VISCOELASTIC-PLASTIC DEFORMATION-BEHAVIOR OF POLYPROPYLENE AFTER CYCLIC PRELOADINGS

Stress-relaxation behavior is studied in polypropylene samples subject ed to different cyclic preloadings and to simple tension. The relaxati on tests are performed under different sets of strain amplitude, numbe r of cycles, and strain rate, using a closed-loop, electrohydraulic, s ervocontrolled testing machine. The calculated stress-strain curves ar e determined from a constitutive equation based on an overstress theor y in which an equilibrium stress and a viscosity function are treated. The calculated results agree well with the experimental ones. It is c oncluded that the overstress theory explains the nonlinear viscoelasti c-plastic behavior of polypropylene.