A Closer Look at Effects of the Loading Rate on Fracture Toughness in the Ductile-to-Brittle Transition Regime of a Ferritic Steel

An increased loading rate causes not only a shift of the ductile-to-brittle transition curve, but also a slight change of the shape of this curve. The latter tends to become steeper as the loading rate increases. This effect was observed even at loading rates that are considered to be quasi-static according to ASTM E1921 [ASTM E1921-13: Standard Test Method for Determination of Reference Temperature, T-0, for Ferritic Steels in the Transition Range, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2013. Actually, the coefficient 0.019 in the exponent of the MC turned out to be substantially higher at elevated loading rates. This means that a basic assumption of the evaluation procedure of ASTM E1921 is not met, which leads to an increased uncertainty of T-0 or T-0,T-x, respectively. This effect of is most pronounced in testing at elevated loading rates using the single-temperature option. An improved method to determine the reference temperature is proposed, where the exponent in the median transition curve in ASTM E1921 is considered to be an open parameter. The resulting reference temperatures, denoted as T-100 and T-100,T-x, respectively, are expected to be more accurate than standard T-0, which is confirmed by comparison with experimental data. Furthermore, the validity requirements for T-100 are less restrictive and more suitable than those in ASTM E1921, since they do not depend on the outcome of the tests. Based on the improved data, an improved estimation formula for the rate-induced shift of T-0 is derived. Suggestions are made for improvement of ASTM E1921.