Master Curve Evaluation Using the Fracture Toughness Data at Low Temperature of T − T₀ < −50 °C

The Master Curve (MC) fracture toughness evaluation methodology standardized by ASTM E1921 and JEAC4216 specifies an acceptable test temperature, T, to be selected within the range of ±50 °C from the evaluated reference temperature, T₀. If small size specimens, such as 4 mm thickness Mini-C(T), are used to measure fracture toughness, K_Jc, temperatures lower in this range should be selected to avoid an excessive number of K_Jc values being censored due to plastic strain evolution that leads to specimen failure above the K_Jclimit prescribed by both standards. The combined constraints imposed by the T₀ ± 50 °C range and the K_Jclimit may cause difficulty in selecting an appropriate test temperature, especially for smaller specimens. The present study proposes a criterion based on T_ave, the average of test temperature of uncensored valid K_Jc values in a dataset, to judge whether a dataset including data obtained T − T₀ < −50 °C can be used for estimation of T₀. Multiple analyses with synthetic K_Jc datasets, which explore different minimum toughness and temperature dependence characteristics than the classical Master Curve assumption as sensitivity studies, demonstrated that if T_ave − T₀ ≧ −50 °C, the resultant T₀ value predicts as Master Curve that reasonably represents the fracture toughness of the data even if the dataset includes some data tested at temperatures below T − T₀ = −50 °C. Master Curve analyses performed on recursively sampled datasets from an experimental fracture toughness dataset of over 200 values demonstrated that the concept of T_ave works reasonably well for T₀ values determined from a realistic dataset size (e.g., 12–16). Allowing use of some data tested below T − T₀ = −50 °C created a higher percentage of valid T₀ evaluations than can be achieved using the existing requirements of either ASTM E1921 or JEAC4216.