Isotropic to distortional hardening transition in metal plasticity

The present paper aims to discuss the transition from isotropic to distortional hardening behavior of metallic materials, based on the Homogeneous Anisotropic Hardening (HAH) model. Furthermore, the effect of yield locus distortion on the evolution of the strain increment, under the assumption of associated flow, is theoretically discussed and exemplified. Special cases, such as coaxial and orthogonal stress states, are analyzed to provide better insight into the model. Particular emphasis is put on the monotonic loading case, which is compared to isotropic hardening. Finally, the evolution equations of the state variables are examined and their properties are discussed. (C) 2014 Elsevier Ltd. All rights reserved.111512Ysciescopu

A dual-mesh strategy for the 3d simulation of fineblanking processes

Fineblanking technology is used to produce blanked metal components which show outstanding surface quality and part flatness. The defining characteristics of the process are, besides the use of a counter punch and a V- Ring, the tiny die clearance and a rounded cutting edge. The 3D FE simulation of the process proves to be thus very challenging. This is mainly because in comparison to the part dimensions (which are of the order of 10mm) a very small mesh size needs to be chosen on the cutting edge (~0.01mm), which leads to a very big number of elements and also tiny time steps. This paper aims to show a solution to the problem using the Arbitrary Lagrangian Eulerian FE formulation, applied on two different levels of refinement. First a relatively coarse mesh (element size of about 0.1mm around the cutting edge) is applied to solve the full size 3D problem. The flow information is subsequently used on a much finer mesh (size ~0.005) defined around a small region on the cutting line to accurately compute the stress-strain distribution around the radi

A strain rate dependent anisotropic hardening model and its validation through deep drawing experiments

In the present work, a modified version of the widely used Yld2000-2d yield function and its implementation into the commercial FE-code LS-Dyna is presented. The difference to the standard formulation lies in the dependency of the function parameters on the equivalent plastic strain. Furthermore, strain rate dependency is incorporated. After a detailed description of the model and the identification of the parameters, the numerical implementation i.e., the stress-update algorithm used for the implementation is explained. In order to validate the model, two different materials, namely Formalex™5x, a 5182-based aluminum alloy and a DC05 mild steel were characterized. The results of the tensile and hydraulic bulge tests are presented and used for the parameter identification. The experimental curves are reproduced by means of one element tests using the standard and modified model to demonstrate the benefit of the modifications. For validation purposes, cross die geometries were drawn with both materials. The outer surface strains were measured with an optical measurement system. The measured major and minor strains were compared to the results of simulations using the standard and the modified Yld2000-2d model. A significant improvement in prediction accuracy has been demonstrated

On the role of Anisotropy and Bauschinger-Effect in Sheet Metal Spinning

Incremental sheet forming is known for its local forming character where the material is continuously bent and unbent in a multitude of tool passes. By this nature, anisotropy and Bauschinger-Effect might play a significant role in numerical modelling of the process. This paper aims to assess different material modelling techniques by comparing the resulting stress and strain histories of the FEM simulations. Amongst these are Barlats non-quadratic yield locus Yld2000-2d yield criterion and the homogeneous anisotropic hardening model.ISSN:1742-6588ISSN:1742-659

Significance of the local sheet curvature in the prediction of sheet metal forming limits by necking instabilities and cracks

The industrial based prediction in sheet metal forming bases still on the Forming Limit Diagrams (FLD) as formally proposed by Keeler 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 influences of bending on the FLC as well as the later crack limits will be discussed. Both criteria will be combined to an extended FLC concept (X-FLC). 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 new local thinning method is proposed and tested for special 6xxx Al-alloys

Assessment of anisotropic hardening models for conventional deep drawing processes

Assessing the predictive capabilities of recent advanced constitutive modelling approaches for processes with industrial complexity is a challenging task. Real process conditions such as blankholder pressure distribution, friction and tool elasticity sensitively affect experimental observations, making the isolation of constitutive effects difficult. A systematic approach is proposed in this work to assess the performance of anisotropic hardening models with the least possible disturbance from process conditions. Two deep drawing examples were used for these purpose (“cross die” and “lackfrosch”) in conjunction with a mild steel (DC05). Optically measured strain distributions have been compared to corresponding simulations, which have been calibrated to accurately match the measured blank draw-in. The effect of initial yield locus shape as well as anisotropic hardening effects have been discussed.ISSN:1960-6206ISSN:1960-621

Numerical modelling, validation and analysis of multi-pass sheet metal spinning processes

Conventional sheet metal spinning is an incremental forming process which typically involves the cost-effective and high-quality manufacturing of axissymmetric parts. The process is usually executed by highly skilled and experienced personnel which is able of optimizing the process parameters during production. Numerical simulation of the process can substantially help discovering systematic methodologies for optimal parameter determination and thus enable the full automation of the process using CNC machines. The present work aims to assess the quality of numerical modelling techniques by a direct comparison with metal spinning experiments. Based on the geometry and thickness distribution of intermediate and final stages of a spinned component, which are measured using the Optical 3D Digitization technique, the quality and validity of different numerical modeling approaches are assessed. Subsequently, deformation mechanisms occurring during process are identified, analysed and discussed.ISSN:1960-6206ISSN:1960-621

A Fourier series based generalized yield surface description for the efficient modelling of orthotropic sheet metals

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.ISSN:1742-6588ISSN:1742-659