Determination of temperature rise during high strain rate deformation

The energy converted to heat during high strain rate plastic deformation is measured directly using an infra-red method for Ta-2.5% W alloy and, indirectly, using UCSD's recovery Hopkinson bar technique for the same alloy, as well as for commercially pure Ti, 1018 steel, 6061 Al and OFHC Cu. The infra-red measurement yields a 70% conversion of work to heat for Ta-2.5% W and generally underestimates this factor for all tested materials. The final temperature at a given strain can be determined indirectly, based on the calculated plastic work. For this, three separate measurements are made: First, a sample is deformed at a high strain rate to a total strain of, say, 60%; this is essentially an adiabatic test. Then a second sample is deformed at the same strain rate to about 30% strain; this should reproduce the first half of the previous adiabatic stress-strain curve and in our test it does. This sample is then allowed to cool down to the initial room temperature. This sample may then be heated to the temperature as was measured by the infra-red detectors and then deformed at the same strain rate to check if the adiabatic curve is traced. It is observed that only when the sample temperature is increased based on 100% conversion of the plastic work to heat, that the original adiabatic stress-strain curve is traced. It is thus concluded that the infra-red detection system records a lower (surface) temperature than the actual temperature of the sample. (C) 1998 Elsevier Science Ltd.