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See detailForming forces in single point incremental forming: prediction by finite element simulations, validation and sensitivity
Henrard, Christophe; Bouffioux, Chantal ULg; Eyckens, P. et al

in Computational Mechanics (2011), 47

The aim of this article is to study the accuracy of finite element simulations in predicting the tool force occurring during the single point incremental forming (SPIF) process. The forming of two cones ... [more ▼]

The aim of this article is to study the accuracy of finite element simulations in predicting the tool force occurring during the single point incremental forming (SPIF) process. The forming of two cones in soft aluminum was studied with two finite element (FE) codes and several constitutive laws (an elastic–plastic law coupled with various hardening models). The parameters of these laws were identified using several combinations of a tensile test, shear tests, and an inverse modeling approach taking into account a test similar to the incremental forming process. Comparisons between measured and predicted force values are performed. This article shows that three factors have an influence on force prediction: the type of finite element, the constitutive law and the identification procedure for the material parameters. In addition, it confirms that a detailed description of the behavior occurring across the thickness of the metal sheet is crucial for an accurate force prediction by FE simulations, even though a simple analytical formula could provide an otherwise acceptable answer. [less ▲]

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See detailStrain Evolution in the Single Point Incremental Forming Process: Digital Image Correlation Measurement and Finite Element Prediction
Eyckens, P.; Belkassem, B.; Henrard, Christophe et al

in International Journal of Material Forming (2011)

Incremental Sheet Forming (ISF) is a relatively new class of sheet forming processes that allow the manufacture of complex geometries based on computer-controlled forming tools in replacement (at least ... [more ▼]

Incremental Sheet Forming (ISF) is a relatively new class of sheet forming processes that allow the manufacture of complex geometries based on computer-controlled forming tools in replacement (at least partially) of dedicated tooling. This paper studies the straining behaviour in the Single Point Incremental Forming (SPIF) variant (in which no dedicated tooling at all is required), both on experimental basis using Digital Image Correlation (DIC) and on numerical basis by the Finite Element (FE) method. The aim of the paper is to increase understanding of the deformation mechanisms inherent to SPIF, which is an important issue for the understanding of the high formability observed in this process and also for future strategies to improve the geometrical accuracy. Two distinct large-strain FE formulations, based on shell and first-order reduced integration brick elements, are used to model the sheet during the SPIF processing into the form of a truncated cone. The prediction of the surface strains on the outer surface of the cone is compared to experimentally obtained strains using the DIC technique. It is emphasised that the strain history as calculated from the DIC displacement field depends on the scale of the strain definition. On the modelling side, it is shown that the mesh density in the FE models plays a similar role on the surface strain predictions. A good qualitative agreement has been obtained for the surface strain components. One significant exception has however been found, which concerns the circumferential strain evolution directly under the forming tool. The qualitative discrepancy is explained through a mechanism of through-thickness shear in the experiment, which is not fully captured by the present FE modelling since it shows a bending-dominant accommodation mechanism. The effect of different material constitutive behaviours on strain prediction has also been investigated, the parameters of which were determined by inverse modelling using a specially designed sheet forming test. Isotropic and anisotropic yield criteria are considered, combined with either isotropic or kinematic hardening. The adopted constitutive law has only a limited influence on the surface strains. Finally, the experimental surface strain evolution is compared between two cones with different forming parameters. It is concluded that the way the plastic zone under the forming tool accommodates the moving tool (i.e. by through-thickness shear or rather by bending) depends on the process parameters. The identification of the most determining forming parameter that controls the relative importance of either mechanism is an interesting topic for future research. [less ▲]

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See detailForming Forces in Single Point Incremental Forming, Prediction by Finite Element Simulations
Henrard, Christophe; Bouffioux, Chantal ULg; Eyckens, P. et al

in Computational Mechanics (2010)

The aim of this article is to study the accuracy of the nite element simulations to predict the tool force during the Single Point Incremental Forming process. The forming of two cones in soft aluminum ... [more ▼]

The aim of this article is to study the accuracy of the nite element simulations to predict the tool force during the Single Point Incremental Forming process. The forming of two cones in soft aluminum was studied with two Finite Element (FE) codes and several constitutive laws (an elastic-plastic model coupled with different hardening approaches). The parameters of these laws were identi ed using tensile and shear tests, as well as an inverse approach taking into account a test similar to the incremental forming process. Comparisons between measured and predicted force values are performed. This article shows that three factors have an in uence on the force prediction: the type of nite element, the constitutive law and the identi cation procedure for the material parameters. In addition, it con rms that a very detailed description of the behavior occurring across the thickness of the metal sheet is crucial for an accurate force prediction by FE simulations, even though a simple analytical formula could provide an otherwise acceptable answer. [less ▲]

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See detailMulti-Step toolpath approach to overcome forming limitations in single point incremental forming
Verbert, J.; Belkassem, B.; Henrard, Christophe ULg et al

in Boisse, P. (Ed.) Proceedings of the 11th International ESAFORM Conference on Material Forming (2008)

