Impact of elevated loading rates on the shape of the Master Curve (ASTM E1921) for a German RPV steel

The Master Curve Methodology (ASTM E1921) experimentally assesses a materials temperature-dependent fracture toughness, predominantly for quasi-static testing conditions. The treatment of elevated loading rates is described by the annex A1 of ASTM E1921 and A14 of ASTM E1820. This paper presents results of the evaluation of a very large and standard-conforming database in order to verify the procedures recommended by the standard.
Testing involved C(T)- and SEN(B)-specimens of the RPV-steel 22NiMoCr3-7 (A508 Grade 2) for loading rates of 100 MPa√m/s ≤ dK/dt ≤ 104 MPa√m/s for the entire ductile to brittle transition region. It was found that the shape and underlying distribution of the Master Curve show stronger shortcomings with increased loading rate. The optimal shape factor p increases with dK/dt, yet the methodology can still show strong shortcomings regarding the underlying distribution. It is argued that the cause for the observations are linked to adiabatic heating and local crack arrest that severely influence macroscopic fracture behavior. It is recommended that fracture mechanics testing at elevated loading rates is performed close to or below T0 in order to minimize the influence of dynamic loading conditions on the assessment.