Julien Férec

• Professor
• Professor at University of Southern Brittany

In material extrusion, while maintaining a constant mass flow rate, operations involving temperature-sensitive expandable materials show an increase in the volumetric flow rate at the nozzle exit. This research uses foamable filaments made of thermoplastic elastomers and thermally expanded microspheres (TEMs) to examine this phenomenon and focuses...

A new kinetic macromodel based on moments of the probability distribution function is proposed to investigate the flow of rodlike Brownian particle suspensions. The rods concentration-orientation coupling is taken into account. A numerical study is presented for rods through the planar channel, with and without introducing a circular obstacle which...

In 1922, Jeffery derived the motion of a triaxial-ellipsoidal particle immersed in Stokes flow as well as its associated stress tensor, which laid the foundation to describe the rheological behavior of non-spherical suspensions and has been used in many applications. In this work, we propose an original derivation of Jeffery equations for the parti...

The addition of rigid axisymmetric particles, like spheroids or rods, to a liquid leads to a noticeable increase of the viscosity of the mixture. The stress tensor in such suspensions can be described by the transversely isotropic fluid (TIF) equation. The TIF equation depends on three rheological parameters which are functions of the particle aspe...

This paper presents an innovative approach towards controlling the material properties of 3D-printed thermoplastic elastomer foam specimens by adjusting the material extrusion process parameters. A foamable filament made of a thermoplastic elastomer and thermally expanded microspheres (TEMs), which is suitable for producing foamed parts, is used to...

Fluids-containing particles are called suspensions and are encountered in natural, biological, and industrial materials. A large number of examples are blood, paint, slurries, mineral concentrates, mine tailings, clay, cement, and fiber-reinforced polymeric resin. One speaks of a suspension when the typical dimension of the suspended particles diff...

In this article, we provide a general description of a random stack of solid particles. We consider fine particles of any shape distributed evenly on a flat surface. The angular distribution of the particles is arbitrary. In the absence of constraints imposed on the medium, the volume fraction is expressed analytically according to the parameters o...

The perturbation technique based on the retardation-motion expansion is a simple method to obtain flow solutions at low Weissenberg number. In this context, this perturbation analysis is used to develop simple expressions for the motion of fibers suspended in viscoelastic fluids. In particular, the suspending fluid is characterized by a second-orde...

The collapse/expansion of a spherical bubble in a Newtonian fluid filled with non-Brownian fibers is numerically investigated. An experimental test was conducted to observe that the spherical geometry of the bubble is maintained during the foaming process. The constitutive equation for the fiber suspension arises from the slender body theory and is...

An innovative hybrid thermoplastic manufacturing process has been investigated, which consists in carbon fibres reinforced poly(ether-ether-ketone) (PEEK) prepreg sandwiched between two amorphous polyetherimide (PEI) films. By using a laser-assisted automated fibre placement (AFP), a laminate is produced by the deposition of hybrid prepreg tapes, l...

The chapter focuses on fiber-reinforced thermoplastics or polymer composites which are mainly produced by injection and compression molding using particles having mostly cylindrical shape (short or long fibers made up of glass, carbon, or flax). The aim of this fiber addition is to improve the mechanical properties of final parts. However, these pr...

A comprehensive numerical model is developed for the simulation of the laser-assisted automated tape placement process of carbon fiber/thermoplastic composites. After being heated with a laser, the thermoplastic is welded with the help of a consolidation roller onto a substrate made up of layers of tapes bonded onto one another. Under the pressure...

Laser-assisted automated fiber placement (AFP) is highly suitable for an efficient production of thermoplastic-matrix composite parts, especially for aeronautic/aerospace applications. Heat input by laser heating provides many advantages such as better temperature controls and uniform heating projections. However, this laser beam distribution can b...

Numerical models are developed to examine fiber suspension flows through axisymmetric geometries, such as a circular pipe, a center-gated disk and a die exit. The fiber orientation micro-structure is fully described by using the entire probability distribution function (PDF) in 3D instead of the second and fourth moments of the PDF, which introduce...

