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.