Fracture of polybutylene terephthalate (PBT) film

Combined effects of thickness and temperature on essential work of fracture (EWF) of polybutylene terephthalate (PBT) film were studied using single edge notched tension (SENT) and double edge notched tension specimens. It is found that specific essential work of fracture (w_e) for PBT is independent of temperature below T_g (≈80 °C), but decreases above T_g. Between temperatures 25 and 100 °C, w_e was independent of film thickness in the range 0.125-0.375 mm. The specific non-essential work of fracture (βw_p) was temperature and thickness dependent, being greater for the SENT type specimens. Specimen orientation had no influence on w_e but strongly affected βw_p. It was found that βw_p is greater for cracks propagating normal to the extrusion direction as compared to the parallel direction.