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 pressure bar (or Kolsky bar) is being installed in a NIST high-current pulse-heating facility. By heating the material sample rapidly with a controlled current pulse immediately before the mechanical impact in the bar, structural changes in the sample are inhibited, thus better simulating conditions during machining. A stress-strain relationship can be determined at various temperatures for test materials. We describe the design and the development of a millisecond-resolution split-Hopkinson apparatus, where the sample is resistively heated by the passage of a sub-second-duration electric current pulse. The impact bar is constructed out of maraging steel and the sample is a cylinder of AISI 1045 steel. The current is transmitted through the oiled-bronze sleeve bushing of the impact bar. The temperature measurements are performed using a near-infrared micro-pyrometer (NIMPY). The NIMPY consists of a refractive 5x microscope objective with a numerical aperture of 0.14 attached to a traditional microscope body. The thermal measurement is performed with an InGaAs detector with ～1 μs response time. The procedure used to calibrate the pyrometer with a variable temperature blackbody is described. A brief description of a model of the pulse heating process is given and the predicted sample temperature history is compared with measured temperature data.
展开▼
