Chantal Bouffioux

Forming Limit Predictions for Single-Point Incremental Sheet Metal Forming

A characteristic of incremental sheet metal forming is that much higher deformations can be achieved than conventional forming limits. In this paper it is investigated to which extent the highly non‐monotonic strain paths during such a process may be responsible for this high formability. A Marciniak‐Kuczynski (MK) model is used to predict the onset of necking of a sheet subjected to the strain paths obtained by finite‐element simulations. The predicted forming limits are considerably higher than for monotonic loading, but still lower than the experimental ones. This discrepancy is attributed to the strain gradient over the sheet thickness, which is not taken into account in the currently used MK model.

Validation of a new finite element for incremental forming simulation using a dynamic explicit approach

Abstract: A new method for modeling the contact between the tool and the metal sheet for the incremental forming process was developed based on a dynamic explicit time integration scheme. The main advantage of this method is that it uses the actual contact location instead of fixed positions, e.g. integration or nodal points. The purpose of this article is to compare the efficiency of the new method, as far as accuracy and computation time are concerned, with finite element simulations using a classic static implicit approach. In addition, a sensitivity analysis of the mesh density will show that bigger elements can be used with the new method compared to those used in classic simulations.

Model identification and FE simulations: Effect of different yield loci and hardening laws in sheet forming

The bi‐axial experimental equipment developed by Flores enables to perform Baushinger shear tests and successive or simultaneous simple shear tests and plane‐strain tests. Such experiments and classical tensile tests investigate the material behavior in order to identify the yield locus and the hardening models. With tests performed on two steel grades, the methods applied to identify classical yield surfaces such as Hill or Hosford ones as well as isotropic Swift type hardening or kinematic Armstrong‐Frederick hardening models are explained. Comparison with the Taylor‐Bishop‐Hill yield locus is also provided. The effect of both yield locus and hardening model choice will be presented for two applications: Single Point Incremental Forming (SPIF) and a cup deep drawing.

Effect of the Kinematic Hardening in the Simulations of the Straightening of Long Rolled Profiles

Residual stresses and lack of straightness appear during the cooling of sheet piles where the initial temperature field is not homogeneous. To meet the standards, the long hot rolled pieces are straightened using a series of rollers placed alternately above and below the pieces with shifts which create a succession of bendings. The process is modeled to study the impact of the industrial parameters (the duration of the cooling and the rollers positions), to improve the final geometry and to reduce the residual stresses. Tests are carried out on this structural steel to observe the material behavior, then material laws are chosen and the parameters of these laws are defined using an inverse method. Two sets of material data are obtained: for the first one, the hardening is supposed to be isotropic, and for the second one, additional tests are performed to describe isotropic and kinematic hardenings. The cooling followed by the straightening is then simulated by the finite element method with these two sets of data. The comparison of the rollers forces, the deformation and the residual stresses show the impact of the kinematic hardening on such a process where the material undergoes a succession of tensions and compressions. The real forces applied by the rollers, the real curvature of the interlocks at the end of the straightening process and the distribution of the residual longitudinal stresses measured on the web using the ring core method are used to validate the numerical model.

Study of the cooling process of an extruded aluminium profile

The prediction of the final axial stresses and the residual strains of complex extruded aluminium profiles requires a good knowledge of the material behavior and of the industrial process. This paper is focused on the methods required to provide the whole set of data: material ones and process ones. Scanning differential calorimetry, dilatometry and diffusivity tests identify thermophysic material properties and hot tensile tests identify parameters of the elasto‐visco‐plastic Norton‐Hoff law. The description of the industrial process and its simulations are described. Then a sensitivity analyzis provides the cooling key parameters causing the undesired final curvature during the industrial process.