Nicolas Triantafyllidis - Academia.edu (original) (raw)

Papers by Nicolas Triantafyllidis

Journal of The Mechanics and Physics of Solids, Mar 1, 2022

International Journal of Solids and Structures, Oct 1, 2022

Journal of Elasticity, Jan 2, 2018

In this communication we present the work by Pampolini and Triantafyllidis [1], in which an elect... more In this communication we present the work by Pampolini and Triantafyllidis [1], in which an electro-mechanical theory for nematic continua is proposed. The theory is based on a variational approach and the equilibrium relations plus the Maxwell's equations are obtained as the Euler-Lagrange equations of a specific potential energy. The variational formulation is applied to the study of a 2D boundary value problem, termed in the literature as Freedericksz transition, where a nematic liquid crystal layer is confined between two plates and an electric field is applied perpendicular to the plates. This boundary value problem is treated as a bifurcation problem and an asymptotic analysis of the bifurcated equilibrium path is carried out.

HAL (Le Centre pour la Communication Scientifique Directe), May 9, 2011

-Magnetorheological elastomers (MREs) are ferromagnetic particle impregnated elastomers whose mec... more -Magnetorheological elastomers (MREs) are ferromagnetic particle impregnated elastomers whose mechanical properties are altered by the application of external magnetic fields. Due to their magnetoelastic coupling response MREs are finding an increasing number of engineering applications. The objective of this work is : (a) the experimental study of transversely isotropic MREs (i.e., the particles form chains along a certain direction) that are subjected to prestressing and arbitrary magnetic fields and (b), the phenomenological modeling of these materials using transversely isotropic energy functions. Mots clés-magneto-mechanics, multi-physics, elastomers.

Journal of The Mechanics and Physics of Solids, 1994

AESTRACI THE STABILITY OF il geological two-layer system composed of a frictional material layer ... more AESTRACI THE STABILITY OF il geological two-layer system composed of a frictional material layer of finite thickness. called the overburden, resting on a viscous half-space of lower density is investigated. The salient features of this study are a realistic description of the stiffness of the overburden and its state of (in sitzl) prestress. and the USC of the viscosity of the substratum to define a characteristic time for the stability analysis. A general variational formulation for the linearized. non-selfadjoint stability problem is prcsentcd, followed by asymptotic analyses for the cases of large and small perturbation wavelengths and by an analytical solution in the absence of gravity. Results obtained by a finite-element method are compared with the analytical and asymptotic predictions: they permit the detection of various modes of instability : interfaceand beam-type modes in the compressive range of deformation. and neck-type modes in the tensile ransc. It is found that the system's stability is not only governed by geometry and density contrast. as expected from the conclusions of earlier studies on viscous and viscoelastic models, but is also scnsitivc to the state of in .viru stress. A complete parametric study reveals that the overburden material cohesion and workhardening properties have more influence on stability than the friction angle. Furthermore, it is found that critical stresses at neutral stability predicted by deformation theory. which is an appropriate model for studying the initiation of faulting in rocks. are smaller in magnitude than those obtained by the corresponding flow theory with a smooth yield surface. Implications of this work for the interpretation of various laboratory analogue model experiments pertaining to geological two-layer systems arc also discussed.

Journal of The Mechanics and Physics of Solids, Nov 1, 1996

Of interest here is the scale size effect on the stability of finitely strained, rate-independent... more Of interest here is the scale size effect on the stability of finitely strained, rate-independent solids with periodic microstructures. Using a multiple scales asymptotic technique, we express the critical load at the onset of the first instability and the corresponding

Journal of Elasticity, Apr 25, 2023

HAL (Le Centre pour la Communication Scientifique Directe), 2008

Of interest here is the stability of a rectangular block subjected to a uniform magnetic field pe... more Of interest here is the stability of a rectangular block subjected to a uniform magnetic field perpendicular to its longitudinal axis. The two ends of the block are frictionless and kept parallel to each other. This boundary value problem is motivated by the classical problem of magnetoelastic buckling in which a cantilever beam subjected to a transverse magnetic field buckles when the applied field reaches a critical value. This work presents a finite strain continuum mechanics formulation of the stability problem of a homogeneous, compressible, magnetoelastic rectangular block in plane strain subjected to a uniform transverse magnetic field. The applied variational approach employs an unconstrained energy minimization recently proposed by the authors. The analytical solution for the critical buckling fields for both the antisymmetric and symmetric modes are obtained for three different constitutive laws. The corresponding result for thin beams is extracted asymptotically for a special material and the solution is compared to previously published results. The critical magnetic field is shown to increase monotonically with the block's aspect ratio for each material and mode type. Antisymmetric modes are always the critical buckling modes for stress saturated and neo-Hookean materials, except for a narrow range of moderate aspect ratios (about 0:25) where symmetric modes become critical. For strain-saturated solids no buckling is possible above a maximum aspect ratio.

