Fig. 6 3D plot of the true fracture strain versus temperature at fracture and reference true strain rate of the test
The fracture true strain at static rates clearly decreases with the temperature, from 1.48 at room temperature to 1.06 at 300 °C. This can also be indirectly caused by the temperature anticipating the necking onset, which, in turn, causes the necking-induced stress triaxialty to evolve much sooner and much largely than it does for tests at room temperature. In fact, the postnecking strain range up to failure is nearly identical for static tests from Troom up to 200 °C and slowly decreases at 300 °C.
Dynamic tests exhibit a material temperature at fracture close to 300 °C, similar to the quasi-static tests at the highest temperature, but the true strains at failure are lower, close to 0.93 for the tests at nominal strain rate of 1800 s-1. This means that the strain rate too tends to anticipate failure of the A270 steel together with temperature, by further decreasing the fracture strain.
The dynamic tests at nominal 800 s-1, progressively heating from Troom up to 300 °C at incipient failure, exhibits a failure strain close to that of static 300 °C: the fracture-delaying effect of the initially low temperature of the dynamic test with respect to the static one (at 300 °C since first yield) is compensated by the fracture-anticipating strain rate effect. Therefore, for the A2-70 steel at hand, both temperature and strain rate have a decreasing effect on the fracture true strain.
Comparing the necked specimen shapes from dynamic tests to those from the quasi-static tests at 300 °C, it is also possible to see that they show different degrees of strain localization at fracture, fully reflecting the maximum postnecking strain in the last row of Table 4.
Fig. 7 shows the comparison between the last frames before fracture of the S-T300 and the D-26, where the difference between the two diameters is highlighted. The two tests series show a similar fracture engineering strain (0.3 and 0.35 respectively) but different fracture true strain (1.06 and 0.93 respectively), i.e. different diameter. In other words, the test S-T300 shows a greater strain localization than the dynamic test at 1800 s-1, despite a comparable overall engineering deformation.
Similarly, in Fig. 8 it is shown the comparison between the last frames before fracture of the S-T300 and the D-15, in which is highlighted the difference between the two gage lengths. In this case, the two group of tests show a similar fracture true strain (1.06 and 1.07 respectively), i.e. similar diameter, but different fracture engineering strain (0.3 and 0.45 respectively), i.e. different overall gage length. Therefore, the S-T300 shows a similar fracture true strain in respect to the D-15 with a lower overall engineering deformation.
Both comparisons highlight that the high temperature quasi-static tests show a greater strain localization with respect to the dynamic tests. This is due to the earlier necking onset of static high temperature tests with respect to the dynamic tests at room temperature, leading to larger postnecking strains at the same overall true strain which either means more pronounced shrinking at given elongation or lower elongation at given diameter contraction.