Measurement of foam properties and modeling of layered foam systems
Foam is an important engineering material used in cushions of car seats, pillows, packaging, acoustic absorption and upholstery. It is the primary means used in most modern automobile seats to achieve static comfort and vibration isolation which also happens to be the application area of the research presented in this thesis. It is non-linear and viscoelastic in nature. Its increasing importance as an engineering material has led to a detailed study of its structure and properties. The static and dynamic behavior is sensitive to compression level, strain rate, and the amplitude and frequency of excitation. Previous investigators used hyperelastic models commonly used in study of rubber materials to characterize quasi-static behavior under compressive loads. The memory behavior was also considered resulting in the development of a nonlinear elastic, linear viscoelastic model for predicting the response under uniaxial compression. One of the main objectives of this investigation was to analyze one such continuum model developed by Widdle Jr.  for its structure and the associated system identification procedure for its robustness. The local Poisson’s ratio of foam was also studied to facilitate better understanding of the behavior of open celled polyurethane foam. Another important focus of the work done here was to develop a methodology to predict the characteristic behavior of multi-layered flexible polyurethane foam system, as the seat cushion in most of the automobiles is an array of foam layers. The zero Poisson’s ratio model developed by Widdle Jr. was analyzed for its structure. The inability of this model to predict stress-response to inputs at different strain-rates was investigated. The effect of addition of a strain-rate term to the model and order of the elastic model on the fit to the experimental data was studied. The robustness of the system identification procedure developed then was also analyzed by addition of white noise to the simulated data. The local Poisson’s ratio was measured in different regions by conducting relaxation tests on foam. This information was utilized in the non-zero Poisson’s ratio model developed by Widdle Jr. for further analysis. Compression tests were then performed at different strainrates on foams of different relative densities individually as well as on the layered foam systems and a methodology was developed to predict the stress-response of layered foam systems using stress-responses of the individual foam layers. This approach will also be useful while studying the combined compressive and dynamic behavior of human tissue, muscle and fat as well. The results of this research will help in improving the process of seat design and hence the comfort of the occupant.^
Anil K. Bajaj, Purdue University, Patricia Davies, Purdue University.