13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- grain size gradient between the weldment (left side) and the base material (right side) is clearly seen. The average grain size of the base material is about 50µm. In the fusion zone, the microstructure is composed of columnar grains, while original grain structure no longer exits. The columnar grains orient perpendicular to the direction of the radial fusion boundary. The grain differences between fusion zone and base material would lead to different mechanical behavior. (a) (b) Figure 2. (a) Macroscopic view of the weld joint and (b) Microstructure of the fusion boundary. 2.3 Nano-indentation test For nano-indentation test, a nano indenter G200 from Agilent Technologies was used. The depth resolution is 0.01nm and the force resolution is 50nN. Berkovich indenter was used for the identation tests. During the test, the displacement of the indenter has been controlled. The maximum depth limit was 2µm and peak hold time was 10 s. Indentation tests were carried out in five times and on a longitudinal section of weldment. The measured hardness shows a normal distribution and listed in Table 2. Due to larger grain size in the base material compared with the indentation depth (2µm), larger scattering of measured hardness were obtained. For the weldment material, the scatter band was smaller. Generally the hardness of weldment material is higher than base material. Table 2. Measured indentation hardness Mean Value (GPa) Standard Deviation Base Material 3.48 0.53 Weldment material 3.10 0.33 3. Nano-indentation Simulation and Results In order to determine the local stress-strain behavior of the weldment, nano-indentation technology was used. Through inverse analysis, the nano-indentation data can help to evaluate the elastoplastic properties of weldment. 3.1 FEM modeling The indentation process was simulated in the commercial FEM code ABAQUS. Four-node axisymmetric elements were used in the simulation. Large strain feature was considered. Due to the
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