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Nonlinear thermo-mechanical behavior of polymers under multiaxial loading


Lu, Hongbing (1997) Nonlinear thermo-mechanical behavior of polymers under multiaxial loading. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/SH42-W153.


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In order to better understand phenomena related to "yield-like" behavior of polymers, the nonlinear thermo-mechanical behavior of Polymethyl Methacrylate (PMMA) under combined axial (tension, compression) and shear stress states (torsion) is investigated on thin walled cylindrical specimens at temperatures between 22[degrees] and 100[degrees]C. A non-contact measurement technique based on digital image correlation method is developed to determine the surface deformations on curved cylindrical specimens at various temperatures, including temperatures close to the glass transition when the specimens become soft. The study indicates that in contrast to tile mutual independence of shear and dilatational response under conditions appropriate for linearized viscoelasticity, one observes an increasingly strong coupling between all deformation or stress invariants as assessed in creep experiments for strains in excess of' 0.4%. While shear stresses alone elicit nonlinear response in creep (rates) as "intrinsically" nonlinear shear response, the superposition of small positive dilatation accelerates shear deformations while negative dilatation retards it in quantitative agreement with free volume arguments. Passing below the glass transition still produces greater creep acceleration from positive dilatation than from a decrease in specific volume, but the dominance of the intrinsic shear nonlinearity vis-a-vis the dilatational ilifluence increases below [...] as the temperature drops. A constitutive model based oil free volume consideration is modified from the models proposed by Losi and Knauss (1992). Comparison of experimental results with model prediction indicates a good qualitative agreement, and reasonably good quantitative agreement.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Aeronautics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Knauss, Wolfgang Gustav
Thesis Committee:
  • Unknown, Unknown
Defense Date:29 April 1997
Record Number:CaltechETD:etd-11072003-103136
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4436
Deposited By: Imported from ETD-db
Deposited On:10 Nov 2003
Last Modified:21 Dec 2019 04:40

Thesis Files

PDF (Lu_h_1997.pdf) - Final Version
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