Key Engineering Materials Vols. 554-557

Abstract: This paper presents two crystal plasticity based computational constitutive models for the intrinsic formation of plastic microstructure during monotonic loading and its altered evolution under strain path changes in metal forming operations. The formation step is modeled via a non-convex strain gradient crystal plasticity framework which could simulate the intrinsic development of the plastic microstructures. The evolution under strain path changes is modeled via phenomenologically based constitutive equations incorporated into crystal plasticity framework. The latter is capable of simulating the transient anisotropy effects (e.g. cross hardening, Bauschinger effect) depending on the change in the strain path. The paper discusses the unification of such models for the continuous modeling of microstructure formation and evolution processes.

Abstract: State-of-art models for mechanical joints in large scale structures typically consider only the linear behavior of the joint zones with lower complex approaches, such as rigid or elastic beams or a merge of opposite sheet metal nodes. In the present study several feasible methods to model nonlinear joint behavior and the connection between sheets and joint are investigated and evaluated. A preferred combination based on nonlinear springs was chosen, which meets the requirements for application in large scale structure models: low computation time, mesh independence and availability in several FEM software packages. For the calibration of the joint zone models a 2-point-tension-specimen was used. Five different joint types and the two sheet material combinations aluminium/aluminium as well as steel/steel were investigated. With the calibrated models a more complex 5-point-tension-specimen was used to consider the local interoperation of the joints. Some deviations were determined especially for highly stressed joint zones. Hence an average function was defined to consider both, the local deformations in the joint zone and additionally the more global sheet deformations. Finally, the simplified joint models were used in a complex specimen model with 22 joints. The comparisons between experimentally and numerically determined results show a good accordance. The nonlinear joint behavior is captured very well. A method is presented, which uses 2-point-specimens to calibrate simplified joint models with nonlinear deformation characteristics. The efficient application in large scale structure models is possible due to simplicity, stability, low computation times and mesh independent implementation.

Abstract: Modeling anisotropic behavior of fiber reinforced rubberlike materials is actually of a great interest in many industrials sectors. Indeed, accurately description of the mechanical response and damage of such materials allows the increase of the lifecycle of these materials which generally evolve under several environment conditions. In this paper theoretical study and finite element analysis of anisotropic biomaterials is presented. The mechanical model adopted to achieve this study has been implemented into the finite element code Abaqus using an implicit scheme. This constitutive law has been utilized to perform some numerical simulations. The material parameters of the model have been determined by numerical calibration. One fiber family is considered in this work. Effects of the fiber orientation on the mechanical response and stiffness change of biomaterial is studied. Both the compressible and incompressible states have been taken into account. The results show firstly the capability of the model to reproduce the known results and that optimal fiber orientation can be found.

Abstract: The recently developed SSH3D solid-shell element [1], which is based on the Enhanced Assumed Strain (EAS) and the Assumed Natural Strain (ANS) techniques, is utilized for the modeling of a severe bending sheet forming process. To improve the element's ability to capture the through thickness gradients, a specific integration scheme was developed. In this paper, the performances of this element for the modeling of the T-bent process were assessed thanks to comparison between experimental and numerical results in terms of the strain field at the outer surface of the sheet. The experimental results were obtained by Digital Image Correlation. It is shown that a qualitative agreement between experimental and numerical results is obtained but some numerical parameters should be optimized to improve the accuracy of the simulation predictions. In this respect, the influence of the penalty coefficient of the contact modeling was analyzed.

Abstract: Fracturing by ductile damage occurs quite naturally in metal forming process due to the development of microcracks associated with large straining or due to plastic instabilities associated with material behavior and boundary conditions. Metal forming processes generally introduce a certain amount of damage in the material being formed. Predictions of the damage formation and growth in a series of forming steps may assist in optimizing the individual operations and their order. This is particularly true for operations such as cutting and blanking, which rely on the nucleation of damage and cracks in order to separate material. In this work numerical simulation of the blanking process, using Deform 2D, taking in account the damage, has been performed. In order to evaluate the accuracy of the numerical solution, experimental test have been performed. Furthermore a numerical – experimental correlation has been carried out.

