INFLUENCE OF STRAIN-RATE ON THE STRESS-STRAIN CURVE IN THE RANGE OF LUDERS STRAIN

The influence of strain rate on the stress-strain curve of materials s howing yield point phenomenon are determined in the range of small str ains and high strain rates by means of the indirect impact tension tes t. With increasing deformation velocity, the Luders strain is found to increase considerably. The stress reduction after reaching the upper yield point, is not abrupt and takes place gradually with increasing s train. These phenomena are investigated on Armco iron, on the tempered steel 42 CrV 6, on pure tantalum and tungsten alloys. A simple mechan ical model is introduced which shows that the Luders front velocity in creases with strain rate approaching asymptotically the value of the p lastic wave velocity as an upper limit. With this restriction, the loc al strain within the Luders band must increase by increasing the rate of elongation of the tensile specimen. The Luders strain could be esti mated quantitatively as a function of strain rate. The model describes also the stress-strain function after reaching the upper yield point considering the limited stiffness of the test arrangement.