Due to extensive development of personal computers in last years and icreasing of their calculation power and data storage capacities, we can observe apid growth of computer programs and systems of programs which are irreplaceable help for engineers involved in almost all field of engineering. The mechanics of structure and materials is a very good example of real revolution in methodology of engineering calculations. This is due to continuous development of numerical methods applied in analyzing of structural response during its service.
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Various existing constitutive equations aiming at the description of the time-dependent deformation behavior for a wide range of stress below and over the elastic limit, i.e. the yield stress, are reviewed in detail. It is suggested that the plastic stretching and the creep stretching have to be treated as independent quantities since they have substantially different physical properties, and that a stress goes out from the yield surface at a high rate of deformation since a plastic deformation is suppressed by a high viscous resistance, and then the yield surface is kept unchanged. The subloading surface model [1 - 3] would satisfy these requirements since it does not premise that a stress is on the yield surface even in the plastic loading process. In this article, based on the extended subloading surface model [4] which is capable of describing not only monotonic but also cyclic loading behavior, the generalized time-dependent elastoplastic constitutive equation is formulated allowing the stress go out from the yield surface by letting the plastic deformation be suppressed at a high rate of deformation and introducing the creep stretching which proceeds with time in addition to the elastic and the plastic stretching.