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Rôle de la texture microstructurale et du champ d'onde élastiques sur la réponse en rupture dynamique des solides fragiles

Abstract : Dynamic crack propagation drives catastrophic material failures. A key aspect in the problem is the stress concentration at the tip of cracks, which makes the failure behavior observed at the macroscopic scale very sensitive to inhomogeneities in the material micro-structure, or, in local loading, down to very small scale. Stress concentration and redistribution of stresses during crack growth, indeed, couple many space and time scales. They stretch from the macroscopic dimension of the sample, L, down to the size of the so-called fracture process zone, which embeds all the dissipative process involved in the creation of new fracture surface area). Elastodynamics and continuum fracture theory a priori provide the relevant tools to describe dynamic crack growth. Still, at high enough speed several instabilities are observed. In brittle polymers like polystyrene or polymethylmethalcrylate (PMMA) for instance, micro-cracking starts occurring in the vicinity of the crack tip, in a zone of length-scale depending on the crack speed and larger than the fracture process zone. The space and time organization of these micro-cracking events evolves with crack speed and can present patterns up to the millimeter scale Since the pioneering work of Gao and Rice, 1986, it has been proposed to model crack propagation in a 3D solid as an elastic line moving through obstacles. The distortion in the crack front, in turn, increases locally the stress intensity factor. Such model gained momentum when the first statistical studies of crack surfaces revealed self-affine features for the roughness. A whole zoology of roughness exponent was measured since then. The elastic line model meet great success in the quasi static crack front modeling.In this context, it is now timely to extend it to dynamic fracture. This may should bring new insight on the dynamic crack problem and the instabilities mentioned above. The main objective of this work is to use a model experiment to study systematically the microscopic and macroscopic properties of a dynamic crack front. This thesis aims at understanding to what extend such a line model is valid experimentally despite the complex environment seen by the crack front due to the micro-cracks of the damage zone and the presence of micro-branches. Along these lines, here is a set of questions to address. Fractography is a promising method to probe the fracture history and to measure the fracture toughness of the material. How could the method evolve to be applicable to dynamic crack surfaces? What would be the available information on the material properties and the crack velocity? Dynamic fracture in PMMA is known to undergo a brittle to quasi brittle transition above a certain critical speed, which is smaller than the micro-branches threshold. What mechanical parameters control the statistical distribution of micro-crack nucleation centers? Dynamic crack instabilities are believed to be a 3D effect, no 2D model could justify them. What is the role of the crack front thickness in their organization and nucleation? In dynamic fracture, instabilities travel along the front, the in plane crack front waves (Morrissey and Rice, 1998) and the out of plane corrugation waves (J.R. Willis, 2012). Can we find, in the behavior of the crack front, some experimental evidences of these front waves? What is their impact on the damage organization?
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Alizee Dubois. Rôle de la texture microstructurale et du champ d'onde élastiques sur la réponse en rupture dynamique des solides fragiles. Physique [physics]. Université Paris Saclay (COmUE), 2018. Français. ⟨NNT : 2018SACLX114⟩. ⟨tel-02445849v2⟩

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