13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 2. Fraunhofer cruciform specimen with five-point bending to simulate biaxial loading of crack is only 10 mm x 20 mm (B = 10 mm and W = 20 mm). The distance between the roller supports in the five point bending test rig is 80 mm, while the overall dimensions of the specimen are 90 mm x 90 mm. Also, smaller scale 3PB specimens are incorporated as well. The Fraunhofer study used pre-cracked Charpy test specimens rather than standard 3PB specimens specified in ASTM E1921 [2]. The 3PB specimens contained shallow cracks (a/W = 0.18). Standard 1T-C(T) specimens (a/W = 0.5) were also tested. Testing of such small specimens is completed in an effort to determine if the fracture toughness results are applicable to much larger scale components, including reactor pressure vessels. 3.2. Material Properties The Fraunhofer specimens were machined from a section of A508 Class 2 steel, commonly used to fabricate reactor pressure vessels. Material properties for A508 were obtained from another recent study conducted by Scibetta et al. [12]. The researchers provided the following equation to correct the yield stress (in MPa) based on temperature (T, in degrees Celsius) (7) In addition, Scibetta et al. [12] also presented a temperature correction for E (in GPa) again where the temperature is in degrees Celsius, (8) Thus, at -85°C, the yield stress and Young’s modulus were determined to be 550 MPa and 212.1 GPa, respectively. Poisson’s ratio was assumed to be 0.3 as typical with many steels. The Fraunhofer study [11] provided true stress vs. true strain curves for the material. A curve fitting procedure was used to determine the strain hardening exponent necessary for the elastic-plastic finite element analysis using the Ramberg-Osgood equation. The strain hardening exponent was determined to be n = 8. The angular stress functions and the stress power exponents
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