Skulls flex, damage brain, under battlefield explosions

When a person’s head strikes, or is struck by, another object, it accelerates. As it begins to decelerate, the brain slams into the skull, then bounces off and oscillates until the impact energy dissipates. The resulting shear and compressive strains can lead to brain damage. But in battlefield explosions, the acoustic waves alone can cause soldiers traumatic brain injuries. To better understand that process, Lawrence Livermore National Laboratory’s William Moss and Michael King and the University of Rochester’s Eric Blackman compared numerical simulations of a head colliding with a wall to one being struck by an explosion’s blast waves. Despite accelerating the head at less than half the rate of the wall collision, the simulated blast produced on the brain surprisingly comparable pressure spikes—ranging up to 3 bars—and even larger pressure gradients. Apparently, those mechanical loads are delivered by the skull, which ripples under the pressure of blast waves—the rippling motion is indicated in the image by velocity vectors. The researchers confirmed the role of the skull’s elasticity by making it 1000 times stiffer in their simulations and observing a fivefold drop in the pressure spikes. The simulations also revealed that helmets in contact with the head can impart an additional mechanical load to the skull and helmets that allow for an air cushion geometrically focus and increase the magnitude of blast waves. (W. C. Moss et al. , Phys. Rev. Lett. , in press.)