A numerical simulation for foaming a melted thermoplastic by extrusion process is conducted, where thermo-expandable microcapsules are used as a blowing agent. Firstly, a growth model for an encapsulated bubble is developed to match experimental results. Then, the growth model is used to numerically solve a 2D non-isothermal die-swell problem, maki...

A smoothed particle hydrodynamics method is employed to study the mechanical and thermal behaviors of a fiber-filled composite with an anisotropic thermal conductivity (which is coupled to the orientation of the fibers) in a three-dimensional printing process for one- and two-layer deposition. Using a microstructure-based fiber suspension model wit...

Polymer melts filled with rod-like particles like glass and carbon fibers have high practical importance. Here we numerically investigate the properties of power-law fluids filled with rigid rods of different aspect ratios. For that we compute the rheological coefficients of the transversely isotropic fluid (TIF) equation and compare our results wi...

Thermoplastic Elastomer Vulcanized (TPE-V) syntactic foams made by Thermo-Expandable Microcapsules (TEMs) are becoming of a large interest for automotive industry. TPE-Vs combine advantages of class rubber compound properties with an easier manufacture process and are recyclable. Therefore, TPE-Vs are more and more used to manufacture automotive se...

The mechanical and thermal behavior of nonisothermal fiber-filled composites in a three-dimensional printing process is studied numerically with a smoothed particle hydrodynamics method. A classical microstructure-based fiber suspension model with a temperature-dependent power-law viscosity model and a microstructure constitutive model is implement...

A numerical model is developed to examine the die swell for fiber suspensions, which occurs in 3D printing extrusion processes. More specifically, it focuses on the fiber orientation distribution in a Newtonian suspending fluid and its effects on the shape of the free surface. In a first step, the flow in a 2D axisymmetric capillary die is explored...

A numerical study is presented for fiber suspension flows through a parallel plate channel and a planar 4:1 contraction. Besides examining a Newtonian suspending fluid, a non-Newtonian matrix exhibiting a pseudoplastic behavior and describing a power-law model is also investigated. Furthermore, instead of using orientation tensors for the macroscop...

When non-spherical particles, like rods or discs, are added to a liquid the increase in viscosity of the mixture is often described by the Lipscomb model. One important result of Lipscomb’s model is that it predicts a strong increase in the viscosity of the suspension with increasing aspect ratio of the filler particles. Despite the fact that this...

We present a numerical study of the fused deposition modeling 3D printing process of fiber-reinforced polymers by means of Smoothed Particle Hydrodynamics (SPH). For this purpose, a classical microstructure-based fiber suspension model coupled with a constitutive model for the suspending polymer is implemented within an SPH framework. The chosen mo...

The cell model (or self-consistent scheme) has been largely used to estimate the bulk thermal properties for dilute slender rods embedded in a matrix by applying a thermal gradient perpendicular to the main axis of the rod. This approach is then extended by considering a thermal gradient along the particle, or in other words a thermal flux through...

Continuous fibre reinforced thermoplastic polymer composites exhibit superior properties like mechanical performance, recyclability and the potential for light-weight structures. These composites are used in a wide range of high-performance structure applications nowadays, and high-quality parts are produced thanks to improvements in processing tec...

A computational model is developed for simulating the motion of a single ellipsoid suspended in a Newtonian and power-law fluid, respectively. Based on a finite element method (FEM), the approach consists in seeking solutions for the linear and angular particle velocities using a minimization algorithm, such that the net hydrodynamic force and torq...

A rheological constitutive law is developed for a suspension of rigid rods in a Bingham fluid for volume fractions ranging up to the semiconcentrated regime. Based on a cell model approach, which allows expressing the shear stress on the particle surface, the particle stress contribution is derived and involves additional yield stress terms related...

APC-2 is a carbon fiber filled poly(ether ether ketone) (PEEK), it is a composite used mainly for aeronautical applications. It can be processed with an automated tape placement process and the material can be melted with a laser. The laser is powerful and can easily degrade the material. Experiments were performed on APC-2 using different laser ir...

Following Leal who gave the motion of a slender axisymmetric rod in a second-order fluid, we derived a complete rheological constitutive equation for dilute and semidilute slender rod suspensions in a viscoelastic solvent based on a cell model. Numerical solutions for the Fokker-Planck equation are obtained for simple shear flows at low and large P...

