Laboratoire de Mécanique, Multiphysique, Multiéchelle

International audience

In recent years, the advancement of artificial intelligence techniques has led to significant interest in reinforcement learning (RL) within the traffic and transportation community. Dynamic traffic control has emerged as a prominent application field for RL in traffic systems. This paper presents a comprehensive survey of RL studies in dynamic traffic control, addressing the challenges associated with implementing RL-based traffic control strategies in practice, and identifying promising directions for future research. The first part of this paper provides a comprehensive overview of existing studies on RL-based traffic control strategies, encompassing their model designs, training algorithms, and evaluation methods. It is found that only a few studies have isolated the training and testing environments while evaluating their RL controllers. Subsequently, we examine the challenges involved in implementing existing RL-based traffic control strategies. We investigate the learning costs associated with online RL methods and the transferability of offline RL methods through simulation experiments. The simulation results reveal that online training methods with random exploration suffer from high exploration and learning costs. Additionally, the performance of offline RL methods is highly reliant on the accuracy of the training simulator. These limitations hinder the practical implementation of existing RL-based traffic control strategies. The final part of this paper summarizes and discusses a few existing efforts which attempt to overcome these challenges. This review highlights a rising volume of studies dedicated to mitigating the limitations of RL strategies, with the specific aim of enhancing their practical implementation in recent years.

This research investigates the fire resistance of novel geopolymer (GP) foams, based on alkali-activated metakaolin and silica fume (SF). Fresh GP foams are applied as coatings on steel plates. After one week curing, the foams are subjected to a flame burn-through test. Changes in their physico-chemical properties are characterized before and after fire test, mainly with XRD , quantitative MAS NMR, electron probe micro-analysis, quantitative X ray micro-computed tomography and heat conductivity . Results show that GP foams are excellent thermal barriers, providing up to 251 °C less than for uncoated steel plate. Their porosity ranges between 25 and 81%, for typical pore sizes d 50 from 0.5 to 3.0 mm 29 Si MAS NMR shows that the proportion of GP cement only decreases from 60-68% to 53–58% after fire. SF expands and creates small pores in the coating, which is favorable to decrease heat conductivity by a factor of 2 whatever the foam. • Novel GP foams based on MK + SF are formulated and characterized for fire resistance. • H 2 O 2 and CTABr foam stabilizing agent provide a controlled porosity of 81%. • GP foams provide superior fire protection to steel in a burn-through scenario. • The amount of GP cement only decreases from 60-68% to 53–58% after fire test. • Due to SF expansion, heat conductivity of foams is reduced by a factor of 2.

This study is part of numerical simulations performed on an in situ heating test conducted by the French National Radioactive Waste Management Agency (Andra) at the Meuse/Haute-Marne Underground Research Laboratory (URL) to study the thermal-hydromechanical behavior of the host Callovo-Oxfordian COx claystone in quasi real conditions, through the international research project DECOVALEX. The emphasis is put on the pore pressure increase generated by the heat released from high level radioactive waste and the consequence on the damage evolution of the host rock. A phase-field method is proposed to describe the evolution of damaged zones around the heating borehole. Both tensile and shear cracks are taken into account. The evolution of damage is coupled with temperature variation, rock deformation, and fluid pressure change. Moreover, the structural anisotropy of elastic properties, permeability, and heat conductivity of the host rock is taken into account. Numerical results are compared with in-situ measurements.

The anisotropic mechanical behavior of rocks under high-stress and high-temperature coupled conditions is crucial for analyzing the stability of surrounding rocks in deep underground engineering. This paper is devoted to studying the anisotropic strength, deformation and failure behavior of gneiss granite from the deep boreholes of a railway tunnel that suffers from high tectonic stress and ground temperature in the eastern tectonic knot in the Tibet Plateau. High-temperature true triaxial compression tests are performed on the samples using a self-developed testing device with five different loading directions and three temperature values that are representative of the geological conditions of the deep underground tunnels in the region. Effect of temperature and loading direction on the strength, elastic modulus, Poisson's ratio, and failure mode are analyzed. The method for quantitative identification of anisotropic failure is also proposed. The anisotropic mechanical behaviors of the gneiss granite are very sensitive to the changes in loading direction and temperature under true triaxial compression, and the high temperature seems to weaken the inherent anisotropy and stress-induced deformation anisotropy. The strength and deformation show obvious thermal degradation at 200 °C due to the weakening of friction between failure surfaces and the transition of the failure pattern in rock grains. In the range of 25 °C–200 °C, the failure is mainly governed by the loading direction due to the inherent anisotropy. This study is helpful to the in-depth understanding of the thermal-mechanical behavior of anisotropic rocks in deep underground projects.

