Issue 39

O. Daghfas et alii, Frattura ed Integrità Strutturale, 39 (2017) 263-273; DOI: 10.3221/IGF-ESIS.39.24 264 mechanical strength of these alloys is increased by structural hardening phenomenon [ 5] . This type of alloy is mainly used in the automotive industry, in transport and aeronautics especially in the design of the fuselage of the airbus [ 6] . Metal sheets, that have undergone extensive plastic deformation by rolling or extrusion, exhibit a significant anisotropy of mechanical properties. Thus, they have a particular texture, characterized by a preferred orientation of the grains constituting the material [ 7 ]. This texture gives the sheet a special plastic behavior. For modeling the plastic behavior, two aspects of the anisotropy are taken into account: the initial anisotropy due to the initial texture of the metal sheets and the anisotropy induced by cold working [8], mainly due to the development of dislocation structures in the material [ 9 ; 10] . Recently, researches on aluminum alloy are focused on mechanical properties, texture and anisotropic behavior that give rise from processing of aluminum alloy sheet [6- 7, 10 -11 ] . There has been little research on formability and anisotropic behavior of commercialized 7075 aluminum alloy. However, the influences of loading orientations on aluminum alloy plate are still an open question. The purpose of the present study is to describe and characterize the mechanical properties, anisotropic behavior of high- strength aluminum alloy loaded at 0°, 45°and 90° to the rolling direction of the 3 mm thick plate, and provide direction for obtaining the optimized parameters for 7075 aluminum alloy in metal forming. As the initial anisotropy is taken into account through a yield criterion [ 9] , the Yld91 anisotropic yield function proposed by Barlat et al. [ 12 ] is chosen to model the elastoplastic behavior of the 7075-T7 aluminum alloy. The plastic parameters were determined using an experimental database from uniaxial tensile tests. Numerical simulations of the experimental tensile tests were performed using the anisotropic elastoplastic model. Predicted stress-strain curves were in very good agreement with the experimental curves for three loading directions. The results of simple tensile test were used subsequently to show the evolution of plastic anisotropy called Lankford coefficient and load surface for several tests. E XPERIMENTAL PROCEDURE Material he 7075 aluminum alloy with structural hardening is used. This alloy is a thin rolled sheet with a thickness of 3.5 mm. The material used in this investigation is a commercially produced 7075 aluminum alloy with the following chemical compositions: Al- Zn (6.1%)-Mg (2.1%)-Cu (1.2%) and balance Al (all in mass pct) [ 13 ]. The heat treatment process for this material is T7351. T7 temper is achieved by solution-treatment at 465°C (  5°C), quenching in water (<40°C), maturation at room temperature during 4 days and tempering (135°C  5°C 12h) and then 2% pre-stretching to release residual stress [ 13 ]. Dimensions and form of the test specimens The uniaxial test is ensured by a specific geometry defined by the standard NF A 03-151 [14]. Schematic tensile specimen used for this study is shown in Fig. 1. The current dimensions of useful part are L0=50mm and b0=12.5mm. The specimens are cut in three directions relative to the rolling direction (RD) in the plane of the sheet (see Fig. 2 (a)). In the following, the rolling direction is referred to as RD, the transverse direction as TD and the direction (45° from the RD) as DD. The angle between the loading direction and the rolling direction will be noted subsequently  . Three samples were prepared for each loading direction to verify repeatability. Each specimen was machined in different directions of the plate to enlighten the anisotropy of the material. Figure 1 : Tensile specimen used in the present study, the useful area (L0=50mm, b0 = 12.5mm). T R20 RD y x T 