This work is focused on shear thinning behavior of suspensions of rigid non-Brownian fibers dispersed in a Newtonian liquid. The work consists of developing a new theoretical model and conducting accurate experimental measurements. The shear thinning is expected to be caused by adhesive interactions between fibers. Experiments on polyamide (PA) fib...

Transient shear flow data of untreated multiwalled carbon nanotubes (MWCNTs) dispersed in a Newtonian epoxy matrix are analyzed. A sequence of shearing and rest steps was applied to characterize the transient responses of the suspensions. Stress overshoots appeared at very small deformation during forward and reverse flow experiments and their inte...

Various industries, such as automotive and leisure, show a great interest in foaming thermoplastic polyurethane (TPU) with physical blowing agents. Hence, this study aims to investigate the effects of absorption of carbon dioxide and nitrogen by melt TPU on solubility and interfacial tension. Using a pressure-volume-temperature apparatus for high p...

This work is focused on the modelling of the shear and normal stresses in fibre suspensions that are subjected to a simple shear flow in the presence of short-range lubrication forces, van der Waals and electrostatic forces, as well as solid friction forces between fibres. All of these forces are weighed by the contact probability. The theory is de...

In the present article, the cell model (or self-consistent scheme) is used to derive constitutive equations for rod suspensions in non-Newtonian viscous matrices such as power-law, Ellis and Carreau fluids. It is found that the shear-thinning character of the matrix influences considerably the rod contribution to the stress tensor, but has no impac...

Direct numerical simulations allow a detailed physical description at the microscopic scale • A statistical description included the number of interactions and the forces involved • Using beam theory it was possible to introduce the elasticity in the suspensions • Develop coarser models for higher interactions (clusters)

Composite materials making use of dispersed and elongated particles in a continuous liquid medium are fundamental for understanding many types of flow studied in physics, biology and engineering. Among other applications, these attractive composites are encountered in coatings, biological fluids, pulps, ceramics and structural materials. However, t...

When fiber suspensions become concentrated, standard mesoscale theories do not allow a precise description of the fine physics involved at the microscopic level. Thus one way of studying the kinematics and the rheological effects of such suspensions is to make a direct numerical simulation (or DNS for abbreviation). In previous works authors propos...

In this work, the rheo-optical response of multiwalled carbon nanotube (MWCNT) suspensions was analyzed. Dichroism was obtained using a polarization-modulation technique in parallel disks and for the first time for these particles in a Couette flow geometry. MWCNTs were dispersed in a Newtonian epoxy matrix, at different concentrations covering the...

Synopsis In this work, the rheo-optical response of multiwalled carbon nanotube (MWCNT) suspensions was analyzed. Dichroism was obtained using a polarization-modulation technique in parallel disks and for the first time for these particles in a Couette flow geometry. MWCNTs were dispersed in a Newtonian epoxy matrix, at different concentrations cov...

SIMULATION DIRECTE DE SUSPENSIONS CONCENTRÉES DE FIBRES SOUMISES A DES EFFETS DE FLEXION (BEST POSTER AWARD)

The motion of an ellipsoidal particle immersed in a flow of a Newtonian fluid was obtained in the pioneering work of Jeffery in 1922. Suspensions of industrial interest usually involve particles with a variety of shapes. Moreover, suspensions composed of rods (a limit case of an ellipsoid) aggregate, leading to clusters with particular shapes that...

In this work a numerical simulation of fiber suspensions in transient and steady shear flow is presented. Concentrated suspensions are considered along with interactions between fibers: short-range hydrodynamic interaction via lubrification forces, contact forces and hydrodynamic forces. The kinematic of a population of fibers is then calculated us...

In this work a numerical simulation of fiber suspensions in transient and steady shear flow is presented. Concentrated suspensions are considered along with interactions between fibers: short-range hydrodynamic interaction via lubrification forces, contact forces and hydrodynamic forces. The kinematic of a population of fibers is then calculated us...

Recently, F,rec et al. (2009a) proposed a model for nondilute rod-like suspensions, where particle interactions are taking into account via a micromechanical approach. The derived governing equation used the well-known second- and fourth-order orientation tensors (a (2) and a (4) ) and novel second- and fourth-order interaction tensors (b (2) and b...

