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

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.