STRESS-RELAXATION BEHAVIOR AFTER CYCLIC PRELOADING IN POLYPROPYLENE

The viscoelastic behavior of polypropylene before and after cyclic pre loading was investigated by stress relaxation tests. The relaxation te sts were performed after a simple uniaxial tension (number of cycles N = 0) and after the cyclic preloading (N = 50) by use of a closed loop , electrohydraulic, servocontrolled testing machine. The tests were co nducted under different sets of strain rate, number of cycles, and str ain amplitude. The experimental data were compared with theoretical re sults analyzed by use of a linear viscoelastic model. The three-elemen t model consists of a Maxwell unit and a Hookean spring in parallel. T he calculated results agree well with the experimental ones; in partic ular, in the relaxation tests after the cyclic preloading (N = 50), th e calculated results agree very well with the experimental ones at bot h the predetermined strain rates of 1,000 Als and 10,000 mu/s, at a st rain amplitude of +/-5%. It can be seen that the linear viscoelastic m odel explains the viscoelastic characteristics of polypropylene despit e the solution of the constitutive equation constructed by the simple three-element model.