The effects of specimen size and testing conditions on fracture toughness evaluation of polypropylene homopolymer

Abstract

The results of an experimental test program performed to investigate the influenceof specimen dimensions and testing rate conditions on room temperature fracturetoughness of PP homopolymer are presented. The material displayed nonlinearload-displacement behavior and exhibited small amounts of slow crack growth(ductile tearing) prior to cleavage instability, which invalidates the direct applicationof the current standards for the determination of KIC in polymers. Data points wereconsiderably scattered consistently with typical ductile-brittle transition patterns.The resulting load-displacement diagrams were analyzed in terms of KQ, KMAX, andthe ratio PMAX/PQ. The fracture toughness at instability, JC, was also computed andplotted against the amount of stable crack growth in order to construct J-R curvesas a function of size and strain rate. From these curves an alternative critical fractureparameter, KJIC, was calculated. Weibull statistics were used as a tentative approachfor treating the scatter of fracture toughness of polypropylene homopolymerin large samples, which exhibited restricted ductile tearing. The significance of eachof these methods and the validity and size independence of the corresponding parametersare discussed.