Sainte-Foy-lès-Lyon, France
Sainte-Foy-lès-Lyon, France

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Yang J.,ENSTA | Lacroix R.,ESI France | Bergheau J.-M.,CNRS Tribology and Dynamic Systems Laboratory | Leblond J.-B.,Alembert Boite 162 | Perrin G.,AREVA
MATEC Web of Conferences | Year: 2016

A "micromorphic", second-gradient model applicable to ductile porous materials has been proposed, as an improvement from the fundamental work of Gurson that take into account the physical mechanisms responsible for ductile damage. The model has been applied to the study of fracture of the decarburized layer of a Dissimilar Metal Weld. The model successfully reproduces the crack path experimentally observed in a notched tensile sample extracted from this weld, different from the one predicted by the first gradient model. © The Authors, published by EDP Sciences, 2016.


Michalski A.,Architectural Office RaschBradatsch | Kermel P.D.,ESI France | Haug E.,ESI France | Lohner R.,George Mason University | And 2 more authors.
Journal of Wind Engineering and Industrial Aerodynamics | Year: 2011

The sensitivity of membrane structures to wind loads due to their flexibility and small inertial masses raises the question of their behavior under natural wind conditions. Particularly transient wind loads could lead to dynamic amplification of the structural response. The assessment of the dynamic response of membrane structures is complex due to their special load carrying behavior, their material properties, and their distinct structural interaction with flow induced effects. Computationally intensive fluid-structure interaction simulation could overcome simplifications and limitations of existing approaches, especially small scale wind tunnel tests, and allow the assessment of all relevant structural and fluid phenomena. This paper outlines a virtual design methodology for lightweight flexible membrane structures under the impact of fluctuating wind loads and provides results on the unique validation of the method at real-scale tests of a highly flexible 29. m umbrella. © 2010 Elsevier Ltd.


Lohner R.,George Mason University | Baqui M.,George Mason University | Haug E.,ESI France | Muhamad B.,Institute for Scientific Architecture
Engineering Computations (Swansea, Wales) | Year: 2016

Purpose - The purpose of this paper is to develop a first-principles model for the simulation of pedestrian flows and crowd dynamics capable of computing the movement of a million pedestrians in real-time in order to assess the potential safety hazards and operational performance at events where many individuals are gathered. Examples of such situations are sport and music events, cinemas and theatres, museums, conference centres, places of pilgrimage and worship, street demonstrations, emergency evacuation during natural disasters. Design/methodology/approach - The model is based on a series of forces, such as: will forces (the desire to reach a place at a certain time), pedestrian collision avoidance forces, obstacle/wall avoidance forces; pedestrian contact forces, and obstacle/wall contact forces. In order to allow for general geometries a so-called background triangulation is used to carry all geographic information. At any given time the location of any given pedestrian is updated on this mesh. The model has been validated qualitatively and quantitavely on repeated occasions. The code has been ported to shared and distributed memory parallel machines. Findings - The results obtained show that the stated aim of computing the movement of a million pedestrians in real-time has been achieved. This is an important milestone, as it enables faster-thanreal- time simulations of large crowds (stadiums, airports, train and bus stations, concerts) as well as evacuation simulations for whole cities. Research limitations/implications - All models are wrong, but some are useful. The same applies to any modelling of pedestrians. Pedestrians are not machines, so stochastic runs will be required in the future in order to obtain statistically relevant ensembles. Practical implications - This opens the way to link real-time data gathering of crowds (i.e. via cameras) with predictive calculations done faster than real-time, so that security personnel can be alerted to potential future problems during large-scale events. Social implications - This will allow much better predictions for large-scale events, improving security and comfort. Originality/value - This is the first time such speeds have been achieved for a micro-modelling code for pedestrians. © Emerald Group Publishing Limited.


