Christophe Binetruy

The interactions between flows occurring inside and outside fabric tows during rtm

Resin-transfer molding is finding use in structural and complex-shaped polymer composites. Simultaneously, preform technology improvement leads to the design of more intricate and heterogeneous fibrous reinforcements. This architectural design favors the increase of flow scales in the fibrous materials, making impregnation modeling more difficult. Hence, the micro-flows are often neglected in current fluid-flow models without real scientific justification. The interactions between microscopic and macroscopic flow in woven fabrics have been theoretically and experimentally investigated. This work indicates that, in the region close to the flow front, tow impregnation leads to a lag in the fluid motion in the more open regions bounding these fiber bundles. A model to predict this velocity drop is proposed. This effect is found to be significant, and should be taken in account in developing more accurate models, especially for high fiber-content preforms.

Tow Impregnation Model and Void Formation Mechanisms during RTM

A simple analytical model has been developed for the impregnation of fiber bundles where the macroscopic flow is parallel to the fiber axis. Based on the study of the contribution of the axial and transverse flow mechanisms inside tows, this model shows that the main tow impregnation process is transverse to the fiber axis. A criterion has been established to indicate when the axial flow can be neglected to simplify the tow impregnation model. This case represents the majority of situations in the RTM of woven fabrics and the model predicts that the flow front in tows has a pointed meniscus shape whose length depends on the effective permeability of the large pores formed between the tows, transverse permeability of tows and thickness. A new boundary condition at the unsaturated tow surface is proposed. It conveys the interactions between the flows occurring inside and outside axial tows during a constant pressure driven impregnation. Two air entrapment mechanisms have been observed. Although the source of these void formation processes is the local difference in fiber arrangement, the dependence of the amount of formed bubbles on the impregnation front length has been clearly identified. In addition, a few means for voids mobilization or bubbles dividing have been investigated.

Gas transport in fibrous media: Application to in-plane permeability measurement using transient flow

This article introduces a methodology to measure in-plane permeability of fibrous media using a transient one-dimensional air flow with absolute pressures ranging from 103 to 105 Pa. The method, based on the measurement of gas pressure at the boundaries throughout the transient flow, is convenient, clean and fast, avoids usage of a gas flow meter and offers a way to study the gas transport within fibrous media. The transport of a compressible fluid is described by several models to comply with different flow regimes which can occur during the experimental measurements. A thermal analysis is given to verify the validity of isothermal conditions during the tests. The permeability, only depending on the fibrous structure, is determined by inverse method, fitting the simulation results to the experimental data obtained using raising or dropping pressure methods. The deviation from Darcys law caused by gas sliding effect is analysed and a relative parameter of fabric material shows a dependence in permeability, with a similar trend as the Klinkenberg sliding parameter in soils and rocks.

Determination of the Thermal Dispersion Coefficient During Radial Filling of a Porous Medium

Resin Transfer Molding is one of the Liquid Composite Molding processes in which a thermoset resin is infiltrated into a fibrous porous media in a closed mold. To reduce the curing time of the resin, the mold may be heated, influencing other filling parameters such as the resin viscosity. Analysis of the non-isothermal effects during filling will help to understand the manufacturing process. One of the issues of non-isothermal filling in porous media is the variation of the velocity profile at the micro scale level, which as it is averaged, cannot be included in the convective term To account for it, the thermal conductivity tensor is modified and a thermal dispersion coefficient K d is introduced to model the micro convection effects. In this paper we explore the temperature profile under non-isothermal conditions for radial injection during Resin Transfer Molding in order to determine the thermal dispersion coefficient. An approximate solution is derived from the series solution and validated with a numerical method. Experiments using carbon fibers and polyester resin were conducted. The triermial dispersion coefficient is determined by comparing experimental results with the steady state analytical solution. The comparison between radial and linear injection results shows that the same degree of dispersion is present in isotropic fibrous porous media.

All-Thermoplastic Composite Sandwich Panels – Part I: Manufacturing and Improvement of Surface Quality

All-thermoplastic sandwiches with composite faces and thermoplastic foam as core material present great potential for future automotive mass applications. They offer many advantages such as high specific bending stiffness, good insulation properties, high impact resistance,... Furthermore, the use of one single polymer throughout the sandwich brings on optimal face–core compatibility, enhanced recyclability and post-formability. The aim of this first part is to investigate the processing of thermoplastic sandwich structures with improved surface quality manufactured by isothermal compression moulding.

