The document describes an experimental procedure used to understand sheet metal formability. Forming limit curves (FLC) were obtained for different steel sheets using different strain path testing methods like the Nakajima test and Marciniak test. The FLCs were influenced by the mathematical method used to calculate them as well as the testing method. The different strain paths from each test led to differences in the FLCs, especially for the TRIP800 steel sheet. The strain path was found to affect advanced high strength steels more than other steels, which could explain why FLCs are not always valid for these types of steels.

1. EXPERIMENTAL PROCEDURE D. Gutiérrez 1 , A. Lara 1 , D. Casellas 1 and J.M. Prado 1,2 1 CTM-Centre Tecnològic, Av. Bases de Manresa, 1, 08240 Manresa, Barcelona, Spain 2 Department of Materials Science and Metallurgical Engineering, ETSEIB, Universitat Politècnica de Catalunya, Av. Diagonal 647, 08028 Barcelona, Spain david.gutierrez@ctm.com.es, toni.lara@ctm.com.es, daniel.casellas@ctm.com.es, jm.prado@ctm.com.es RESULTS AND DISCUSSION CONCLUSIONS INTRODUCTION Effect of Strain Paths on Formability Evaluation of TRIP steels ACKNOWLEGMENTS Used to understand sheet metal formability Forming Limit Curve (FLC) Obtained by diferent linear strain paths ( ε 2 /ε 1 ) Parameters affect FLC Strain hardening exponent Plastic anisotropy Strain paths Modification of FLC by changes in strain paths (Graf and Hosford, 1994) Nakajima Test Marciniak Test Sample geometries Strain measurement: Stochastic pattern GOM/ARAMIS Testing device: Double acting press Materials TRIP800 2.0mm DC03 1.5mm Mathemathical method: ISO standard Bragard modified method The authors acknowledge financial support from the Spanish Ministerio de Industria, Turismo y Comercio (Programa de Proyectos Consorciados, FIT 170300-2007-1) and from Generalitat of Catalonia Departament d’Innovació Universitats i Empresa for the FIE financial support. FLCs are influenced by mathematical method used to calculate them. Bragard method more conservative. Influence of mathematical method Influence of stretching test: Bigger differences in TRIP steel, especially in FLD 0 and biaxial tension areas. Marciniak test  more linear strain paths, especially in FLD 0 . DC03  different strain paths, but similar FLCs. The different strain paths provides differences FLCs with Nakajima and Marciniak tests. TRIP800 steel have more differences in FLCs than DC03. The strain paths affect more in AHSS steel, it could be an explanation why FLC are not valid in these steels.