Fracture Modeling in Elastomeric Materials: Relating the Macroscopic Response to Microscopic Processes

• Author / Creator

  Shawn Ryan Lavoie

• Inelastic material behavior is known to be able to enhance the fracture toughness of elastomers.  However, how to optimally exploit this toughening mechanism is still being investigated.  In this thesis a series of continuum models are developed to describe such inelastic behaviors, with the goal of gaining greater understanding of the fracture process which can then be exploited to produce tougher elastomers.  This is accomplished by relating the mechanical response of the macroscopic elastomer to the extension and rupture of the microscopic polymer chains which comprise the material.  
  

  The peeling of a viscoelastic double cantilever beams by rupturing a layer of polymer chains along the plane of fracture was first investigated. The fracture energy consists of two components: the adhesive fracture energy arising from interfacial bond rupture, and viscous dissipation within the bulk material. Faster crack propagation requires larger adhesive fracture energy whereas the rate-dependence of bulk viscous dissipation is non-monotonic. The coupling between bulk and adhesive behavior was found to be weak in this problem because the strain in the beam and the adhesive stress on the interface are perpendicular.
  
  Motivated by experimental observation of chain rupture in a large area surrounding crack tips, kinetic modeling of polymer chain scission was incorporated into the constitutive model of the bulk elastomer. When the material is assumed to contain a distribution of chains with different lengths the model produces two important results during uniaxial loading. First, the polydispersity causes the maximum stress to decrease because chains with different lengths attain their maximum force and rupture at different deformations. Second, progressive damage occurs in the material, which results in hysteresis between first loading and unloading under cyclic condition. This elastomeric damage model was subsequently applied to establish a constitutive relation for multinetwork elastomers. These materials are synthesized by swelling a primary elastomeric network with filler network(s), a process which prestretches the chains of the network formed in the previous step(s). The effect of such a process is captured by basing the strain energy of the material on the combined effect of swelling and subsequent deformation of the completed multinetwork elastomers. The model provides a good match to experimental data on uniaxial extension, including cyclic loading, for a variety of prestretches.
  The relationship between the force applied on a chain and its extension plays an important role in the mechanical properties of elastomers. Classical models for the chain force-extension relationship are too stiff at large extension due to the lack of consideration of bond deformation on the polymer chains backbone. A new model was developed to correct this, which is accomplished by considering free energy contributions from not only the configurational entropy, but also bond stretching and bond angle distortion. The model was also extended to include the consideration of active polymer chains containing mechanophores or molecules which have reactions that are triggered by mechanical force.

• Subjects / Keywords

  • Elastomer
  • polymer chain
  • fracture toughness
  • constitutive model

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R31J97Q4B

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.