13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- The GTN damage model could be used to analyze the ductile fracture behavior under tension loads. In this paper, we also used the model to predict the damage behavior, as shown in Eq. (1), (1) where p the hydrostatic pressure, σeq the effective Von Mises stress, σ0 the yield stress of the matrix (function of the plastic deformation), q1, q2 are material parameters and f* is the effective porosity. Material grade API 5L X80 was adopted in the models and the parameters are listed in Table 1. Table 1 Main material parameters Parameters Value Density, kg/m3 7850 Young’s Modulus, GPa 206 Yield stress, MPa 610 f* 0.06 q1,q2 1.5,1.0 As specified in the ASTM E436 [2], the DWTT specimen is loaded in three-point bending by a drop hammer with 400kg weight and loading span of 254 mm. The simulated fracture test comprises two steps employed, as shown in Fig. 2: (1) pressing the notch and, (2) fracturing the sample under the action of the hammer. A three-dimensional geometrical model of the DWTT process was created with a pressed crack was set up with the above parameters and computational meshes with 8-node elements. The elements around the expected fracture zone are much finer than elsewhere in order to accommodate steeper gradients in parameters. In the models, there are 124864 elements and 127002 nodes. In the DWTT process, the hammer descends with an initial impact speed of 7 m/s. Fig. 2 Geometry of the full simulation process 3. Results and discussion Fig.3 shows the equivalent stress distribution around the pressed notch position. After the pressed notch was introduced (step 1) at the centre of the specimen, stress concentration in the notch tip
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