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.