A Pulse-Heated Kolsky Bar Technique for Measuring the Flow Stress of Metals at High Loading and Heating Rates

Abstract

The National Institute of Standards and Technology (NIST) has developed an electrical pulse-heated Kolsky Bar technique for measuring the constitutive response of metals at heating rates of up to 6,000 K/s and strain rates up to 10 4 s −1 . Under these conditions, which are approaching those found in high speed machining, thermally activated microstructural processes such as grain growth, solid state phase transformation and dislocation annealing can be bypassed, leading to unique non-equilibrium superheated microstructural states. Flow stresses can thus differ significantly from equilibrium high temperature conditions. This paper describes the NIST pulse-heated Kolsky bar technique in detail, including a thorough assessment of uncertainties in temperature and flow stress measurement.

High-rate Characterization of 304L Stainless Steel at Elevated Temperatures for Recrystallization Investigation

“…In order to heat the specimen, one may use a furnace [10,11], an induction-coil heater [12], a radiant heating system [13], an infra-red spot heater [14], or an electrical pulse heating system [15]. If the bars are heated together with the specimen, the mechanical properties of the heated portions of the bars will be changed.…”

“…A shorter CCT is of course desired for elevated-temperature SHPB experiments. Frantz et al [11] developed an electric screw driven system to bring the bars into contact with the specimen pre-heated in a furnace. Their study also indicates less than 5°C decrease in the specimen temperature for CCT up to 400 ms. Lennon and Ramesh [14] used an infrared spot heater to heat the specimen individually.…”

“…Up to date, investigations of dynamic recrystallization have been limited to low and intermediate strain rates. High-rate mechanical characterization of alloys at elevated temperatures was performed in the past [5][6][7][8][9][10][11]. In this work, we have integrated several experimental techniques in one dynamic experiment to investigate recrystallization under high rate and elevated temperature conditions for the first time.…”

“…The SHPB has been explored to dynamically characterize materials including alloys at elevated temperatures [5][6][7][8][9][10][11]. Different researchers have developed various elevated-temperature SHPB techniques to obtain high-rate stress-strain behavior at elevated temperatures.…”

Temperature-Dependent Mechanical Response of an UFG Aluminum Alloy at High Rates

“…Two complications arise when conducting high temperature experiments at high strain rates: implementing proper heating techniques and reducing the thermal effects on wave propagation. Dynamic testing techniques at high temperatures have been refined over the last 15 years, predominantly using modified Kolsky bar systems [14][15][16][17][18][19].…”