designed position, the assembly works like a flat spring against the clamping device force in all spatial directions. The active surface does not reach the position which was measured before. In order to take these facts in clamping simulations into account, the end position of the active surfaces should be known. A clamping device concept on the basis of a measuring probe for optical measurement systems was developed. It is possible to determine the position of active surfaces with high reproducibility while the parts are clamped. It can be shown, that the presented clamping device concept contributes to significantly better results of clamping simulations. Thus a better starting basis for further simulations along the process chain is offered.

Abstract: The aim of this work is to examine two specific finite strain elastoplasticity models in terms of their applicability in metal forming processes, namely a hyperelastic-based model which relies upon a multiplicative decomposition of the deformation gradient into elastic and plastic parts and a hypoelastic-based model which makes use of an additive elastic-plastic split of the rate of deformation tensor. Both models allow for nonlinear isotropic and kinematic hardening and were implemented as user material subroutines (UMAT) into ABAQUS/Standard. Various sample calculations were performed to assess the respective properties and capabilities of the models. The FE simulation of a deep drawing process produced nearly congruent results for both models which suggests that they are equally well-suited for modeling metallic materials in metal forming processes.

Abstract: Sheet metals exhibit anisotropic plastic behavior due to the large plastic deformations that occur during the rolling of the sheet and which induce texture and are responsible for the initial anisotropy. There exist various possibilities to introduce plastic anisotropy into the finite element modelling of sheet metal forming. The initial yield anisotropy can be incorporated either through an anisotropic yield surface or directly by means of a crystallographic texture model. Here, one basically differentiates between empirical and phenomenological anisotropic yield function equations, where the anisotropy coefficients can be obtained from mechanical tests, and texture-based models the coefficients of which are directly determined based on experimentally obtained orientation distributions. Another type of anisotropy that can be usually found in anisotropic materials is the elastic anisotropy. In metal plasticity one often considers the effect of elastic anisotropy significantly smaller than the effect of plastic anisotropy. Consequently, elastic isotropic expressions are often used for elastic stored energy functions with anisotropic yield criteria. However, the influence of elastic anisotropy in the elastoplastic behavior can be very important especially during elastic recovery processes during unloading after forming and springback. This research focuses, therefore, on the study of the influence of elastic anisotropy on the amount of springback in bending processes such as e.g. unconstrained bending. 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 applied to the simulation of springback of unconstrained bending.

Abstract: Numerical modeling of the heat treatment is developed over the past 30 years and connected to the industrial applications. This requires a good insight in the thermal, mechanical and metallurgical behavior of steels and the coupling effects among these physical phenomena. To describe these effects, a suitable material model is developed. Additionally, the thermo-mechanical behavior of multi-phase steel during the phase transformation is investigated. Furthermore, the material model is applied to describe the plastic behavior of the steel during the cooling process in process-integrated powder coating which is a new kind of ring-rolling process. It takes advantages of the high temperatures and high forces of the ring rolling process. This is not only to increase the ring's diameter, but also to integrate powder metallurgical multi-functional coatings within the same process. In order to increase the strength and wear properties of investigated steels, an appropriate heat treatment should be done. Therefore, the heat treatment after the rolling process is discussed which goes along with phase transformations. The paper is concluded by a detailed description of the process simulation and a comparison of its results with experimental data.

Abstract: Liquid Composite Molding processes are characterized by the impregnation of a dry fibrous perform by means of injection or infusion of a catalyzed resin. In recent years computational flow and cure models allowed for a remarkable time and cost compression in process planning with respect to trial and error procedures. In this contest multi-scale simulative approaches are gaining considerable attention and intriguing results have been recently presented. Most of the proposed models, however, rely on deterministic hypothesis, assuming perfect fiber packing and neglecting dimensional variations between fibers, in strong contrast with experimental observations. In this paper the influence of the stochastic variability of the fiber packing on tow permeability has been investigated by means of a CFD micro scale model. The variability of the microstructure defining the Representative Volume Element has been considered introducing random perturbations of the fiber packing. The components of the permeability tensor, in each case, have then been derived applying the Darcy model to flow simulations through the representative cell.

