Thermal effects on fracture of biaxial-oriented poly(ethylene terephthalate) (BOPET) film

The effect of temperature on the fracture behaviour of biaxial-oriented poly(ethylene terephthalate) (BOPET) film was studied using the Essential Work of Fracture (EWF) approach. Fracture tests were performed over the temperature range +25 to +160 degrees C at the speed of 5 mm/min using double edge notched tension (DENT) specimens. The length of the specimens was either along the machine direction (MD) (0 degrees), transverse direction (TD) (90 degrees) or at 45 degrees to either MD or TD. Ductile tearing of the ligament region was noted over the entire temperature range in all three directions. A linear relationship was found between the specific total work of fracture and the ligament length at all test temperatures. Values of the specific essential work of fracture (w(e)) in the MD and TD were similar and smaller than in the 45 degrees direction. Within temperature range 25-140 degrees C, w (e) showed little variation if any with respect to temperature. As expected, the Specific Non-Essential Work of Fracture (beta w(p)) was temperature dependent. This parameter increased with increasing temperature and reached a maximum around the glass transition temperature of BOPET (T-g approximate to 80 degrees C). The values of the maxima are respectively 16.15, 20.38 and 17.8 MJm(-3) for the 0 degrees, 45 degrees and 90 degrees.