Numerical investigation and experimental validation of physically-based advanced GTN model for DP steels

in Materials Science & Engineering : A (2013), 569

This numerical investigation of an advanced Gurson-Tvergaard-Needleman (GTN) model is an extension of the original work of [Ben Bettaieb, M., Lemoine, X., Bouaziz, O., Habraken, A.-M., Duchêne, L ... [more ▼]

This numerical investigation of an advanced Gurson-Tvergaard-Needleman (GTN) model is an extension of the original work of [Ben Bettaieb, M., Lemoine, X., Bouaziz, O., Habraken, A.-M., Duchêne, L., Numerical modeling of damage evolution of DP steels on the basis of X-ray tomography measurements, Mechanics of Materials 43 (2011) 139-156]. The current damage model extends the previous version by integrating the three damage mechanisms: nucleation, growth and coalescence of voids. Physically-based void nucleation and growth laws are considered, including an effect of the kinematic hardening. [less ▲]

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 ▲]

Towards fracture prediction in single point incremental forming

in Key Engineering Materials [=KEM] (2013), 554-557

The stress state in metal forming processes usually implies low values of triaxiality. It is well known that damage models based only on triaxiality fails to capture the damage behavior properly, and ... [more ▼]

The stress state in metal forming processes usually implies low values of triaxiality. It is well known that damage models based only on triaxiality fails to capture the damage behavior properly, and recent articles have stressed the effect of the Lode parameter in describing damage. Moreover, in some process like incremental forming, the through thickness shear could dominate the rupture mechanism making the description, using solely the triaxiality, inaccurate. In this paper, a preliminary study of the stress state is carried over a near-to-failure single point incremental forming (SPIF) formed cone, through finite elements simulations using a newly developed solid-shell element. The results provide a basis for further studies into damage development in SPIF. [less ▲]

Prediction of residual stresses by FE simulations on bimetallic work rolls during cooling

in Computer Methods in Materials Science (2013), 13(1), 84-91

Bimetallic rolls used in the roughing stands of the Hot Strip Mill require mixed properties as a high wear resistance for the shell material and an enhanced toughness for the core material. The bimetallic ... [more ▼]

Bimetallic rolls used in the roughing stands of the Hot Strip Mill require mixed properties as a high wear resistance for the shell material and an enhanced toughness for the core material. The bimetallic roll studied in this paper is obtained from a vertical spin casting process followed by cooling and subsequent heat treatments. Failure of the compound roll sometimes occurs during the cooling stage of the casting route or later during the thermal treatments. It requires to deeply investigate the thermo mechanical metallurgical interactions generated during cooling and heat treatment in order to find the origin of cracks. For this purpose, a thermo metallurgic mechanical finite element model is used. However these numerical simulations require a high amount of mechanical, thermal and metallurgical parameters. In order to determinate these parameters, a study of available data for estimation of mechanical parameters was performed. Thermo physical parameters were obtained by DTA and DSC methods. Metallurgical characterization by inverse numerical method based on available CCT diagrams was performed to determine the TTT diagrams. First cooling numerical simulations are presented, allowing a rough estimate of residual stresses values and the identification of key parameters for predicting accurate residual stresses by sensitivity analysis. [less ▲]

Study of the geometrical inaccuracy on a SPIF two-slope pyramid by finite element simulations

in International Journal of Solids and Structures (2012)

Single Point Incremental Forming (SPIF) is a recent manufacturing process which can give a symmetrical or asymmetrical shape to an undeformed metal sheet by using a relative small tool. In this article, a ... [more ▼]

Single Point Incremental Forming (SPIF) is a recent manufacturing process which can give a symmetrical or asymmetrical shape to an undeformed metal sheet by using a relative small tool. In this article, a two-slope SPIF pyramid with two different depths, which suffers from large geometric deviations when comparing the intended and final shapes, is studied. The article goal is to detect if these divergences are due to new plastic strain while forming the second angle pyramid by using finite elements simulations. To validate the numerical results, both the shape and the forces are compared with experimental measurements. Then, an analysis of the material state is carried out taking the equivalent plastic strain, von Mises effective stress and yield stress distribution through a cut in the mesh. It is noticed that there is plastic deformation in the center of the pyramid, far from the tool neighbourhood. Also, high values of stresses are observed under the yield stress in other parts of the sheet. As a strong bending behaviour plus membrane tension is found in some sheet elements, these elastic stresses are due to a bending action of the tool. It is concluded that the main shape deviations come from elastic strains due to structural elastic bending, plus a minor contribution of localized springback, as no plastic deformation is observed in the angle change zone. Future developments in toolpath designs should eventually consider these elastic strains in order to achieve the intended geometry. [less ▲]

