HAL-Ecole des Ponts ParisTech
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Residual stress control in large-format additive manufacturing of polylactic acid via a digital twin and in-operando imaging
No full textInternational audiencePolymer-based Large Format Additive Manufacturing (LFAM) is an extrusion-based technology that deposits large-diameter polymer beads using a robotic arm-mounted nozzle. However, slow cooling rates and heat accumulation generate technical challenges, including significant deformation that requires nozzle path adjustments and the buildup of residual stresses from thermo-chemical shrinkage that may cause debonding. This study integrates two fast modeling approaches, ScanFast (thermal) and QuadWire (mechanical), to reduce the number of degrees of freedom compared to conventional methods while maintaining accuracy. A computationally efficient digital twin of the process is developed and validated experimentally on a thin wall printed with polylactic acid. Anisotropic material properties are characterized, and in-operando temperature and displacement fields are measured using infrared thermography and backward Digital Image Correlation. The results show correlation coefficients greater than 0.80 between experimental and numerical data. The validated digital twin is then applied to assess the influence of process parameters on three key aspects: (i) the number of layers above the glass transition temperature, (ii) residual stress development, and (iii) positional offset between the nozzle and the structure. The proposed approach provides an efficient tool to optimize process parameters and nozzle trajectories, thereby enhancing the quality and manufacturability of LFAM-produced parts
Online computation of normalized substring complexity
No full textThe normalized substring complexity of a string is defined as , where is the number of \textit{distinct} substrings of length . This simply defined measure has recently attracted attention due to its established relationship to popular string compression algorithms. We consider the problem of computing online, when the string is provided from a stream. We present two algorithms solving the problem: one working in amortized time per character, and the other in worst-case time per character. To our knowledge, this is the first polylog-time online solution to this problem
I can't forget about U: lifetime unemployment and retirement wellbeing
No full textInternational audienceIt is well known that unemployment leaves scars after re-employment, but does this scarring effect persist even after retirement? We analyse European data on retirees from the SHARE panel, and show that the wellbeing of the retired continues to reflect the unemployment that they experienced over their working life. These scarring effects are somewhat smaller for older retirees, but larger for those who arguably had higher expectations regarding the labour market when they were active. The lower wellbeing from lifetime unemployment does not reflect lower retirement income. This long-run scarring for those who have left the labour market underlines that contemporaneous correlations significantly underestimate the wellbeing cost of unemployment
Energy Transition Pathways: From Resource Depletion to the Rise of Recycling
No full textInternational audienc
Finding a Shortest Curve that Separates Few Objects from Many
No full textInternational audienceWe present a fixed-parameter tractable (FPT) algorithm to find a shortest curve that encloses a set of k required objects in the plane while paying a penalty for enclosing unwanted objects.The input is a set of interior-disjoint simple polygons in the plane, where k of the polygons are required to be enclosed and the remaining optional polygons have non-negative penalties. The goal is to find a closed curve that is disjoint from the polygon interiors and encloses the k required polygons, while minimizing the length of the curve plus the penalties of the enclosed optional polygons. If the penalties are high, the output is a shortest curve that separates the required polygons from the others. The problem is NP-hard if k is not fixed, even in very special cases. The runtime of our algorithm is O(3^k.n^3), where n is the number of vertices of the input polygons.We extend the result to a graph version of the problem where the input is a connected plane graph with positive edge weights. There are k required faces; the remaining faces are optional and have non-negative penalties. The goal is to find a closed walk in the graph that encloses the k required faces, while minimizing the weight of the walk plus the penalties of the enclosed optional faces. We also consider an inverted version of the problem where the required objects must lie outside the curve. Our algorithms solve some other well-studied problems, such as geometric knapsack
Coffrages architecturés gonflables : conception, expérimentation, réalisation et modélisation
No full textThis doctoral thesis contributes to the ongoing reflection on the evolution of construction practices in response to the climate emergency. The building sector, responsible for nearly 37% of global greenhouse gas (GHG) emissions, must transform profoundly to reduce its environmental impact. One identified lever involves limiting the use of concrete, an omnipresent material with a high carbon footprint due to the production of one of its key components, cement. Optimizing design strategies is a relevant approach, within the reach of architects and engineers, to reduce the volume of concrete used. Thin reinforced concrete shells, capable of spanning large distances with minimal material through efficient structural performance, represent significant potential. However, their application remains marginal today, hindered by the high costs of manufacturing the temporary formworks required, often custom-made for curved geometries.This research aims to develop an innovative, cost-effective formwork system to make this structural typology more accessible. Inflatable structures have been identified as promising solutions for the basis of formwork. Made from standard thin and flexible materials shaped and stiffened by air pressure, these structures are lightweight, compact, and potentially reusable. Inflatable formwork emerges as a cost-effective alternative to traditional formwork systems.This research is structured in four stages. First, an overview of existing formwork solutions for concrete shells is presented, highlighting current technical and