The static and dynamic low-temperature crack-toughness performance of seven structural steels☆

Abstract To properly describe the crack-toughness behavior of steels in a quantitative manner, a study was undertaken to establish the effects of strain rate and low temperature on the K ic values of seven structural steels. Steels having room-temperature yield strengths ranging from 40 to 250 ksi-ABS-C, A302-B, HY-80, A517-F, HY-130(T), 18Ni(180), and 18Ni(250) steels-were evaluated for static and dynamic loading over the range of temperatures for which K ic values were attainable. The results indicate that for the ABS-C, A302-B, HY-80, and A517-F steels, an increase in strain rate of approximately six orders of magnitude caused a decrease in the K ic values measured at the same test temperatures. No significant effect was observed for the HY-130T and 18NI(250) steels. However, the most significant effect of the increased strain rate was the increase in the threshold temperature below which plane-strain behavior occurred. When all steels — except the 18Ni(180) maraging steel, for which insufficient valid data were obtained were compared on the basis of equivalent critical flaw-size behavior, the crack-toughness performance in terms of ic / σ ys for dynamic loading could be separated into three groups. The HY-80 and HY-130(T) steels were best, the ABS-C, A302-B, and A517-F steels were intermediate in performance, and the 18Ni-(250) maraging steel was the poorest. These groupings of performance prevailed over a relatively wide range of test temperatures. As a means of accounting for the differences in strain rate, the K ic / σ ys values for all steels investigated were plotted in terms of the rate-temperature parameter, Tln A / ϵ , which superimposed most of the static crack-toughness performance data into these same levels of performance. In addition, the results of the investigation substantiated the interpretation that the nil-ductility-transition temperature measured in the drop-weight test is the upper limit of dynamic plane-strain crack-toughness behavior for 1-in.-thick plates. In general, the results of the present investigation provide a quantitative comparison of the plane-strain crack-toughness performance of 1-in.-thick plates of seven structurel steels under both static and dynamic loading conditions. Because of the increase in temperature range over which K ic behavior occurs with increased strain rate, dynamic loading can be an especially significant factor in the performance of structural steels, particularly those having yield strengths less than approximately 140 ksi.