13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- of 2024 Al-alloy was studied experimentally and numerically using FEM by Al-Khazraji et al. [18]. Effect of plastic predeformation by bending to create deep residual compressive stresses on the fatigue strength of steel specimens and compressor blades was studied by Ezhov and Sidyachenko [19]. It was found that plastic predeformation increases the fatigue strength by about 20%. In other work, effect of residual stress induced by plastic predeformation was investigated by Mokhdani [20] on API 5L pipeline steel and Benachour [21] and Jones [22] on 2024 T351 Al-alloy using Four bent specimen. It was found that the fatigue life was influenced by the plastic preload. An increasing in fatigue life was shown by increasing of the level of plastic preload. The fatigue crack growth rates at low stress intensity factor were decreased by the presence of compressive residual stress. In study conducted by Jones and Dunn [23], fatigue crack growth from a hole with residual stress introduced by tensile preload was predicted using linear elastic fracture mechanics and the principle of superposition. O'Dowd et al. [24] introduced residual stresses in compact tension (CT) specimen by mechanical compression. The level of the compressive load was determined by finite element method (FEM). The compressive residual stresses present a beneficial effect on fatigue lifetime. Additionally fatigue life and fatigue crack growth rate (FCGR) were affected by stress ratio. Many researchers [25-28] have studied effect of this parameter on some Al-alloy with and without residual stress. The main aims of the present investigation is to studied effect of residual stress on fatigue life and fatigue crack growth around hole, determined by plastic preload in tension of samples using finite element method. 2. Finite element model and analysis procedure 2.1. Modeling The FE model used in simulation of plastic preload (PP) was a plate assumed to be made from Al-alloy 2024 T351 and 6061 T6. The mechanical properties of the both materials are shown in Table 1. In order to analyze the respect of elasto-plastic behavior, a true stress–true strain curve as shown in Figure 1 was used as an input property of FE analysis. As shown in Figure. 2, the dimensions of the plate containing Ø 6 diameter holes and thickness (t) = 4 mm. I have varied the level of applied preload characterized by non dimensional ratio σp/σy, where σp is applied preload and σy is yield stress for specified material, in order to investigate the level of the residual stress variation on fatigue crack growth behavior. The finite element mesh is shown in Figure 3. Only four quart of the entire plate has been modeled considering of the symmetry. More finite elements than those in other regions are put closer to the boundary of holes. Since we are interested of the residual stress variation according to the X axis from hole edge to free surface, two-dimensional analysis has been carried out with uniform distributed plastic preload σp. The program used in the FE analysis was ANSYS, Ver. 11. The mesh element type was “PLANE183”. Table 1. Mechanical properties for Al-alloys Al-alloys E (GPa) σy (MPa) UTS (MPa) ν 2024 T351 [44] 74.08 363 477 6061 T6 [45] 69.04 252 360 0.33
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