Véronique Michaud

Infiltration processing of fibre reinforced composites: governing phenomena

All three classes of fibre reinforced composite materials (polymer, metal and ceramic matrix) may be produced by flow of liquid matrix into the open spaces left within pores of a fibre preform. Even though several specific issues arise from the nature of each composite matrix class, governing phenomena apply to all infiltration processes, and include in particular: (i) capillary phenomena, (ii) transport phenomena, and (iii) the mechanics of potential fibre preform deformation. These phenomena and their governing laws are reviewed for the case of isothermal infiltration with no phase transformations. Four basic functional quantities, which need to be known to model the processes, are identified, and addressed in turn. The paper concludes with some examples of modelling methodologies and comparison with experimental data.

An Impregnation Model for the Consolidation of Thermoplastic Composites Made from Commingled Yarns

A model for the consolidation of thermoplastic composites based on commingled yarns was developed. The evolution of composite void content during consolidation was related to the processing parameters and the material properties. Additionally, the analysis relied on experimental investigation of the yarn structure, taking into account a distribution in dry fiber bundle sizes. Residual porosity was considered to result from pore closing at a given stage during fiber bundle impregnation. The accuracy of the model was assessed by conducting consolidation experiments using a unidirectional commingled yarn fabric made of polyamide 12 resin and carbon fibers. Excellent correlation was found between predicted and experimental void contents. The model aided prediction of suitable processing conditions and identification of the main physical and geometrical parameters influencing the consolidation rate.

Dynamic properties of sandwich structures with integrated shear-thickening fluids

The integration of shear-thickening fluids (STFs) into composite structures has been investigated with the aim of tuning part stiffness and damping capacity under dynamic deformation. Results from oscillatory rheological measurements for a STF based on concentrated fused silica in polypropylene glycol were correlated with results from vibrating beam tests on model sandwich structures containing layers of the same STF sandwiched between polyvinyl chloride (PVC) beams. Above a critical amplitude, the relative motion of the PVC beams provoked shear thickening of the silica suspensions, and the vibration and damping properties were significantly modified. These changes were related to the rheological response of the STF through analytical calculations of strains in the STF layers, an approach that was verified experimentally by replacing the STF with a slow-curing epoxy resin. The potential for integrating STFs into structures exposed to dynamic flexural deformation, with the aim of controlling their vibrational response, has thus been demonstrated.

Commingled yarn composites for rapid processing of complex shapes

Abstract Commingled yarns of reinforcing and thermoplastic fibres offer a potential for low-cost manufacturing of complex-shaped composite parts, due to reduced impregnation times and applied pressures during processing. In order to benefit from this competitive advantage, the process parameters governing consolidation must be controlled. In this study, a consolidation model, previously validated for unidirectional commingled yarn fabrics processed isothermally in a flat matched-die mould, is applied to three other processing techniques capable of producing complex-shaped composites. Tubes of braided commingled yarns were manufactured by bladder inflation moulding. Selectively reinforced polymeric parts were processed by compression–injection moulding. Stamp forming was also used to allow high-speed processing of commingled yarn-based laminates. Besides particular stamp forming cases, for which part deconsolidation occurred, the model predictions were in good agreement with the void content values obtained from specimens consolidated under different processing conditions. This suggests that the consolidation model can be successfully applied to a wide range of yarn architectures and processing techniques.

Effect of silane coupling agent on the morphology, structure, and properties of poly(vinylidene fluoride-trifluoroethylene)/ BaTiO3 composites

Micron- and submicron-sized barium titanate (BaTiO3) particles, untreated and surface modified with aminopropyl triethoxy silane, were incorporated in poly(vinylidene fluoride–trifluoroethylene) to fabricate composites with up to 60 vol% of ceramic phase. The morphology and structure of solvent cast and compression-molded films, and their thermal, viscoelastic, and dielectric properties were investigated. When surface-modified BaTiO3 was used, it was possible to decrease both the viscoelastic and the dielectric losses of highly filled solvent cast films, while their storage modulus and relative permittivity either increased or remained equal, owing to reduced porosity and improved matrix-filler compatibility. The effect of BaTiO3 surface modification on the morphology of compression-molded films was less marked, leading to unchanged viscoelastic properties, and lower permittivity and dielectric losses. For all composites the frequency dependency of the dielectric properties at low frequencies was suppressed with modified BaTiO3.

