Article: Effect of mean strain on the cyclic deformation and stress relaxation in polypropylene.

INTRODUCTION

Cyclic deformation and stress relaxation after cyclic preloadings in polypropylene have been studied in a push-pull tension-compression mode by using a closed-loop, electrohydraulic, servocontrolled testing machine (1, 2). The data were compared with theoretical results analyzed by using a linear viscoelastic model (3, 4) and an overstress model (5, 6). An equilibrium stress and a viscosity function were treated in the overstress theory. The calculated results based on the overstress theory agree well with experiment in both the stress-strain and the stress-relaxation curves. It was concluded that the overstress theory explained the nonlinear ...

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