Crash Testing for Dummies: Bark Mulch vs. Concrete
Thea Rogak, Jaime Yee
Sir Winston Churchill
Floor Location : M 225 D

Our goal was to design and build a helmet drop-testing apparatus in order to measure and compare accelerations of drops onto bark mulch and concrete. To achieve this, we bolted a mannequin head, filled with plaster so it was the correct weight, onto a long wooden beam. This "arm" was hinged to a base, so that it could be raised up then released, dropping the helmet and head in an ark onto our test surface. We measured acceleration in G force using an accelerometer attached securely to the head. The values were transfered to a computer where they could be graphed using Excel via an Arduino data acquisition board.
The main difference we observed between the two surfaces was the contrast between the "rebound" accelerations, or bounces, following the helmets initial impact onto the ground.
The graphs of the drops onto concrete showed very distinct, abrupt second spikes a few hundred microseconds after the first acceleration, while acceleration of drops onto bark mulch consistently returned to a steady 0G after only a small second acceleration. The shape of this, when graphed, which was that of a gradually sloped "valley" suggested the slow compression of the bark mulch as it absorbed shock, then its return to its original shape.
This helps us to better understand why a fall onto concrete would more likely result in a concussion than a fall onto bark mulch. Not only would the acceleration of the initial impact be greater, but the many subsequent spikes of acceleration would shake the brain around more within the skull. A fall onto bark mulch would softly return the head and brain to a neutral position and minimize dammage.
Further experiments would include drops onto other surfaces such as ice, grass, Astro-turf, etc. In addition to testing different surfaces, the accelerometer technoloy could be further refined to better study the magnitude of the inital impact.