Influence of temperature on plane stress ductile fracture of poly(ethylene terephthalate) film

Double edge notched poly(ethylene terephthalate) (PET) specimens of varying ligament lengths and 0.125 mm thickness have been pulled to complete fracture between 23 and 160°C. Within this temperature range, propagation of the crack was always stable, producing load-displacement curves at various ligament lengths that were geometrically similar to one another. Essential work of fracture (EWF) analysis was used to study the effect of temperature on fracture toughness. A linear relationship was obtained between specific total work of fracture W(f) and ligament length over the entire temperature range under consideration. The slope of the line, which is termed specific nonessential work of fracture (beta)Wp, showed a maximum near the glass transition temperature of the material (Tg=93°C). Beyond this point, (beta)Wp decreased sharply with increasing temperature. The intercept of the line at zero ligament length, which is referred to as specific essential work of fracture w(e), showed three types of variation with respect to temperature. Below Tg, w(e) was found to be more or less independent of temperature; above Tg it increased with temperature and reached a maximum value at the end of the leathery region (~ 120(degree)C); beyond which it decreased steadily.