Chapter 35 Dynamic Flow Stress Measurements for Machining Applications Steven Mates, Eran Vax, Richard Rhorer, Michael Kennedy, Eric Whitenton, Stephen Banovic, and Tim Burns Abstract Metals undergo a combination of rapid loading and rapid heating during normal and high speed machining processes. Constitutive models for these materials, however, generally lack any information regarding kinetics of thermallyinduced transformations, such as austenite formation in carbon steels, that can have profound effects on their mechanical viscoplastic behavior. The NIST electrically-pulse-heated Kolsky bar was developed specifically to probe material response under conditions approaching those present during machining operations. We have achieved heating rates in excess of 1,000 C/s combined with strain rates above 1,000 s 1 with this system. This paper presents recent experimental results for AISI 1045 and AISI 1075 steel using the pulse-heated Kolsky bar, and examines some aspects of the uncertainty of the method. Keywords Kolsky Bar • Machining • Steel • High strain rate • High heating rate 35.1 Introduction Machining models remain unable to predict temperatures or cutting forces very accurately, which prevents their use for optimizing machining processes to extend tool life, maximize material removal rate and boost overall efficiency [1, 2]. Virtually all viscoplastic constitutive models are inappropriate for machining conditions, which combine high strain rates with high temperatures and high heating rates. In response to this need, the National Institute of Standards and Technology (NIST) developed a specialized pulse-heated Kolsky bar that combines traditional Kolsky bar mechanical methods with a rapid heating capability using direct current (DC) electrical heating of the specimen as it sits sandwiched between the bars. Heating rates above 1,000 C/s have been achieved with this technique. Fast heating rates can have a number of different effects on the viscoplastic response of metals. Of primary importance to the present work is the effect of heating rate on phase transformations and the resulting effect they may have on plastic flow stress. When machining steel, for example, shear zone temperatures may exceed the austenite formation temperature [1, 2], which renders the workpiece material thermodynamically unstable as the chip forms. Because heating rates are fast, however, there is some question as to whether or not this transformation has time to occur. If it does, the proper flow stress for a machining model is thermodynamically stable austenite at the given shear zone temperature. If not, the model must use the strength of the thermodynamically unstable, pre-transformed steel to get a correct answer, or better yet a combination of the two that accounts for the transformation kinetics. The NIST pulse-heated Kolsky bar method was developed to study this question and provide data under conditions closer to actual cutting processes than are possible with conventionally heated Kolsky bar techniques. S.Mates (*) • M. Kennedy • E. Whitenton • S. Banovic • T. Burns National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD, USA e-mail: smates@nist.gov E. Vax Nuclear Research Center Negev, Negev Desert, Israel R. Rhorer Rhorer Precision Engineering LLC, 118 Summit Hall Rd., Gaithersburg, MD, USA B. Song et al. (eds.), Dynamic Behavior of Materials, Volume 1: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06995-1_35, #The Society for Experimental Mechanics, Inc. 2015 235
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