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BioMechanics and Materials Laboratory


Blast Wave Interaction With Helmet and Head

Overview

 The role of protective gear such as helmet is often been questioned in mitigiating asymmetric combat scenarios like IED attacks. Though helmets provide protection against ballistic and impact loading, it is unlikely that they provide protection against blast. The main goal of this project is to understand role of helmet/s under blast loading conditions.

Main Studies

  • Evaluate the helmet's role under various configurations and orientations of the head and helmet.
  • Analyze the pressure amplification under the helmet, an effect called the underwash that is currently studied as a function of the size of the gap between the head and the helmet.
  • Using computational studies of the human head developed from MRI datasets with and without helmet, study the stress and strain fields inside the brain in relation to orientations of the helmet and the head.
  • Measure the response of head-helmet system at various blast intensities.
  • Design, develop, and validate effective blast mitigation strategies.

Computational Modeling of Shock Mechanics

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The figure below shows FE discretization of the computational model used to study the head and helmet response under blast loadings. The head and the helmet are modeled with Lagrangian elements and the surrounding fluid medium in which shock wave propagates is modeled with Eulerian elements. The head model is generated from the segmentation of high resolution MRI data ( 192 mm x 256 mm x 256 mm ) obtained from the Visible Human Project. The head is segmented into the skin, skull, csf and the brain (both grey and white matter). The model of the helmet is generated by digitizing Advanced Combat Helmet (ACH). These geometric models are discretized in the HyperMesh. This head, helmet, upper body assembly is immersed into Eulerian domain which essentially models the surrounding atmosphere in which shock wave propagates.

Experimental Modeling of Shock Experiments

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Numerical modeling allows us to predict the stresses experienced by the brain. Experiments were carried out with the dummy head, both with and without helmet, and kept 22.2 cm outside the 9" inch shock tube as shown in figure. The simulations were carried out to replicate these experiments. Pressure history in the test section from the experiments was input into the simulations. Simulation setup and FE discretization is shown in the figure.

Underwash Effect Between Head and Helmet

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The figure shows the blast front after encountering the head-helmet assembly. It is divided into two fronts: One front traveling around the outer perimeter of the helmet; another front penetrating the gap between the head and the helmet and traveling underneath the helmet towards the back of the head as shown in (a). The shock front traveling outside the helmet reaches the rear of the helmet before the shock front traversing through the gap (b-i), and when these two blast fronts meet they focus on a region on the backside of the head (b-ii). This process has been termed the underwash effect of the helmet. This underwash produces the higher peak pressures on the head, away from the direction of the incident wave when the location is shielded by the helmet. After this high pressure is generated, the high pressure air in the head-helmet subspace expands in all the directions (b-iii).

Theory and Experiments

The figure shows the pressure experienced by the forehead sensor at different blast intensities. The simulations are in good agreement with the experiments.

Flow Fields Outside the Shock Tube

The figure shows the flow fields outside the shock tube. The flow at the exit of the shock tube no longer remains one dimensional due to the expansion of the shock waves. The flow evolves three dimensionally into various waves and vortex rings as shown in figure.