Temperature control of pulse heated specimens in a Kolsky bar apparatus using microsecond time-resolved pyrometry

Analysis of machining processes is important in the understanding and improving of manufacturing methods. The modeling of machining processes relies on high-strain-rate, high-temperature material properties. A Split-Hopkinson ( or Kolsky) bar has been developed at NIST for this purpose. By heating the material specimen rapidly with a controlled current pulse prior to the mechanical impact of the bar, structural changes in the specimen are inhibited, thus better simulating conditions during machining. A stress-strain relationship can be determined at various temperatures for a range of materials. For the elevated temperature Kolsky experiments it is essential for the specimen to be maintained at a constant and uniform temperature prior to dynamic loading. The development and implementation of a near-infrared micro-pyrometer (NIMPY) for the precision control of the Kolsky specimen temperature by using a pulse heating system preceding the mechanical impact are described. The pulse-heating system can be operated either in the transient mode, where the current to the specimen is switched off at a preset temperature or time; or in the brief steady-state mode, where the specimen is heated rapidly to achieve the preset temperature ( in the range from 400 to 1300 K) in a short time ( about 200 ms) and then held isothermally for a brief period (< 2 s). A brief description of a model of the pulse heating process is provided, and the predicted specimen temperature history is compared with measured temperature data.