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Jeong J.,Special School for Public Works, Building and Industry | Neff P.,TU Darmstadt
Mathematics and Mechanics of Solids | Year: 2010

We investigate the weakest possible constitutive assumptions on the curvature energy in linear Cosserat models still providing for existence, uniqueness and stability. The assumed curvature energy is μL2c ∥dev sym ∇axl A∥2 where axl A is the axial vector of the skewsymmetric microrotation A ∈ so(3) and dev is the orthogonal projection on the Lie-algebra sl(3) of trace free matrices. The proposed Cosserat parameter values coincide with values adopted in the experimental literature by R. S. Lakes. It is observed that unphysical stiffening for small samples is avoided in torsion and bending while size effects are still present. The number of Cosserat parameters is reduced from six to four. One Cosserat coupling parameter μc > 0 and only one length scale parameter L c > 0. Use is made of a new coercive inequality for conformal Killing vectorfields. An interesting point is that no (controversial) essential boundary conditions on the microrotations need to be specified; thus avoiding boundary layer effects. Since the curvature energy is the weakest possible consistent with non-negativity of the energy, it seems that the Cosserat couple modulus μc > 0 remains a material parameter independent of the sample size which is impossible for stronger curvature expressions. © 2010 SAGE Publications.

Ramezani H.,French National Center for Scientific Research | Jeong J.,Special School for Public Works, Building and Industry
Acta Mechanica | Year: 2011

In the present paper, environmentally motivated numerical modeling for the limestone layered masonry structures has been investigated involving the continuous hygric state field variable, i.e. relative humidity Π. By taking advantage of the Lin et al. assumption pertaining to the relative humidity (independency of the relative humidity and temperature) (Lin MW et al. Build. Environ. 41(5):646-656, (2006); Khoshbakht M et al. Finite Elem. Anal. Des. 42(5):414-429, (2006); Khoshbakht M, Lin MW Meas. Sci. Technol 2989-2996, (2006); Khoshbakht M Finite Elem. Anal. Des.; 45(8-9):511-518, (2009)), we have provided a mathematical model involving hygro-thermo-mechanical aspects as well as the water vapor transfer across the porous limestone masonry walls. The numerical study substantiates the impact of hygric effects as the major key point in the thermo-hygro-mechanical degradation and effect of geometry in the real brick-line of mortar assembly. Furthermore, we have obtained the moisture entrapment at the intersection of the lines of mortar through the layered masonry wall by means of the multidisciplinary nonlinear finite element method (NFEM) for variably saturated porous media. The new outlooks and fresh departure in durability and aging have been briefly discussed. © 2011 Springer-Verlag.

Jeong J.,Special School for Public Works, Building and Industry | Ramezani H.,French National Center for Scientific Research
Computer Methods in Applied Mechanics and Engineering | Year: 2010

A non-linear Cosserat theory involving the Arbitrary Lagrangian-Eulerian (ALE) method has been introduced into the brittle isotropic materials (amorphous glass and cement mortar) using the Modified Brazilian Disk (MBD) under an uni-axial compressive loading. These numerical experiments shed light on the nature of the Cosserat-based media and material moduli determination which are difficult to acquire using the most well-known experimental viewpoints. By using the identical micro-rotation constants α=β=γ=μLG2andΨ=2/3, the Cosserat moduli reduce to only four constants for the 3D cases. According to the results obtained in this paper, the present methodology substantiates that the Cosserat theory would be readily applied to the wide range of materials from the full amorphous materials to the heterogeneous materials by changing the length scale parameter. Some fresh routes and new outlooks are discussed afterwards. © 2010 Elsevier B.V.

Eddhahak-Ouni A.,Special School for Public Works, Building and Industry | Dony A.,Special School for Public Works, Building and Industry | Colin J.,Special School for Public Works, Building and Industry
Road Materials and Pavement Design | Year: 2012

This work deals with the experimental characterization of the mechanical behavior of bituminous binders used in road pavement. First, conventional tests are used to determine the grade of four bituminous products with a Polymer modified one. Then, rheological experiments were performed to compare the viscoelastic properties of the different binders. Tests were carried out at least three times so that one can take advantage of the statistical collected data to define the available information used in the stochastic modeling. The parametric probabilistic model highlights the necessity to include experiments uncertainties into modeling for a safe and accurate prediction of bitumens ultimate properties. © 2012 Taylor & Francis.

Ramezani H.,French National Center for Scientific Research | Steeb H.,Ruhr University Bochum | Jeong J.,Special School for Public Works, Building and Industry
European Journal of Mechanics, A/Solids | Year: 2012

In the present study, the micro-dilatation theory has been investigated. The analytical investigations on the penalized cases using a novel total energy density concept have been achieved via quadratic, affine and linear vector-to-vector and vector-to-scalar mappings. The total energy density has divided into four counterparts which are explicitly linked into the constitutive parameters. This novel energy density concept leads to four explicit constitutive laws. By taking advantage of the strongly ellipticity for the total strain energy density, a freshly defined coupling number was introduced. This issue yields two interesting outcomes, the first one is that the micro-dilatation coupling modulus b is bounded β2 > Kx and negative (β > 0) and the second one is that the pore pressure must be less than a specific threshold (P2 > 42Kx). As a matter of fact, the micro-dilatation theory holds a rigorous restriction on the pore-fluid pressure comparing to Biot's theory in which there is no restriction. We have analytically scrutinized the case limits and particularly bulk (case1) and conformal case (case2) in which the heterogeneous deformations occur. The numerical simulations of the micro-dilatation theory have been brought afterwards using Galerkin FEM under an Augmented LagrangianeEulerian (ALE) setting. The aforementioned numerical simulations substantiate that one can obtain the physically acceptable results for moderate and low coupling numbers. Based upon these results about the micro-dilatation theory some new routes in determining the micro-dilatation theory constants have been triggered. © 2011 Elsevier Masson SAS.

