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

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

A discussion of the associated flow rule based on the FAY model and Nakajima tests

Constitutive models based on a non-associated flow rule (non-AFR) have received increased attention due to their flexibility in capturing laboratory experiments. On the other hand, the recently proposed Fourier Asymmetric Yield (FAY) model enables the definition of convex yield functions with arbitrary complexity, which can be used in conjunction to an associated flow rule and nevertheless accurately match experimental data. The present contribution aims comparing AFR and non-AFR based approaches with respect to their ability in capturing measured strain fields in controlled Nakajima experiments. It is shown that a sufficiently flexible yield function is able of delivering accurate results without having to renounce to restrictions such as flow rule association or convexity. The analysis is carried out using two common deep drawing materials, namely an AA6016 and a steel grade DC05.ISSN:1742-6588ISSN:1742-659

Numerical Tool Path Optimization for Conventional Sheet Metal Spinning Processes

To this day, conventional sheet metal spinning processes are designed with a very low degree of automation. They are usually executed by experienced personnel, who actively adjust the tool paths during production. The practically unlimited freedom in designing the tool paths enables the efficient manufacturing of complex geometries on one hand, but is challenging to translate into a standardized procedure on the other. The present study aims to propose a systematic methodology, based on a 3D FEM model combined with a numerical optimization strategy, in order to design tool paths. The accurate numerical modelling of the spinning process is firstly discussed, followed by an analysis of appropriate objective functions and constraints required to obtain a failure free tool path design.ISSN:1742-6588ISSN:1742-659

A new optimization procedure for the accurate characterization of thermal phase transformation curves based on controlled quenching experiments

Precise hardness and phase content prediction for quenched steel with the finite element method requires optimal material data, which is usually obtained from measured continuous cooling transformation (CCT) diagrams. However, most software packages that are able to predict final phase composition require time temperature transformation (TTT) diagrams. TTT diagrams can be calculated from the chemical composition of the material. With this methods the numerical prediction often result in deviations to reality. A newly developed optimization method can improve the accuracy of phase content and hardness prediction after quenching by optimizing the TTT diagram with measured data of controlled quenching experiments

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.ISSN:1960-6206ISSN:1960-621