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A new look at tensile yielding in isotactic polypropylene: role of strain rate and thermal softening

Farahani, M. F ; Sharif University of Technology | 2022

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  1. Type of Document: Article
  2. DOI: 10.1007/s00289-021-03997-z
  3. Publisher: Springer Science and Business Media Deutschland GmbH , 2022
  4. Abstract:
  5. Polypropylene (PP) is a semi-crystalline polymer with a very wide range of industrial applications. This is why its mechanical properties, including yielding, have been the subject of numerous studies. The current study has examined the influence of strain rate and thermal softening on yield stress and post yielding behavior of an isotactic PP. A two-process Ree and Eyring model and the heat equation in adiabatic deformation were coupled with a physical parameter described as the temperature sensitivity of the flow stress at yielding, to simulate the effect of gradual temperature rise in adiabatic heating on yielding behavior. Along with that, tensile bars were subjected to tests at different strain rates experimentally. Results showed considerable influence of strain rate on yielding behavior of PP. Two transition points in the yield stress versus strain rate diagram were noticed. The first one occurring at the strain rate of about 7*10–5 s−1 is associated with a shift in deformation mechanism from Process I, i.e., side chain motions, to main chain motions, named Process II. Strain hardening rate increases significantly by moving from Process I to Process II region. The other transition point, equal to 3*10–2 s−1, is the strain rate value at which adiabatic heating surpasses strain hardening. As a result, the positive influence of strain rate on yield stress due to strain hardening stops at this point and temperature starts to rise by further increase in the strain rate. This change from isothermal to adiabatic deformation, causing approximately 2 °C increase in temperature, is associated with the decline of the yield stress and also variations in the post yielding behavior of PP. Stress softening slope and drop after yielding go through a maximum by passing the transition point of 3*10–2 s−1. The effect of strain rate and thermal softening were successfully simulated in terms of decrease in the yield stress and gradual temperature rise above the transition strain rate using the proposed model. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature
  6. Keywords:
  7. Polypropylene ; Isotherms ; Polypropylenes ; Strain hardening ; Strain rate ; Yield stress ; Eyring equation ; Isothermal to adiabatic deformation ; Post yield behavior ; Post yields ; Ree and eyring equation ; Strain-rates ; Thermal softening ; Transition point ; Yield behaviors ; Yielding behavior ; Deformation
  8. Source: Polymer Bulletin ; Volume 79, Issue 12 , 2022 , Pages 11157-11176 ; 01700839 (ISSN)
  9. URL: https://link.springer.com/article/10.1007/s00289-021-03997-z