Determination of Simple Shear Material Properties of the Brain at High Strain Rates
Split Hopkinson Pressure Bars are used to study dynamic material response in uniaxial compression, tension or torsion. Modifications of traditional test techniques have permitted the successful measurement of highly elastic soft biological tissue properti
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Determination of Simple Shear Material Properties of the Brain at High Strain Rates Kyle A. Ott, R.S. Armiger, A.C. Wickwire, A.S. Iwaskiw, and Andrew C. Merkle
Abstract Split Hopkinson Pressure Bars are used to study dynamic material response in uniaxial compression, tension or torsion. Modifications of traditional test techniques have permitted the successful measurement of highly elastic soft biological tissue properties. Testing biological materials, specifically brain material, using the modified Split Hopkinson Pressure Bar method is effective in characterizing the tissue response at strain levels and strain rates that are greater than traditional tissue testing modalities. The high strain rate material properties are of particular interest for use in constitutive models that govern material response and injury predictions for computational models of the human body exposed to dynamic impact events. Since the use of viable brain material is critical, and the viability largely depends on post mortem interval, harvesting fresh, non-frozen brain tissue is critical. In this paper, we present the shear response of fresh human brain tissue under high rate loading. The investigated strain rates ranged from 25 to 248 strain/s. Specimen were prepared from various orientations and locations within the brain across two specimen. The average elastic modulus was 13.0 10 kPa computed across all trials. The response was also characterized based on specimen, loading rate, and strain level to analyze contributing factors. This information is critical for targeting surrogate material properties and generating parameters for computational model constitutive equations. Keywords Brain • High rate • Shear • Modulus • Material properties • Tissue characterization • Split hopkinson pressure bar • Computational modeling • Finite element
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Introduction
Accurately characterizing the mechanical response of biological tissues is required to understand the response and injury potential that results from high-rate loading events. These types of events range from automotive crashes to ballistic impacts and blast overpressure loading. It has been shown that biological materials respond differently at these high loading rates. Therefore, accurate modeling of these events through computational or physical modeling necessitates the understanding of how the response of these materials change under these unique loading conditions. For physical surrogates, knowledge of biological material response at high loading rates allows the development and verification of more accurate simulant materials. For computational models, material response data allows for the development of constitutive equations that model the response of these biological materials. Specifically, many of the commonly utilized viscoelastic material models require an understanding of the bulk and shear material response in both the quasi-static and high strain rate loading regime. The requirement of understanding how a particular material responds at both low and high strain
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