13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 2. Experimental investigations In this paper, a low-alloyed structural steel S355NL was adopted as the base material (BM) for producing the weldments. After the electron beam welding process, a butt joint is obtained from two S355NL plates with the thickness of 60 mm. The chemical components of S355NL are shown in Table 1, which is obtained from spectrometric analysis. Table 1: Chemical composition of the steel S355NL, mass contents in % Steel C Si Mn P S Cr Mo Ni Al Co S355NL 0.198 0.260 1.386 0.026 0.013 0.020 <0.005 <0.005 0.013 0.006 After the hardness test, the dimensions of the fusion zone (FZ) and the heat affected zone (HAZ) are found to be 2.8 mm and 3.1 mm, respectively. The mechanical properties of different weld regions of S355 EBW joints are obtained from flat specimens along the weld line of which the gauge length is 50 mm. These stress-strain curves [12] are used as finite element model input data. Table 2 shows the mechanical properties of base material (BM) and fusion zone (FZ) of the welded joints containing yield strength Re, tensile strength Rm, uniform elongation Ag and strain at rupture A. Table 2: Mechanical properties of different weld regions of S355NL EBW joint BM eR (MPa) FZ eR (MPa) BM mR (MPa) FZ mR (MPa) BM gA FZ gA BM A FZ A 348 513 533 687 0.151 0.037 0.246 0.052 Fracture toughness tests of S355 electron beam welded joints were performed using compact tension (C(T)) specimens. The specimens were manufactured and tensile tested according to ASTM standard [21] which have a thickness of B=25 mm, a net thickness of Bn=20 mm due to 20% side grooves, the width of specimen is W=50 mm. After the C(T) test, the experimental results are shown in terms of force vs. Crack Opening Displacement (COD) as well as fracture resistance JR curves. The F-COD curves of compact tension (C(T)) specimens with initial crack located in the BM (C(T)-BM), in the center of FZ (C(T)-FZ) and at the interface between the FZ and HAZ (C(T)-HAZ) can be found in Fig. 2(a). For a C(T) specimen with the crack in the FZ, the specimen suddenly ruptures, showing a rather brittle fracture behavior. The C(T)-FZ specimen broke suddenly without stable crack propagation, no fracture resistance (JR) curves were obtained during the test process. The JR curves for a C(T)-BM and a C(T)-HAZ specimen can be found in Fig. 2(b). 3. Numerical calculation Before the application, the cohesive parameters must be fixed first. According to the discussion of Cornec and Scheider [22], for mode I situation, the cohesive parameter T0 is equal to the projection of the applied force on a plane perpendicular to the specimen cross section. The notched round specimen extracted from the BM is used for the determination of T0. For the notched round specimen, as the geometry and loading are axisymmetric and symmetric to the cross section, only one quarter of the structure is used for the modeling. The finite element (FE) mesh of the notched round specimen and the detailed mesh can be found in Fig. 3. The comparison of axial stress versus the diameter reduction curve from the FE simulation and the experiment as well as the maximum true axial stress in the center of the specimen can be found in Fig. 4. The simulated axial stress
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