Lohner R.,George Mason University | Britto D.,SL Rasch GmbH | Michailski A.,SL Rasch GmbH | Haug E.,ESI France
Engineering Computations (Swansea, Wales) | Year: 2014

Purpose - During a routine benchmarking and scalability study of CFD codes for typical largescale wind engineering runs, it was observed that the resulting loads for buildings varied considerably with the number of parallel processors employed. The differences remained very small at the beginning of a typical run, and then grew progressively to a state of total dissimilitude. A "butterfly-effect" for such flows was suspected and later confirmed. The paper aims to discuss these issues. Design/methodology/approach - A series of numerical experiments was conducted for massively separated flows. The same geometry - a cube in front of an umbrella - was used to obtain the flowfields using different grids, different numbers of domains/processors, slightly different inflow conditions and different codes. Findings - In all of these cases the differences remained very small at the beginning of a typical run, they then grew progressively to a state of total dissimilitude. While the mean and maximum loads remained similar, the actual (deterministic) instantiations were completely different. The authors therefore suspect that for flows of this kind a "butterfly effect" is present, whereby even very small (roundoff) errors can have a pronounced effect on the actual deterministic instantiation of a flowfield. Research limitations/implications - This implies that for flows of this kind the CFD runs have to be carried out to much larger times than formerly expected (and done) in order to obtain statistically relevant ensembles. Practical implications - For practical calculations this implies running to much larger times in order to reach statistically relevant ensembles, with the associated much higher CPU time requirements. Originality/value - This is the first time such a finding has been reported in the numerical wind engineering context. © Emerald Group Publishing Limited.


Cuilliere J.-C.,University of Quebec at Trois - Rivieres | Francois V.,University of Quebec at Trois - Rivieres | Lacroix R.,ESI France
CAD Computer Aided Design | Year: 2016

Through our research on the integration of finite element analysis in the design and manufacturing process with CAD, we have proposed the concept of mesh pre-optimization. This concept consists in converting shape and analysis information in a size map (a mesh sizing function) with respect to various adaptation criteria (refining the mesh around geometric form features, minimizing the geometric discretization error, boundary conditions, etc.). This size map then represents a constraint that has to be respected by automatic mesh generation procedures. This paper introduces a new approach to automatic mesh adaptation around circular holes. This tool aims at optimizing, before any FEA, the mesh of a CAD model around circular holes. This approach, referred to as "a priori" mesh adaptation, should not be regarded as an alternative to adaptive a posteriori mesh refinement but as an efficient way to obtain reasonably accurate FEA results before a posteriori adaptation, which is particularly interesting when evaluating design scenarios. The approach is based on performing many offline FEA analyses on a reference case and deriving, from results and error distributions obtained, a relationship between mesh size and FEA error. This relationship can then be extended to target user specified FEA accuracy objectives in a priori mesh adaptation for any distribution of circular holes. The approach being purely heuristic, fulfilling FEA accuracy objectives, in all cases, cannot be theoretically guaranteed. However, results obtained using varying hole diameters and distributions in 2D show that this heuristic approach is reliable and useful. Preliminary results also show that extension of the method can be foreseen towards a priori mesh adaptation in 3D and mesh adaptation around other types of 2D features. © 2016 Elsevier Ltd. All rights reserved.


Gilles P.,AREVA | Zhang G.,AREVA | Madou K.,ESI France
American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP | Year: 2014

In fracture mechanics, several J-estimation schemes are based on the reference stress approach. This approach has been developed initially in the frame of the R5 rule for creep and R6 rule for elasto-plastic fracture assessments. Later other methods, based on the reference stress concept, where derived like the Js method introduced in the French RSE-M code en 1997 and the Enhanced Reference Stress (ERS) method in Korea around 2001. However these developments are based on the J2 deformation plasticity theory and well established for a pure power hardening law. Even in this latter case, the reference stress depends on the hardening exponent. Js and ERS attempt to minimize this dependence and propose some corrections for recorded behavior laws which cannot be fitted by a power law. However their validation has been established mainly on cases where the material behavior is governed by a Ramberg-Osgood (R0) law. The question may be raised, as for the bilinear hardening law case, of the existence of a reference stress for non RO laws. Copyright © 2014 by ASME.


Robin V.,AREVA | Gilles P.,AREVA | Brosse A.,ESI France | Chaise T.,INSA Lyon
American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP | Year: 2015

Component manufacturing may induce cold work, residual stresses, microstructure changes and even surface defects. This initial condition is usually ignored in component integrity assessments, but can strongly affect its lifetime. For instance, it is well-known that a rough surface finish associated to the presence of tensile residual stresses may favor fatigue damage. In the same manner, cold work and tensile residual stress will assist initiation of Stress Corrosion Cracking (SCC) for susceptible materials. As the manufacturing process can affect the lifetime of the structure, mitigation treatments such as precompressive loadings, chemical treatments, film deposits or coatings may be applied to sensitive areas. The objectives of these complementary operations are to avoid or compensate negative effects of manufacturing consequences. In the industry surface mechanical treatments such as Ultrasonic Shot Peening (USP) are then used in order to improve surface integrity. Even if these mitigation treatments are well known to increase component lifetime regarding corrosion and fatigue damages, a good understanding of their consequences is required to assess their efficiency and perpetuity under operating conditions. Numerical modelling of USP is one solution to simulate the motion of beads in the peening chamber and to predict the level of stresses in the peened part as shown in this paper. This model which gives a better understanding of the effect on surfaces should help the manufacturers to select the best process parameters. Copyright © 2015 by ASME.


