Analysis of maching 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 in 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 the dynamic loading. We describe the development and implementation of a near-infrared micro-pyrometer (NIMPY) to the precision control of Kolsky specimen temperature preceding the mechanical impact. The pulse-heating system can be operated either in the transient mode, where the current to the Kolsky 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 desired temperature (in the range from 400 K to 1300 K) in a short time (about 200 ms) and then held isothermally for a brief period (<2 s). The sensing signal for the feedback is provided by the NIMPY. Based on a feedback control algorithm, a dedicated computer operates a solid-state switch, consisting of field-effect-transistors (FETs), with a fast response time (<5 ms), which controls the current to the Kolsky specimen to achieve isothermal condition. 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.
Citation: International Journal of Thermophysics
Pub Type: Journals
Kolsky bar apparatus, PID control, pulse heating, pyrometry, temperature control