AN INTEGRAL CONSTITUTIVE EQUATION FOR NONLINEAR PLASTO-VISCOELASTIC BEHAVIOR OF HIGH-DENSITY POLYETHYLENE

In the present paper an effort is made to model the time-dependent beh avior of high-density polyethylene (HDPE) with a one-dimensional integ ral representation. Owing to the plasto-viscoelastic behavior of the m aterial, we assume that the total strain can be decomposed into a reco verable viscoelastic strain and an irrecoverable plastic strain. The v iscoelastic deformation is represented by the Schapery thermodynamic t heory. The plastic deformation is assumed to be accumulated during the loading history. An effective time concept is introduced for the plas tic deformation, so that the response due to complex loading can be ac counted for. The present representation gives a very good prediction o f the responses of creep and recovery, two-step creep, and constant st ress rate loading and unloading. It is also applied successfully to de scribe the process of preconditioning of semicrystalline polymers.