  Constitutive equations are derived for the viscoelastoplastic response of
amorphous glassy polymers at isothermal loading with small strains. A polymer
is treated as an ensemble of cooperatively relaxing regions (CRR) which
rearrange at random times as they are thermally agitated. Rearrangement of CRRs
reflects the viscoelastic response of the bulk medium. At low stresses, CRRs
are connected with each other, which implies that the macro-strain in a
specimen coincides with micro-strains in individual relaxing regions. When the
average stress exceeds some threshold level, links between CRRs break and
relaxing domains begin to slide one with respect to another. Sliding of
micro-domains is associated with the viscoplastic behavior of polymers. Kinetic
equations are proposed for viscoplastic strains and for the evolution of the
threshold stress. These equations are validated by comparison with experimental
data in tensile relaxation tests and in tests with constant strain rates. Fair
agreement is demonstrated between results of numerical simulation and
observations for a polyurethane resin and poly(methyl methacrylate).
