COMPARISON OF THE FRACTURE AND FAILURE BEHAVIOR OF INJECTION-MOLDED ALPHA-POLYPROPYLENE AND BETA-POLYPROPYLENE IN HIGH-SPEED 3-POINT BENDING TESTS

The fracture and failure mode of alpha- and beta-isotactic polypropyle ne (alpha-iPP and beta-iPP, respectively) were studied in high speed ( 1 m/s) three-point bending tests on notched bars cut from injection-mo lded dumbbell specimens and compared. The fracture response of the not ched Charpy-type specimens at room temperature (RT) and -40 degrees C, respectively, was described by terms of the linear elastic fracture m echanics (LEFM), namely fracture toughness (K-c) and fracture energy ( G(c)).K-c values of both iPP modifications were similar, while G(c) va lues of the beta-iPP were approximately twofold of the reference alpha -iPP irrespective of the test temperature. It was demonstrated that be ta-iPP failed in a ductile and brittle-microductile manner at RT and - 40 degrees C, respectively. By contrast, brittle fracture dominated in alpha-iPP at both testing temperatures. Based on the fracture surface appearance, it was supposed that beta-to-alpha (beta alpha) transform ation occurred in beta-IPP. The superior fracture energy of beta-iPP t o alpha-iPP was attributed to a combined effect of the following terms : morphology, mechanical damping, and phase transformation. Results in dicate that their relative contribution is a function of the test temp erature. (C) 1997 John Wiley & Sons, Inc.