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On the Study and Comparison of the Post-yield Strain Softening in Polymer and Cellular Solid under Compression Loading

Goodarzi Hosseinabadi, Hossein | 2017

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 49888 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Bagheri, Reza
  7. Abstract:
  8. Strain softening after the yield point is a prevalent phenomenon in the stress-strain response of cellular solids, consisting of porosities in micro/macro-meter scale, as well as polymers, consisting of free volumes in sub-nanometer molecular scale. Although the underlying micromechanisms for softening in cellular solids are already known, the molecular origin for softening in polymers is still unknown. In this thesis, the micromechanisms of the softening in both materials are investigated. Then, a hypothesis is raised and evaluated to connect the nano-mechanics of softening in polymers to the micro-mechanics of softening in cellular solids. Experiments of compression test, positron-based Doppler broadening spectroscopy, and digital image correlation (DIC) are used to investigate the correlation between the total free volume variations and strain softening response in poly-styrene (PS) samples of different thermal histories. Additionally, numerical routes such as finite elements and non-linear numerical homogenization are used to study the role of cell’s architecture on softening response of 2D cellular solids, i.e. honeycomb models. Finally, a coupled Molecular Dynamics-Finite Elements (MD-FE) approach is developed to simulate the structural role of free-volume spatial arrangement in 3D on the softening response of poly-vinyle-alcohol (PVA) at different cooling rates. Experimental results proved the inability of the total free volume concept for describing the post-yield softening behavior in PS. Numerical results on honeycomb models, reported with 80% confidence, showed that a systematic variation of cellular ordering in a cellular solid can monotonously change the slope and height of the softening response in stress-strain curve. The trends obtained by 3D simulations of PVA were in accordance to the 2D simulations of honeycomb models, and were different from the experimental results in polymers. It was found that a scaling phenomenon governs on the softening behavior from sub-nano up to macroscale. Two distinct mechanisms for softening behavior in polymers and cellular solids were found to be active in the different length scales. At last, the potential molecular origin for strain softening behavior in a polymer is described based on the energy landscape analysis and the changes of molecular mobility in polymeric systems
  9. Keywords:
  10. Digital Image Correlation ; Poly Vinyl Alcohol ; Molecular Dynamics ; Polystyrene ; Post-Yield Softening ; Three Dimentional Printing ; Free Volume Spatial Analysis

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