Khemais Saanouni

Numerical improvement of thin tubes hydroforming with respect to ductile damage

Fully coupled constitutive equations, formulated in the framework of the thermodynamics of irreversible processes with state variables, accounting for isotropic hardening as well as the isotropic ductile damage are used to simulate numerically, by the finite element analysis, 3D metal hydroforming processes with damage occurence. An implicit integration scheme for local time integration of the constitutive equations and a dynamic explicit resolution scheme to solve the associated dynamic equilibrium problem are used. The effects of friction coefficient, material ductility and hydro bulging condition, on the hydroformability of various thin tubes are discussed.

Numerical Prediction of Ductile Damage in Metal Forming Processes Including Thermal Effects

The paper is dedicated to the study of the multiphysical coupling in metal forming. Attention is paid to the coupling between thermal exchange, small strain elasticity, finite plasticity with nonlinear hardening, ductile damage, and contact with friction. The standard framework of the thermodynamics of irreversible processes with state variables is used to derive fully coupled thermo-elastoplastic constitutive equations accounting for mixed nonlinear hardening and ductile damage. The related numerical aspects concerning both the local integration scheme of the constitutive equations, as well as the global resolution strategies of the associated Initial and Boundary Value Problem (IBVP) are shortly discussed. For the local integration, an asymptotic iterative scheme is used together with a reduction in the number of the integrated constitutive equations. This model is implemented into ABAQUS/Std using the Umat and Umatht user subroutines. A special care is given to the exact calculation of the consistent stiffness matrix required by the Newton-Raphson implicit resolution strategy of the coupled mechanical and thermal IBVPs. Applications are made to simple examples and the interactions between hardening plasticity, ductile damage, and thermal fields are carefully analyzed.

Numerical aspects of finite elastoplasticity with isotropic ductile damage for metal forming

ABSTRACT This work is devoted to the study of an efficient numerical algorithm for evaluating damaged-plastic response of a material submitted to large plastic deformations. Fully coupled constitutive equations accounting for both combined isotropic and kinematic hardening as well as the ductile damage are formulated in the framework of Continuum Damage Mechanics (CDM). The associated numerical aspects concerning both the local integration of the coupled constitutive equations and the (global) equilibrium integration schemes are presented and implemented into a general purpose Finite Element code (ABAQUS). For the local integration of the fully coupled constitutive equations an efficient implicit and asymptotic scheme is used. Special care is given to the consistent tangent stiffness matrix derivation as well as to the reduction of the number of constitutive equations. Some numerical results are presented to show the numerical performance of the proposed stress calculation algorithm and the capability of the approach to predict the damage initiation and growth during a given metal forming process.

2D Adaptive FE Simulations in Finite Thermo-Elasto-Viscoplasticity with Ductile Damage: Application to Orthogonal Metal Cutting by Chip Formation and Breaking

Fully coupled thermo-elasto-visco-plastic-damage constitutive equations based on the state variables under large plastic deformation are developed for metal forming simulation. Relevant numerical aspects concerning both the global resolution strategy as well as the local integration scheme are discussed. The model is implemented into ABAQUS/Explicit using the Vumat user subroutine and used in connection with a 2D adaptive mesh facility. Application is made to the orthogonal metal cutting resulting in chip formation and segmentation or breaking. This example is studied in order to examine the ability of this adaptive fully coupled approach to predict qualitatively the formation and the segmentation of the chip compared to the classical procedure, which neglects the damage effect.

Transfert de champs plastiquement admissibles

This paper presents a new approach to interpolate the mechanical fields associated to a given mesh of the computational domain which satisfy the equilibrium equations together with the mechanical criteria which are quadratical in terms of these fields. The method is based on the diffuse approximation techniques. These allow us to construct a field of globally arbitrary order of continuity which approximates accurately the initial discrete mechanical fields. Indeed, the construction is based locally on the resolution of a quadratical optimisation problem under degenerate quadratical constraints for which we propose an analytical solution. The method is applied, in particular, to an equilibrium problem of elastoplastic solid with non linear hardening. To cite this article: P. Villon et al., C. R. Mecanique 330 (2002) 313–318.

Identification of fully coupled anisotropic plasticity and damage constitutive equations using a hybrid experimental–numerical methodology with various triaxialities

A hybrid experimental–numerical methodology is presented for the parameter identification of a mixed nonlinear hardening anisotropic plasticity model fully coupled with isotropic ductile damage accounting for microcracks closure effects. In this study, three test materials are chosen: DP1000, CP1200, and AL7020. The experiments involve the tensile tests with smooth and notched specimens and two types of shear tests. The tensile tests with smooth specimens are conducted in different directions with respect to the rolling direction. This helps to determine the plastic anisotropy parameters of the material when the ductile damage is still negligible. Also, in-plane torsion tests with a single loading cycle are used to determine separately the isotropic and kinematic hardening parameters. Finally, tensile tests with notched specimens and Shouler and Allwood shear tests are used for the damage parameters identification. These are conducted until the final fracture with the triaxiality ratio η lying between 0 and 1 / 3 (i.e. 0 ≤ η ≤ 1 / 3 ). The classical force–displacement curves are chosen as the experimental responses. However, for the tensile test with notched specimens, the distribution of displacement components is measured using a full field measurement technique (ARAMIS system). These experimental results are directly used by the identification methodology in order to determine the “best” values of material parameters involved in the constitutive equations. The inverse identification methodology combines an optimization algorithm which is coded within MATLAB together with the finite element (FE) code ABAQUS/Explicit. After optimization, good agreement between experimental and numerically predicted results in terms of force–displacement curves is obtained for the three studied materials. Finally, the applicability and validity of the determined material parameters are proved with additional validation tests.

