Unstable fracture of rubber‐toughened polymers

The goal of this investigation has been to obtain a useful criterion for the onset of ductile instability. Notched 3‐point bend samples of 4 impact‐modified plastics were tested under load‐controlled conditions to enhance the instability. Neither plastic collapse of the remaining ligament nor large scale yielding at the crack tip was observed, although significant damage zone development and sub‐critical crack growth were observed prior to crack instability. The stress intensity factor at instability (Kc) was found to be independent of crack length for the plastics tested. More surprisingly, when Kc was normalized by the corresponding yield stress, this normalized value was found to be experimentally the same for these four impact modified polymers. Calculations based on ductile tearing instability theory were ambiguous and inconclusive. These results suggest that linear fracture mechanics criteria in load controlled tests may still be applicable for engineering purposes under what has normally been considered ductile conditions.