In this paper, we study a class of active materials named magneto-rheological elastomers (MREs) w... more In this paper, we study a class of active materials named magneto-rheological elastomers (MREs) with a main focus on their coupled magneto-mechanical response. Based on the theoretical framework proposed by Triantafyllidis [1], we develop the coupled magneto-mechanical constitutive laws in order to identify the corresponding model's material parameters. The experimental data, obtained with a novel experimental setup allowing tensile tests up to large strains and under high magnetic fields, provide the constitutive parameters needed as input for subsequent numerical simulations.

HAL (Le Centre pour la Communication Scientifique Directe), Aug 19, 2012

International audienc

Siam Journal on Applied Mathematics, 2020

Transverse wrinkles are known to appear in thin rectangular elastic sheets when stretched in the ... more Transverse wrinkles are known to appear in thin rectangular elastic sheets when stretched in the long direction. Numerically computed bifurcation diagrams for extremely thin, highly stretched films...

HAL (Le Centre pour la Communication Scientifique Directe), May 11, 2008

ABSTRACT The present work studies the connections between microstructural instabilities and their... more ABSTRACT The present work studies the connections between microstructural instabilities and their macroscopic manifestations defined as the loss of rank-one convexity of the effective properties in finitely strained porous elastomers with a) random iso-disperse and b) periodic microstructures. The powerful second order homogenization (SOH) approximation technique, initially developed by P. Ponte Castaneda for random media, is also used here to study the onset of failure for periodic microstructures and the results are compared to more accurate finite element method (FEM) calculations. The influence of microgeometry (random and periodic with square and hexagonally arranged pores), initial porosity, matrix constitutive law (neo-Hookean and Gent) and macroscopic load orientation on the microscopic instability (loss of uniqueness of the principal solution for the case of periodic microstructures) and the macroscopic instability (loss of rank-one convexity of the homogenized energy density for all microstructures) is investigated in detail. In addition to the above-described stability-based onset of failure mechanisms, limitations to the elastomers response at finite strains (such as void surface instability, percolation, pore closure and strain locking) are also addressed, thus giving a complete picture of the different possible failure mechanisms present in finitely strained porous elastomers.

29th European Photovoltaic Solar Energy Conference and Exhibition, Nov 7, 2014

The influence of mechanical stress on PECVD thin film layers (250 to 500 nm) of hydrogenated amor... more The influence of mechanical stress on PECVD thin film layers (250 to 500 nm) of hydrogenated amorphous and microcrystalline silicon (both intrinsic, p-and n-doped) as well as indium tin oxide (ITO) and aluminum doped zinc oxide (ZnO:Al) was examined via uniaxial tension and compression tests and a simultaneous measurement of the resistivity both parallel and perpendicular to the applied stress (piezoresistivity). The resistivity for intrinsic hydrogenated amorphous layers is increasing for tensile strain whereas it is decreasing for intrinsic hydrogenated microcrystalline layers. P-doped layers of microcrystalline silicon have an increasing resistivity with increasing tensile strain which is in opposition to the behavior of n-type layers that show a decrease in resistivity for the same strain. Both ITO and ZnO:Al show a strong increase in resistivity with strain. The experiments for every material type show that the effect is reversible up to a certain strain and that a permanent damage remains if the sample is subjected to a strain exceeding this limit. In-situ mechanical tests in a scanning electron microscope demonstrate that such irreversible changes are due to crack formation when a certain strain is exceeded. These cracks can be observed perpendicular to the direction of the applied stress.

Journal of Applied Mechanics, Mar 1, 1999

Surface bifurcation is an instability mechanism which appears in the form of surface waviness on ... more Surface bifurcation is an instability mechanism which appears in the form of surface waviness on traction-free surfaces in ductile solids subjected to large strains. In sheet metal forming, the practical interest in this phenomenon stems from the fact that it occurs past the onset of localization, i.e. the forming limit, but prior to the local fracture failure in a quasi-static, monotonic loading process. In this work, we apply the general theory for surface bifurcation in a homogeneously strained, anisotropic, rate-independent, elastoplastic half-space, to study the influence of material anisotropy on the onset of surface instabilities. In particular, we calculate the critical principal strains ε c 1 , ε c 2 and the corresponding eigenmode orientation angle Ω c when the principal strain axes are at a fixed angle α with respect to the rolling direction of the solid. The presented calculations are for a 2024-T3 aluminum alloy, whose constitutive properties have been determined experimentally. It is found that by varying the strain orientation angle α, the surface bifurcation strains can vary up to an order of 80% for in-plane principal strains of a different sign, but only up to an order of 10% for principal strains of the same sign. The eigenmode orientation angle Ω c is calculated for a particular strain orientation (α = π/6), for which case it is found that Ω c is close to the forming limit angle ψ c only for positive principal strains. The presentation is concluded by a discussion of the influence of the anisotropy and the yield surface parameters of the constitutive model on surface bifurcation.

