Topic: Advances in constitutive modelling and formability of sheet metals

Speaker: Professor Dorel Banabic
　　 Technical University of Cluj Napoca, Romania

Time: 10:00-12:00, (Fri.) May 29, 2015

Venue: Room 403, Shi Changxu Building, IMR CAS

Abstract:
In the last decades, numerical simulation has gradually extended its applicability in the field of sheet metal forming. Constitutive modelling and formability are two domains closely related to the development of numerical simulation tools. This paper is focused, on the one hand, on the presentation of new phenomenological yield criteria developed in the last decade, which are able to describe the anisotropic response of sheet metals, and, on the other hand, on new models and experiments to predict/determine the forming limit curves.

During the last years, the competition in the automotive and aeronautical industry has become more intense. This fact has led to the development of new steel alloys (Bake Hardenable, Dual Phase, Complex Phase, Transformation Induced Plasticity-TRIP, Martensitic Steels, Hot-stamping boron-alloyed steels), aluminium alloys having better performances and increased interest on the use of magnesium and superplastic alloys. Since 2000, the modelling of the anisotropic behaviour of these materials has encouraged research activities focused on the development of yield criteria. The accuracy of the simulation results is given mainly by the accuracy of the material models. Several new models have been proposed during the last years (Barlat 2000, Barlat 2003, Banabic 2000, Banabic 2005, Banabic 2008, Cazacu-Barlat 2001, Cazacu –Barlat 2004, Plunkett- Cazacu-Barlat 2008, Vegter 2006, Aretz-Barlat 2013 etc.). These models allow a very good description of the anisotropic behaviour both of steel alloys (BCC crystallographic structure), aluminium alloys (FCC structure) and magnesium alloys (HCP structure). The new yield criteria incorporate a large number of coefficients (usually, at least 8 coefficients). Due to this fact, they are able to give an accurate description of the yield surface and follow closely the planar variations of the uniaxial yield stress and the coefficient of plastic anisotropy. Some efforts have been made during the last years to formulate macroscopic level models that account for the evolution of anisotropy due to evolving texture (Savoie 1996, Plunkett-Cazacu 2006, 2008, van Houtte 2009, An-Vegter 2011, Gawad 2010, Gawad-Banabic 2014, He 2014 etc.).

It is well known the fact that the position and shape of the FLC is influenced by the shape of the constitutive models (yield criteria, hardening models) adopted in the computational model. A sensitivity analysis regarding the influence of different material parameters and constitutive models upon the limit strains is needed in the pre-processing stage. Such an analysis is also useful for the sheet metal producers when trying to obtain materials having desired formability characteristics. Aiming to meet these requirements, in the CERTETA centre has been developed a software package named FORM-CERT able to calculate FLC’s. One of the most important facilities offered by FORM-CERT consists in the possibility to adopt different yield criteria (Hill’48, Barlat’89, BBC2005 etc.) and strain hardening laws (Swift, Voce, Ghosh, Hockett-Sherby, Autoform, etc.) when computing the FLC’s. The author proposed a new procedure for the experimental determination of the FLCs. The methodology is based on the hydraulic bulging of a double specimen. The upper blank has a pair of holes pierced in symmetric positions with respect to the centre, while the lower one acts both as a carrier and a deformable punch. By modifying the dimensions and position of the holes, it is possible to investigate the entire deformation range of the FLC. The main advantage of the new experimental procedure consists in the reduction of the frictional effects that may alter the values of the limit strains.