Issue 39

O. Daghfas et alii, Frattura ed Integrità Strutturale, 39 (2017) 263-273; DOI: 10.3221/IGF-ESIS.39.24 268 yy zz r       (7) where yy   and zz   are the plastic strain rates in-plane and through the thickness, respectively. The subscript specifies the angle between the axis of the specimen and the rolling direction (Fig 2 a). In the case of orthotropy r  varies depending on the off axis angle  .This scalar quantity is used extensively as an indicator of the formability. I DENTIFICATION PROCEDURES n this section we focus on the phenomenology of plastic behavior; especially modeling plasticity and hardening based on experimental data represented as families of hardening curves, and Lankford coefficient data. In order to simplify our identification process, the following assumptions are adopted: Identification through “small perturbations” process, the tests used are treated as homogeneous tests, we neglect the elastic deformation; the behavior is considered rigid plastic incompressible, the plasticity surface evolves homothetically (isotropic hardening) and all tests are performed in the plane of the sheet resulting in a plane stress condition. The identification of this constitutive law requires the identification of the hardening function, the anisotropy coefficients c 1 , c 2 , c 3 , c 4 , c 5 , c 6 of the Barlat criterion (Eq. 2), the shape factor m and the Lankford coefficients r ( )  . The Barlat criterion or Yield 91 is proposed by Barlat et al [ 12 ] as a non quadratic criterion for anisotropic materials. Respecting to plane stress condition, the anisotropy coefficients are reduced to 4 (c 1 , c 2 , c 3 , c 4 ). First identification step By smoothing the experimental tensile curves the Hollomon and the Voce parameters are determined for three loading directions. Tab. 2, Tab. 3 and Tab. 4 illustrate respectively the identified parameters of Hollomon and Voce laws. Knowing that the coefficient n is the same for all tests [ 18-19 ], by convention we choose n for traction in direction ψ = 00° as reference. For n=0.0718, we present different values of k (see Tab. 3). The different values of Voce parameters are illustrated in Tab. 4. ψ k n 00° 606.8441 0.0718 45° 622.0848 0.0766 90° 671.2648 0.0853 Table 2 : Identification of the constants of Hollomon law for different loading directions. ψ k 00° 606.8441 45° 613.0297 90° 640.4611 Table 3 : Identification of the constant hardening law for fixed n. ψ s    00° 508.0623 0.2111 -36.8421 45° 511.1316 0.2161 -39.3294 90° 531.9995 0.2613 -49.5677 Table 4 : Identified parameters of Voce law.