Nonlinear viscoelasticity of an epoxy thermoset in uni-axial and multi-axial deformation

Jaewoo Kim, Purdue University

Abstract

This dissertation presents the research on nonlinear viscoelasticity of a glassy polymer and its underlying physics via experimental, theoretical, and analytic investigations for a single well characterized material. The findings attained through four different systematic studies are as follows: (i) development of longitudinal deformation that is dilatationally dominated - Through a specially designed experimental set-up, for the first time yield of a glassy polymer is observed in a nearly triaxial extension deformation. This novel technique also enables decoupling dilatational and deviatoric contributions, resulting in the validation of pressure-modified von Mises yield criterion for much wider pressure range than has been previously accessible. (ii) mobility evolution during deformation via stress relaxation technique - Mobility changes as the material is deformed up to yield, but remains nearly constant from yield through the post-yield flow region. The mobility either increase or decrease during deformation critically depending on the strain rate but not temperature or aging time, seriously challenging the simple picture that yield is the straightforward consequence of deformation-induced mobility enhancement. (iii) dependence of mobility evolution on deviatoric vs. dilatational deformation - Even under experimental conditions where the mobility decreases with the deformation in uniaxial extension, in case of a dilatationally dominated longitudinal deformation the mobility increases during deformation, indicating strong dependence of relaxation time on deformation type. Furthermore, the relaxation time calculated for longitudinal deformation exhibits a dramatically different dependence on the excess volume than the relaxation time obtained in structural relaxation experiments. (iv) nonlinear viscoelastic relaxation process via stress memory experiment - A stress memory experiment has been developed to picture the nonlinear viscoelastic relaxation processes in a glassy polymer, which can be regarded as an analogue to the well-known specific volume memory experiment of Kovacs. The most important observation is a strong dependence of the location of the maximum of the stress overshoot on the loading/unloading strain rate, which clearly shows the nonlinear nature of the stress memory effect and indicates that the strain rate is an important physical variable for describing the deformation induced evolution of the relaxation spectrum of a glassy material.

Degree

Ph.D.

Advisors

Caruthers, Purdue University.

Subject Area

Mechanics|Polymer chemistry|Chemical engineering

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