Non-linear dynamic response of the seated person to whole body vibration

Mansfield, Neil James (1998) Non-linear dynamic response of the seated person to whole body vibration. University of Southampton, Doctoral Thesis.

Abstract

Recent studies have established many of the dynamic properties of the seated human at specific vibration magnitudes. Fairley and Griffin (1989) proposed a linear single degree-of-freedom model of the mechanical driving point impedance of the seated body based on measurements of 60 subjects at 1.0 ms^-2 r.m.s. Current international standards (e.g. ISO 5982, 1981; ISO 7962, 1987) also define linear models of the biodynamic response of the seated body. Although previously reported data give some insight into the dynamics of the body, this thesis presents data which show that the body responds as a non-linear system.

Preliminary studies and a series of five main experiments have shown non-linearities in apparent mass, absorbed power and transmissibility for seated human subjects. The first experiment investigated the effect of vibration magnitude on the apparent mass and transmissibility of the seated body. The apparent mass resonance frequency decreased from abut 5.5 Hz to 4.5 Hz when vibration magnitudes were increased from 0.25 to 2.5 ms^-2 r.m.s. Similarly, the resonance frequencies of the seat-to-abdominal wall, seat-to-spine and seat-to-pelvis transmissibilities reduced with increases in vibration magnitude. A second experiment studied the effects of vibration duration and vibration magnitude on the apparent mass of the body. The response of the body was not dependent on vibration duration or the magnitude or previous vibration exposures. Experiment three showed that changing the posture of subjects did not significantly change the resonance frequency or the non-linearity in the apparent mass. The frequency of the peak in the apparent mass reduced with increases in vibration magnitude for all postures. Likewise, the peak in the seat vertical to pelvis pitch transmissibility occurred at a lower frequency for higher vibration magnitudes. A fourth experiment used sinusoidal motion to show non-linearities in the acceleration waveform at the pelvis and in the driving point force response. Distortion in the waveform was shown to be greatest at resonance and to increase with vibration magnitude. The fifth experiment investigated the effect of vibration spectrum on the apparent mass of seated subjects. The apparent mass peak frequency reduced with increased acceleration magnitude, regardless of whether the vibration spectrum changed at low or high frequencies.

Measurements of the power absorbed by the seated body when exposed to vibration showed a peak at 5.9 Hz when measured using vibration at 0.25 ms^-2 r.m.s. The median frequency of the peak reduced with increased vibration magnitude to 4.5 Hz at 2.5 ms^-2 r.m.s.

This record has no associated files available for download.

Contributors

Download statistics