Portail HAL Ensta
Not a member yet
11080 research outputs found
Sort by
Size effects in the toughening of brittle materials by heterogeneities: A non-linear analysis of front deformations
No full textInternational audienceTraditional computational approaches in simulating crack propagation in perfectly brittle materials rely on the estimate of stress intensity factors along the rupture front. This proves highly challenging in 3D when the crack geometry departs from very specific cases for which analytical solutions are available, like e.g. the penny-shaped crack geometry. Here, we extend the first-order theory of Gao and Rice (1987), and predict the distribution of the mode I stress intensity factor KI along the front of a tensile coplanar crack that is slightly perturbed from a reference penny-shaped configuration, up to second order in the perturbation amplitude. Our theory is validated against analytical solutions available for embedded elliptical cracks, and its range of validity is further assessed using numerical simulations performed on cosine front perturbations of varying mode and amplitude. It is then used to develop a homogenization framework for the toughness of weakly disordered media. The effective toughness and its fluctuations are bridged quantitatively to the intensity of the toughness fluctuations and their spatial structure. Our theoretical predictions are compared to the results of ~1 million simulations of crack propagation building on our second-order theory and Fast Fourier Transforms. We show that the impact of toughness heterogeneities is size-dependent, as they generally weaken the material when the crack size is lower or comparable to the typical heterogeneity size, but reinforces it otherwise. It results in an apparent R-curve behavior of the brittle composite at the macroscale
Robot self-recognition via facial expression sensori-motor learning
No full textInternational audienc
Inverse Scattering Using a Kohn-Vogelius Formulation and Shape Optimization Method
No full textInternational audienceThis paper presents a method to retrieve the form of a metallic object given partial electromagnetic measurements. We propose a numerical resolution of this inverse problem based on a shape optimization method. More precisely, we aim to minimize the so-called Kohn-Vogelius functional, which is numerically more stable than the least squares functional, by computing its shape gradient. The optimization problem is then solved using a Nesterov inertial scheme to accelerate the descent algorithm. Numerical simulations in 2D are provided to highlight the efficiency of the proposed method
Fracture properties evaluation of structural adhesives using the dual actuator test frame and digital image correlation technique
No full textInternational audienceThis paper implements a dual actuator testing protocol to evaluate the fracture behavior of a toughened epoxy adhesive under mode I and mixed mode I+II loading conditions. Digital image correlation is used to monitor crack propagation and assess adherend and adhesive deformations. A good repeatability between the test results is observed for both failure modes (I and I+II). The constitutive law is obtained by evaluating the J-integral while measuring the crack tip opening displacement. Different mixed mode test strategies are developed in this work. First, an actuator load control protocol is implemented so as to propagate the crack under constant mixed mode I+II ratio and evaluate the fracture envelope of the studied adhesive, which turns out to be adequately described by the Gong-Benzeggagh failure criterion. A second strategy is proposed with a variable mixed mode ratio during the experiment for faster investigation. Using this technique, stable fracture energy values are recorded along the crack propagation path, thus leading to very fast and accurate results
Mechanical characterisation of Aerosil-polycarbonate-based ceramic nanocomposites: 3D printing versus injection moulding technology
No full textInternational audienceThe 3D printing of composite materials especially, nanocomposites, is an essential step in exploring new perspectives for the applications of organic matrix composite materials for industrial applications. In this work, the effect of ceramic nanofillers on the mechanical properties of a thermoplastic matrix is studied using different wt% of nanofillers (AEROSIL). The results showed that the increase in the wt% of AEROSIL resulted in an increase in the mechanical properties in terms of hardness, stiffness, ductility, and tensile strength. Moreover, these samples were 3D printed and were compared with the samples prepared by conventional injection moulding. The comparison characterized the influence of the manufacturing method on the mechanical performance of materials. Although there was very little behavior difference in both samples, the 3D-printed samples showed a weight reduction. This has broadened the possible applications of this material and technique in areas where weight is of utmost significance
Palladium-Catalyzed Regioselective Halogenation of 1-Naphthaldehydes: Direct Access to Natural Product Skeletons
No full textInternational audienc
Spectrum of the Dirichlet Laplacian in a thin cubic lattice
No full textInternational audienceWe give a description of the lower part of the spectrum of the Dirichlet Laplacian in an unbounded 3D periodic lattice made of thin bars (of width ε ≪ 1) which have a square cross section. This spectrum coincides with the union of segments which all go to +∞ as ε tends to zero due to the Dirichlet boundary condition. We show that the first spectral segment is extremely tight, of length O(e−δ/ε), δ > 0, while the length of the next spectral segments is O(ε). To establish these results, we need to study in detail the properties of the Dirichlet Laplacian AΩ in the geometry Ω obtained by zooming at the junction regions of the initial periodic lattice. This problem has its own interest and playing with symmetries together with max–min arguments as well as a well-chosen Poincaré–Friedrichs inequality, we prove that AΩ has a unique eigenvalue in its discrete spectrum, which generates the first spectral segment. Additionally we show that there is no threshold resonance for AΩ, that is no non trivial bounded solution at the threshold frequency for AΩ. This implies that the correct 1D model of the lattice for the next spectral segments is a system of ordinary differential equations set on the limit graph with Dirichlet conditions at the vertices. We also present numerics to complement the analysis
Voicing Things in Space in a Post-Planetary Age: The Scientific, the Hard, and the Weird
No full textInternational audienceThis essay offers a re-interpretation of the notion of secularization as disenchantment by focusing on the history of extraterrestrial music within American literature and culture. It argues that if modernity disenchanted the Earth, the extraterrestrial remained magical. Yet with the coming of the Space Age, the now historical heavens did enter a process of disenchantment. This made itself felt within the attunements of lyrical writing. As humans and robots visited the moon and other planets, near space lost its magic and became available to extractivist endeavors and imaginaries, even as magical exoplanets appeared deeper in space and the song of the Earth as heard from space became more distinct. Thinking beyond inherited oppositions between earth and heaven, this essay seeks to highlight how the astropastoral strains in American lyric can elicit critical resistance to Anthropocene’s the new space economy
