H. Laurent

Asynchronous interface between a finite element commercial software ABAQUS and an academic research code HEREZH

The aim of this paper is to describe an efficient method to connect two independent softwares so as to jointly use best qualities of each software around a complex problem solved by the finite element method (FEM). This connection makes it possible to extend quickly and easily the applicability of new models developed in academic softwares, by their simultaneous use with commercial softwares. This is particularly interesting when these models are very difficult to implement directly in commercial softwares. Most of the commercial FEM applications allow users to add additional features, physical models or boundary conditions via a programming interface. Within these user routines, access to internal data structures is possible, either through subroutine parameters and global variables, or via internal modules for reading and storing data. We use these capabilities to link the commercial software ABAQUS and an academic object-oriented C++ software HEREZH++, via the user-defined mechanical material behaviour (Umat). In this interface, HEREZH++ computes the mechanical behaviour of material and the code coupling performs a communication procedure between ABAQUS and HEREZH++. This paper describes this architecture which allows to implement easily original behaviour law in the commercial ABAQUS code. The asynchronous code coupling is made with a named piped interprocess communication method and an interface written in c/C++. Several test samples are presented to show the efficiency and accuracy of the proposed implementations concerning the computational time. In particular, an industrial test is carried out with an original behaviour model of elasto-visco-hysteresis which would have been very difficult to implement directly in ABAQUS.

Mechanical Behaviour and Springback Study of an Aluminium Alloy in Warm Forming Conditions

This study deals with the mechanical behaviour and material modelling of an AA5754-O alloy at elevated temperature. Experimental shear tests were performed from room temperature up to 200°C, and the material behaviour has been identified with both shear and tensile tests, as a function of temperature. To analyse the influence of temperature during forming over springback, a split-ring test is used. Experimental results are obtained and compared to numerical simulations performed with the finite element in-house code DD3IMP. The numerical process of ring splitting is performed with the in-house code DD3TRIM. The main observed data are force-displacement curves of the punch during forming, cup thickness at the end of forming, and ring gap after splitting. It is shown that all these parameters are strongly dependent on the forming temperature. A correlation is obtained between experimental data and numerical simulation for the evolution of punch force and opening after springback as a function of temperature. The distribution of the tangential stress in the cup wall is the main factor influencing the springback mechanism in warm forming condition.

A simplified methodology to identify material parameters of a hyperelasto-visco-hysteresis model: application to a fluoro-elastomer

This paper presents a method to identify material parameters of a hyperelasto-visco-hysteresis (HVH) model and its application for the simulation of a fluoro-elastomer behaviour. This 3D-phenomenological model is based on the additive decomposition of three stress components. Each of these constitutive stresses is related to a physical phenomenon that occurs during mechanical loading: a hyperelastic equilibrium stress response, an irreversible pure hysteresis stress contribution and a rate-dependent viscoelastic stress behaviour.In order to independently identify these parts of the model, an experimental campaign, including multi-step relaxation in traction and compression tests and simple relaxation in tension and compression tests, is used. The hysteretic and hyperelastic contributions are identified considering only the state at the end of the relaxation periods of the multi-step relaxation tests. The viscoelastic response is analytically calculated with the simple relaxation test. As an advantage, the developed identification scheme gives the possibility to discriminate all the stress components of the model. Finally, the numerical simulation of a seal in relaxation is carried out to verify the capability of the proposed HVH model by reproducing the mechanical response of the studied material.

Springback study in aluminum alloys based on the Demeri Benchmark Test : influence of material model

Springback is a serious problem in sheet metal forming. Its origin lies in the elastic recovery of materials after a deep drawing operation. Springback modifies the final shape of the part when removed from the die after forming. This study deals with Springback in an Al5754‐O aluminum alloy. An experimental test similar to the Demeri Benchmark Test has been developed. The experimentally measured Springback is compared to predicted Springback simulation using Abaqus software. Several material models are analyzed, all models using isotropic hardening of Voce type and plasticity criteria such as Von Mises and Hill48’s yield criterion.