Improvement of forging process of a 3D complex part with respect to damage occurrence

Abstract This work is devoted to the study of a numerical (FE-based) methodology developed in order to improve the cold 3D forging process with respect to the ductile damage occurrence. This methodology is based on advanced constitutive equations accounting for the “strong” coupling between the elastoplastic behavior, the mixed isotropic and kinematic hardening and the isotropic ductile damage. Both the mechanical and numerical aspects related to the associated initial and boundary values problem (IBVP) are briefly outlined. Application is made to be the cold forging of a 3D part (spider) by studying the influence of the material ductility as well as the friction nature between the part and the die on the damage occurrence. The proposed methodology is shown to be very useful and helpful when dealing with a “virtual” improvement of any forging processes with respect to damage occurrence.

Prediction of serrated chip formation in orthogonal metal cutting by advanced adaptive 2D numerical methodology

In this work a complete numerical methodology combining ‘advanced’ thermo-elasto-viscoplastic constitutive equations accounting for mixed (isotropic and kinematic) non-linear hardening, thermal effects, isotropic ductile damage and contact with friction is proposed to simulate the 2D orthogonal cutting process of AISI4340 steel. First, the fully coupled constitutive equations are presented and their specificities highlighted. The relevant numerical aspects concerning both the local integration scheme as well as the global resolution strategy together with the 2D adaptive remeshing facility are discussed. This model is implemented into ABAQUS/EXPLICIT using the Vumat user subroutine and connected with an adaptive 2D meshing program. Application is made to the 2D orthogonal metal cutting by chip formation. A special care is put in the prediction of the primary shear band where the temperature, the strain and the damage are highly localised giving the serrated shape and possible segmentation of the ship.

Numerical Design of Extrusion Process Using Finite Thermo-Elastoviscoplasticity with Damage. Prediction of Chevron Shaped Cracks

The influence of the initial temperature and its evolution with large plastic deformation on the formation of the fully coupled chevron shaped cracks in extrusion is numerically investigated. Fully coupled thermo-elasto-viscoplastic constitutive equations accounting for thermal effects, mixed and nonlinear isotropic and kinematic hardening, isotropic ductile damage with micro-cracks closure effects are used. These constitutive equations have been implemented in Abaqus/Explicit code thanks to the user subroutine vumat and used to perform various numerical simulations needed to investigate the problem. It has been shown that the proposed methodology is efficient to predict the chevron shaped cracks in extrusion function of the main process parameters including the temperature effect.

Anisotropic ductile damage fully coupled with anisotropic plastic flow: Modeling, experimental validation, and application to metal forming simulation

The main goal of this paper is the modeling, numerical simulation, and experimental validation of the anisotropic ductile damage effects on initially anisotropic plastic flow with mixed (isotropic and kinematic) nonlinear hardening under large plastic strains for metal forming processes simulation. A symmetric second-rank damage tensor together with a symmetrized fourth-rank damage-effect tensor is used to describe the anisotropic ductile damage evolution and its effect on the large plastic flow with hardening. Following the concept of effective state variables in the framework of the total energy equivalence assumption, the “Murakami” fourth-rank damage-effect tensor is chosen to describe the anisotropic damage effect on the elastic-plastic behavior including the mixed hardening. The “Lemaitre” ductile anisotropic damage evolution relationships, where the principal directions of the damage rate tensor are governed by those of the plastic strain rate tensor, are used. As difference with the works cited above, the nonlinear mixed isotropic and kinematic hardening is taken into account considering the full and strong damage effects through the effective state variables deduced from the total energy equivalence assumption initially proposed by Saanouni et al. The non-associative plasticity theory is considered, and the “Hill 1948” quadratic (equivalent) stress norm is used to describe the large plastic anisotropic flow accounting for mixed isotropic and kinematic hardening with anisotropic damage effects. The formulation is performed assuming finite plastic strains and small elastic strains through the so-called rotated frame formulation. The obtained model was implemented into ABAQUS/Explicit® FE software thanks to the user’s developed subroutine VUMAT. The numerical aspects related to the time discretization of the fully coupled anisotropic constitutive equations are carefully described. Finally and for the validation purpose, the model is identified using an appropriate experimental data base concerning the grade 316L stainless steel to simulate numerically some metal forming processes.