Abstract: The stress state in metal forming processes usually implies low values of triaxiality. It is well known that damage models based only on triaxiality fails to capture the damage behavior properly, and recent articles have stressed the effect of the Lode parameter in describing damage. Moreover, in some process like incremental forming, the through thickness shear could dominate the rupture mechanism making the description, using solely the triaxiality, inaccurate. In this paper, a preliminary study of the stress state is carried over a near-to-failure single point incremental forming (SPIF) formed cone, through finite elements simulations using a newly developed solid-shell element. The results provide a basis for further studies into damage development in SPIF.

Abstract: In the field of sheet metal forming traditional forming processes are used. However, a quasi-static forming process combined with a high speed forming process can enhance the forming limits of a single one. In this paper, the investigation of the process chain quasi-static deep drawing – electromagnetic forming by means of a new coupled damage-viscoplasticity model for large deformations is performed. The finite strain constitutive model, used in the finite element simulation combines nonlinear kinematic and isotropic hardening and is derived in a thermodynamically consistent setting. The anisotropic viscoplastic model is based on the multiplicative decomposition of the deformation gradient in the context of hyperelasticity. The kinematic hardening component represents a continuum extension of the classical rheological model of Armstrong–Frederick kinematic hardening. Hill-type plastic anisotropy is modelled by expressing the yield surface as a function of second-order structure tensors as additional tensor-valued arguments. The coupling of damage and plasticity is carried out in a constitutive manner according to the effective stress concept. The constitutive equations of the material model are integrated in an explicit manner and implemented as a user material subroutine in the commercial finite element package of LS-Dyna with the electromagnetical modul. Aim of the work is to show the increasing formability of the sheet by combining quasi-static deep drawing processes with high speed electromagnetic forming process.

Abstract: Dual phase (DP) steels consisting of two phases, ferrite and dispersed martensite, offer an attractive combination of strength and stretchability, which is a result of the strong distinctions of these constituents in mechanical properties. However, the damage behavior in DP steels exhibits a rather complex scenario: voids are generated by the debonding of the hard phase from the matrix and the inner cracking of the hard phase in addition to by inclusions. The target of this study is to describe the initiation and evolution of damage in DP steel and develop a microstructure-based model which is capable of reflecting the underlying damage mechanisms. Both uniaxial and biaxial tensile tests are performed and the subsequent metallographic investigations are executed to reveal the mechanisms of damage initiation and evolution under different stress state condition and attention will be paid on the influence of various microstructural features on the initiation of damage. In finite element (FE) simulations, the microstructural features are taken into account by the representative volume elements (RVE). Different treatments of the constitutive behaviour of each constituent including isotropic hardening rule and crystallographically dependent configuration with crystal plasticity finite element method are investigated. Several numerical aspects are also discussed, such as RVE size, mesh size, element type, and boundary connections. In the end, the study is attempting to achieve a quantitative assessment of the cold formability of the investigated steel in a microscopic level based on microstructure information of material as well as to understand the damage mechanisms under different stress states condition which cause the macroscopic failure during plastic deformation.

Abstract: After roll forming processes, metallic coils show several flatness imperfections and residual stresses that must be minimized when high quality components are manufactured by means of sheet metal forming processes. The equipments typically used for this purpose are roll leveling facilities. In the present work, a uniaxial cyclic tension-compression test has been used to determine the mechanical response of steel sheet under the different loading modes. After this, the Chaboche and Lemaitre nonlinear mixed hardening model has been fitted to the material behavior. This hardening model is able to reproduce some phenomena which occur during low cyclic deformation such as Bauschinger effect and workhardening. During the fitting of the model, the number of tension-compression cycles performed in the material characterization and the number of backstresses used for the model definition have been analyzed. Finally the influence of the material model in the roll leveling process results has been numerically analyzed. Different simulations have been performed by introducing initial defects with the objective of predicting residual stresses, residual curvatures, leveling force and torque force at the end of the process.