Dynamic Response of Wheelchair Cushions to the ISO Impact Damping Test
Chung, B. M.
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The International Organization for Standardization defines tests that characterize the properties of wheelchair cushions (ISO 16840-2). The impact damping test (IDT) characterizes cushions' abilities to reduce impact loading on tissues and to help maintain postural stability. The ISO test reports the number of rebounds greater than 10% of the peak impact acceleration and the ratio of the second to first rebound acceleration in order to determine the damping properties. We performed IDTs on three different wheelchair cushions: 3" elastic foam (EF), 3" viscoelastic foam (VEF), and a 3" laminar cushion (LC); a viscous fluid bladder on the top of an elastic foam. The LC had two rebounds greater than 10 % of the peak acceleration while the EF and VEF had one. The LC had high-est ratio (0.48) followed by the EF (0.29) and the VEF (0.25). According to ISO, these results indicate that EF and VEF dampen impacts quicker than LC and have superior abilities to help maintain postural stability. Based upon this analysis, three critical issues about the ISO IDT have been identified. The first is the ISO should incorporate impact magnitude in the analysis, since impact loading on tissues is a stated purpose. The second is the ISO should use impact instead of re-bound accelerations. Results show that nonlinear responses are evident in rebounds in all three cushions. The third issue is that oscillation from impact is not a simple second order damped harmonic. Analysis showed that three to five natural frequencies are embedded in the damped harmonic oscillation, so simple ratios of accelerations do not represent the damping properties of cushions. In conclusion, the ISO IDT should utilize impact accelerations and employ more complex analysis to better characterize the damping properties of wheelchair cushions.
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