Lacroix R.,ESI France | Leblond J.-B.,CNRS Jean Le Rond d'Alembert Institute | Perrin G.,AREVA
European Journal of Mechanics, A/Solids | Year: 2016

Experiments have shown that in porous ductile materials, cyclic loadings lead to lower fracture strains than monotone ones. The effect has been tentatively ascribed to a continued increase of the mean porosity during each cycle with the number of cycles ("ratcheting of the porosity"). In this work, we first perform finite-element-based micromechanical simulations of elementary hollow cells. These cells are initially spherical, contain an initially spherical void and are loaded cyclically through conditions of homogeneous boundary strain rate; the triaxiality is held constant throughout in absolute value. These simulations fully confirm the interpretation of the reduced fracture strains under cyclic loadings just mentioned. The modelling of the ratcheting of the porosity is then discussed. Gurson (1977)'s classical model is shown not to be able to predict such an effect, the evolution of the porosity being stabilized right from the first semi-cycle. The so-called LPD model due to Leblond et al. (1995), an improved variant of Gurson (1977)'s model with a more refined description of strain hardening, makes a better job but fails to accurately reproduce the results of the micromechanical simulations. One explanation of the discrepancy is the assumption of positively proportional straining made in this model, which is basically inadequate for cyclic loadings. An improved version of the LPD model is introduced; this version discards this assumption, at the expense of introduction and radial discretization of an underlying spherical "microcell" at each material point. It is not significantly more computationally expensive than the old one and permits a satisfactory reproduction of the results of the micromechanical simulations. This paves the way to simulations of ductile rupture under cyclic loadings within the framework of Gurson-like models. © 2015 Elsevier Masson SAS. All rights reserved.


Valiorgue F.,LTDS | Brosse A.,ESI France | Naisson P.,LTDS | Rech J.,LTDS | And 2 more authors.
Applied Thermal Engineering | Year: 2013

This paper will present the infrared thermography principles applied to the thermal fields recording during orthogonal cutting of 316L stainless steel. This paper is divided in three parts. First, emissivity curve of 316L is extracted by warming up a sample and dividing recorded grey levels by black body ones. This first step requires the design of special equipment that allows controlling temperatures and atmosphere while recording. Next, the IR camera equipped with a microscope is integrated in a CNC lath to record grey levels while orthogonal cuttings of 316L samples. To finish, the recorded grey levels fields are then numerically post treated using homemade emissivity curve to plot the thermal gradient created during machining. All these works are important to increase the cutting analytical and numerical models accuracy especially in the thermal field prediction. © 2013 Elsevier Ltd. All rights reserved.


Michalski A.,SL Rasch GmbH | Gawenat B.,SL Rasch GmbH | Gelenne P.,ESI France | Haug E.,ESI France
Journal of Wind Engineering and Industrial Aerodynamics | Year: 2015

The sensitivity of membrane structures to transient wind loads becomes severe at wide spans and low pre-stress levels of the membrane. At stationary wind loads, the elastic behaviour of the flexible membrane leads to deformations with an associated change of the flow conditions and wind pressure distributions. This effect can be enhanced by time dependant fluid fluctuations such as atmospheric or building induced turbulences. Common methods in wind engineering practise like small scale wind tunnel experiments do not fully cover non-linear structural behaviour, contact interaction between membrane and structural elements and the interaction of the flow field with the structural response. Therefore numerical tools are used for the structural design of lightweight membranes. This paper presents results of the first industrial application of the fully coupled fluid structure interaction simulation for aerodynamically sensitive membrane structures situated in a built environment. The dynamic behaviour for gust induced wind loads has been investigated and reaction forces were determined. The application of the fully computational wind engineering method, in time domain, allowing as well for non-linear structural effects as for fluid structure interaction effects, makes the discovery of large dynamic amplification factors possible. The described procedure and results were reviewed and approved by Buro Happold, one of the world leading structural engineering offices. © 2015 Elsevier Ltd.

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