Foam Core Deformation During Liquid Molding of Sandwich Structures: Modeling and Experimental Analysis

The rapid manufacturing of composite parts by liquid molding containing foam cores introduces potential processing issues that could lead to degradation in part quality and performance. This paper addresses the key issue of the flow-induced foam core deformation and its effect on the molding pressure. A set of molding experiments are carried out to investigate mechanisms linked to the foam core and fabric deformation during the filling stage. A simplified two-dimensional model is proposed to predict the pressure evolution resulting from the combined deformation of foam core and the fabrics. Computed results show reasonable agreement with the experimental results.

Mechanical behavior of municipal solid waste incinerator bottom ash: Results from triaxial tests

Bottom ash resulting from the incineration of various domestic wastes can be viewed as a typical granular material. It is mainly used in civil engineering as a substitute for traditional natural aggregates. The purpose of this paper is to characterize their mechanical behavior and evaluate their mechanical properties for engineering applications. First, results of triaxial tests confirm that bottom ash behaves like dense sand. Second, the deformation and strength characteristics of bottom ash, such as the secant modulus, Poisson ratio, characteristic angle, dilation angle, effective cohesion and effective friction angle, are determined. It is found that these mechanical parameters are in close agreement with those of road aggregates and are influenced by the effective confining pressure. Third, the evolution of the deformation modulus according to the axial strain and the variation of the deviator stress according to the mean effective pressure are analyzed. Finally, a set of points of the yielding state is determined from triaxial tests to represent the shape of the yielding surface of bottom ash.

Experimental Characterization and Modeling of Bending Properties of Woven Fibrous Preforms

The properties of final composite parts depend on properties of dry preforms often being formed over doubly-curved shapes. In this case the fibrous preforms exhibit intricate large deformations, including shear, tension, and bending modes. Although the bending stiffness of fibrous materials is small, in shaping of preforms, when wrinkling occurs, its influence is important, not negligible and responsible for the wrinkles shape. Because of the structural and mechanical peculiarities, the experimental determination of bending properties of fibrous materials is rather complex, and there is no unique generally adopted test. A set of cantilever tests was chosen to be carried out in this study, in the form of sequence of different loading cases for one material that permits to reveal the eventual non-linear and non-elastic behavior of the material in bending. The tests were realized for glass fabrics with different types of weaving patterns and different areal weights. The effect of these parameters on the bending response is studied. The analysis of the data on bending of fabrics and bending of yarns, extracted from fabrics with the preserved undulated shape, is performed as well. The regions of the deformed specimens characterized by the largest scatter of experimental data are identified and analyzed. Besides, the analytical model based on corrugated plates theory, taking into account the undulated architecture of fabrics, is employed to characterize its bending properties, and to make a future comparison with the test results.

All-Thermoplastic Composite Sandwich Panels – Part II: Modelling of Bending Behaviour

This second part aims to investigate the modelling of the bending behaviour of all-thermoplastic sandwich structures with improved aesthetic properties, manufactured by isothermal compression moulding. The aesthetic thermoplastic sandwiches exhibit specific features such as thick multi-layered faces and significant core properties variation due to processing conditions. Considering these specific features, a three-step calculation methodology has been developed to determine an accurate analytic model, to predict the equivalent shear properties of the core after manufacturing and to take into account the influence of the glass veil layer used to improve the surface quality of the part. Finally, a finite element modelling of the linear elastic behaviour of a simply supported sandwich plate under a concentrated load is achieved to validate the proposed methodology.

Three-dimensional mechanical properties of dry carbon fiber tows subjected to cyclic compressive loading:

Characterizing the response of compressed dry fibrous reinforcements is a key feature to control liquid composite molding processes. Considering preforms manufactured by automated dry fiber placement, supplementary information are needed, since cyclic loading occurs, during which the mechanical properties of tows evolve. In this way, an extensive characterization procedure is proposed in this article. Each mechanical feature, such as nonlinearity, springback, irreversibility, stabilization, and strain rate dependence, is analyzed, and when possible, related to microstructural observations. Particular attention is paid to the response in other directions than the compressed one. Finally, from this characterization, a route to modeling is drawn: analogies with other classes of materials are proposed, leading to choices on the modeling of each mechanical property.