Urease is involved in the formation of carbonate sediments by microbial-induced calcium carbonate precipitation (MICP), and Sporosarcina pasteurii used extensively in this technique owing to its high urease production. In this study, a simple two-step culture method with the appropriate medium was developed to enhance the urease activity of S. pasteurii. Urea played an important role in the culture process, particularly during the pre-cultivation step and the newly developed method improved both urease activity and specific urease activity. Furthermore, the increase in urease activity by MICP resulted in increased production of calcium carbonate and better strength of bio-cemented sand.

Nowadays, additive manufacturing of metallic materials is most often carried out using expensive and complex tools that leave the user with limited control and no possibility of modification. In order to make the printing of metal parts more accessible to small structures but also better suited for academic research, the use of a mixture of thermoplastic polymer and metal powder is a good solution as many granular feedstocks already exist for Metal Injection Molding applications. To perform the shaping process, the Fused Granular Fabrication 3D printing technology is set up by diverting the use of a feedstock in the form of pellets that are directly inserted into the print head. This solution, which is less costly, is implemented here by modifying a mid-range printer, the Tool Changer from E3D, and by making the hardware and software adaptations to mount a compact granulates extruder on it, which is also available on the market. The polymer portion present in the green part can then be removed in order to perform the heat treatments that will densify the powder by sintering and give a fully metallic dense object.

This paper presents an economic analysis of manufacturing geopolymer bricks for use in the construction sector. The manufacturing processes of both geopolymer bricks and traditional fired bricks were investigated. For this study, we collected and analyzed all phases of geopolymer brick production from the extraction of raw materials to storage. Seven formulations of geopolymer bricks based on clay and waste bricks were analyzed. We considered the cost of raw materials and logistics operations in the production line of brick manufacturing. The results of this study prove that the manufacturing cost of geopolymer bricks based on clay provides an economic gain of 5% compared to fired bricks for the same compressive strength of 20 MPa. In the case of waste bricks, for the same production cost, the compressive strength of the geopolymer bricks is double that of fired bricks. Hence, this study shows the economic interest in the industrial production of geopolymer bricks. It also confirms that future research is needed that focuses on necessary changes to the current industrial production chain required for the manufacture of geopolymer bricks.

Abstract Cracking property and brittleness are critically important to the drillability of injection and production wells of enhanced geothermal systems. This paper is devoted to evaluating the cracking property and brittleness of the Gonghe granite under high-temperature true triaxial compression conditions through a series of laboratory tests. Thermal–mechanical coupled true triaxial compression tests were conducted on Gonghe granite samples under four representative temperatures (20 °C, 120 °C, 180 °C, and 240 °C) and three different minimum (5, 10, and 30 MPa) and intermediate (40, 60, and 75 MPa) principal stresses that correspond to the in-situ conditions of Gonghe geothermal reservoir. The strength, deformation, and cracking characteristics of the Gonghe granite are quantitatively evaluated from macro- to micro-scales based on the experimental results. Meanwhile, a novel brittleness index evaluation method considering both energy and failure surface roughness is proposed to accurately assess the brittleness strength of the Gonghe granite. We found that the strength of Gonghe granite is reduced by more than 20% when the temperature increases from room temperature to 240 °C. Higher temperature contributes to a smoother fracture surface and reduced brittleness of the Gonghe granite under thermal–mechanical coupled true triaxial compression. Our findings provide new insights for brittleness evaluation of the Gonghe granite formation and assist in efficient wells drilling design of enhanced geothermal systems.

