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

The Master Curve 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 4mm thickness Mini-C(T) are used to measure fracture toughness, KJc, temperatures lower in this range should be selected to decrease the number of censored KJc due to plastic strain evolution that leads to specimen failure above the KJclimit. The combined constraints imposed by the T₀ ± 50°C range and the KJclimit may cause difficulty in selecting an appropriate test temperature. The present study proposes a criterion based on Tave, the average of test temperature of uncensored valid KJc values in a data set, to judge whether a dataset including data obtained T-T₀< −50°C can be used for estimation of T₀. Analyses with synthetic KJc datasets demonstrated that if Tave-T₀> = −50°C the resultant Master Curve reasonably represents the fracture toughness distribution even if the dataset includes data tested below T-T₀ = −50°C. Analyses on recursively sampled datasets from an experimental fracture toughness dataset of over 200 values demonstrated that the concept of Tave reasonably works for T₀ evaluation of such datasets even if a realistic limited number of specimens (e.g., 12-16) are tested. Allowing some data tested below T-T₀ = −50 °C showed a higher percentage of valid T₀ evaluations than can be achieved using the existing requirements of either ASTM E1921 or JEAC4216.