Impact of Imperfect Kolsky Bar Experiments Across Different Scales Using Finite Elements

Typical Kolsky bars are 10–20mm in diameter with the lengths of each main bar being on the scale of meters. To push 104 and higher strain rates smaller diameter bars, accompanied by shorter lengths, are needed. As the diameters of the bars decreases the precision in the alignment of the system must increase to maintain the same relative tolerance as the larger experimental systems. Conversely, as the size of the bars decreases so does the magnitude of gravity based frictional forces due to the decreased mass of the system. Finite Element (FE) models are typically generated assuming a perfect experiment with exact alignment and no gravity. Additionally, these simulations tend to take advantage of the radial symmetry of an ideal experiment which removes any potential for modeling non-symmetric effects but has the added benefit of a reduced computational load. In this work, we discuss some of the results of these fast-running symmetry models to establish a baseline and demonstrate the first-order use case of such methods. We then take advantage of high-performance computing techniques to generate several three-dimensional, half symmetry simulations using Abaqus® allowing modeling of gravity and misalignment. The imperfection is initially modeled using the static general process followed by a dynamic explicit simulation in which the impact portion of the test is conducted. This multi-step simulation structure creates a system that can properly investigate the impact of these real-world, non-axis symmetric effects. These simulations fully explore the impacts of these experimental realities and are described in detail to allow other researchers to implement a similar FE modeling structure to aid in their experimentation and diagnostic efforts. Both a 12.7 mm and 3.16 mm diameter bar system are evaluated to quantify the degree that these various experimental imperfections have across two size scales of Kolsky bar systems.