Water-filled containers are used as building blocks in a new generation of containment systems for testing high-speed flywheels. Such containment systems are needed to ensure safety by trapping high-speed debris in the event of centrifugal breakup or bearing failure. Traditional containment systems for testing flywheels consist mainly of thick steel rings. While steel rings are effective for protecting against fragments from conventional and relatively simple metal flywheels, they are also expensive. Moreover, it is difficult and expensive to configure steel-ring containment systems for testing of advanced flywheel systems that can include flywheels made of composite materials, counter-rotating flywheels, and/or multiple flywheels rotating about different axes. In contrast, one can quickly, easily, and inexpensively stack water-filled containers like bricks to build walls, (and, if needed, floors, and ceilings) of sufficient thickness to trap debris traveling in any debris traveling in any possible direction at the maximum possible kinetic energy that could be encountered in testing a given flywheel system.
Water is remarkably effective in decelerating high-speed debris: In an analysis performed in 1998, it was found that for a fragment that has a characteristic dimension L and an initial kinetic energy E0, the kinetic energy E after traveling a distance d through water is approximated by E = E0e–d/L. For typical fragment sizes and speeds expected to be encountered in tests of advanced flywheel systems, this equation leads to the expectation that a wall of water only 0.6 m thick would suffice to dissipate practically all the kinetic energy.
The effectiveness of this approach to shielding against high-speed debris was demonstrated in a series of tests, including one in which a bullet was fired into a stack of two cubic cardboard boxes, 0.28 m on each side, containing water-filled bladders see figure). The bullet had a mass of 9.7 g and an initial speed of 790 m/s. Upon impact, the first container was split, the water in the container was widely dispersed, and the bullet was deformed. The bullet did not reach the second container. In other words, the bullet was stopped by less than 0.28 m of water. Limited composite fragment testing was also performed in the Ballistics Impact Laboratory at Glenn Research Center, which demonstrated the ability to stop a 2.5 × 2.5 × 1.0 cm fragment with a velocity of approximately 730 m/s within 30.5 cm of water.
This work was done by Larry Trase and Dennis Thompson of Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Mechanics category.
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