The pulse heated Kolsky bar method, described in detail elsewhere [3], runs DC current through the ends of the bars and sample to rapidly heat metal specimens under controlled conditions. Feedback temperature control is provided by a fastresponse, near-infrared spot pyrometer, which is linked to a rapid current switch made from a bank of solid-state field effect transistors. To facilitate uniform heating and good temperature control, thin conductive foils of graphite are placed on either side of the specimen, as shown in Fig. 35.1. With this method, specimen temperature as uniform as 20 C has been demonstrated from thermal camera measurements, also shown in Fig. 35.1. In this paper, we describe continuing efforts to evaluate the accuracy of stress–strain measurement results obtained with this technique, which involves modeling the mechanical deformation of the graphite foils around the specimen to extract an accurate stress–strain response of the specimen via the strain wave analysis methods fundamental to Kolsky bar metrology [4]. Further, we describe measurement results on 1045 and 1075 steel. The 1045 results demonstrate that the technique is fast enough to interrupt austenite transformation and therefore it can be used to probe non-equilibrium plastic flow stresses in ferritic–pearlitic steels, for example, at high strain rates. 35.2 Results 35.2.1 Graphite Foil Model Evaluation Graphite foil is used in the pulse heated compression Kolsky Bar method to facilitate uniform heating and to prevent arcing that can weld the specimen to the bar. Because the foil thickness (0.13 mm, Fig. 35.1) is appreciable compared to the initial thickness of the specimen (2 mm), we have to take into account the deformation of the foil to accurately determine the straintime history of the specimen using strain wave analysis methods common to Kolsky bar metrology [4]. A one-dimensional mechanical model for the foil, based on separate dynamic compression tests on foil pads themselves, is used to determine the stress-deflection response of the foil during a pulse heated test [3]. The model is used to calculate the foil deflection as a function of the stress transmitted through the specimen and both foil layers. The stress is obtained from the transmitted strain pulse obtained during a pulse-heated Kolsky bar test. Subtracting the foil deformation from the total deformation between the bars using the 1-wave method [4] yields the deformation of the specimen: Fig. 35.1 Left: Graphite foil placement in pulse-heated Kolsky bar tests. Top right: High speed visible light camera images during heat-up and hold periods. Bottom right: Infrared thermal camera measurement of specimen temperature uniformity during temperature hold period 236 S. Mates et al.
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