This paper describes a constitutive model for long-term behavior of plastics. The Struik theory of physical aging is adapted for use with the earlier proposed generalization of the Schapery model. Experimental data on creep and recovery behavior for different elapsed times are necessary to build the model. The parameter identification procedure is based on a minimization of the total error between experimental data and model prediction.

The model has been verified using extensive experimental data on long-term creep and recovery of polypropylene, reported by the National Physical Laboratory (NPL, Teddington, U.K.). The model has shown the ability to work with the independent data; it sufficiently describes the variation of strains in time for loading history, which contains several loading steps. The maximum deviation of the model prediction from the experimental data in a control set is less than 11% to 12%. The average deviation is less than 4%.