Kolsky Bar Facility

Experimental Capabilities

The NIST Kolsky Bar lab has both compressive and direct tensile Kolsky Bars (Split Hopkinson Pressure Bars) with electrical pulse-heating capability to a maximum temperature of 1200 °C and controlled heating rates above 1000 °C/s. Instrumentation includes a 5 MHz data acquisition system with 8 differential channels with 24 bits per channel, two near-infrared spot pyrometers, a mid-wave infrared thermal camera with a maximum capture rate of 870 Hz with 160 by 128 pixel resolution, and a three-dimensional digital image correlation system with maximum deformation (strain) measurement rate of 180 kHz (128 by 128 pixel resolution) and maximum (rigid body) displacement measurement rate of 360 kHz (16 by 64 pixel resolution). Experiments are modeled using the explicit finite element technique, including coupled electric-thermal-mechanical solutions for pulse-heated experiments, to assist in uncertainty estimation and enhance data value for stakeholders.

Significance

The mechanical behavior of most materials varies with applied strain rate and temperature. Dynamic experiments are needed to characterize this behavior so that accurate models can be developed to both interpret and design complex engineering systems and processes to improve performance and decrease costs for products throughout the US economy.

The NIST Kolsky bar facility was developed with a unique electrical pulse-heating capability to explore the behavior of metals under rapid heating and loading combined. Rapid heating to temperatures that exceed the thermal stability of a metal will result in non-equilibrium plastic behavior since the microstructure through which dislocations flow during plasticity will begin to evolve in a time-dependent way. Thermal stability can be defined as the lowest temperature where microstructural changes can be anticipated, such as grain or precipitate growth, annealing, or wholesale phase transformation. This temperature threshold is usually about half the melting temperature, but for many engineering alloys the stability threshold is substantially lower. Many important industrial processes, such as machining or friction stir welding, easily achieve these temperatures through adiabatic and frictional heating in the workpiece material, in a fraction of a second. This is also true in ballistic impacts on ductile metal armor. In order to model material behavior in these processes, the thermal softening behavior of the material must be specified. However, the few relevant data in the literature that can be used to specify thermal softening in constitutive models are unable to match the heating conditions in most dynamic processes, and thus their value is in question for situations where temperatures exceed the material stability threshold.

The NIST Kolsky bar was designed to probe dynamic metal plasticity by subjecting the test material to a rapid temperature excursion followed immediately by a high strain rate mechanical test. Heating rates exceeding 1000 °C/s are produced using direct electric current heating of the sample in a controlled manner (figure below shows an animation of electrical heating followed by impact of a specimen in the NIST Kolsky bar). With this technique, the thermal history of the specimen can be varied to inform relationships between time, temperature, microstructure and plastic behavior that are needed to develop more accurate material models for machining, friction stir welding, and ballistic impact.