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Top 20 Most Read Articles
April 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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AIP Conf. Proc. 907, pp. 3-8; doi:http://dx.doi.org/10.1063/1.2729479 (6 pages) Online Publication Date: 13 April 2007
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Coupling Finite Element (FEM) simulations to mathematical optimisation techniques provides a high potential to improve industrial metal forming processes. In order to optimise these processes, all kind of optimisation problems need to be mathematically modelled and subsequently solved using an appropriate optimisation algorithm. Although the modelling part greatly determines the final outcome of optimisation, the main focus in most publications until now was on the solving part of mathematical optimisation, i.e. algorithm development. Modelling is generally performed in an arbitrary way. In this paper, we propose an optimisation strategy for metal forming processes using FEM. It consists of three stages: a structured methodology for modelling optimisation problems, screening for design variable reduction, and a generally applicable optimisation algorithm. The strategy is applied to solve manufacturing problems for an industrial deep drawing process. © 2007 American Institute of Physics |
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AIP Conf. Proc. 748, pp. E1-E1; doi:http://dx.doi.org/10.1063/1.2405718 (1 page)
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The robust optimisation of metal forming processes AIP Conf. Proc. 907, pp. 9-14; doi:http://dx.doi.org/10.1063/1.2729480 (6 pages) Online Publication Date: 13 April 2007
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Robustness, reliability, optimisation and Finite Element simulations are of major importance to improve product quality and reduce costs in the metal forming industry. In this paper, we review several possibilities for combining these techniques and propose a robust optimisation strategy for metal forming processes. The importance of including robustness during optimisation is demonstrated by applying the robust optimisation strategy to an analytical test function: for constrained cases, deterministic optimisation will yield a scrap rate of about 50% whereas the robust counterpart reduced this to the required 3σ reliability level. © 2007 American Institute of Physics |
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A Practical Approach To Preform Design For Different Materials AIP Conf. Proc. 907, pp. 15-20; doi:http://dx.doi.org/10.1063/1.2729481 (6 pages) Online Publication Date: 13 April 2007
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To forge an H‐shaped cross section, various preform designs have been tested for steel 42CrMo4, aluminum 7075 and nickel base alloy 80 A (Bohler L306). The influence of different boundary conditions like temperature and friction on the preform and hence on the forming process have been investigated by means of two dimensional finite element analyses. Furthermore, the influence of the preform on the microstructure was computed and the structural damage evolution in the forged parts depending on the preform design has been considered for alloy 80 A. © 2007 American Institute of Physics |
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AIP Conf. Proc. 907, pp. 47-52; doi:http://dx.doi.org/10.1063/1.2729486 (6 pages) Online Publication Date: 13 April 2007
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A large strain elastic‐plastic single crystal constitutive law, based on dislocation annihilation and storage, is implemented in a new self‐consistent scheme, leading to a multiscale model which achieves, for each grain, the calculation of plastic slip activity, with help of regularized formulation drawn from visco‐plasticity, and dislocation microstructure evolution. This paper focuses on the relationship between the deformation history of a BCC grain and induced microstructure during monotonic and two‐stage strain paths. © 2007 American Institute of Physics |
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Meta‐Model Based Optimisation Algorithms for Robust Optimization of 3D Forging Sequences AIP Conf. Proc. 907, pp. 21-26; doi:http://dx.doi.org/10.1063/1.2729482 (6 pages) Online Publication Date: 13 April 2007
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In order to handle costly and complex 3D metal forming optimization problems, we develop a new optimization algorithm that allows finding satisfactory solutions within less than 50 iterations (/function evaluation) in the presence of local extrema. It is based on the sequential approximation of the problem objective function by the Meshless Finite Difference Method (MFDM). This changing meta‐model allows taking into account the gradient information, if available, or not. It can be easily extended to take into account uncertainties on the optimization parameters. This new algorithm is first evaluated on analytic functions, before being applied to a 3D forging benchmark, the preform tool shape optimization that allows minimizing the potential of fold formation during the two‐stepped forging sequence. © 2007 American Institute of Physics |
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AIP Conf. Proc. 907, pp. 94-99; doi:http://dx.doi.org/10.1063/1.2729494 (6 pages) Online Publication Date: 13 April 2007
