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

Abstract

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, KJc, temperatures lower in this range should be selected to avoid an excessive number of KJc values being censored due to plastic strain evolution that leads to specimen failure above the KJclimit prescribed by both standards. The combined constraints imposed by the T0 ± 50 °C range and the KJclimit may cause difficulty in selecting an appropriate test temperature, especially for smaller specimens. The present study proposes a criterion based on Tave, the average of test temperature of uncensored valid KJc 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 KJc datasets, which explore different minimum toughness and temperature dependence characteristics than the classical Master Curve assumption as sensitivity studies, demonstrated that if Tave − 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 Tave 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.