DETERMINATION OF THE INTRINSIC STRESS-STRAIN EQUATION OF THIN POLYMER-FILMS FROM STRETCHING EXPERIMENTS UNDER PLANE-STRAIN TENSION

Thin films of amorphous poly(ethylene terephthalate) were stretched under plane-strain tension at temperatures from 20-degrees-C to 100-degrees-C and at various elongation-rates. It was observed that under such a deformation regime a neck is likely to form perpendicular to the tensile axis and to propagate steadily towards the gripped ends of the sample. The kinetics of the neck propagation was recorded carefully during the course of the tests by means of a photographic technique and the profile of the neck was analysed by quantitative microscopy during the propagation stage. It is shown that, under simple assumptions, the plastic constitutive equation of the material can be determined from this procedure in terms of the local effective stress versus the local effective strain and effective strain-rate at each of the temperatures investigated. It is demonstrated that the method employed is generally applicable to most ductile polymers.