Helmet-Mounted Displays: - USAARL - The - U.S. Army

Biodynamics 199
been fabricated into helmet shells. These constructions often perform well
in impact attenuation, but the fabrication cost and relative low production
volume are detrimental to successful implementation.
Helmet design for impact attenuation is based on the laws of physics.
To bring a test head form, traveling at an initial velocity of 20 feet per
second (6.1 meters per sec), to a stop requires a deceleration. If the
deceleration magnitude is not to exceed 175G, sufficient stopping distance
must be provided. This required stopping distance is dependent on the
acceleration’s pulse shape which results from the impact. Three basic pulse
shapes are the square, triangular, and half sine pulses. The triangular pulse
shape can vary with location of the peak value, with the two extremes
having the peak located at the very beginning, a zero rise time, or at the
very end, a zero offset time, of the acceleration pulse. These pulse shapes
are illustrated in Figure 7.7. As a comparison, the acceleration time history
trace of an Army aviation helmet impact result is provided in Figure 7.8.
Calculation of the required stopping distance for these pulse shapes are
based on the following equations (Zimmermann, et al., 1989):
2 S = Vo / 2gG (square pulse) Equation 7.4
2 S = (0.7854)(Vo )/ gG (half sine pulse) Equation 7.5
2 S = Vo / gG (triangular pulse, symmetrical ) Equation 7.6
2 S = 2Vo / (96.6)gG (triangular pulse, zero rise time) Equation 7.7
2 S = 4Vo / (96.6)gG (triangular pulse, zero offset time) Equation 7.8
where S is the minimum required stopping distance, V o is the initial
velocity, g is the gravitation acceleration, and G is the “not to exceed”
number of multiples of gravity. For these equations, the required
theoretical stopping distances are plotted in Figure 7.9 for various G levels.
The theoretical stopping distance can be used to help determine the
required energy liner thickness used in helmet construction. Additional
factors, such as energy liner material efficiency, contact area, and impact
surface shapes, must be considered. Material efficiency represents the
percentage of useful crush distance available for a given thickness. During
the crushing process, the material compacts and occupies a percentage of
the total thickness. The space required for the compacted material must be
considered during the thickness determinations. If not, then a “bottoming