Open Access Dissertation
Date of Award
Typical polymers have a time-dependent response to loading which results in stress relaxation or creep. Models using springs/dashpots or Volterra integrals are capable of predicting the material response, but place little or no emphasis on the reasoning behind the response. This research proposes a microscopic reasoning behind polymer chain movement, while developing a model to predict the creep and stress relaxation of a polymer foam. Based on the theorized slip/stick of polymer chains as they slide past each other, this model successfully predicts the behavior of a PMI polymer foam under tensile loads. This model lends insights into polymer microscopic behavior, which may be used for the development of future polymer materials.
When possible, industry standard test methods are used to obtain tensile creep and stress relaxation results from rectangular specimens of Rohacell 31 IG foam. A common set of material parameters is fitted to the data, validating the micromechanic reasoning to polymer chain movement. To gain insight into observed test result variability, an investigation of the elastic modulus and material density relationship is performed using nominal foam densities of 31 kg/m³, 51 kg/m³, 71 kg/m³.
Additional testing and modeling is performed to validate the model under load/partial-unload/hold, load/unload/recovery, and load/instantaneous-unload test cycles. The model successfully captures the observed material nuances during these more complex loading cycles.
Herman, John, "Rheology Of Cross-Linked Polymers And Polymer Foams: Theory And Experimental Results" (2015). Wayne State University Dissertations. 1138.