A constitutive model for long-term behavior of polymers

This paper describes a constitutive model for long-term behavior of plastic s. The Struik theory of physical aging is adapted For use with the earlier proposed generalization of the Schapery model. Experimental data on creep a nd recovery behavior for different elapsed times are necessary to build the model. The parameter identification procedure is based on a minimization o f 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 Labo ratory (NPL, Teddington, U.K.). The model has shown the ability to work wit h the independent data; it sufficiently describes the variation of strains in time for loading history, which contains several loading steps. The maxi mum deviation of the model prediction ham the experimental data in a contro l set is less than 11% to 12%. The average deviation is less than 4%.