<p>Most of the attention to the seismic performance of tunnels has been devoted to shear waves propagating in a direction perpendicular to the tunnel axis, with motion perpendicular to the tunnel axis, causing the so-called “ovaling or racking response”. Body waves, however, can travel through the ground and intersect the tunnel at different angles, thus inducing a complex seismic response that requires a comprehensive understanding of all modes of deformation. This study provides analytical solutions to capture the behavior of the liner and the surrounding ground, for a deep circular tunnel subjected to body waves, for all five possible modes of deformation: (i) axial compression-extension; (ii) transverse compression-extension; (iii) ovaling; (iv) axial shear; and (v) axial bending or snaking. The main assumptions used to derive the analytical solutions include: (i) the tunnel is deep and very long and has a circular cross section; (ii) the ground and the support are homogeneous and isotropic, and their response remains elastic; (iii) the interface between the ground and the liner is either no-slip or full-slip; (iv) the pseudo-static approach, i.e. inertia forces can be neglected, is acceptable to estimate seismic deformations; (v) for the transverse compression-extension and ovaling deformation modes, plane strain conditions in the direction of the tunnel axis apply at any cross section; and (vi) for the axial compression-extension and axial bending deformation modes, the wavelength of the seismic motions is much larger than the tunnel radius. Two and three-dimensional numerical simulations with the finite element codes ABAQUS, for static drained/undrained loading and dynamic drained loading conditions, and MIDAS GTS NX, for dynamic undrained loading conditions, are carried out to validate the analytical solutions and further investigate the seismic response of the tunnel. All the comparison results show good agreement between the analytical and numerical solutions.</p> <p>Dynamic amplification effects on the tunnel cross section are studied for the axial compression-extension, transverse compression-extension, and axial bending deformation modes, through a set of dynamic time-history models where the input frequency of the far-field seismic motion is changed. The results reveal the limits of the analytical solutions, in the form of minimum wavelength-to-tunnel diameter (/D) ratios such that the errors are less than twenty percent, including: (i) 25 (drained) and 20 (undrained) for axial compression-extension; (ii) 25 (drained) and 12.5 (undrained) for transverse compression- extension; and (iii) 7.5 (unsupported tunnel), 7.5 (supported tunnel with no-slip interface), and 12.5 (supported tunnel with full-slip interface) for axial bending or snaking. These ratios are also the limits of applicability of quasi-static (instead of dynamic) numerical simulations to estimate the seismic behavior of the liner and the surrounding ground.</p>

Concevoir des moyens de mobilité plus sûrs et plus légers est un défi majeur des constructeurs automobiles. Dans ce contexte, l'intégration des matériaux dans le pneumatique a elle aussi été soumise à de nouvelles exigences : concilier performances et économie d'énergie. Les renforts traditionnels à forte densité comme les tréfilés d'acier sont peu à peu et en partie remplacés par des matériaux polymères hautes-performances plus légers tel que le para-aramide. Le Kevlar® est le nom industriel du composé polyamide aromatique : poly(paraphénylène téréphtalamide), intégré à l'architecture du pneumatique sous forme d'un fil torsadé. La structure rigide et fortement orientée de l'aramide confère à ce polymère de hautes performances mécaniques, telles qu'un module élevé dans la direction longitudinale, de l'ordre de 85GPa, et une grande résistance mécanique de plus de 2.8GPa. Les hautes performances de cette fibre de 15 micromètres de diamètre sont dues à son organisation multi-échelles isotrope transverse avec des liaisons covalentes dans la direction longitudinale et des liaisons de plus faibles énergies dans la direction radiale. L'objectif de cette étude est de comprendre, à l'échelle de la fibre unitaire, les corrélations entre l'architecture microstructurale et la réponse mécanique dans les directions longitudinale et transverse. Une approche expérimentale multi-échelle a été adoptée (Extensometrie Raman, DRX, MEB, caractérisation mécanique sur fibre unitaire), couplée à l'outil numérique afin d'apporter des nouveaux éclairages sur les micro-mécanismes de dissipation. Ce travail a mené à une identification expérimentale du comportement mécanique anisotrope ainsi qu'à établir une limite de plasticité transverse. De plus, grâce à l'approche numérique, une architecture cœur/peau a été mise en avant en modélisant le comportement par une loi viscoélasto-viscoplastique anisotrope. Enfin, des éléments sur le couplage thermo-mécanique sont apportés en vue de mieux comprendre le cycle de vie de la fibre au sein du pneumatique. / Designing safer and lighter vehicles is a major challenge for manufacturers. Nowadays, a vehicle needs to be eco-friendly and conciliate efficiency and energy-saving. Considering these requirements, tire materials are subject to change: high performance polymers are a good replacement, in terms of weight and dissipation, for traditional reinforcements such as drawn steel. For instance, aramid strands (1000 fibres) are used because the single fibre exhibits good mechanical properties such as its high modulus (85 GPa) conferred by its anisotropy or its high temperature resistance. The mechanical performance of a Kevlar® fibre is due to its different scale organisation : the primary (molecular chains held by covalent bonds), the secondary (pleated sheets held by interactions) and the tertiary structure (sheets stacked together). Because of the cooling thermodynamics during the fabrication process, the 15 microns diameter fibre seems to have a skin/core structure with punctual more or less critical defects. To go further in the understanding of the complex structure, the contribution of the skin/core structure in the mechanical performance in the longitudinal and the transverse directions is investigated through a multi-disciplinary approach made of a numerical and an experimental study. During its use, an aramid single fibre undergoes cyclic multiaxial loading and harmful thermal treatments, at the origin of structural and mechanical properties modifications but also dissipative behaviour evolution, still misunderstood. To deal with these change in depth, an experimental and numerical multi-scale characterisation is used. Mechanical and thermal treatments are realised and their impact on the microstructure, on the deformation micromechanisms and on the mechanical properties including the dissipative behaviour are investigated. Limiting use values in terms of temperature, longitudinal and transverse stresses are highlighted in this work in order to understand modifications enhanced by the fibre life cycle.

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