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Damage mechanisms under tensile stress of amorphous and low semi-crystalline polymers

Abstract : In the last few years, a new class of polyamides has been developed for specific applications requiring better mechanical and thermal properties, such as electronics or automotive industry. Polyphthalamides (PPA) are semi-aromatic polyamides containing aromatic rings in their main chain. Recently, the damage mechanisms have been studied in the case of semi-crystalline (PA66) and amorphous (cellulose acetate) polymers. The aim of the PhD thesis was to study the damage mechanisms of amorphous and semi-crystalline polymers. In addition, few studies have been conducted to characterize the properties of pure PPA. More detailed data regarding their mechanical properties are needed. The study of the properties of this class of polymers is important, especially since their applications are different from aliphatic polyamides. We first characterized these polymers and then studied their mechanical behavior in traction, compression and Charpy impact strength. We have thus been able to highlight the appearance of a necking phenomenon and a strain hardening regime from 20% deformation for three PPA studied. Strain hardening has also been observed in other amorphous ductile polymers such as polycarbonate (PC) or poly(methyl methacrylate) (PMMA). The strain hardening stabilizes the deformation by avoiding the localization of the damage. In order to describe the microscopic mechanisms associated with the initiation and propagation of damage under tensile deformation of our polymers, we have carried out observations by scanning transmission electron microscopy (STEM) and optical microscopy as well as by Ultra-small angle X-ray scattering (USAXS). The analysis of these different polymers (amorphous and semi-crystalline PPA, PC, PMMA) by USAXS highlights different modes of damage. The simultaneous nucleation of nanometric crazes around the pre-existing defects (defects related to the injection process), then the limited growth of these crazes are observed for the amorphous PPA, the PC and the PMMA studied, in a mechanism similar to that studied in the case of cellulose acetate. The damage is blocked in the first place by the strain hardening. However, in the case of polycarbonate and PMMA, when the stress applied becomes sufficiently high, a small fraction of these crazes grows faster to cause the rupture of the sample, which allows to observe the evolution of a second family of larger crazes. We also observe the growth of a second family of crazes for one of the amorphous PPA, but their growth stops at the appearance of necking. No failure of the sample is observed. The second amorphous PPA is damaged by cavitation up to about 5% deformation. The cavities stop to grow at the appearance of shear bands. When the shear bands propagated on either side of the sample, the failure is observed. The two semi-crystalline PPA deform by necking without failure. No cavitation was observed by USAXS. The damage of the polymers studied can be classified into three categories. The first category concerns polymers that behave similarly to cellulose acetate. Crazing nucleation is observed, which growth is initially blocked by strain hardening. When the stress applied becomes sufficiently high, a fraction of these crazes increases more rapidly until the sample is broken, which makes it possible to observe the evolution of a second family of larger crazes. This category concerns polycarbonate and PMMA. The second category concerns one of the amorphous PPA with crazes whose growth is initially blocked also by strain hardening. The growth of a second family of crazes stops at the appearance of necking, and no failure of the specimen is observed. The last category concerns polymers that do not show any damage. These are the two semi-crystalline PPA. We propose in this thesis an interpretation of these different mechanisms of damage
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Stéphanie Djukic. Damage mechanisms under tensile stress of amorphous and low semi-crystalline polymers. Material chemistry. Université de Lyon, 2020. English. ⟨NNT : 2020LYSE1041⟩. ⟨tel-02797014⟩

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