Application of Master Curve Technology to Biaxial and Shallow Crack Fracture Data for A533B Steels
Authors: Joyce JA , Tregoning RL , Zhang XJ 
Published: April 2008

The Master Curve approach has recently been proposed by Wallin [1] to define the expected toughness of ferritic steels in the ductile-to-brittle transition. This method has been standardized in ASTM E1921-98 using deeply cracked fracture toughness specimens tested quasistatically in the lower transition regime. However, the commercial nuclear power industry will often use this method to infer structural performance where the critical crack of interest is shallow, rather than deep, and the loading is biaxial rather than uniaxial. Research has been funded by the U.S. Nuclear Regulatory Commission (NRC) and conducted at the Oak Ridge National Laboratory (ORNL), the U.S. Naval Academy (USNA), and the Naval Surface Warfare Center (NSWC) to address potential shortcomings of the present ASTM Standard Master Curve method for such applications.

Low-constraint specimens of A533B steel were tested to measure performance under more realistic crack tip constraint conditions, and results were analyzed using the Master Curve approach. The A533B steel plate was heat treated to simulate the elevation of the material yield strength and reduction in toughness which results from extended aging due to neutron irradiation. ORNL conducted shallow crack (a/W = 0.1), uniaxially and biaxially-loaded cruciform specimen testing, while USNA performed shallow crack (a/W = 0.1), uniaxially-loaded SE(B) testing. Both laboratories also utilized additional deeply cracked (a/W &gt; 0.5) specimens to fully characterize the A533B steel plate tested as per ASTM E1921. ORNL conducted 1/2T C(T) specimens, while USNA tested both 1/2T C(T) and 1T SE(B) specimens. The standardized master curve approach has been used for analysis in most cases, but this technique was augmented with the maximum likelihood, multi-temperature method [2] when data at a single temperature was sparse.

Results indicate that larger variability in KJc (fracture toughness at onset of cleavage initiation) is present in the shallow cracked specimens compared with deeply cracked specimens tested at the same temperature. This result is expected due to interaction between the gross section and crack-tip plasticity which lowers constraint and reduces the opening mode stress magnitude within the crack-tip process zone. Surprisingly, however, the 1T SE(B) shallow crack specimens exhibited a distinct reduction in the median KJc in comparison with the deeply cracked specimens tested using the same material. This trend is counter to historical short-crack data which Zexhibits an increase in the median KJc [3, 4].

Stereo-Section fractography was performed to reveal the microstructure near the cleavage crack initiation sites of several shallow and deeply cracked A533B specimens. More intense carbide bands are present in the USNA shallow cracked SE(B) specimens than in the other specimens tested. This implies that the local carbon content and hardness is higher in these specimens and lead to the low median KJc [5–8]. This effect has recently been more rigorously quantified [9]. The variation of the carbide band density in the A533B material tested is a cause of considerable concern. The ability of surrogate, surveillance capsule specimens to accurately capture the reduced toughness due to banding is questionable in light of the relatively small amount of material available for testing, the highly localized nature of the banding, and the inability to predict where it will occur.