AbstractThe biomechanical responses (i.e. apparent mass and transmissibility) to whole-body vibration have been found to be nonlinear - resonance frequency decreases with increasing vibration magnitude. The main mechanism causing the nonlinearity has been recently found to be caused by the soft tissue at the excitation-subject interface (i.e. buttock). Despite the experimental evidence of the cause, mathematical formulation of the dynamic forces at the interface and the prediction of the body movement at different magnitudes of excitation remain absent. The principal objective of the research reported in this thesis was to examine the dynamic behaviour of excitation-subject interface of the whole-body vibration using a scaled rigid mass-soft tissue system. The research was also designed to investigate the single degree of freedom (SDOF) linear and nonlinear models in time domain to predict the responses of the whole- body vibration at different magnitudes.
The preliminary study looked into the time domain modelling of whole-body vibration (WBV) responses using four different SDOF linear and nonlinear viscoelastic models found that all four models failed to predict the responses at different magnitudes.
In the first experiment, the dynamic characterisation of the silicone rubber was accomplished by means of uni-axial cyclic compression test. The experiment was conducted using different excitation frequencies (i.e. 2, 5, 10, 15, 20 and 40 Hz) and magnitudes (i.e. 25, 50, 75 and 100 N). The effect of the excitation frequency and the magnitude on the mechanical properties was found to be significant. The stiffness and elastic modulus was observed to be increased with increasing excitation frequency and the magnitude. The thixotropic or memory dependent behaviour observed in WBV is missing in this experiment with silicone rubber.
In the second experiment, scaled rigid mass-silicone rubber system was studied using base excitation with broadband (2 to 80 Hz) random vibration at four different magnitudes (i.e. 0.5, 1.0, 1.5 and 2.0 ms-2 r.m.s.). The silicone rubber specimens with three different thickness (i.e. 10, 15 and 20 mm) and three different diameters (i.e. 50, 75 and 100 mm) were tested with three different sprung masses (i.e. 1.5 kg, 2.5 kg and 5.0 kg) and two different sprung mass contact contour (i.e. flat and hip-borne). A dominant single resonance was observed for the rigid mass-silicone rubber system in the present study and also in most whole-body vibration studies with vertical vibration. Although the frequency range at which the resonance occurred were different: 20 to 33 Hz for current study and below 10 Hz for WBV studies, the mode of sprung mass might be similar in both scenarios. The effect of thickness, diameter, sprung mass and sprung mass contact contour on resonance frequency and stiffness was found to be significant. However the magnitude dependency was observed to be absent in the present study.
In the third experiment, Impulse responses of the scaled rigid mass-soft tissue system were studied for the first time using silicone rubber and the porcine muscle. A linear SDOF viscoelastic model was utilised to extract stiffness and damping from the measured accelerance and receptance frequency response functions. With a porcine muscle exposed to vertical impact, a repeatable bimodal resonance was observed at 25 Hz and 40 Hz and the similar size silicone rubber specimen showed a single resonance at 20 Hz for 20 mm, 22 Hz for 15 mm and 26Hz for 20 mm thick specimens - a higher frequency range than those observed in vertical WBV. A repeatable principal resonance is observed for the horizontal impact tests at around 3 Hz in both silicone rubber and porcine muscle specimens – similar to that observed in horizontal WBV. The effect of thickness, diameter, sprung mass and sprung mass contact contour on resonance frequency and stiffness of the silicon specimens was found to be significant in both vertical and horizontal impacts. However the porcine specimen thickness has no clear effect on the parameters extracted.
It is concluded that the thixotropic or memory effect of the human body is missing in the SDOF rigid mass-soft tissue system studied in this thesis.
|Date of Award||Sep 2016|
|Supervisor||Ya Huang (Supervisor) & Jie Tong (Supervisor)|