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The purpose of the present work is to analyze several aspects related to the connection between the constitutive models, their identification and the FEM predictions. Several issues are addressed: the experimental data base that should be used in the identification procedure, the choice of the mechanical tests involved (monotonous and/or non‐proportional loading, homogeneous or heterogeneous tests…), the identification strategies (direct or inverse FE optimization, simultaneous or sequential material parameters identification…). Besides its obvious interest, such study aim to find a good balance between the number and the type of relevant involved mechanical tests in material behavior characterization. This is an important issue for industrial applications. © 2007 American Institute of Physics |
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A finite strain isotropic/kinematic hardening model for springback simulation of sheet metals AIP Conf. Proc. 907, pp. 139-144; doi:http://dx.doi.org/10.1063/1.2729501 (6 pages) Online Publication Date: 13 April 2007
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Crucial for the accurate prediction of the blank springback is the use of an appropriate material model, which is capable of modelling the typical cyclic hardening behaviour of metals (e.g. Bauschinger effect, ratchetting). The proposed material model combines both nonlinear isotropic hardening and nonlinear kinematic hardening, and is defined in the finite strain regime. The kinematic hardening component represents a continuum extension of the classsical rheological model of Armstrong‐Frederick kinematic hardening. The evolution equations of the model are integrated by a new form of the exponential map algorithm, which preserves the plastic volume and the symmetry of the internal variables. Finally, the applicability of the model for springback prediction has been demonstrated by performing simulations of the draw‐bending process. © 2007 American Institute of Physics |
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Numerical Prediction of Elastic Springback in An Automotive Complex Structural Part AIP Conf. Proc. 907, pp. 211-216; doi:http://dx.doi.org/10.1063/1.2729513 (6 pages) Online Publication Date: 13 April 2007
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The occurrence of elastic springback phenomena in sheet metal processing operations determines a relevant issue in the automotive industry. The routing and production of 3D complex parts for automotive applications is characterized by springback phenomena affecting the final geometry of the components both after the stamping operations and the trimming ones. In the present paper the full routing of a automotive structural part is considered and the springback phenomena occurring after forming and trimming are investigated through FE analyses utilizing an explicit implicit approach. In particular a sensitivity analysis on process parameter influencing springback occurrence is developed: blank holder force, draw bead penetration and blank shape. © 2007 American Institute of Physics |
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AIP Conf. Proc. 907, pp. 100-105; doi:http://dx.doi.org/10.1063/1.2729495 (6 pages) Online Publication Date: 13 April 2007
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Reverse simple shear tests are used to analyse the Bauschinger effect and the evolution of the kinematic hardening for a wide range of equivalent von Mises strain [0.025 – 0.3]. This work is carried out on two high strength low‐alloyed steels. In order to investigate the effect of the precipitates on the macroscopic behaviour, a ferritic mild steel is used as a reference. Different phenomenological descriptions of the back‐stress tensor are examined in order to analyse their ability to describe the experimental behaviour. © 2007 American Institute of Physics |
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AIP Conf. Proc. 907, pp. 33-38; doi:http://dx.doi.org/10.1063/1.2729484 (6 pages) Online Publication Date: 13 April 2007
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Biaxial properties of materials (polymer or steel) used in many industrial processes are often difficult to measure. However, these properties are useful for the numerical simulations of plastic‐processing operations like blow moulding or thermoforming for polymers and superplastic forming or single point incremental forming for steels. Today, Optical Full Field Measurements (OFFM) are promising tools for experimental analysis of materials. Indeed, they are able to provide a very large amount of data (displacement or strain) spatially distributed. In this paper, a mixed numerical and experimental investigation is proposed in order to identify multi‐axial constitutive behaviour models. The procedure is applied on two different materials commonly used in forming processes: polymer (rubber in this first approach) and steel. Experimental tests are performed on various rubber and steel structural specimens (notched and open‐hole plate samples) in order to generate heterogeneous displacement field. Two different behaviour models are considered. On the one hand, a Money‐Rivlin hyperelastic law is investigated to describe the high levels of strain induced in tensile test performed on a rubber open‐hole specimen. On the other hand, Ramberg‐Osgood law allows to reproduce elasto‐plastic behaviour of steel on a specimen that induces heterogeneous strain fields. Each parameter identification is based on a same Finite Element Model Updated (FEMU) procedure which consists in comparing results provided by the numerical simulation (ABAQUS™) with full field measurements obtained by the DISC (Digital Image Stereo‐Correlation) technique (Vic‐3D®). © 2007 American Institute of Physics |
