Numerical simulation of a pyramid steel sheet formed by single point incremental forming using solid-shell finite elements

in Key Engineering Materials [=KEM] (2013)

Single Point Incremental Forming (SPIF) is an interesting manufacturing process due to its dieless nature and its increased formability compared to conventional forming processes. Nevertheless, the ... [more ▼]

Single Point Incremental Forming (SPIF) is an interesting manufacturing process due to its dieless nature and its increased formability compared to conventional forming processes. Nevertheless, the process suffers from large geometric deviations when compared to the original CAD profile. One particular example arises when analyzing a truncated two-slope pyramid. In this paper, a finite element simulation of this geometry is carried out using a newly implemented solid-shell element, which is based on the Enhanced Assumed Strain (EAS) and the Assumed Natural Strain (ANS) techniques. The model predicts the shape of the pyramid very well, correctly representing the springback and the through thickness shear (TTS). Besides, the effects of the finite element mesh refinement, the EAS and ANS techniques on the numerical prediction are presented. It is shown that the EAS modes included in the model have a significant influence on the accuracy of the results. [less ▲]

Numerical simulation of T-bend of multilayer coated metal sheet using solid-shell element

in Steel Research International (2012)

The main aim of this paper is to model the T-bend test performed on multilayer coated metal sheets in order to measure the coating flexibility. Because of important uses of polymer coatings in many ... [more ▼]

The main aim of this paper is to model the T-bend test performed on multilayer coated metal sheets in order to measure the coating flexibility. Because of important uses of polymer coatings in many industrial applications and higher requirement on the quality of products, an accurate modeling of the T-bend process is therefore essential. During the modeling with the finite element method, the large thickness ratio between the different layers is likely to produce elements with an unfavorable aspect ratio. Therefore, to avoid obtaining inaccurate results linked to the shape of the elements, solid-shell elements are used in this study. These elements are based on the Enhanced Assumed Strain (EAS) technique and the Assumed Natural Strain (ANS) technique. These techniques permit to avoid locking problems even in very bad conditions (nearly incompressible materials, very thin elements conducting to large aspect ratios, distorted element geometry…). The EAS technique artificially introduces additional degrees of freedom (DOFs) to the element. They permit to increase the flexibility of the element which is very efficient for several locking issues. On the other hand, the ANS technique modifies the interpolation scheme for particular strain components. The ANS technique proved to eliminate the transverse shear locking from the element in bending dominated situations. Besides, a numerical integration scheme dedicated to Solid-Shell elements was implemented. It uses a user-defined number of integration points along the thickness direction, which permits to increase the element accuracy with a mesh containing a reduced number of elements along the thickness direction. The results obtained from numerical simulations are compared with some analytical results in order to check the strain predicted in the coated layer by FEM. This information helps to investigate the coating layer ductility in the real process. [less ▲]

On the numerical integration of rate independent single crystal behavior at large strain

in International Journal of Plasticity (2012), 32-33

This paper presents a new numerical algorithm for the integration of the constitutive equations of a single crystal for finite rate-independent elastoplastic strains. The algorithm addressed in this paper ... [more ▼]

This paper presents a new numerical algorithm for the integration of the constitutive equations of a single crystal for finite rate-independent elastoplastic strains. The algorithm addressed in this paper is dedicated to face-centered-cubic (FCC) crystal structures. Its first feature is a much more efficient and more accurate integration scheme of the constitutive equations compared to previous attempts. This scheme is based on a fully implicit integration procedure, yet it may be transformed easily into an explicit scheme. Determining the set of active slip systems is performed by the use of a combinatorial search procedure, and the determination of the slip rates of the different active slip systems is based on the fixed point method. The second feature of this algorithm stems from the original method used to solve the ambiguity of the possible non-uniqueness of the set of active slip systems. A robust method, based on a small positive perturbation of the critical shear stresses, is proposed to overcome this difficulty. It is worth mentioning that the algorithm developed in this paper is not limited to one particular hardening law or to FCC crystal structures. Rather, it can be used and extended to various hardening laws and crystal structures (e.g. BCC or HCP. . .) in a straightforward manner. The authors demonstrate the performance of the proposed algorithm and illustrate its accuracy and efficiency through various numerical simulations at the single crystal and polycrystal scales. The predicted results obtained from those simulations were compared with those obtained using other numerical techniques and algorithms (i.e., a pseudo-inversion technique and an explicit algorithm). Our numerical predictions are also compared with some numerical and experimental results from other papers. The response of the polycrystal was computed by using the proposed algorithm combined with Taylor’s homogenization scheme, which is used to compute the overall polycrystalline behavior. The paper ends with a statistical study of the influence of the perturbation technique on the response prediction for a single crystal and a polycrystal. [less ▲]

Finite element analysis of the free surface effects on the mechanical behavior of thin nickel polycrystals

in International Journal of Plasticity (2012), 29

The miniaturization of metallic samples has been proved to deeply affect their mechanical properties leading to a softening or a hardening effect depending on the order of the dimension reduction. The ... [more ▼]

