13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- 3. Results The results from the three tensile tests at room temperature are summarised in Table 2. The average yield stress was 446 MPa and the ultimate tensile stress was 594 MPa. The fracture toughness properties of the A508 Class 3 steel are illustrated as a J R-curve in Figure 1 which includes data from both the unloading compliance tests and monotonically loaded tests presented together. The data from both test types are in agreement and the initiation toughness, measured by the intersection of the blunting line including 0.2 mm tearing and the power-law curve fit to the data is ~ 475 kJ/m2. The specimens for tomography analysis were extracted from test samples B, G and C as these specimens were subjected to the most ductile tearing. Table 2: Tensile Test results Figure 1: J R-curve for A508 Class 3 material tested in the hoop-radial direction at 23oC. Figure 2 illustrates the general upper bainitic microstructure of the ferritic steel under the optical microscope and SEM respectively. The average grain size was estimated at 11µm. But the microstructure is interspersed with clusters of very small grains and regions where very large grains are present. Figure 2: General microstructure of the bainitic steel under optical microscope. Figure 3Figure 4 illustrate selected microstructural observations of voids in the material. Microvoids 200µm
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