Some micromechanics-based constitutive models are presented in this study for porous geomaterials. These micro-macro mechanical models focus on the effect of porosity and the inclusions on the macroscopic elastoplastic behaviors of porous materials. In order to consider the effect of pores and the compressibility of the matrix, some macroscopic criteria are presented firstly for ductile porous medium having one population of pores with different types of matrix (von Mises, Green type, Mises–Schleicher and Drucker–Prager). Based on different homogenization techniques, these models are extended to the double porous materials with two populations of pores at different scales and a Drucker–Prager solid phase at the microscale. Based on these macroscopic criteria, complete constitutive models are formulated and implemented to describe the overall responses of typical porous geomaterials (sandstone, porous chalk and argillite). Comparisons between the numerical predictions and experimental data with different confining pressures or different mineralogical composites show the capabilities of these micromechanics-based models, which take into account the effects of microstructure on the macroscopic behavior and significantly improve the phenomenological ones.

In this study, through severe reduced-scale braking tests, we investigate the wear and integrity of organic matrix brake pads against gray cast iron (GCI) discs. Two prototype pad materials are designed with the aim of representing a typical non-metal NAO and a low-steel (LS) formulation. The worn surfaces are observed with SEM. The toughness of the pad materials is tested at the raw state and after a heat treatment. During braking, the LS-GCI disc configuration produces heavy wear. The friction parts both keep their macroscopic integrity and wear appears to be homogeneous. The LS pad is mostly covered by a layer of solid oxidized steel. The NAO-GCI disc configuration wears dramatically and cannot reach the end of the test program. The NAO pad suffers many deep cracks. Compacted third body plateaus are scarce and the corresponding disc surface appears to be very heterogeneous. The pad materials both show similar strength at the raw state and similar weakening after heat treatment. However, the NAO material is much more brittle than the LS material in both states, which seems to favor the growth of cracks. The observations of crack faces suggest that long steel fibers in the LS material palliate the brittleness of the matrix, even after heat damage.

Rock mechanical property testing under high-temperature true triaxial compression conditions is significantly important to understanding the stability of deep underground engineering rock masses where high temperature exists. This article presents the development, calibration and application of a self-designed true triaxial test system for investigating rock mechanical behaviors under coupled high-temperature and high-pressure conditions with cuboid rock samples. The test system is consisted of four independent parts, respectively for mechanical loading in three axis, thermal loading up to 250 °C, deformation and temperature measurement, and the data acquisition center. The key technology and calibration method of the measurement sensors were introduced. The primary testing results of Jinping marble, Bayu granite and Longchang sandstone under high-temperature and high-pressure coupled true triaxial conditions were obtained. It shows that the peak strength, elastic modulus and fracture mode vary with rock types and temperatures. The mechanical property of Jinping marble increases with the increase of temperature in the tested temperature range, while Longchang sandstone shows a decreasing trend, and the change of Bayu granite is relatively small. The fracture mode of Jinping marble, Bayu granite Longchang sandstone at different temperatures is compressive-shear fracture. The results verify the reliability of the test system, and provides a new platform for understanding the mechanical properties of rocks of deep underground engineering with high temperatures.

Today, the development of brake pads is carried out by empirical feedback validated by experimental bench tests. Nevertheless, these test campaigns are time-consuming, costly and ultimately do not help to understand phenomena for improving performances (coefficient of friction, durability, noise, etc.). To overcome these limitations, numerical braking models have been developed for several years. They need to be representative of the experimental reality in order to predict the current performance of brake systems and ultimately improve them. The difficulties, especially for high energy dissipation applications such as high-speed train (HST) braking systems, lie in the fact that thermomechanical coupling is important to consider, which particularly affects the behaviour of brake linings. According to the literature, this thermomechanical evolution is not sufficiently taken into account. In this work, a complete methodology is proposed for the identification of the behaviour of friction materials under coupled mechanical and thermal loads . In a second step, the properties obtained are injected into a simulation of a real and representative braking system. An experimental test with enriched instrumentation on a HST braking system configuration is separately conducted. A comparison between numerical and experimental results is carried out to validate the complete approach. Finally, based on the good results provided by the model, an optimization on the brake pas design is proposed, which allows a better distribution of the thermo-mechanical sollicitations and wear reduction.

