Dependence of polyethylene terephthalate crack-tip temperature on stress intensity and notch sensitivity

Polyethylene terephthalate (PET) is often thought to be extremely notch sensitive, as other amorphous or slow-crystallizing polymers. However, our previous studies demonstrated three distinct fracture modes during tensile loading of PET when notch depth was gradually increased, namely ductile, semiductile and brittle. Therefore, the notch sensitivity of PET is thought to depend on its morphology and notch depth, whereas linear elastic fracture mechanics would be inadequate to characterize the fracture toughness of PET, especially when the material is injection molded and possesses the characteristic skin-core structure. This study revealed that the temperature at the crack tip significantly increased because of local plastic deformation when the crack tip was situated within the skin region. As such, full ligament yielding was obtained, which allowed characterization of the fracture toughness of PET using the principles of essential work of fracture (EWF). However, no significant temperature increase occurred when specimens fractured in a brittle manner as the notch penetrated through the skin and into the core. At this point, the material is said to be notch sensitive and is no longer able to undergo any plastic deformation before failure.