Although Incremental Forming offers distinct advantages over traditional forming processes, such as short lead times and low setup costs, the process still has some drawbacks. Besides the obtainable ... [more ▼]

Although Incremental Forming offers distinct advantages over traditional forming processes, such as short lead times and low setup costs, the process still has some drawbacks. Besides the obtainable accuracy, one of the main challenges of the process are the process limits. Many workpiece geometries cannot be manufactured due to the fact that the maximum wall angle that can be formed is limited for a certain sheet material and thickness to a given angle. Different solutions to this approach have been proposed and this paper further investigates one of those solutions, the multi step approach for single point incremental forming. Experiments were performed and compared with simulations to better understand the phenomena underlying the improved process performance. [less ▲]

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See detailComparison of the tests chosen for material parameter identification to predict single point incremental forming forces
Bouffioux, Chantal ULg; Henrard, Christophe ULg; Eyckens, P. et al

in Asnafi, Nader (Ed.) Proceedings of the International Conference of International Deep Drawing Research Group (IDDRG 2008) (2008)

Single Point Incremental Forming is a sheet forming process that uses a smooth-ended tool following a specific tool path and thus eliminates the need for dedicated die sets. Using this method, the ... [more ▼]

Single Point Incremental Forming is a sheet forming process that uses a smooth-ended tool following a specific tool path and thus eliminates the need for dedicated die sets. Using this method, the material can reach a very high deformation level. A wide variety of shapes can be obtained without specific and costly equipment. To be able to optimize the process, a model and its material parameters are required. The inverse method has been used to provide material data by modeling experiments directly performed on a SPIF set-up and comparing them to the experimental measurements. The tests chosen for this study can generate heterogeneous stress and strain fields. They are performed with the production machine itself and are appropriate for the inverse method since their simulation times are not too high. [less ▲]

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See detailAnalysis of speckle patterns for deformation measurements by DIC
Lecompte, David; Sol, H.; Vantomme, J. et al

in Slangen, Pierre; Cerruti, Christine (Eds.) Proceedings of SPIE Vol. 6341 (2006)

Digital Image Correlation (DIC) – also referred to as white light speckle technique – is an optical-numerical full-field measuring technique, which offers the possibility to determine in-plane ... [more ▼]

Digital Image Correlation (DIC) – also referred to as white light speckle technique – is an optical-numerical full-field measuring technique, which offers the possibility to determine in-plane displacement fields at the surface of objects under any kind of loading. For an optimal use of the method, the object of interest has to be covered with a speckle pattern. The present paper studies the efficiency of a random speckle pattern and its influence on the measured in-plane displacements with respect to the subset size. First a randomly sprayed speckle pattern is photographed three times. Each picture is taken with a different zoom, yielding three speckle patterns, which are different by the size of the speckles. Secondly a number of speckle patterns are generated numerically using a given speckle size and image coverage. Subsequently, each speckle pattern image undergoes a numerically controlled deformation, which is measured with digital image correlation software. Both imposed and measured displacements are compared and it is shown that the size of the speckles combined with the size of the used pixel subset, clearly influences the accuracy of the measured displacements. Furthermore it is shown that it is possible to create an optimal speckle pattern when a given subset size is chosen. [less ▲]

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See detailComparison between homogeneous and heterogeneous field information for plastic material identification
Lecompte, D.; Sol, H.; Vantomme, J. et al

in Grédiac, Michel; Huntley, Jonathan (Eds.) Proceedings of Photomecanics Conference 2006 (2006)

The accuracy of a Finite Element Simulation for plastic deformation strongly depends on the chosen constitutive laws and the value of the material parameters within these laws. The identification of those ... [more ▼]

The accuracy of a Finite Element Simulation for plastic deformation strongly depends on the chosen constitutive laws and the value of the material parameters within these laws. The identification of those mechanical parameters can be done based on homogeneous stress and strain fields such as those obtained in uniaxial tensile tests and simple shear tests performed in different plane material directions. Another way to identify plastic material parameters is by inverse modeling of an experiment exhibiting a heterogeneous stress and strain field. Experimental forces and strains are in this case compared to the simulated ones and it is tried to reduce the difference in a least-squares sense by optimizing the model parameters. The optimization technique used is this case is gradient based, which means that at every iteration a sensitivity calculation has to be performed in order to indicate the direction in which the parameters are to be identified. The basic principle of the inverse modeling procedure as it is used for parameter identification is the generation of a complex and heterogeneous deformation field that contains as much information as possible about the parameters to be identified. One way of obtaining such a non-homogeneous deformation is by altering the geometry of the specimen for a uniaxial test. Another possibility is to make the loading conditions more complex. In this paper both options are actually combined by using a biaxial tensile test on a perforated cruciform specimen. In the present paper, the work hardening of the material is assumed to be isotropic and it is described by a Swift law. The yield locus is modeled by the anisotropic Hill48 criterion. A comparison is made between the identification of the Hill48 parameters based on the one hand on the Lankford coefficients [1] and on the inverse modeling of a biaxial tensile test on the other hand [less ▲]

