Static and Dynamic Low-Temperature K_Ic Behavior of Steels

To determine the crack-toughness behavior of steels subjected to high strain rates, a study was undertaken to establish the effects of strain rate and temperature on the K_Ic-values of steels. As an initial step, a procedure was developed to obtain dynamic K_Ic-values of steels by impact loading a fatigue-cracked bend specimen in a drop-weight machine. Steel plates having widely different yield strengths (40, 140, and 250 ksi) were studied initially to establish testing techniques. The results indicate that the procedures and analysis developed were satisfactory for determining static and dynamic K_Ic-values. For the 250 ksi yield strength steel [18Ni(250) maraging] the K_Ic linearly increased with increased temperature over a 400 F temperature range with no significant effect of strain rate. The strain rate had no effect on the K_Ic-values of the 140 ksi yield strength steel [HY-130(T)] between −320 and −150 F. However, in the 1-in-thick specimens the increase in strain rate which occurred in the dynamic test increased the temperature range for which plane-strain crack extension occurred. The K_Ic behavior of the 40 ksi yield strength steel (ABS-C) showed a marked sensitivity to strain rate with the same crack toughness being measured dynamically at a temperature 190 F higher than for the static test. Correlations of ABS-C steel K_Ic-values with its yield strength and a rate parameter, T In A/ε demonstrated the equivalency between decreasing temperature and increasing strain rate as influencing the crack toughness. The agreement of an estimate of K_Id from the dynamic yield strength at the NDT with the measured dynamic K_Ic-value showed that the six orders of magnitude increase in strain rate for this dynamic test appears sufficient to give K_Ic-values within 10 percent of the crack-toughness value estimates for a running crack in rate and temperature-sensitive steels. In general, the results of this investigation showed that a dynamic K_Ic test has been developed which can be used to establish the strain-rate sensitivity of the crack toughness of steels as well as corresponding design information.