Abstract: Yoshida-Uemori model (Y-U model) can be used with any types of yield functions. The calculated stress strain response will be, however, different depending on the chosen yield function if the yield function and the effective strain definition are inappropriate. Thus several modifications to Y-U model were proposed in the 10th International Conference on Technology of Plasticity. It was ascertained that in the modified Y-U model, the same set of material parameters can be used with von Mises, Hill’s 1948, and Hill’s 1990 yield function. In this study, Yld2000-2d and Yoshida’s 6th-order polynomial type 3D yield function were examined and it was clarified that the same set of Y-U parameters can be used with these yield functions.

Abstract: In recent decades,the forming area advanced both in terms of material used as well as in flexibilityand process cost reduction. New processes are been studied, including theIncremental Sheet Forming – ISF. The ISF is a process characterized by theproduction of small batches of parts, rapid prototyping, and manufacturingflexibility with reduced operational cost. This study aims to compare thecomputer simulation with real experiments from ISF. The results of strain pathsof the three main strains simulated were consistent with the experimentalmanufacture of a symmetrical sample.

Abstract: Tailored heat treated blanks (THTB) is the generic term for an innovative approach to enhance the formability of blanks made out of high strength steel or aluminum alloys. Key idea of the technology is the adaption of the mechanical properties by a local heat treatment. Based on the new property distribution, the material flow during the forming operation can be improved and the forming limit can be enhanced. In comparison to conventional temperature assisted approaches the forming is performed at room temperature and therefore all advantages of a cold forming process can be used. Most challenging within the application is the definition of the heat treatment layout. Up to now the layout is dimensioned in a time-consuming trial and error procedure. In this paper a new approach for the automatic optimization of the heat treatment layout and the blank outline is presented.

Abstract: This paper presents an efficient knowledge based method using data mining and multilayer perceptrons (MLP) for the prediction of the technical feasibility of sheet metal forming processes. The stored forecast models can be applied to similar geometries and forming processes using the digital fingerprint to identify the most suitable MLP. Moreover, we establish an algorithm to detect extrapolation which is a priori applied to each new design in order to avoid probable high errors in the forecast model caused by extrapolation. We demonstrate the benefits of the method on a model problem specially designed to investigate a wide range of industrial relevant forming characteristics.

Abstract: Abstract. The application of modern high strength low alloyed steels (HSLA) and advanced high strength steels (AHSS) for structural and safety relevant components in the automotive industry offers the advantage of combining low specific weight with high material strength. Typical manufacturing processes for these steel grades are bending and cutting operations. The forming and cutting potential of these innovative steel grades is different to conventional steels as the process and the damage behaviour is changing. In bending operations cracks occur at the outer bending edge, whereas in cutting operations delamination can appear at the sheared edge. These damages, even though they are small, can initiate the component to fail. For a reliable use of such materials in industrial application a method for the process design is essentially needed. In particular, damages have to be predicted at an early stage. In industrial application damage is detected by a trial-and-error approach causing significant work and a high failure rate. A system for an offline assessment of the risk of failure is unknown so far. In the scope of this work, a method is presented to describe the damaging behaviour of both, bending and cutting operations, by theoretical metamodels. In order to generate a database experiments were carried out using different high strength steels. The main influence factors have been varied, such as the rolling direction, the punch-to-die clearance and the cutting contour in the cutting operation. The bending was investigated using an air-bending process varying the bending angle, the bending radius and the rolling direction. To calculate further sampling points a finite element model has been developed and validated against the experimental data. The damage criterion of Lemaître has been applied. The necessary parameters were determined by reverse identification by means of the major strain for the bending operation and by the punch force-punch stroke curve for the cutting operation. To build up a system for the prediction of the damage the gained data basis was approximated by mathematical functions. An error analysis was carried out showing good accordance. In doing so, a metamodel for the occurrence of damages could be established. The functions are implemented in a software tool which allows the user to determine the failure probability for a given parameter set.