High strain rate, high temperature deformation of stainless steel.

In order to clarify the dependence of strain localisation on different
parameters, a literature survey is presented covering the theory and
observations of instability formation during deformation, dynamic recovery
processes, mechanical testing, adiabatic heating and shear band formation.
Several series of experiments were carried out, on titanium bearing steel
and on an austenitic stainless steel , to determine the reasons of the inversion
of the dependence of the strain to the peak in flow stress on strain rate
observed at high strain rates. The materials were deformed under conditions of
plane strain and axisymmetric compression, and torsion. Special techniques were
developed in order to measure the distribution of strain and temperature within
deforming plane strain specimens.
In the strain distribution analysis, it was observed that the range of
local strains is dependent on nominal strain rates, but the average value of the
local strain either in the whole deforming region or the slip line bands depends
only on the value of nominal strain. When the plane strain specimens were
deformed with inserted thermocouples, it was found that both the overall
temperature increase and the difference in temperature between different local
regions of the specimen, increased with the nominal strain rate.
It was shown that, at high strain rates, the stress-strain curve of any
material will develop a peak in stress as a result of the increase of
temperature due to deformational heating, without the occurrence of any strain
instability.
The results emphasise the complexity of deformation under plane strain
compression, particularly at high strain rate, but it was possible to compare
stress-strain curves obtained under plane strain and axisymmetric compression
conditions when the effects of strain and stress distribution and of
temperature localisation were taken into account.