  During severe impact conditions, brain tissue experiences a rapid and complex
deformation, which can be seen as a mixture of compression, tension and shear.
Moreover, diffuse axonal injury (DAI) occurs in animals and humans when both
the strains and strain rates exceed 10% and 10/s, respectively. Knowing the
mechanical properties of brain tissue in shear at these strains and strain
rates is thus of particular importance, as they can be used in finite element
simulations to predict the occurrence of brain injuries under different impact
conditions. In this research, an experimental setup was developed to perform
simple shear tests on porcine brain tissue at strain rates < 120/s. The maximum
measured shear stress at strain rates of 30, 60, 90 and 120/s was 1.15 +/- 0.25
kPa, 1.34 +/- 0.19 kPa, 2.19 +/- 0.225 kPa and 2.52 +/- 0.27 kPa, (mean +/-
SD), respectively, at the maximum amount of shear, K = 1. Good agreement of
experimental, theoretical (Ogden and MooneyRivlin models) and numerical shear
stresses was achieved (p = 0.7866 - 0.9935). Specimen thickness effects (2.0 -
10.0 mm thick specimens) were also analyzed numerically and we found that there
is no significant difference (p = 0.9954) in the shear stress magnitudes,
indicating a homogeneous deformation of the specimens during simple shear
tests. Stress relaxation tests in simple shear were also conducted at different
strain magnitudes (10% - 60% strain) with the average rise time of 14 ms. This
allowed us to estimate elastic and viscoelastic parameters (mu = 4942.0 Pa and
Prony parameters: g1 = 0.520, g2 = 0.3057, tau1 = 0.0264 s, tau2 = 0.011 s)
that can be used in FE software to analyze the hyperviscoelastic behavior of
brain tissue.
