Abstract :
[en] Chapter 2 contains the continuum mechanical notions for the description of the plastic behavior of sheet metal under large deformations at room temperature. As such, it includes the kinematics of a continuum body, strain and stress definitions, and a general elasto-plastic constitutive model description. This last point iscomplemented by the definition of anisotropy, as well as the description of some yield criteria and hardening laws.Next, Chapter 3 defines the stress – strain states required to be experimentally reproduced inorder to describe the initial yield locus and its displacement in the stress axis representationduring plastic deformation. A review of the available experimental equipment capable of reproducing the required stress – strain states is presented in order to choose the best forfurther construction. After consideration, those deemed the highest performing are the Miyauchi device, able to perform simple shear tests and the bi-axial testing machine, able toperform plane strain and simple shear tests separately or simultaneously. This chapter presents the mechanical features of the Miyauchi device and the bi-axial test machine that were built at the M&S Laboratory, followed by a description of the optical strain gaugechosen that allows the computation of the strain field throughout the specimen’s deformation area.Chapter 4 focuses on the validation of the experimental equipment. First, the homogeneity of the stress and strain fields is verified. Then, the availability of performing the plane strain, simple shear, Bauschinger and orthogonal tests is checked. The repeatability (precision) of thetests is corroborated and the accuracy is validated by comparison with finite elements simulations.In Chapter 5, the identification methods are proposed and DC06 (0,8mm thick), DP1000 (1,6mm thick) and S320GD (0,69mm thick) steels are identified according to those methods.The initial yield surface for DC06 is identified by two methods (one using the strain measurements, the other using stress measurements) for two yield criteria, which are thencompared with a texture-based yield criterion and the experimental points. The initial yield surface for the other two materials is described by the Hill 1948 yield criterion identifiedusing strain measurements. The yield surface evolution (hardening) for DC06 and S320GD is described by the Teodosiu and Hu hardening law due to the observed mechanical behavior,i.e., the Bauschinger effect and strong influence of the pre-strain when strain-path changes take place. DP1000 exhibits a high Bauschinger effect and its flow stress is not influenced bythe amount of pre-strain when reversing the load; hence, its behavior is described by a kinematic hardening law.Finally, in Chapter 6, conclusions about the present work are established and equipment improvement and further topics for research are proposed, namely, the study of textureevolution, the material axis rotation and the experimental validation of new yield criteria.
