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Investigating the Effect Strain Rate on the Poisson’s Ratio of Auxetic Polyurethane Foams

Behbahani, Ali | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55256 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Nourani, Amir
  7. Abstract:
  8. Auxetic materials have a negative Poisson ratio (NPR), which is their most distinguishable characteristic as compared to regular materials. Test specimens were made of conventional polyurethane (PU) foams according to ASTM ; then, with a thermo-mechanical process, the foam was converted to auxetic PU foam. The reproducibility of specimens' fabrication was checked with CoV= for the final foam thickness. With regard to the fact that changing loading conditions can vary Poisson's ratio, in this study, the variations occurred in Poisson's ratio due to changing loading conditions are investigated. The applied strain and strain rate were altered in experiments and statistical analysis was conducted. A finite element model (FEM) was developed for changing Poisson's ratio as a consequence of changing loading circumstances. Reputability of testing was approved by statistical analysis (e.g., CoV= for the Poisson's ratio with a strain of and a strain rate of ). Experiments were performed for the applied strain ranging from to , under three strain rate regimes; i.e., quasi-static ( ), moderate ( ), and dynamic ( ). Displacements were tracked during the tests by digital image correlation (DIC). ANOVA statistical analysis with confidence interval shows that both strain and strain rate can play significant roles in changes of Poisson’s ratio. It was also found that transient strain auxeticity (TSA) depend on the strain rate. Finally, by a linear simplification, a model based on Hooke's law, termed the adaptive hook's model, was developed in ABAQUS software. The validity of this model was investigated by the experiments, showing a maximum difference of compared to the recorded stress-strain curves.
  9. Keywords:
  10. Poisson Ratio ; Foam ; Strain ; Strain Rate ; Experiments Design ; Statistical Analysis ; Auxetic Materials

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