Fracture Toughness Estimation for Small CT Specimen of Austenitic Stainless Steels Using Stretched Zone Width

Austenitic stainless steels in boiling water reactor (BWR) core structures can experience significant fracture toughness reductions at high fluence levels of neutron irradiation. To understand the fracture toughness reductions of irradiated austenitic stainless steels is very important for integrity estimation of core internals. Larger size specimen is needed to obtain the valid J_IC of high fracture toughness material. However, it is generally difficult for irradiated material to prepare the enough big specimens for taking valid fracture toughness in ASTM E1820–06 since the material volume is restricted. Stretched zone which is generated between fatigue crack and dimple surface on the CT specimen fracture surface is said to have good linear relationship with J_IC. New method for J_IC estimation from small CT specimen using stretched zone width was investigated. Cold-worked SUS304 was used for investigation. 0.25–1.5TCT specimens with similar figure of standard dimensions and the CT specimens with different thickness from standard dimensions were prepared for J_IC tests. The correlations of specimen width, thickness, critical stretched zone width, J_Q and J_IC were investigated. J_Q obtained from J-R curve tended to decrease with specimen width in the case of similar figure. However, J_Q tended to increase with thickness decrease in the case of CT specimen with different thickness from standard dimensions. On the other hand, critical stretched zone width showed the tendency to decrease with thickness decrease in the case of CT specimen with different thickness from standard dimensions. The equation for J_IC estimation of invalid small CT specimens was designed by regression analysis using the above relationship. The prediction accuracy achieved within plus-minus 30%.