Session: 03-01-01: Mechanics of Penetration, Shockwaves, and High-Strain-Rate Events: Modeling and Experiments

Paper Number: 96816

96816 - Impact of Imperfect Kolsky Bar Experiments Across Different Scales Using Finite Elements 

As the need for high-rate material properties has grown in recent decades, so too has the use of the Split Hopkinson Pressure Bar (SHPB) or Kolsky Bar experimental system.  Typical Kolsky bars probe material properties in the 10² to 10³ strain rate regimes, using bars with diameters of 12.7 mm and up with the lengths of each main bar being on the scale of meters.  To push 10⁴ and higher strain rates smaller diameter bars, usually accompanied by shorter lengths, are needed to drive samples to such high rates while maintaining a valid experiment.  These smaller experimental systems are prone to a higher level of sensitivity to mechanical tolerances due to their size.  As the diameters of the bars decreases, the precision in the alignment of the system must increase in order to maintain the same relative tolerance as the larger experimental systems.  Conversely, as the size of the bars decreases so does the impact of gravity based frictional effects due to the decreased mass of the system. Typically, Finite Element (FE) models are generated assuming a perfect experiment.  For example, alignment of the entire experiments is assumed to be exact and gravitational effects which result in friction between the bars and supports are negated.  Additionally, these simulations tend to take advantage of the radial symmetry of an ideal experiment which totally 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 full three-dimensional simulations using Abaqus® which allows us to incorporate real world experimental imperfections.  The imperfection is initially modeled using the static general process to solve for the initial deformed configuration caused by the imperfection.  These results are then used as the initial conditions for 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 imperfections and effects.  These simulations fully explore the impacts of these experimental realities and are described in great 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 3.18 mm diameter bar system are evaluated to quantify the degree that these various experimental imperfections have across two size scales of Kolsky bar systems. 

Presenting Author: Thomas Hannah The Pennsylvania State University

Presenting Author Biography: Thomas Hannah holds a BS and MS degree in mechanical engineer from Penn State where he is currently pursuing his PhD. Thomas specializes in solid mechanics and rate dependent material testing, and is an expert in small scale or miniature Kolsky Bars. Thomas' work is sponsored by Los Alamos National Lab with which Thomas has worked closely to design and implement several high speed impact tests to characterize metals and polymer based composites. Previously, Thomas worked as an engineering program manager for the United State Navy overseeing flight, guidance, fire control, and navigation sub-systems for advanced submarine launched missile systems.

Authors:

Thomas Hannah The Pennsylvania State University
Reuben Kraft The Pennsylvania State University
Valerie Martin The Pennsylvania State University
Steve Ellis Los Alamos National Laboratory