Sensitivity Analysis of Process Parameters in the Drawing and Ironing Processes

Deep drawing is one of the most important operations used in sheet metal forming. Within this, forming of cylindrical cup is one of the most widely studied deep drawing processes since it allows analysing the effect of different process parameters in phenomena such as earing, springback and ironing. In fact, during the deep drawing of a cylindrical cup the blank thickness gradually increases as the blank outer diameter is reduced to the die inner diameter, resulting in a thickness increase from a point near the bottom radius until the maximum value at the top of the cup. Therefore, if the gap between the punch and the die is not sufficiently large to allow the blank material to flow, ironing of the cup wall will occur. The ironing process typically imposes high contact forces, normal to the surface of the punch and the die, which can lead to the occurrence of galling, particularly for aluminium alloys. In this work an experimental device, adopted in previous studies, was used to analyse the influence of the lubricant conditions in the deep drawing of a cylindrical cup. The study considers an AA5754-O aluminium alloy blank with a diameter of 60 mm, which is fully deep drawn with a 33 mm diameter punch. Due to the forming conditions, the cup is deep drawn and ironing of the cup wall also occurs. The experimental tests were performed considering different amounts of lubricant in the blank surfaces in the contact with the die and with the blank-holder in order to better understand the influence of these tools on the process. The experimental study was complemented with numerical simulations, exploring the conditions induced by the ironing operation, quite challenging for the numerical simulation of the process using the finite element method. Besides the influence of the contact with friction conditions in the forming process (i.e. punch force evolution, thickness distribution along the cup wall and contact pressure), the influence of the die shoulder and inner radius were also analysed.

Natural aging effect on the forming behavior of a cylindrical cup with an Al-Mg-Si alloy

Natural Aging of EN AW 6016-T4 is experimentally evaluated under uniaxial tensile test and forming of a cylindrical cup. The uniaxial tensile tests were performed 4 days, 1, 4, 7 and, 18 months after the alloy quenching. The results shows an increase of the proof and tensile strengths, while the in-plane anisotropy remains globally invariable with the increase of the storage time. The forming of cylindrical cups was performed too, with specimens at 1 and 18 months of natural aging. The increase of the proof and tensile strengths leads to an increase of the punch force during the deep drawing process. However, the effect on the thickness evolution along the cup’s wall and on the cup’s height is negligible. In fact, the numerical simulation results indicate that these parameters are more sensitive to the initial sheet thickness (considering the mean value of 1.047mm or the approximated one of 1.000mm) than to the changes induced by aging in the hardening behavior.

Numerical study of mechanical behaviour of a polypropylene reinforced with Alfa fibres

The main purpose of this work consists of the mechanical characterization of a copolymer polypropylene (PP) filled with natural Alfa fibres. The elaboration of the PP-based composite reinforced with these natural fibres is explained. The mechanical behaviour of these composite blends has been analysed. Contrary to classical studies of the mechanical characterization of these kinds of materials which focus only on monotonic tensile tests, cyclic loading/unloading tests and loading/unloading tests interrupted by relaxation steps have been investigated. These tests enable us to compare the mechanical response of the virgin PP and the PP filled with Alfa fibres (Alfa/PP). Reinforcement by these natural fibres shows an impact in the improvement of the mechanical properties of this composite. All these experimental results constitute a data base used to identify the material parameters of a phenomenological constitutive hyperelasto-visco-hysteresis model. These tests also provide indications about the hyperelastic, viscous and hysteretic stress contributions of the hyperelasto-visco-hysteresis model with these materials during cyclic loading.

Simulations of joule effect heating in a bulge test

This work focuses on the integration of an electrical conduction heating of circular blank in a bulge test device. This device will be used to characterize the thermomechanical behaviour of Usibor®1500 under biaxial deformation at very high temperature (to 930°C). First a thermoelectric model using COMSOL Multiphysics® was developed to study the heating of a rectangular blank. This model is validated by comparing the calculated temperatures with thermocouples measurements. Secondly electrical field optimization is approached to obtain a fast and uniform heating of a circular blank.

A staggered coupling strategy for the finite element analysis of warm deep drawing process

The thermomechanical finite element analysis of warm forming processes enables an improved comprehension of the process parameters affecting the material formability. However, the thermal and mechanical coupling problem is still a challenge from the computational standpoint. A staggered strategy for the thermomechanical coupling problem is presented in this study, which is based on an isothermal split approach and allows the treatment of the two problems separately. The exchange of information between the mechanical and the thermal problem is performed to achieve a compromise between computational cost and accuracy. The proposed algorithm was implemented in DD3IMP in-house finite element code. Its performance is analysed and compared with a classical strategy commonly employed for solving thermomechanical problems.

Influence of prestrain on the occurrence of PLC effect in an Al-Mg alloy

In the present work, the jerky flow in an Al-Mg alloy during simple shear tests for various strain paths and temperatures is studied. Direct observations of the sample surface using digital image correlation is used to investigate the type and the dynamics of Portevin-Le Châtelier (PLC) bands as a function of shear strain and temperature. The influence of strain path changes on the occurrence of PLC effect is evidenced through cyclic shear tests, composed of a loading up to several values of the shear strain followed by a reloading in the opposite direction. It is shown that the occurrence of PLC effect modifies the transient behavior after strain path changes.