Smart composites with embedded shape memory alloy actuators and fibre Bragg grating sensors: activation and control

This paper describes the production of an adaptive composite by embedding thin pre-strained shape memory alloy actuators into a Kevlar-epoxy host material. In order to combine the activation and sensing capabilities, fibre Bragg grating sensors are also embedded into the specimens, and the strain measured in situ during activation. The effect of manufacturing conditions, and hence of the initial stress state in the composite before activation, on the magnitude of the measured strains is discussed. The results of stress and strain simulations are compared with experimental data, and guidelines are provided for the optimization of the composite. Finally, a pilot experiment is carried out to provide an example of how a strain-stabilizing feedback mechanism can be implemented in the smart structure.

Effect of aging on the performance of solvent-based self-healing materials

Solvent healing of thermoset polymer matrices was suggested as an efficient and cost-effective alternative to other encapsulated self-healing systems. However, self-healing efficiency is not guaranteed over time. We report here on the autonomic self-healing of under-cured epoxy with ethyl phenylacetate-filled microcapsules under different aging conditions. We carried out quasi-static fracture tests and observed a decrease of healing efficiency from 77% for fresh samples down to 13% for samples aged at room temperature for 77 days. DSC results of pure epoxy matrix curing showed that the residual heat of reaction only decreased from 161 J g−1 to about 135 J g−1; this decrease was largely due to water absorption and could only partly explain the large decrease in healing efficiency. Furthermore, FTIR spectra did not indicate a change in conversion as observed by monitoring the associated oxirane peak in the pure epoxy matrix over time, confirming the above findings. While the microencapsulated solvent has good shelf life properties, evidence was found that solvent diffusion into the epoxy matrix prior to cracking was responsible for the reduced healing capability. DSC and FTIR results of aged self-healing epoxy confirmed the presence of the EPA solvent in the matrix even though no damage was introduced to the microcapsules. Therefore two phenomena responsible for the decreased solvent-healing capability in under-cured epoxy were identified: ambient moisture uptake and premature solvent diffusion through the microcapsule walls.

Passive damping of composite blades using embedded piezoelectric modules or shape memory alloy wires: a comparative study

Emission reduction from civil aviation has been intensively addressed in the scientific community in recent years. The combined use of novel aircraft engine architectures such as open rotor engines and lightweight materials offer the potential for fuel savings, which could contribute significantly in reaching gas emissions targets, but suffer from vibration and noise issues. We investigated the potential improvement of mechanical damping of open rotor composite fan blades by comparing two integrated passive damping systems: shape memory alloy wires and piezoelectric shunt circuits. Passive damping concepts were first validated on carbon fibre reinforced epoxy composite plates and then implemented in a 1:5 model of an open rotor blade manufactured by resin transfer moulding (RTM). A two-step process was proposed for the structural integration of the damping devices into a full composite fan blade. Forced vibration measurements of the plates and blade prototypes quantified the efficiency of both approaches, and their related weight penalty.

In-plane conduction of polymer composite plates reinforced with architectured networks of Copper fibres

Model composite plates composed of highly conductive slender copper fibres impregnated with a poorly conductive and transparent PMMA matrix were processed with different fibrous architectures, i.e. with various controlled fibre contents and orientations. Their microstructure was characterised using both optical observations and X-ray microtomography. Their in-plane thermal conductivity was measured by using a specific testing apparatus combined with an inverse modelling method. Results point out the strong link between the anisotropy of the in-plane conductivity and of the microstructure. The role of the fibre content on the conductivity is also emphasised. An analytical conduction model which accounts for the influence of the fibre content, the orientation, the aspect ratio and the thermal resistances at fibre-fibre contacts, was proposed and its predictions were compared with the experimental results. Using only one fitting parameter, namely the conductance at fibre-fibre contacts, this model shows a good prediction of all the experiments.

In situ Strain and Temperature Monitoring of Adaptive Composite Materials

An optical fiber sensor is designed to simultaneously measure strain and temperature in an adaptive composite material. The sensor is formed by splicing two fiber Bragg gratings (FBGs) close to each other, which are written in optical fibers with different core dopants and concentrations. Their temperature sensitivities are hence different. The sensor is tested on an adaptive composite laminate made of unidirectional Kevlar-epoxy prepreg plies. Several 150 μm diameter prestrained NiTiCu shape memory alloy (SMA) wires are embedded in the composite laminate together with one fiber sensor. Simultaneous monitoring of strain and temperature during the curing process and activation in an oven is demonstrated.