economic barriers. The opportunities offered by inflatable structures are then emphasized. The second stage focuses on these structures' design and fabrication challenges through a structural-morphological analysis involving five parameters: form, force, structure, material, and technology. Based on this analysis, a specific design and manufacturing principle is selected for the following stages: inflatable structures composed of superimposed planar membranes sealed according to regularly repeated patterns, forming a padding once inflated. In the third stage, these structures are then used to design and fabricate dome-shaped inflatable formwork prototypes. Small-span concrete shells are also cast on these formworks to assess the system's feasibility. This experimentation highlights the flexibility of the inflatable structures, prompting a fourth and final stage dedicated to mechanical and feasibility studies. Inflatable tubes are tested, characterized, and reinforcement strategies are proposed to increase their stiffness.Combining theoretical, experimental, and construction-based approaches, this exploratory research offers an original contribution to civil engineering. It opens a promising development path for an innovative, cost-effective alternative formwork solution compatible with contemporary environmental requirements.Cette thèse s’inscrit dans une réflexion sur l’évolution des pratiques constructives face à l’urgence climatique. Le secteur du bâtiment, responsable de près de 37 % des émissions mondiales de gaz à effet de serre (GES), doit engager une transformation profonde pour réduire son impact. Un des leviers identifiés consiste à limiter l’usage du béton, matériau omniprésent et fortement émetteur de GES dû à la fabrication de l’un de ses constituants, le ciment. L’optimisation dans la conception des projets apparaît alors comme un moyen pertinent à la portée des architectes et ingénieurs pour réduire les volumes de béton utilisés. Les coques minces en béton armé, capables de franchir de grandes portées avec une quantité minimisée de matière grâce à une utilisation efficace, représentent un réel potentiel. Toutefois, leur usage est aujourd’hui marginal, freiné par les coûts de fabrication liés aux structures temporaires que sont les coffrages de forme courbe souvent réalisés sur mesure. L’objectif de ce travail est de développer un coffrage économique innovant pour rendre à nouveau accessible cette typologie de structure. Les structures gonflables sont identifiées comme pertinentes pour constituer la base du coffrage. Conçues à partir de matériaux standard fins et souples mis en forme et raidis grâce à la pression de l’air, elles sont légères, compactes et potentiellement réutilisables. Le coffrage gonflable est alors une bonne alternative économique aux coffrages traditionnels. Ces travaux s’organisent en quatre temps. Dans un premier temps, un panorama des solutions de coffrage existantes pour coques est dressé, mettant en lumière les verrous techniques et économiques actuels. Les opportunités offertes par les structures gonflables sont ainsi mises en avant. Le second s’attache aux enjeux de conception et de fabrication de ces structures, à travers une analyse morpho-structurelle mobilisant cinq paramètres : forme, force, structure, matériau et technologie. Pour la suite des travaux, un principe de conception et de fabrication est privilégié : des structures gonflées constituées de membranes superposées à plat et liaisonnées selon des motifs régulièrement répétés pour former un capitonnage un fois gonflées. Ces dernières sont utilisées pour, dans un troisième temps, concevoir et réaliser des prototypes de coffrages gonflés en forme de dômes. Des coques de petite portée sont également fabriquées sur ces coffrages gonflés pour évaluer la faisabilité du système. Cette expérience met en évidence la souplesse des structures gonflées, une étude mécanique est alors menée dans un quatrième et dernier temps. Des tubes gonflés sont testés, caractérisés et des stratégies de renforcement sont proposées pour accroître leur raideur. Cette recherche exploratoire, à la fois théorique, expérimentale et constructive, propose une contribution originale au champ du génie civil. Elle ouvre une voie de développement pour un coffrage alternatif innovant et économique, compatible avec les exigences environnementales contemporaines
Creep of rock salt under a large range of deviatoric stresses: Insights from tests in mines and rock mechanics laboratories
No full textInternational audienceThe mechanical behavior of rock salt has been most often predicted using phenomenological laws fitted on laboratory tests conducted on centimeter-scale samples. To minimize the impact of stress and temperature extrapolation, such experiments should be conducted along loading paths relevant to underground operations. However, the viscoplastic response of rock salt has been primarily investigated using confined creep tests covering a differential stress range between 5 and 20 MPa, and varying temperatures. More recently, lower deviatoric levels have been investigated, primarily through unconfined creep tests in remote drifts in underground mines. The joint analysis of experimental data in the two differential stress ranges is often delicate due to the use of (i) different salt facies/geographical origins, (ii) different sample preparation and preconditioning methods, and (iii) different scales of displacement measurements. We present results of two creep tests conducted using very similar loading conditions on two salt samples of the same provenance and prepared using the same protocol. One test is performed in a mine drift and the other is performed in a rock mechanics laboratory. The results are consistent with each other, which is promising to enlarge the differential stress range used to develop constitutive models for rock sal
The characterisation of thermal creep for energy pile systems
No full textInternational audienceThe characterisation of thermal creep for energy pile system
Laboratory and numerical investigation on the poro-elasto-visco-plastic behaviour of in-situ heated Callovo-Oxfordian claystone
No full textInternational audienceLaboratory and numerical investigation on the poro-elasto-visco-plastic behaviour of in-situ heated Callovo-Oxfordian clayston
Effects of poro-mechanical coupling on fracture propagation around a pressurized excavation
No full textInternational audienceEffects of poro-mechanical coupling on fracture propagation around a pressurized excavatio