Finite element investigation of size effects on the mechanical behavior of nickel single crystals

in Materials Science & Engineering : A (2012), 550(30), 342-349

The influence of dimensions on the mechanical behavior of f.c.c. single crystals with dimensions larger than a few micrometers has been the topic of many experimental investigations and controversies ... [more ▼]

The influence of dimensions on the mechanical behavior of f.c.c. single crystals with dimensions larger than a few micrometers has been the topic of many experimental investigations and controversies during the 1970s and this question is still open. The objective of this article is to shed new light on this point by performing finite element simulations thanks to a strain gradient crystal plasticity model. Based on the model identification for nickel, several single crystal samples with various thicknesses and orientations were tested numerically in tension. The effect of dimensions was then analyzed considering the spatial distribution of dislocation densities. Near loading boundaries, dislocation density gradients appeared perpendicular to the Burgers vector direction of the primary activated slip system which modified the mechanical behavior. These gradients are discussed in terms of boundary conditions, crystal orientation and dislocation interactions with surfaces. [less ▲]

Evaluation of the Enhanced Assumed Strain and Assumed Natural Strain in the SSH3D and RESS3 Solid Shell Elements for Single Point Incremental Forming Simulation

in Key Engineering Materials [=KEM] (2012), 504-506

Single Point Incremental Forming (SPIF) is a recent sheet forming process which can give a symmetrical or asymmetrical shape by using a small tool. Without the need of dies, the SPIF is capable to deal ... [more ▼]

Single Point Incremental Forming (SPIF) is a recent sheet forming process which can give a symmetrical or asymmetrical shape by using a small tool. Without the need of dies, the SPIF is capable to deal with rapid prototyping and small batch productions at low cost. Extensive research from both experimental and numerical sides has been carried out in the last years. Recent developments in the finite element simulations for sheet metal forming have allowed new modeling techniques, such as the Solid Shell elements, which combine the main features of shell hypothesis with a solid-brick element. In this article, two recently developed elements -SSH3D element [1, 2] and RESS3 element [3]- implemented in Lagamine (finite element code developed by the ArGEnCo department of the University of Liège) are explained and evaluated using the SPIF line test. To avoid locking problems, the well-known Enhanced Assumed Strain (EAS) and Assumed Natural Strain (ANS) techniques are used. The influence of the different EAS and ANS parameters are analysed comparing the predicted tool forces and the shape of a transversal cut, at the end of the process. The results show a strong influence of the EAS in the forces prediction, proving that a correct choice is fundamental for an accurate simulation of the SPIF using Solid Shell elements. [less ▲]

Compression test for plastic anisotropy characterization using optical full-field dis-placement measurement technique

in Steel Research International (2012)

In this work, a constitutive law, its identification and its validation for TA6V titanium alloy are presented. The material characteri-zation is achieved by quasi-static monotonic tensile and compression ... [more ▼]

In this work, a constitutive law, its identification and its validation for TA6V titanium alloy are presented. The material characteri-zation is achieved by quasi-static monotonic tensile and compression tests at several strain rates and in three orthogonal material orientations. Constitutive model as Johnson-Cook which describes the strain rate sensitivity of the plastic material response is used in order to identify the initial yield locus at a common strain rate (10-3 s-1). The macroscopic orthotropic yield criterion chosen is the well-known CPB06 proposed by Cazacu et al., which describes both the tension/compression asymmetry and the anisotropic behavior of hcp metals and alloys. Voce type isotropic hardening model is used for simplicity reasons. A compression test on ellipsoidal cross-section specimen is selected for validation purpose, where 3 Digital Image Correlation 3D systems are used in order to acquire the evolution of the full field displacement/strain during the test. Coulomb friction coefficient between the dies and the specimen is computed by means of inverse modelling. A good agreement on the prediction of the load as a function of axial displacement of the specimen is found. [less ▲]

Numerical simulation of DP steel damage using a physically-based GTN model

Poster (2012)

Experimental and numerical study of TA-6V mechanical behavior in different monotonic loading conditions at room temperature

in Procedia IUTAM (2012), 3

This paper presents an experimental characterization of the quasi-static mechanical response of a TA-6V titanium alloy in sheet form. Monotonic tests, such as uniaxial tension and compression, plane ... [more ▼]

This paper presents an experimental characterization of the quasi-static mechanical response of a TA-6V titanium alloy in sheet form. Monotonic tests, such as uniaxial tension and compression, plane strain and simple shear, were conducted along several orientations in the plane of the sheet in order to investigate the plastic anisotropy and the tension-compression asymmetry displayed by the material at the macroscopic scale. It was observed a moderate anisotropy in yielding while the anisotropy in r-ratios is very pronounced. Initial yielding is described using criteria that account for both plastic anisotropy and strength differential effects. Comparison between experimental data and FE simulation results illustrates the ability of the model to predict the mechanical response of TA-6V. [less ▲]