Neff P.,Universitatsstr 2 | Jeong J.,Special School for Public Works, Building and Industry | Fischle A.,Universitatsstr 2
Acta Mechanica | Year: 2010

We describe a principle of bounded stiffness and show that bounded stiffness in torsion and bending implies a reduction of the curvature energy in linear isotropic Cosserat models leading to the so-called conformal curvature case μ L c 2/2 ∥ rm dev ∇ axl ̄ A 2∥ where ̄ A ∈ so (3) is the Cosserat microrotation. Imposing bounded stiffness greatly facilitates the Cosserat parameter identification and allows a well-posed, stable determination of the one remaining length scale parameter L c and the Cosserat couple modulus μ c . © 2009 Springer-Verlag.

Jeong J.,Special School for Public Works, Building and Industry | Sardini P.,University of Poitiers | Ramezani H.,CNRS Center for Research on Divided Matter | Siitari-Kauppi M.,University of Helsinki | Steeb H.,Ruhr University Bochum
Computational Materials Science | Year: 2013

In the present paper, we focus on the modeling of the CO2 carbonation phenomenon through the cement mortars. The CO2 carbonation will modify not only the pH value of the cement mortars but also the mortar specimen size, i.e. mortar shrinkage. The aforementioned issue is driven via the chemical reactions and it affects the micro-structure and particularly the porosity. Based upon the micro-dilatation theory which was originally proposed by Cowin and Nunziato [1,2], one of the four material moduli dealing with the micro-structure especially, β-ξ relation, has been determined for the first time. To pursue this goal, the carbonation experiments on the cement mortar (CEMI) have been achieved to determine CO2 affected zones by the carbonation process and to measure the corresponding deformations caused by the shrinkage phenomenon. According to our proposed semi-empirical method, it is possible to compute the induced chemical stress as well as the porosity gradient through the mortar. The numerically-obtained porosity distribution across the cement mortar has been afterwards compared to the outcomes coming from the 3D-connected porosity profile which is extracted by means of 14C-PMMA experiments. The latter experiments allow us not only to evaluate the computations for the porosity, porosity gradient and the induced stress but also to determine the micro-dilatation material moduli (coupling number, N = -0.316), in conjunction with the pore stiffness or the so-called void stiffness of the cement mortars. © 2012 Elsevier B.V. All rights reserved.

Ramezani H.,French National Center for Scientific Research | El-Hraiech A.,French Institute of Health and Medical Research | Jeong J.,Special School for Public Works, Building and Industry | Benhamou C.-L.,French Institute of Health and Medical Research
Computer Methods in Applied Mechanics and Engineering | Year: 2012

In the present study, the method of size effect was used to describe the hierarchical multi-scale behavior of trabecular human bone using micro-CT image. The micro-CT images extracted from a 98-year old male patient were used to provide the 3D-model of cancellous bone, which involves one of the most complex natural pore network. The non-linear or so-called geometrically exact Cosserat elasticity including Augmented Lagrangian-Eulerian (ALE) method has been used to handle the multi-scale behavior of trabecular human bone involving size effect in explicit manner. We have chosen the conformal energy density within micro-randomness issue in which the heterogeneous deformations at small scales are inherently sustained. © 2012 Elsevier B.V.

Braham-Bouchnak T.,Arts et Metiers ParisTech | Germain G.,Arts et Metiers ParisTech | Morel A.,Arts et Metiers ParisTech | Lebrun J.L.,Special School for Public Works, Building and Industry
International Journal of Advanced Manufacturing Technology | Year: 2013

The Ti533-3 alloy is a new titanium alloy which is starting to see increased use in the aeronautical domain to improve the durability of components and to optimize the weight/resistance ratio. This alloy is characterized by greater resistance compared to the more commonly used titanium alloys such as Ti6Al4V. However, a disadvantage of the Ti533-3 alloy is that it is very difficult to machine. In this work, the use of laser-assisted machining has been tested to improve chip formation by a thermal softening phenomenon and to improve the machining productivity of the alloy. A parametric investigation of laser assistance on the machinability of the Ti555-3 titanium alloy shows that: (1) the cutting forces can be greatly decreased if the surface temperature is high; (2) the thermal gradient induced by laser heating modifies the surface integrity in terms of strain hardening and residual stresses in the workpiece; and (3) the chip formation mechanisms are also changed, by increasing the sawteeth frequency when using laser assistance. © 2013 Springer-Verlag London.

Ramezani H.,CNRS Center for Research on Divided Matter | Mounanga P.,University of Nantes | Jeong J.,Special School for Public Works, Building and Industry | Bouasker M.,CNRS Center for Research on Divided Matter
Cement and Concrete Composites | Year: 2013

The self-induced stress in mortars caused by autogenous shrinkage phenomenon at very early age has been investigated by taking advantage of the experimental results and application of hygro-Cosserat theory. Three cement matrices, prepared with three different types of cement, have been considered to analyze the role of cement paste composition on the development of these initial stresses surrounding the aggregates. The induced bulk deformations around the aggregates have been thereafter scrutinized by means of hygro-Cosserat elasticity taking into account the size effect in an explicit manner and a newly defined parameter called Cosserat size effect number (CS). By taking advantage of the autogenous shrinkage experiments on the cement pastes, the parallel time-dependent finite element analyses have been achieved considering the aggregate-to-cement ratio. The aforementioned parameters are put into practice to create an analytical pseudo-stochastic geometry using a spherical-packing algorithm. The numerical outcomes were analyzed and compared to the experimental outcomes coming from the Scanning Electronic Microscopy observations at 48 h after hydration. © 2013 Elsevier Ltd. All rights reserved.

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