Modeling And Analysis On The Vertical Dynamics Of An Occupant And Seat Cushion System

Vehicle ride comfort has gradually become one of the key indicators of theautomotive design. The objective evaluation on the ride comfort is mainly based on thefield test with real human beings. Numerical prediction for objective index to assess theride comfort is an important research trend. Modeling on vehicle occupant and the seatcushion and developing methods for objective assessment on the ride comfort canaccelerate and economize the design process of vehicle ride comfort. The verticalvibration response of a seated occupant and the pressure distribution on the interfacebetween the occupant and the seat are two key factors in the objective evaluation on theride comfort. A fairly reasonable biomechanical model on the vertical vibration of aseated human body is necessary for prediction of the occupant’s vertical vibrationresponse. Polyurethane foams have been extensively used as vehicle seat cushionnowadays. An accurate description on both static and dynamic properties of the foammaterials is thus also important to calculate the ride comfort indices.Vertical vibration tests were conducted in this dissertation on28Chinesevolunteers subjected to excitations of various levels. Vertical apparent mass data ofthese seated human bodies were obtained in a frequency band from1.0to20.0Hz. Twovertical vibration models commonly adopted internationally were employed tocharacterize the vibration properties of the seated volunteers. Parameter identificationwas implemented by simultaneously using both amplitude and phase data of themeasured apparent mass. By taking average of and normalizing the apparent mass datafrom experiments, model parameters and the corresponding modal parameters wereobtained for the seated Chinese people at ages of20–25with a standard weight. Theapparent masses predicted by the biodynamic models with identified parameters agreevery well with those obtained from experiments. Statistical analysis demonstrates theinfluence of the volunteer’s height and weight on the model parameters for a seatedhuman body. The mean value and the standard deviation of the model parameters arealso derived.Uniaxial compression tests with constant loading and unloading speeds wereconducted at various strain rates on the foam samples, from which four sets ofstress-strain data were obtained with reliable repeatability. Based on a hereditary integral model consisting of a strain polynomial function and a convolution integral, amodified model was then developed to uniformly characterize both the elastic andviscoelastic properties of the foam material at various strain rates. The model wasvalidated by experimental results.The rate-dependent constitutive model was then extended into a three dimensionalone, and a user material model was developed in the commercial software ABAQUS.The user subroutine was verified by a simulation on the compression of a foam bodywith only one element. The validated material model was then used to perform an finiteelement analysis for pressure distribution on the interface between the foam cushion anda standard fake stern. The calculated results were then compared with experimental onesabout the pressure distributionsThe storage and loss modulus in the frequency range between0.5and10.0Hzwere measured by dynamic uniaxial compression tests on the polyurethane foamsamples. A fractional differential equation was employed to describe the dynamicbehavior of the foam materials. Model parameters were identified according to thecomplex modulus data. Combined with the vertical vibration model of a seated humanbody, the fractional derivative model was adopted to predict the vertical apparent massof the seated human body on a cushion under vertical vibration. Applicability of themodel is demonstrated by comparing the predicted apparent masses with experimentallyobtained ones.