Conference proceedings of the Society for Experimental Mechanics, 2016

Magnetorheological elastomers (MREs) are active composite materials that deform under a magnetic ... more Magnetorheological elastomers (MREs) are active composite materials that deform under a magnetic field because they are made of a soft elastomer matrix filled with magnetizable micrometric particles. Along with short response times and low magnetic inputs, not only do MREs alter their viscoelastic properties and stiffness in response to external magnetic fields but they can also undergo very high deformation states. While the former effect can be exploited in controllable-stiffness devices, the latter is of interest for haptic devices such as tactile interfaces for the visually impaired. In the perspective of developing a persistent tactile MRE surface exhibiting reversible and large out-of-plane deformations, the first part of this work focuses on the fabrication of MREs that can sustain large deformations. In particular, we determine the critical strain threshold up to which the interfacial adhesion between particles and matrix is ensured. In the second part of this work, an experimental setup is developed in order to characterize MRE composites under coupled magneto-mechanical mechanical loading. The experiments conducted on this setup will eventually serve as an input for a continuum model describing magneto-mechanical coupling.

Springer eBooks, 2000

Page 129. Stability Analysis of Incipient Folding and Faulting of an Elasto-Plastic Layer on a Vi... more Page 129. Stability Analysis of Incipient Folding and Faulting of an Elasto-Plastic Layer on a Viscous Substratum Yves M. Leroy1 and Nicolas Triantafyllidisz 1 Laboratoire de Mecanique des Solides Ecole Polytechnique. ... is investigated theoretically for an elasto-plastic. ...

Proceedings of SPIE, May 17, 2013

This work investigates the interfacial adhesion between the iron fillers and the silicone matrix ... more This work investigates the interfacial adhesion between the iron fillers and the silicone matrix in magneto-rheological elastomers at high deformations. Carbonyl iron powder, composed of mechanically soft spherical particles with a median size of 3.5 µm and a volume concentration of 3.5%, was mixed in a soft silicone matrix (Shore 00-20); the compound was then degassed and cured under temperature. The presence of a homogeneous magnetic field of 0.3 T during the curing process allowed the formation of particle chains. Tensile tests of these samples under scanning electron microscope showed interfacial slipping and debonding between the two phases. To improve interfacial adhesion, a silane primer was applied to the iron particles, following two different procedures, before the mixing and crosslinking process, thus giving two additional types of samples. In tensile testing lengthwise to the particle alignment, with engineering strains up to 150%, the structural responses of the different types of samples were compared. An enhanced adhesion of the iron fillers to the silicone matrix resulting in a reinforced matrix and increased tensile strength during the first loading path could be observed. Furthermore, scanning electron microscope images show that a more elaborated particle-matrix interface was obtained with the primer additive.

Journal of The Mechanics and Physics of Solids, Jun 1, 2000

The linear stability criterion, proposed for structural models in an earlier paper, is now extend... more The linear stability criterion, proposed for structural models in an earlier paper, is now extended for a general class of elastic±viscoplastic continua. The time-dependent trajectories, whose stability is under investigation, are functions of two characteristic times: the relaxation time of the viscous solid and the rate of the applied loading, with their ratio denoted by T. It is assumed that the loading conditions of the trajectory are not modi®ed by the perturbation. The criterion predicts that a solid is initially unstable if there exists a unit norm perturbation in the velocity ®eld whose time derivative is positive. This condition is equivalent to ®nding a positive eigenvalue in the self-adjoint part of the operator relating the initial ®rst and second rate of the displacement perturbations. If the dominant eigenvalue is obtained from the non self-adjoint operator, the change in sign of its real part is a sucient condition for instability. For solids with an associated¯ow rule, it is shown that the exclusion of instability in a trajectory, in the limit of vanishing T, coincides with stability of the corresponding rate-independent solid in the sense of Hill. The theory is applied to the analysis of a ®nitely strained rectangular block under uniaxial tension and compression, for dierent elastic±viscoplastic solids of the von Mises and Drucker±Prager type.

Proceedings of the Royal Society of London, Aug 8, 1986

Journal of The Mechanics and Physics of Solids, 1985

THE PROBLEM investigated here is the plastic bifurcation of an initially flat circular plate held... more THE PROBLEM investigated here is the plastic bifurcation of an initially flat circular plate held frictionlessly between a blankholder and a die and deformed by a spherically shaped punch. In view of the large deviations of the prebifurcation solution from proportional loading, a recently developed phenomenological corner theory has been employed and an appropriate bifurcation criterion has been developed. The effects of geometry and material properties on the onset of the (nonaxisymmetric) plastic instability have been investigated using a numerical solution of the resulting equations based on the finite element method.