Yield behavior and energy-absorbing mechanisms of single and multiphase glassy thermosets subjected to multiaxial stress states

• Main Author: Kody, Robert Steven
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1999
• Subjects: Materials science|Mechanical engineering|Plastics
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9920618

This thesis describes an investigation into the multiaxial yield behavior of single and multi-phase glassy thermosets. Specifically we evaluated the yield behavior of pure, rubber-modified, and voided epoxies, as a function of: molecular architecture, morphology, stress state, strain rate and temperature. This work resulted in: a phenomenological model that incorporates both molecular and testing parameters to describe the full yield behavior of single-phase thermosets, and an improved understanding of the importance of particle cavitation, inelastic void growth and other local irreversibilities in multi-phase materials. The work on the single and multi-phase epoxy thermosets both began with the design of a biaxial testing facility and a specimen fabrication protocol. Using this testing facility to test thin-walled hollow cylinders has allowed us to evaluate the yield and brittle failure response of materials in stress states ranging from uniaxial compression to biaxial tension. Two of the challenges involved in developing such a facility were: fabricating well aligned hollow cylinder specimens, and testing samples at a constant strain rate independent of stress state. After meeting these challenges, thin walled hollow cylinders were tested in stress states ranging from uniaxial compression to biaxial tension. With regard to describing the yield behavior of single-phase glassy thermosets, we investigated the effects of both test conditions and molecular parameters on the yield/deformation behavior. Using these results, a generalized yield model was developed, evaluated and later modified. The model ties both test conditions and molecular parameters to the yield response of glassy polymer networks. The later chapters of this thesis focus on experimentally investigating the deformation behavior of multi-phase polymer systems and the analytical models that describe their behavior. The hollow cylinder tests were conducted on rubber-modified and voided epoxies, to determine their macroscopic yield/failure envelopes. Models that predict the threshold levels for rubber particle cavitation and the macroscopic yield behavior of a perfectly plastic media containing voids were then evaluated. To study the energy absorbing mechanisms involved with deforming rubber-modified epoxy networks, we also investigated the microstructural changes that occur prior to gross yielding. This was accomplished by loading and unloading hollow cylinders in equi-biaxial tension, and observing changes in hysteresis, loading stiffness, and microstructure. Specifically, we measured both the onset and magnitude of energy dissipation primarily due to irreversible matrix deformation. Similar to past researchers, we found difficulty in separating the effect of rubber particle cavitation from the inelastic deformation of the matrix. Therefore, we then investigated the yield behavior and fracture toughness of voided epoxies.