Fatigue Fracture in Polypropylene with Different Spherulitic Sizes

Abstract

Engineering plastics, because of their characteristics, which combine lightness and corrosion resistance with a good balance of stiffness and toughness over a wide temperature range, were able to penetrate the markets of traditional materials. Semicrystalline polymers were among the main beneficiaries of these developments. Along with an increasing use in a variety of industrial applications there was an emerging need to investigate the fatigue properties of these polymers. The degree of crystallinity and average spherulitic size influence the crack propagation as well as the mechanical properties. The effects of different morphologies on crack propagation and crazing in polypropylene were examined. Although several works have been reported on the mechanical response of polypropylene and microstructural events, at the spherulitic level, the influence of different morphologies on crack propagation has not been sufficiently investigated. The purpose of this chapter is to observe macro behavior and microstructural developments of crack propagation in semicrystalline polypropylene under different morphologies as well. The aim is to expose the various phenomena during fatigue fracture in PP specimens with different spherulitic size. In all morphologies, damage appeared in the form of yielded material (process zone) ahead the crack tip and crazes disseminated around the crack path. In all morphologies, the early phases of fatiguing were similar. Namely, the initial notch remained stationary while the process and damage zones formed ahead of the crack tip. While the size of the process zone increased, voids were observed with the zone and close to the crack tip.