The tensile behaviour of polyethylene terephthalate

Abstract

Measurements of the load/extension curves of polyethylene terephthalate (PET) over a wide range of temperatures showed four regions of behaviour. These were brittle fracture, ductile, cold-drawing, and uniform extension. A particular study was made of the transitions between brittle fracture and ductile failure, and between ductile and cold-drawing, since these define the limits of the three regimes of failure observed at temperatures below the glass transition and softening range. The effects of molecular weight and crystallinity were examined. The brittle strengths measured in tension at low temperatures showed a very large scatter. There was evidence, in spite of this scatter, that the brittle strength falls with decreasing molecular weight. The yield behaviour was not affected, so that the brittle/ductile transition moves to higher temperatures. Crystallinity affects both brittle strength and yield behaviour. The brittle strength falls with increasing crystallinity, whereas the yield stress rises. Both effects combine to raise the temperature of the brittle/ductile and the ductile/cold-drawing transitions. Stress/temperature curves were also constructed for notched specimens. Notching raises the effective yield stress and reduces the brittle strength so that the brittle/ductile transition is moved to higher temperatures. The observed effects are in qualitative agreement with theoretical predictions of the plastic constraint at the tip of a notch, and thus the latter gives a satisfactory qualitative explanation of notch sensitivity. Notching leads to brittle failure at room temperature, and in notched specimens the brittle strength rises as the temperature is decreased. The brittle strength of the lowest molecular weight sample was again significantly less than that of the higher molecular weight samples.