High-Speed Tension Tests at Elevated Temperatures—Parts II and III

Abstract

Abstract The authors discuss the results of an investigation on the resistance to plastic forming of several metals over a wide range of rates of deformation at various temperatures. To carry out the tests, two machines were built: One, a high-speed testing machine, was described in Part I of this report; the other, a constant-strain-rate machine, is dealt with in this presentation. The general trend of the test results for aluminum and copper indicates a continuous increase of the yield stresses with the strain rate. The speed relation for pure iron and the iron alloys seems to be much more complicated. The resistance to deformation at a given speed of straining exhibits a minimum and a maximum at certain temperatures. This maximum, known as “blue brittleness,” shifts to higher temperatures with increasing speeds of straining; it appears at 200 C for short-time tension speeds and shifts to 550 C for the high speeds. In the high-speed tests a local temperature rise of 50 C, due to the conversion of the work of deformation into heat, was observed in a specimen of pure iron. At very rapid rates of deformation remarkably high ultimate stresses were found for aluminum and copper when tested at temperatures approaching their melting points. A theory for the necking of a bar, based on the speed law, predicts the observed shapes of broken bars which were drawn down to a point. Observation data are furnished for an evaluation of the forces required for very rapid plastic forming of the metals at high temperatures, particularly through rolling.