Fig. 8 Comparison at fracture
between S-T300 and D-15 kN with highlighted the difference between the
gage lengths, i.e. between the engineering strains, with a similar
diameter, i.e. similar true strain
Conclusions
In the present work, an in-depth analysis of the necking onset under
quasi-static and dynamic conditions has been carried out for the A2-70
stainless steel.
Firstly the influences of temperature and strain rate together with
their variability have been evaluated on the necking inception strain,
by assuming that strain, strain rate and temperature are uncoupled to
each other. From such qualitative analysis it was shown that, in
standard SHTB tests, the increasing temperature and strain rate should
respectively cause a decrease and an increase of the necking initiation
strain.
The experimental campaign on the A2-70 steel included quasi-static tests
at different temperatures and dynamic SHTB test at room temperatures,
all with cylindrical specimens. The true stress – true strain curves,
derived from the tests by means of fast camera acquisitions of the
evolving diameter, are translated into equivalent stress-strain curves
by using the MLR function.
The necking anticipation found from quasi-static experiments at constant
high temperatures demonstrated the existence of a coupling between
strain and temperature within the thermal softening function. This
necking anticipation was then mathematically demonstrated by simply
introducing a general coupling between strain and temperature within the
thermal softening function.
The evolving temperatures of all the tests, due to the fast adiabatic
conversion of plastic work into heat, were evaluated; then, the triplets
of strain, temperature and thermal softening values were best-fitted by
a general polynomial.
The dynamic necking strains lower than their quasi-static counterparts
at similar temperatures evidenced that the anticipating effect caused by
the growing temperature is greater than the delaying effect of the
growing strain rate.
Lastly, the analysis of fracture strains showed that, for the A2-70
steel at hand, both temperature and strain rate have a decreasing effect
on the fracture true strain. Moreover, comparing the necked specimens
from dynamic tests to those from quasi-static tests at high temperature,
an influence of temperature on the strain localization and on the
maximum postnecking strain range was evidenced and addressed to earlier
necking strains from higher temperatures.
Author contribution statement
All persons who meet authorship criteria are listed as authors, and all
authors certify that they have participated sufficiently in the work to
take public responsibility for the content, including participation in
the concept, design, analysis, writing, or revision of the manuscript.
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