A promising new paradigm for meso-scale modeling. Macroscopic outputs taking into account microscopic physics ... a natural up-scaling procedure. A novel description of population dynamics and evolving micro-structures

To explain the shear-thinning behavior of untreated carbon nanotube (CNT) suspensions in a Newtonian matrix, a new set of rheological equations is developed. The CNTs are modelled as rigid rods dispersed in a Newtonian matrix and the evolution of the system is controlled by hydrodynamic and rod-rod interactions. The particle-particle interactions i...

Most suspension descriptions used nowadays are based on Jeffery’s model or some phenomenological modifications of it that do not take into account size effects: the kinematics and stresses do not involve a micro-mechanical characteristic length and thus, the predicted rheological properties are necessarily independent of rod length. More sophistica...

Suspensions involving nanoparticules - in particular nanofibers and nanotubes - are in much use in the development of functional materials. Thus in order to optimize the usage of these materials and their fabrication, it is essential to have a thorough knowledge of the microstructure and its evolution. In this work, the objective is to develop a tw...

Kinematics and elasticity of high concentrated suspensions

• Kinetic theory based model to describe the evolution of the microstructure of a concentrated suspension • Validation of the micromechanics via comparisons with direct simulation at the microscale • Identification of the model parameters from usual rheological experiments • Introduction of the aggregates’ size effect • Study of the aggregation and...

Suspensions involving nanoparticules -in particular nanofibers and nanotubes -are in much use in the development of functional materials. Thus in order to optimize the usage of these materials and their fabrication, it is essential to have a thorough knowledge of the microstructure and its evolution. In this work, the objective is to develop a two-...

Models encountered in computational physics and engineering, usually involve too many degrees of freedom, too many simulation time-steps, too many iterations (e.g. non-linear models, optimization or inverse identification…), or simply excessive simulation time (for example when simulation in real time is envisaged). In some of our former works diff...

An efficient simulation of thermal and mechanical models involved in thermosetting composites forming needs to overcome some numerical difficulties related to: (i) the multi-scale behaviour; (ii) the complex geometries involved needing too many degrees of freedom; (iii) the large time intervals where the solution has to be computed; (iv) the non-li...

Models encountered in computational physics and engineering, usually involve too many degrees of freedom, too many simulation time-steps, too many iterations (e.g. non-linear models, optimization or inverse identification…), or simply excessive simulation time (for example when simulation in real time is envisaged). In some of our former works diff...

A set of rheological equations is developed for semiconcentrated suspensions of rigid fibers in a Newtonian fluid taking into account hydrodynamic and fiber-fiber interactions. The force generated by the fiber interactions is modeled using a linear hydrodynamic friction coefficient proportional to the relative velocity at the contact point, and wei...

The use of separated space-time representations for the solution of many engineering transient models induces impressive computing time savings. Its use is justified as soon as computed finite element solutions accept one such representation when one applies the SVD on the models solution. In quasi-static models separated representations exhibited...

The probability distribution function for fiber orientation under flow (Fokker–Planck equation) is numerically solved using a finite volume method. Different time and spatial schemes have been tested to reduce considerably the computational time and to cover a wide range of the Peclet (Pe) number. For Pe≤103, the results are compared with data avai...

Small and large amplitude oscillatory shear measurements (SAOS and LAOS) were used to investigate the rheological behavior of short glass fibers suspended in polybutene and molten polypropylene. Raw torque and normal force signals obtained from a strain-controlled instrument (ARES rheometer) were digitized using an analog to digital converter (ADC)...

The behavior of short glass fiber–polypropylene suspensions in extensional flow was investigated using three different commercial instruments: the SER wind-up drums geometry (Extensional Rheology System) with a strain-controlled rotational rheometer, a Meissner-type rheometer (RME), and the Rheotens. Results from uniaxial tensile testing have been...

Un modèle rhéologique pour les suspensions de fibres est proposé. Il prend en compte la présence des interactions entre particules. L’évolution de l’orientation et la contrainte totale dépendent de la matrice, des fibres et des interactions. Des tenseurs d’interaction apparaissent dans ces équations. Ce modèle prévoit une augmentation quadratique d...