ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

A sensitivity study was carried out by subjecting the stationary head model to a range of
10 impacts, with the initial velocity of the impactor varying incrementally from 0.2 to
3.8 m/s. The analytical model derived in [5] were used to predict the peak impact force
of each case, and the mass of the impactor was changed as needed such that the peak
impact force remained constant across all impact cases. The impacts were performed
using LS-DYNA explicit finite-element code (LSTC Inc.). This resulted in a range of
impacts of differing durations (measuring from 0.34 ms to 3.32 ms) with approximately
equal peak force. The simulations showed good agreement with predictions made using
the analytical model. The results of the simulations were non-dimensionalised. This
reduces the influence of any small variations of peak impact force in the simulations
about the predicted constant value, and also allows any qualitative changes in pressure
response to be analyzed and displayed more clearly. This was done by normalizing
recorded peak intracranial pressures over peak pressures predicted using Equation 1
(above), i.e. the pressures one would expect from a quasi-static response, so resulting in
a measure of the pressure “magnification”. Also impact durations were normalized over
the period of the first equivoluminal mode of vibration.
5. SKULL AND BRAIN: STAGE 2
The second model utilized the same material properties and impactor as the fluid-filled
sphere model, but had modified geometry. The spherical shell was replaced with
realistic skull geometry generated from in vivo MRI scan data of a human male
generated using image-based meshing techniques. As previously, the cranial cavity was
filled with a fluid model representing the brain; the fluid must be enclosed, so a
deformable thin soft membrane was artificially included over the foramen magnum so
that any pressure releasing effects of this feature remain intact. A sensitivity study was
carried out in the same manner as Stage 1: 12 impacts of approximately equal peak
force were simulated with impact durations measuring from 0.17 ms to 3.28 ms. 5
additional high energy impacts were performed, to study the influence of any geometric
non-linearities resulting from significantly increased impact forces.
6. REALISTIC MODEL: STAGE 3
The final model is a complex high bio-fidelity model of the whole head and neck,
derived from the same MRI scan data as used in Stage 2. Semi-automated image based
meshing techniques from the ScanIP commercial software (Simpleware Ltd.) were
employed to segment and mesh the skull, vertebrae, intervertebral discs, cerebrum,
cerebellum, brain stem and spinal cord, cerebrospinal fluid (CSF), scalp and
surrounding flesh. Representative linear material properties were applied to these
structures. Structures deemed of particular importance to the impact response were
assigned more complex material models: nervous tissue was represented by a
viscoelastic model, the CSF was an elastic fluid, and the scalp beneath the impact point
is assigned non-linear elastic properties based on in vitro tests by Gadd et al. [7]
This Stage 3 model was validated against cadaveric experimental data by simulating
impact case 37 from Nahum et al.’s 1977 physical tests, which is used widely in the
literature to verify the accuracy of head models. The results showed good agreement:
the Stage 3 model was able to predict well the intracranial pressures observed by
Nahum et al. in [8].
The size and complexity of this finite-element model requires greater computational
resources per simulation. Instead of a full sensitivity study as previous, 3 impacts