On the Possibility of Extending the Crack Length Criterion in the Master Curve Methodology

The Master Curve (MC) methodology is a well-known approach utilized to characterize the ductile-to-brittle transition region (DBTR) of ferritic–pearlitic steels. This methodology was initially standardized in ASTM E1921 in 1997 and has undergone continuous evolution and improvement since its origin. However, the validity criterion for the crack aspect ratio (0.45 ≤ a₀/W ≤ 0.55) has remained unchanged since its inception. It is worth noting that this criterion was originally established in accordance with standard ASTM E399, which characterizes fracture conditions under linear-elastic plane strain conditions, apparently for historical precedents rather than any scientific rationale. Furthermore, ASTM E1820, which is employed to characterize the fracture behavior of metallic materials in elastic–plastic conditions, permits a maximum crack length-to-width ratio of 0.70. In this context and considering that ASTM E1921 measures K_Jc (elastic–plastic) values of the fracture toughness, our research seeks to empirically demonstrate that the crack length-to-width criteria established in ASTM E1921 can be increased up to 0.60, without compromising the accuracy of the reference temperature calculations, at least for the specific datasets used in this work. Such a correction would offer significant advantages, especially when dealing with mini-C(T) specimens. Their subsize dimensions may result in the discarding of numerous specimens that could otherwise be effectively employed for reference temperature calculations. More research is recommended to provide additional validation of the crack aspect ratio upper limit proposed here, and even to explore the possibility of further extensions.