Effect of thermal treatments on the fracture behaviour of notched polyethylene terephthalate ribbons

The application of different thermal treatment procedures to thin polyethylene terephthalate (PET) sheets yields to microstructures of different molecular weight/ degree of crystallinity combination. As a consequence, variations in the mechanical properties, especially the fracture properties and the particular fracture mechanisms occur. This is demonstrated in this paper by measurements of elastic modulus, maximum stress, failure initiation energy, and total work to fracture of notched PET-ribbons. Failure mechanisms analysed by the use of optical and scanning electron microscopy vary between highly ductile via semi-brittle after crazing, to absolute brittle at very low stresses. The results are summarized in terms of a three-dimensional failure energy map divided into regions of particular failure behaviour for particular molecular weight/ degree of crystallinity combination. In addition, the typical values of material strength, defined as the product of resistance to damage initiation (maximum stress) and crack propagation (total work to failure) are given for each region. The optimum fracture resistance was achieved for PET material with moderately low molecular weight and moderately high degree of crystallinity.