Deformation and yield of epoxy networks in constrained states of stress

Abstract

A series of epoxy networks were made with molecular weights between crosslinks, Mc, ranging from 380 to 1790 g mol-1. Resins were cast into thin walled hollow cylinders and tested in stress states ranging from uniaxial compression to biaxial tension. These tests elucidated the effects of stress state, strain rate, and Mc on the yield and fracture response of epoxy networks. Throughout the study, the strain rate along the octahedral shear plane, γoct, was maintained constant independent of stress state, for each failure envelope. The hollow cylinder tests showed that the yield behaviour of epoxy networks can be described by a modified von Mises criterion, τocty=τocty0−μσm where τoctg is the octahedral shear stress at yield, τocty0 is the octahedral shear stress at yield in pure shear, μ is the coefficient of internal friction and Vm is the hydrostatic tensile stress imposed on the sample. Furthermore, these tests showed that changes in γoct and Mc only affect τocty0, while μ remains constant. Standard tensile and compression tests were run to confirm the hollow cylinder result and to test the effect of temperature on the yield and brittle response. Tensile tests showed that changes in Mc only affect the glass transition temperature, Tg, of the materials, and the glassy modulus remained independent of Mc. With regard to the yield strength, changes in Mc cause a shift in the Tg of the materials, and the yield strengths of all the materials collapse together at a constant temperature relative to Tg. Finally, yielding of these epoxies was shown to follow an Eyring type flow model over the range of temperatures and strain rates tested.