13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Ductile to brittle transition concept on fracture behavior of poly(vinylidene fluoride) / poly(methyl methacrylate) blends Lucien Laiarinandrasana1,*, Yannick Nziakou2, Jean-Louis Halary2 1 Centre des Matériaux CNRS UMR 7633, Mines ParisTech, Evry Cedex 91003, France 2 SIMM-Lab. UPMC/CNRS/ESPCI UMR 7615, ESPCI ParsTech, Paris Cedex 05 75231, France * Corresponding author: Lucien.Laiarinandrasana@mines-paristech.fr Abstract The fracture behavior of blends of poly(ninylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) was investigated by gradually increasing the PVDF content. The study focuses on semi-crystalline blends. The trends of net stress versus crack opening displacement curves were analyzed. From these plots, two fracture energies were defined: the fracture energy to crack initiation corresponding to the area under the curve up to the maximum net stress and the fracture energy to crack propagation considering the last part of the curve where the load continuously decreases. Fracture surface inspections confirmed typical semi-crystalline polymer features. Critical values of the degree of crystallinity corresponding to brittle to ductile transition were determined, depending on the selected fracture energy. Keywords Blends, Poly(methyl methacrylate), Poly(vinylidene fluoride), Fracture toughness, Brittle to Ductile transition 1. Introduction Mechanical parameters at failure are key factors to assess the durability of engineering components. For structural polymeric materials, fracture toughness is often characterized by the impact strength (Charpy or Izod tests). These are dynamic tests, accurate for brittle failure characterization. They might be inappropriate when the material exhibits significant ductility. Theoretically, fracture mechanics approaches suggest a methodology to define fracture toughness, whatever the mechanical response of the material. Whereas comprehensive research has been carried out for years on metallic materials concerning the equivalence between impact strength and fracture toughness, there is no such amount of work on polymers. Some attempts were made in this way but failed because, for instance, of the difficulty to fulfill requested plane strain conditions. Nevertheless, impact strength or toughness of polymeric materials was currently plotted as a function of the test temperature, in order to find a ductile to brittle transition temperature. The essential motivation of such a plot is to check whether the modification of the material process actually leads to an improvement of the toughness or the impact strength, by shifting the brittle to ductile transition temperature away from in-service temperature. The present contribution focuses on blends of poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA). Indeed, in recent years, PMMA/PVDF blends got a renewal of interest with studies aiming at assessing their mechanical response. In terms of processing, they exhibited a transition from miscible amorphous to semi-crystalline structures. At room temperature and for approximately the same given crosshead speed, PMMA is basically brittle and neat PVDF exhibits ductile failure. By gradually increasing the PVDF content, toughening effects were evidenced according to a given fracture mechanics parameter based on fracture energy. The motivation here is then to determine a critical degree of crystallinity able to characterize the transition between brittle (low fracture energy) and ductile (high fracture energy) failures, for the same test conditions (room temperature/ crosshead speed). It was demonstrated that depending on the fracture energy definition,
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