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High Velocity Forming of Magnesium and Titanium Sheets AIP Conf. Proc. 907, pp. 157-162; doi:http://dx.doi.org/10.1063/1.2729504 (6 pages) Online Publication Date: 13 April 2007
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Cold forming of magnesium and titanium is difficult due to their hexagonal crystal structure and limited number of available slip systems. However, high velocity deformation can be quite effective in increasing the forming limits. In this study, electromagnetic forming (EMF) of thin AZ31B‐O magnesium and CP grade 1 titanium sheets were compared with normal deep drawing. Same dies were used in both forming processes. Finite element (FE) simulations were carried out to improve the EMF process parameters. Constitutive data was determined using Split Hopkinson Pressure Bar tests (SHPB). To study formability, sample sheets were electromagnetically launched to the female die, using a flat spiral electromagnetic coil and aluminum driver sheets. Deep drawing tests were made by a laboratory press‐machine. Results show that high velocity forming processes increase the formability of Magnesium and Titanium sheets although process parameters have to be carefully tuned to obtain good results. © 2007 American Institute of Physics |
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Experimental Analysis and Modelling of Fe‐Mn‐Al‐C Duplex Steel Mechanical Behaviour AIP Conf. Proc. 907, pp. 76-81; doi:http://dx.doi.org/10.1063/1.2729491 (6 pages) Online Publication Date: 13 April 2007
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A new variety of duplex steels with high content of manganese and aluminum has been elaborated in Arcelor Research. These steels contain two phases: austenite and ferrite combining the best features of austenitic and ferritic steels. In this work, four duplex steels with different chemical composition and phase volume fraction are studied. The evolution of internal stresses for the two phases has been determined by X‐ray diffraction during an in situ tensile test. These measurements results were used to determine the mechanical behaviour of the duplex steel using a micromechanical approach by scale transition for tensile tests. Though a good agreement between experiments and simulations is found at the macroscopic level, the calculated internal stresses of the austenitic phase do not match experimental results. These discrepancies are attributed to (i) a bad estimation of the austenite yield stress or (ii) the presence of kinematic hardening in the austenitic phase. A new step is then proposed to test these two hypotheses. © 2007 American Institute of Physics |
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Research on Softening of A95456 Alloy Deformed Under Elevated Temperatures AIP Conf. Proc. 907, pp. 27-32; doi:http://dx.doi.org/10.1063/1.2729483 (6 pages) Online Publication Date: 13 April 2007
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The present paper describes the results of the research on the softening of aluminium alloy A95456 deformed at elevated temperatures. The investigations were carried out within the temperature range of 310–450 °C and strain rate of 0.01–0.4 s−1. The strain rate was either constant or variable in performed experiments. In case of variable strain rate two different schemes were observed. Firstly, the deformation of alloy A95456 was performed at constant die velocity and so the strain rate increased monotonically. Secondly, the die velocity was changed suddenly during the deformation of A95456 alloy. In turn, it caused the sudden strain rate change. To describe the softening behaviour of A95456 alloy several equations were investigated. The accuracy of each equation was estimated. Some practical recommendations for use of those equations were given. © 2007 American Institute of Physics |
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Ductile Damage Evolution and Strain Path Dependency AIP Conf. Proc. 907, pp. 187-192; doi:http://dx.doi.org/10.1063/1.2729509 (6 pages) Online Publication Date: 13 April 2007
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Forming limit diagrams are commonly used in sheet metal industry to define the safe forming regions. These diagrams are built to define the necking strains of sheet metals. However, with the rise in the popularity of advance high strength steels, ductile fracture through damage evolution has also emerged as an important parameter in the determination of limit strains. In this work, damage evolution in two different steels used in the automotive industry is examined to observe the relationship between damage evolution and the strain path that is followed during the forming operation. © 2007 American Institute of Physics |
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Forming Limit Predictions for Single‐Point Incremental Sheet Metal Forming AIP Conf. Proc. 907, pp. 309-314; doi:http://dx.doi.org/10.1063/1.2729530 (6 pages) Online Publication Date: 13 April 2007