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See detailIdentification of yield locus parameters of metals using inverse modeling and full field DIC
Lecompte, D.; Cooreman, S.; Sol, H. et al

in Proceedings of the 7th national congress on theoretical and applied mechanics (2006)

The basic principle of the inverse modeling procedure as it is used for parameter identification is the generation of a complex and heterogeneous deformation field that contains as much information as ... [more ▼]

The basic principle of the inverse modeling procedure as it is used for parameter identification is the generation of a complex and heterogeneous deformation field that contains as much information as possible about the parameters to be identified. One way of obtaining such a non-homogeneous deformation is by making the geometry of the specimen less regular. Another possibility is to make the loading conditions more complex. In this paper both options are actually combined by using the concept of a biaxial tensile test on a perforated cruciform specimen. In the present paper, the work hardening of the material is assumed to be isotropic and it is described by a Swift law. The yield locus is modeled by the anisotropic Hill48 criterion. The optimization technique used is a constrained gradient based Newton-type routine, which means that in every iteration step, a sensitivity calculation has to be performed in order to indicate the direction in which the parameters are to be optimized. The functional to be minimized is a least-squares expression of the discrepancy between the measured and the simulated strain fields at a certain load. The numerical routines as well as the identification results of the different parameters, based on simulated strain fields, are discussed. [less ▲]

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See detailIdentification of hardening behavior using inverse modeling and image correlation
Lecompte, D.; Cooreman, S.; Sol, H. et al

in Proceedings of the SEM annual conference and exposition 2006 (2006)

The basic principle of an inverse modeling procedure as it is used for parameter identification, is the generation of a complex and heterogeneous deformation field that contains as much information as ... [more ▼]

The basic principle of an inverse modeling procedure as it is used for parameter identification, is the generation of a complex and heterogeneous deformation field that contains as much information as possible about the parameters to be identified. One way of obtaining such a non-homogeneous deformation is by making the geometry of the specimen less regular. Another possibility is to make the loading conditions more complex. In this paper both options are actually combined by using a biaxial tensile test on a cruciform specimen in order to identify the parameters of a Swift isotropic hardening law. The yield criterion is modeled by the isotropic Von Mises criterion. The optimization technique used is a constrained gradient based Newton-type routine, which means that in every iteration step, a sensitivity calculation has to be performed in order to indicate the direction in which the parameters are to be optimized. The functional to be minimized is a least-squares expression of the discrepancy between the measured and the simulated strain fields at a certain load. The numerical routines as well as the identification results, based on simulated strain fields, are discussed. [less ▲]

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See detailExperimental validation of the finite element simulation of the first stroke in single point incremental forming
Watzeels, Ken; Tunckol, Y.; Henrard, Christophe ULg et al

in Banabic, D. (Ed.) Proceedings of the 8th ESAFORM Conference on Material Forming (2005)

Single-point incremental forming (SPIF) is a sheet metal forming technique that has gained particular interest in rapid prototyping and small volume production. The study of the underlying forming ... [more ▼]

Single-point incremental forming (SPIF) is a sheet metal forming technique that has gained particular interest in rapid prototyping and small volume production. The study of the underlying forming mechanisms is supported by new developments in finite element simulations and experimental full field strain measurements. This article aims to describe the possibilities and difficulties encountered during validation of finite element predictions of the incremental forming process. The drawing of a straight line into a metal plate was selected as a first test case for this kind of validation. Results of both finite element simulation and experimental work will be discussed. [less ▲]

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See detailIdentification of elastic orthotropic material parameters by inverse modeling using ESPI
Lecompte, D.; Sol, H.; Vantomme, J. et al

in Proceedings of the 2005 SEM Annual Conference & Exposition on Experimental and Applied Mechanics (2005)

Parameter identification methods, which integrate optimization techniques and numerical methods such as the finite element method (FEM), offer an alternative tool for material characterisation. The most ... [more ▼]

Parameter identification methods, which integrate optimization techniques and numerical methods such as the finite element method (FEM), offer an alternative tool for material characterisation. The most common approach is to determine the optimal estimates of the model parameters by minimizing a selected measure-of-fit between the responses of the system and the model. The possibility is studied of retrieving the four independent elastic engineering constants for an orthotropic medium, based on the measurement of a heterogeneous displacement field. In the present case a tensile test is performed on a perforated specimen. The responses of the system, i.e. the surface displacements are measured with an Electronic Speckle Pattern Interferometer. Strains are subsequently calculated, based on the measured displacement field. A finite element model of the perforated specimen is made. The difference between the experimental and numerical strains is minimized in a least squares sense by updating the values of the parameters. The obtained material (or model) parameters are very well in agreement with the traditionally determined ones. [less ▲]

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