Comparison between Loading Rate and Local Stress Rate When Applying ASTM E1921 at Elevated Loading Rates

ASTM E1921-2019b, Standard Test Method for Determination of Reference Temperature, T₀, for Ferritic Steels in the Transition Range, Annex 1, describes the determination of the reference temperature T_0,X for a loading rate X for ferritic steels in the transition range, where X corresponds to the logarithm of the average loading rate for the tests performed, rounded to the nearest integer. Within the framework of three research projects on dynamic crack initiation and crack arrest, funded by the German government, fracture mechanics tests using specimens of 22NiMoCr3-7 steel (A 508 Grade 2 Cl.1) were performed in a range from 10² MPa√m s⁻¹ to 10⁶ MPa√m s⁻¹ at different temperatures in the ductile to brittle transition region. The Weibull stress rate was calculated from the results of a numerical analysis of tests with 1-inch compact tension (1T C(T)) specimens. The modified three-parameter Beremin model of Petti and Dodds was used, since this approach is congruent with the Master Curve method. In addition to the stress parameters corresponding to K_min and K₀, the Weibull exponent can be calibrated using the experimental results. The analysis shows the effect of the Weibull exponent on the time-dependent Weibull stress rate for a dynamic 1T C(T) test. Contrary to the loading rate calculated from K_Jc divided by the time to cleavage according to ASTM E1921, the Weibull stress rate decreases with increasing plastic behavior. This has to be considered when the dynamic reference temperature T_0,X is determined using results from tests with large plastic contribution.