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A characteristic of incremental sheet metal forming is that much higher deformations can be achieved than conventional forming limits. In this paper it is investigated to which extent the highly non‐monotonic strain paths during such a process may be responsible for this high formability. A Marciniak‐Kuczynski (MK) model is used to predict the onset of necking of a sheet subjected to the strain paths obtained by finite‐element simulations. The predicted forming limits are considerably higher than for monotonic loading, but still lower than the experimental ones. This discrepancy is attributed to the strain gradient over the sheet thickness, which is not taken into account in the currently used MK model. © 2007 American Institute of Physics |
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AIP Conf. Proc. 907, pp. 39-46; doi:http://dx.doi.org/10.1063/1.2729485 (8 pages) Online Publication Date: 13 April 2007
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A macroscopic model based on a viscoplastic constitutive law is presented to describe the sintering process of metallic powder components obtained by injection moulding. The model parameters are identified by the gravitational beam‐bending tests in sintering and the sintering experiments in dilatometer. The finite element simulations are carried out to predict the shrinkage, density and strength after sintering. The simulation results have been compared to the experimental ones, and a good agreement has been obtained. © 2007 American Institute of Physics |
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Micro‐Macro approach for mechanical problems involving microstructure AIP Conf. Proc. 907, pp. 1342-1347; doi:http://dx.doi.org/10.1063/1.2729701 (6 pages) Online Publication Date: 13 April 2007
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In this paper we propose a new coupled two‐scales modelling for mechanical problems involving microstructure. It is based on the use of a microscopic description of the material and a meshless constrained natural element approximation to take into account the large and very localized variations in the nodal density. The domain is divided into microscopic zones and a complementary medium. The union of the microscopic zones does not cover the whole domain. The constitutive relation is defined only at the microscopic scale in each microscopic zone. In the complementary medium the constitutive relation is deduced numerically from the macroscopic response of the microscopic zones. An iterative resolution scheme allows building the displacement field in the whole domain and the extension of the constitutive relation to the complementary medium. This approach is an appealing choice for treating problems requiring fine local descriptions, whose evolution can be accurately described in the macroscopic scale using coarse approximations. It leads to a model where more than 95% of the nodes are in less than 5% of the considered domain volume. In contrast to the vast majority of homogenization techniques, the presented approach allows an accurate description of the boundary conditions, because the microscopic domains can be located on the domain boundary. © 2007 American Institute of Physics |
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AIP Conf. Proc. 907, pp. 396-404; doi:http://dx.doi.org/10.1063/1.2729546 (9 pages) Online Publication Date: 13 April 2007
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In this paper, yield functions describing the anisotropic behavior of textured metals are proposed. These yield functions are extensions to orthotropy of the isotropic yield function proposed by Cazacu et al.. Anisotropy is introduced using linear transformations of the stress deviator. It is shown that if two linear transformations are considered, the proposed anisotropic yield function represents with great accuracy both the tensile and compressive anisotropy in yield stresses and r‐values of materials with hcp crystal structure and of metal sheets with bcc crystal structure that exhibit asymmetry between tensile and compressive behavior. Furthermore, it is demonstrated that the proposed formulations can describe very accurately the anisotropic behavior of metal sheets whose tensile and compressive stresses are equal. © 2007 American Institute of Physics |
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An Advanced Numerical Differentiation Scheme for Plastic Strain‐Rate Computation AIP Conf. Proc. 907, pp. 151-156; doi:http://dx.doi.org/10.1063/1.2729503 (6 pages) Online Publication Date: 13 April 2007
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Modern yield functions for orthotropic sheet metals are quite complex in a mathematical sense, mainly due to their non‐quadratic character and/or the incorporation of eigenvalues of linearly transformed stress tensors (e.g. [5, 6]). In particular, the analytical computation of first and second order yield function gradients, which are, for instance, required in finite element codes, can become a very lengthy task. Thus, the numerical differentiation is a very convenient method to circumvent these difficulties. In the present article an advanced numerical differentation scheme is presented that exploits consequently the homogeneity property of the yield function. © 2007 American Institute of Physics |
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Sandia National Laboratories
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