Time-dependent response of polypropylene after strain reversal

Observations are reported in tensile relaxation tests on specimens subjected to stretching up to various maximum strains max followed by retraction to various minimum strains . Experimental data show that shapes of relaxation curves on pre-loaded samples are strongly affected by strain increment Δ = _max - : (i) when Δ is small, stress monotonically decays with time, (ii) with an increase in Δ, the relaxation process becomes non-monotonic, and (iii) at relatively large increments Δ, stress grows with time. A two-phase constitutive model is derived for the viscoelastic and viscoplastic responses of semicrystalline polymers under arbitrary deformations with small strains. Adjustable parameters in the stress-strain relations are found by fitting the observations. Numerical simulation demonstrates that the model correctly describes the time-dependent behavior of polypropylene subjected to cyclic pre-loading in creep and relaxation tests.