The effect of moisture on the nonlinearly viscoelastic behavior of an epoxy

Abstract

The shear and bulk relaxation moduli required to characterize a homogeneous, isotropic, and linearly viscoelastic material were determined using a confined compression experiment and by introducing a new iterative scheme that accounts for the fact that the hoop and radial strains are not step functions. In addition, the coefficients of thermal and hygral expansion of the epoxy being considered were determined along with its diffusive behavior. Fickian diffusion of moisture was confirmed by coupling radial diffusion in an epoxy disk with optical interference measurements of the out-of-plane displacements. These properties are essential components of the modified free-volume model of nonlinear viscoelasticity established by Popelar and Liechti (J. Eng. Mater. Technol. 119:205–210, 1997, Mech. Time-Depend. Mater. 7(2):89–141, 2003). For the nonlinear component of the model, its distortional parameters were evaluated from Arcan pure shear test results at one strain rate and temperature/humidity state. The nonlinear viscoelasticity model was then used to predict the shear stress-strain response under other conditions. The dilatational parameters were extracted from uniaxial tensile test results at one strain rate and temperature humidity state and predictions under other conditions compared favorably with the results from experiments. This exercise adds to a growing list of glassy polymers whose nonlinear stress-strain behavior can be modeled by this modified free-volume model.