Stress relaxation behavior after cyclic preloading in polypropylene

The viscoelastic behavior of polypropylene before and after cyclic preloading was investigated by stress relaxation tests. The relaxation tests 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 conducted under different sets of strain rate, number of cycles, and strain amplitude. The experimental data were compared with theoretical results analyzed by use of a linear viscoelastic model. The three‐element model consists of a Maxwell unit and a Hookean spring in parallel. The calculated results agree well with the experimental ones; in particular, in the relaxation tests after the cyclic preloading ( N = 50), the calculated results agree very well with the experimental ones at both the predetermined strain rates of 1,000 μ/s and 10,000 μ/s, at a strain amplitude of ±5%. It can be seen that the linear viscoelastic model explains the viscoelastic characteristics of polypropylene despite the solution of the constitutive equation constructed by the simple three‐element model.