Cavitation et rupture du Polyamide 6 sous état de contrainte multiaxial en traction monotone, fluage et fatigue. Dialogue entre imagerie 3D et modélisation par éléments finis.

Abstract : Many industrial structures subjected to quasi-static (creep) or cyclic (fatigue) long-term loadings are made of semi-crystalline polymers. Such is the case, for instance, of pressure vessels and pipes. It is therefore considered critical to study the issues related to their durability in order to be able to anticipate and control their end of life. Furthermore, they generally have complex designs and are subjected to multiaxial stress states.The material which has been studied was a semi-crystalline Polyamide 6. Its structure consisted of amorphous and the crystalline phases and a spherolitic microstructure.As a first step, the links between the mechanical behaviour at the global scale of the specimens and the underlying micro-mechanisms of deformation that lead to failure have been established experimentally for monotonic and creep loadings that show similar results and then for fatigue loadings. The influence of the multiaxiality of the stress state has been studied using circumferentially notched round bars with different notch root radii and Compact Tensile specimens. The cavitation phenomena were characterized using synchrotron radiation tomography and laminography techniques that enabled the observation and quantification of the spatial distributions of the voids and the anisotropy of the cavities. An analysis of the fracture surfaces has shown that the initiation of ductile failure resulted from void growth and coalescence mechanismsA poro-visco-plastic model with two mechanisms (that allow the behaviours of the amorphous and crystalline phases to be distinguished) has been used. Thanks to this model, the global behaviour (loading curves) under steady strain rates and steady loads but also the spatial distributions of the void volume fraction could be reproduced numerically. In addition finite element calculations have permitted the spatial distributions of the stress field to be studied and the influence of the stress state on the cavitation state to be investigated. The temporal evolutions during the deformation of the hydrostatic pressure have been linked to the spatial distributions of void volume fraction. The void anisotropy (and thus the void morphology and shape factors) has been related to the evolutions of the components of the Cauchy stress tensor. Finally, the definition of a rupture criterion based on a critical value of the void volume fraction has enabled crack propagation under steady strain rate and steady load to be simulated.
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Nathan Selles. Cavitation et rupture du Polyamide 6 sous état de contrainte multiaxial en traction monotone, fluage et fatigue. Dialogue entre imagerie 3D et modélisation par éléments finis.. Matériaux. PSL Research University, 2017. Français. ⟨NNT : 2017PSLEM038⟩. ⟨tel-01778515⟩

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