The miniaturization of metallic samples has been proved to deeply affect their mechanical properties leading to a softening or a hardening effect depending on the order of the dimension reduction. The objective of this work is to provide new numerical results which explain the softening mechanisms on the mechanical behavior for nickel polycrystals which have been experimentally characterized by the authors in a previous published paper (Keller et al., 2011). Based on a strain gradient crystalline plasticity model identified for nickel, simulations of tensile tests were performed for samples with different thicknesses and grain sizes. The simulations correctly reproduce the softening effect linked to a decrease in the thickness and in the number of grains across the thickness. The analysis of the plasticity mechanisms shows that the softening is due to surface effects which are discussed in terms of grain orientations, dislocation mean free path and long-range back-stress. The model also predicts a hardening mechanism for further dimension reduction if the samples have only grain boundaries perpendicular to the tensile direction. In this case, the modification of the mechanical behavior is due to strain gradients formation. [less ▲]

Phenomenological and crystal plasticity approaches to describe the mechanical behaviour of Ti6Al4V titanium alloy

in International Journal of Material Forming (2011), 4(2), 205-2015

This paper presents a multiscale study of the quasi-static behaviour of a Ti6Al4V titanium alloy sheet. Tensile and compressive tests were carried out on specimens along several orientations from the ... [more ▼]

This paper presents a multiscale study of the quasi-static behaviour of a Ti6Al4V titanium alloy sheet. Tensile and compressive tests were carried out on specimens along several orientations from the rolling direction in order to characterise the material anisotropy. In parallel, X-Ray diffraction texture measurements were performed before and after deformation in tension. A phenomenological model (CPB06exn) and a multiscale crystal plasticity model (Multisite) were investigated to describe the mechanical behaviour of the tested material. The identification of the material parameters provides good predictions of the plastic anisotropy using both tensile and compressive data. The crystal plasticity model is in good agreement with the experiments in tension but it was observed that some improvements should be done to take into account the tension-compression asymmetry displayed by the material. Moreover both models lead to a good prediction of the Lankford’s coefficients and yield strength. [less ▲]

On the numerical integration of an advanced Gurson model

in International Journal for Numerical Methods in Engineering (2011), 85(8), 1049-1072

This article is focused on a new extended version of Gurson's model (J. Eng. Mater. Technol. 1977; 99:2–15), its numerical integration scheme and its consistent tangent matrix being within an FE code ... [more ▼]

This article is focused on a new extended version of Gurson's model (J. Eng. Mater. Technol. 1977; 99:2–15), its numerical integration scheme and its consistent tangent matrix being within an FE code. First, this new advanced Gurson model is proposed, which is an extension of the original to take into account plastic anisotropy and mixed (isotropic+kinematic) hardening. In this paper, only the growth phase of cavities is considered (the nucleation of new voids is ignored). Second, a new numerical algorithm for the integration of this new Gurson model is presented. The algorithm is implicit in all variables and is unconditionally stable. This algorithm is generic and could be used for other anisotropic yield functions and other hardening laws. Third, the consistent tangent matrix is computed in an explicit way by exact linearization of the constitutive equations. To check its efficiency and robustness, the proposed integration algorithm is compared, under some simplified assumptions and choices, with the algorithms of Aravas (Int. J. Numer. Meth. Engng 1987; 24:1395–1416) and Kojic (Int. J. Numer. Meth. Engng 2002; 53(12):2701–2720). The performance of the developed consistent modulus, compared to other techniques for the computation of the tangent matrix is assessed. The paper ends with numerical simulations of tensile tests on homogeneous and notched specimens. [less ▲]

Investigation of the Efficiency of the Ti555 Compared with TA6V Alloy in the Case of an Aeronautic Application

in Proceedings of the 12th World Conference on Titanium (2011)

Material behavior of the hexagonal alpha phase of a titanium alloy identified from nanoindentation tests

in European Journal of Mechanics A : Solids (2011), 30

This article focuses on the numerical modeling of nanoindentation tests performed on the hexagonal a phase of Ti-5553 alloy in order to identify its mechanical behavior. The main goal consists in ... [more ▼]

This article focuses on the numerical modeling of nanoindentation tests performed on the hexagonal a phase of Ti-5553 alloy in order to identify its mechanical behavior. The main goal consists in determining the relative strength of the slip modes in the a phase of Ti-5553. This work was performed using an elastoviscoplastic crystal plasticity-based constitutive law. The difficulties in determining the slip systems that can be activated and their corresponding critical resolved shear stresses (CRSS) are discussed. Numerical predictions are compared to experimental nanoindentation curves. [less ▲]

Assessment of the enhanced assumed strain (EAS) and the assumed natural strain (ANS) techniques in the mechanical behavior of the SSH3D solid-shell element

in Complas XI (2011)

Investigation of the efficiency of the Ti555 compared with TA6V alloy in the case of an aeronautic application

in The 12th World Conference on Titanium Ti-2011 (2011)