Direct shear tests were performed to study the influence of concrete–rock bonds and roughness on the shear behavior of concrete–rock interfaces. The results of these tests show that the shear behavior of concrete–hardrock interfaces depends on the micro-roughness driving the formation of strong concrete–rock bonds and on the macro-roughness accounting for the influence of the surfaces interlocking. Based on this outcome and recent literature, a cohesive frictional model is used to simulate direct shear tests of bonded concrete–granite interfaces with the explicit representation of naturally rough interfaces. The results of these simulations show that the model has good prediction capability compared to the experimental results, opening up the pathway to numerically based robust statistical analysis.

Abstract In this contribution, we present a space-time formulation of the Newmark integration scheme for linear damped structures under both harmonic and transient excitations. The incremental set of equations of motion and the Newmark approximations are transformed into their corresponding space-time equivalents. The dynamic system is then represented by one algebraic space-time equation only. This equation is projected into a coupled pair of space-time equations, which is solved via the fixed point algorithm. The solution is iteratively assembled by enrichments, each of which is decomposed by a dyadic product of spatial and temporal enrichment vectors. The evolution of the spatial enrichment vectors is investigated during convergence and interpreted by comparing them to the set of linear modes of vibration. The new method is demonstrated by means of four numerical examples, presenting not only the excellent convergence behavior and the numerical efficiency but also the limits of the proposed approach.

Modified static and dynamic shear tests are introduced to study the shear behavior of concrete-rock interfaces under static and dynamic loading in the context of low confinement stresses. Static and dynamic shear tests of concrete-sandstone and concrete-granite interfaces are performed using these techniques. Three levels of interface roughness are considered: smooth, bush-hammered, and rough rock surfaces. The results of these tests show that in both static and dynamic regimes, the shear evolution of concrete-rock interfaces can be described according to three successive stages: the shear stress accumulation, the shear slip, and the residual shear stress stage. The main parameters driving the shear process are the concrete-rock bonds, the interface roughness, and the residual friction. However, unlike in the static shear evolution, in the dynamic shear evolution, the concrete-rock bonds and the roughness seem active in the shear stress accumulation stage. Furthermore, the correlation between the shear strength and the normal stress is stronger in static than dynamic conditions. The significance of the normal stress on the dynamic shear strength appears more important in rough concrete-granite interfaces than in the other two interfaces. Lastly, the dynamic peak shear strengths of all the interfaces tested are three to four times higher than their static counterparts. • A new testing method to investigate the dynamic shear behavior of interfaces under low confinement. • The formation of concrete-sandstone bonds does not require the presence of roughness. • Friction and bond strength drive the pre-peak stage of the dynamic shear evolution. • The dynamic shear strength is independent of the normal stress σ n , when σ n ≤ 2 MPa . • Estimation of the dynamic increase factor (3-4) for shearing concrete-rock interfaces.

The lack of knowledge on the link between the manufacturing process and performance constitutes a major issue in brake lining development. The manufacturing process of organic brake friction composite materials includes several steps (mixing, preforming, hot molding and post-curing), which define their final microstructure, properties and performances. This study focuses on the effect of mixing duration on the microstructure, properties and tribological behavior of organic friction composite materials. The adopted methodology is based on simplified formulations effective in limiting synergistic effects by reducing the number and size distribution of constituents. Two simplified materials are here developed according to the mixing duration of the constituent introduction sequence. The microstructural characteristics are studied using 2D and 3D analyses, and then correlated with the thermophysical and mechanical properties. Wear mechanisms and tribological behavior are studied in relation to the microstructure and properties of the materials. The results show the effect of mixing duration as regards particle distribution and fiber arrangement. The distribution and size of fiber entanglements contribute to the formation of carbonaceous particle clusters, which create bulk bridges improving thermal conductivity. Moreover, the arrangement of rock fibers affects density, porosity and thermo-physical properties. In addition, the mixing disrupts the cohesion of fiber bundles with the matrix, affecting compressive modulus and wear behavior. This microstructural defect also fosters abundant third-body source flow, which disturbs the tribological circuit and behavior. Porosities induced by fiber entanglements, having a large and irregular size and distribution on the frictional surface, result in low wear resistance and alter the frictional stability.

