13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- weldment material, and to predict fatigue life of weldment with stress concentrations. The local elastoplastic properties of weldment material are derived from inverse nano-indentation analysis. From FEM simulation the nominal stress-strain curve of weld joint is obtained and compared with the monotonic tensile curve. A critical plane method based Cruse-Meyer model is applied to predict the fatigue life of weldment. Combined with critical distance method center holed specimen fatigue life was calculated and compared with the experiment. 2. Experiments Inconel 718 is a precipitation-hardenable nickel-chromium alloy, contains large amount of iron, niobium and molybdenum. Due to the outstanding combination of tensile strength, high temperature creep strength and high resistance to fatigue, this alloy is widely used in the high pressure compressor and turbine discs of commercial and military aero engines and recommended for using up to 650°C [13]. The composition of the alloy is given in Table 1. After electron beam welding process, post-weld heat treatment was applied. Specimens were cut from the welded joint material with weldment in the center of the specimen. Surface of raw specimens was removed and machined to a thickness of 4mm, resulting weldment shape changing from bell shape to trapezoid shape. Fig. 1 shows the geometry of the holed specimen. All tests were carried out at the room temperature. Table 1. Chemistry of Inconel 718 Ni Fe Cr Mo Nb Al Ti C Weight% balance 18.5 19.0 3.0 5.1 0.5 0.9 0.04 2.1 Tensile and Fatigue Test Monotonic tensile test was conducted at strain rate 10-4/s. Cracks originated from the weldment. Tensile failure occurred in the center of weldment, while shear failure occurred in the outer side. Yield stress and ultimate tensile stress of weld joint were lower than base material. Fatigue tests were performed under stress control at stress ratio, R = -1, 0.1. The fatigue life was defined as the number of cycles when a specimen was completely separated. Figure 1. Geometry of center holed EB welded specimen. 2.2 Macroscopic and microscopic observation of weld joint Fig. 2a shows the fusion region of EB welding joint by macroscopic observation. The fusion zone is quite narrow, formed in 2-4mm width. The fusion boundary is clearly evident. No significant heat affected zone can be seen. The evolution of microstructure from the base material to the fusion zone can be clearly recognized. Fig. 2b shows a microscopic observation near fusion boundary. Large
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