[en] Better formability, less forming force and satisfactory quality are the most important characteristics of warm forming processes. However, the material models for either cold forming or hot forming cannot be directly adopted for the numerical simulation of warm forming processes. Supplement and modification are necessary. Based on the Zener-Hollomon formulation, additional terms are proposed in the presented work to describe the softening effect observed during warm forming processes as well as the strain hardening effect. The numerical simulation provides detailed information about the history and distribution of both deformation and temperature, the phase transformation can then also be evaluated, provided the experimental data are available

[en] Stainless steels as well as TRIP and TWIP steels show a hardening behavior, which can be described only in dependency on the deformation and temperature history during the real forming process. Because the hardening behavior is the determinate factor for the necking phenomenon, the prediction of rupture becomes also deformation path and temperature dependent. As a consequence, the common FLC-method, using a single curve for the prediction of the failure state is not accurate enough. In this paper, a temperature dependent Forming Limit Surface (FLS) is presented

[en] In recent years the use of the Mohr-Coulomb type fracture criteria has become popular in ductile fracture prediction. In ductile fracture prediction, fracture criteria are often transformed into the mixed stress strain space formulation and applied to numerical simulations in this formulation. This paper will investigate the shape of the Mohr Coulomb fracture criterion in the mixed stress strain space as a function of the used hardening curve formulation, in particular the hardening curve exponent n that determines the slope of the hardening curve approximation for large strains. It is found that the shape of the fracture criterion significantly depends on the slope of the hardening curve. This makes a correct extrapolation of the hardening curve for large strain values essential for a correct ductile fracture prediction using the Mohr-Coulomb fracture model in its mixed stress strain space formulation. (paper)

[en] In the current state of the art, there exist a large number of different yield surface descriptions, which are well established in modelling particular types of materials. The success of a particular model is primarily related to its ability of accurately representing the material behaviour based on a limited number of experiments. In the present work, instead of defining a particular mathematical formulation, a generic yield surface is proposed based on a 2D Fourier series. Yield surfaces of increasing complexity can be effectively generated by increasing the number of terms in the series. The particular properties of the modelled materials are not derived from a predefined formulation, but enforced as a set of constraints. It is shown that both symmetric and asymmetric yield loci can be easily constructed using this approach. Furthermore the accuracy and computational efficiency of the proposed model is discussed in comparison to well established yield surface functions, using deep drawing simulations. (paper)

[en] 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. (paper)

[en] The following topics were examined: (i) Methodology of fracture tests at temperature gradients and pressurized thermal shock (PTS) cooling tests on large specimens (1500x1200x140 mm) with a designed postulated crack and other smaller cracks in the cooled area (test performed on a ZZ 8000 (80MN) loading stand); (ii) simulation of radiation embrittlement of tested material near the end of the RPV designed life, the material is subjected to standard mechanical property tests and fracture tests of standard test specimens modelling the PTS regime of material loading; (iii) 100% NDT tests of a specimen before the beginning of tests and μTOFD before and after each particular test of a specimen; (iv) on-line monitoring of the test conditions based on instrumentation of a specimen with thermocouples, COD and strain gauges together with on-line monitoring of Acoustic Emission during the tests; (v) calculation of K_l at the critical points of the crack front during the test, based on monitored boundary conditions; (vi) fractographic analysis after the fracture of a specimen and evaluation of the whole test. (P.A.)

[en] A mathematical model is presented for a forming limit for non-proportional loading under plane stress condition. The model results in an approach to reduce the number of experiments needed for the Generalized Forming Limit Concept (GFLC) and presents a numerical approach to calculate the linearized FLC from real Nakajima measurements. The mathematical model has been analyzed in comparison to Polar Effective Plastic Strain (PEPS) diagram and enhanced Modified Maximum Force Criterion (eMMFC), discussing both consistency with plasticity modeling and industrial applicability. An experimental setup based on Nakajima specimens is presented and DIC measurements are used to capture loading paths. The measured loading paths are used to validate predictions made by PEPS, eMMFC and the presented mathematical approach. The latter model shows promising results for prediction of failure for non-proportional loading. (paper)

[en] In production of deep drawn sheet metal parts, it is often challenging to achieve a robust process. Especially in the production of kitchen sinks made out of stainless steel, the fluctuation of the process and material properties often lead to robustness problems. Therefore, numerical simulations are used to detect critical regions. To keep a constant product quality, process control is realised based on metamodels, which are computed by means of a series of finite element simulations. In order to enhance the forecast capability of the simulation model and to increase the reliability of quality features, the yield curve, the yield locus and the forming limit curve (FLC) of different suppliers are measured with tensile, bulge and Nakazima experiments. Because of the large deformation capability of stainless steels, large drawing depths can be achieved. However, the classic Nakazima geometries are not distributed homogenously in the forming limit diagram (FLD). To overcome this shortcoming, a shape optimization of the Nakazima specimen has been performed. Furthermore the influence of alloying elements on the hardening behaviour and on the FLC are analysed. In addition, the measured FLC-T (temperature dependent FLC) conducted with heated Nakazima tests is compared with the computed FLC-T with the modified maximum force criterion (MMFC). (paper)

[en] Stainless steel is a complex material and has properties that make it difficult to use in deep drawing processes. Because of its scattering material properties a robust process is difficult to achieve, resulting in the necessity to constantly adjust the drawing process. In order to produce parts at constantly high quality and to minimize scrap production, an intelligent control system, the Q-Guard system is implemented in production, covering the whole process chain from raw material to the finished part. This control system is presented in this contribution, with the main focus on the process control. This system is based on numeric simulations as well as material data, the process settings and draw-in measurements, all of them acquired in-line in production. Part of the data is used for a feedforward control for immediate good parts production, part of the data, like the draw-in, measured with an optical measurement system after the first draw, is used in a feedback loop. The layout of the process control and results from production runs will also be shown in this work. (paper)

[en] Finite element analysis (FEA) was used to model the angular stretch bend test, where a strip of sheet metal is locked at both ends and a tool with a radius stretches and bends the center of the strip until failure. The FEA program used in the study was Abaqus. The FEA model was verified by experimental work using a dual phase steel (DP600) and with a simplified analytical analysis. The FEA model was used to simulate the experimental test for various frictional conditions and various radii of an upward moving tool. The primary objective of the study was to evaluate the concave-side rule, which states that during stretch bending the forming limit occurs when the strains on the concave surface plane of the bent sheet (i.e. bottom plane) reach the forming limit curve (FLC). The verification with experimental data indicates that the FEA model represents the process very well. Only conditions where failure occurred on or near the tooling are included in the results. The FEA simulations showed that the actual forming limit of the sheet occurs when the strains on the bottom plane of the sheet (i.e. concave side of the bend) reach the forming limit curve for high friction and small tool radii. For lower friction and for larger tool radii the actual forming limit occurs when strains on other planes in the sheet (i.e. mid planes or top surface plane) reach the forming limit curve. The implications of these results suggest that care must be taken in assessing forming operations when both stretch and bending occur. Although it is known that the FLC cannot predict the forming limit for small bend radii, the common assumption that the forming limit occurs when the strains for the middle thickness plane of the sheet reach the forming limit curve or that the concave side rule is often made. Understanding the limits of this assumption needs to be carefully and critically evaluated. (paper)