Decellularized matrices are an attractive choice of scaffold in regenerative medicine as they can provide the necessary extracellular matrix (ECM) components, signals and mechanical properties. Various detergent-based protocols have already been proposed for decellularization of skeletal muscle tissue. However, a proper comparison is difficult due to differences in species, muscle origin and sample sizes. Moreover, a thorough evaluation of the remaining acellular matrix is often lacking. We compared an in-house developed decellularization protocol to four previously published methods in a standardized manner. Porcine skeletal muscle samples with uniform thickness were subjected to in-depth histological, ultrastructural, biochemical and biomechanical analysis. In addition, 2D and three-dimensional cytocompatibility experiments were performed. We found that the decellularization methods had a differential effect on the properties of the resulting acellular matrices. Sodium deoxycholate combined with deoxyribonuclease I was not an effective method for decellularizing thick skeletal muscle tissue. Triton X-100 in combination with trypsin, on the other hand, removed nuclear material but not cytoplasmic proteins at low concentrations. Moreover, it led to significant alterations in the biomechanical properties. Finally, sodium dodecyl sulphate (SDS) seemed most promising, resulting in a drastic decrease in DNA content without major effects on the ECM composition and biomechanical properties. Moreover, cell attachment and metabolic activity were also found to be the highest on samples decellularized with SDS. Through a newly proposed standardized analysis, we provide a comprehensive understanding of the impact of different decellularizing agents on the structure and composition of skeletal muscle. Evaluation of nuclear content as well as ECM composition, biomechanical properties and cell growth are important parameters to assess. SDS comes forward as a detergent with the best balance between all measured parameters and holds the most promise for decellularization of skeletal muscle tissue.

<div class="htmlview paragraph">During braking, third-body flows and layers govern friction mechanisms, which are fully responsible of the friction coefficient and wear. In the context of development of brake friction pairs, the involved tribological circuit has to be well understood and mastered. This paper concerns a sintered metal matrix composite used for TGV very high speed train. A series of low-energy stop brakings allows a detailed study of the third-body formation at the pad-disc contact. The pin surface is observed after each test. The evolution of the rubbing-area expansion all along the series is explained, and the friction behaviour, typical of the studied friction material, is related to the formation of a well-established third body at the pad-disc interface.</div>

Abstract Identification of an individual artist’s touch on paintings is studied using surface metrology. Paintings’ topographies were measured using focus variation and stitching, creating 13 × 13 mm maps with 1 μ m sampling intervals, and 169 megapixels, with a 10X objective lens. Topographic characterization parameters were analyzed for their ability to differentiate different painters’ renderings. Statistical treatments from data mining were used to discriminate, by optimization, multiscale topographic signatures characterized by a multitude of areal texture parameters. It appears that a fractal dimension can define 3 characteristic scale ranges. One from 3 to 70 μ m corresponds to brushstroke details. Another, from 70 to 700 μ m, corresponds to the topography of the material of the canvas fabric. Finally, scales greater than 700 μ m correspond to undulations of the canvas. For scales less than 50 μ m, the fractal structure of the topography left by brushstrokes follows a power law characterized by the slopes of the topography. The topography of the clouds painted on the canvas has an Sdq (topographic slopes) increasing with the clarity of the clouds at scales of 3–500 μ m. According to the Torrance-Sparrow theory, the higher the Sdq, the more diffuse the light on the surface. The painter therefore wanted to show, by his brushstroke, that the light clouds diffuse more light giving an impression of local brightness. This study is confirmed by the analysis of the painting of Max Savy, a French painter from Carcassonne (1918–2009), which was measured with a white light interferometer Zygo NewView 7300, a X100 objective lens giving a 517 μ m × 517 μ m stitched surface, with a sampling interval of 0.109 μ m. The box-counting method for estimating the fractal dimension of the topography of an oil painting appears optimal by the fact that it morphologically integrates scale variations of the local slopes of the surface morphology. This method thus characterizes the multiscale aspects, as well as the scale changes, of the topography.