Laurent Bizet

Mechanical Properties of Flax Fibers and of the Derived Unidirectional Composites

Flax fibers were used to process unidirectional composites by two different methods. Their mechanical properties obtained by tensile testing are discussed with respect to the properties of the fibers and those of the matrix (unsatured polyester). The similarity of the tensile curves of the composites and of the elementary fibers is attributed to the good adhesion of the fibers with the matrix. Moreover, as there is almost a linear evolution of the composite properties with the fiber volume fraction, these properties are used to estimate those of the real reinforcement material, that is, the flax bundles: the calculations lead to a fiber strength of 500-800 MPa and a fiber modulus of roughly 30 GPa, which is half the values obtained by tensile testing elementary fibers. These data may be helpful when trying to model the deformation behavior of flax fiber-reinforced composites.

Analysis of the Film-stacking Processing Parameters for PLLA/Flax Fiber Biocomposites

Nowadays, the market demand for environmentally friendly materials is rapidly increasing. Biodegradable fibers and biodegradable polymers, mainly extracted from renewable resources, are expected to be a major contribution to the production of new industrial high performance biodegradable composites, partially solving the problem of waste management. At the end of its lifetime, a structural biodegradable composite can be crushed and recycled through a controlled industrial composting process. Bodros et al. [1] showed that biodegradable L-polylactide acid (PLLA)/flax fibers mat composites exhibiting specific tensile properties equivalent to glass fiber polyester composites can be manufactured by an un-optimized film-stacking process. In our study, the process has been investigated more extensively. Indeed, the compaction of flax mats requires a higher load than for glass mats of similar areal weight. The transverse permeability of flax mats has also been shown to be lower than for glass mats. In both cases, this is due to a higher degree of entanglement of the flax fibers within the mat. However, the range of permeability and compressibility values of the flax mats are well within the values that allow a good through-the-thickness impregnation. Flax fibers cannot sustain long exposures at the impregnation temperature of the mats by PLLA resin. Through-the-thickness impregnation of flax mats processes such as film stacking are more suitable than in-plane impregnation processes such as resin transfer molding because the flow of resin is limited on short distances and allows short times of impregnation.

Analytical modeling and in situ measurement of void formation in liquid composite molding processes

Liquid composite molding processes are widely accepted in the aeronautic industry to manufacture large and complex structural parts. In spite of their cost-effectiveness, void defects created during the manufacturing process are a major issue of these processing techniques because they have detrimental effects on the mechanical performance. The reliable modeling is still a difficult task and experimental observations are usually adopted for the analysis of void formation mechanism, however, because many different physics are simultaneously involved during the mold filling process and the resin curing process. The complexity of the void formation physics implies the need for an in situ measurement of void formation not in the final part but in the mold filling procedure during the manufacturing process to better understand the void mechanism. In this regard, we present a sensor system measuring the electric conductivity for the in situ monitoring of void formation during the mold filling process. We also propose a theoretical model to predict void formation in a quantitative way with the properties of the resin and the fiber reinforcement. The model prediction is compared with the experimental data obtained by the sensor system to validate the model.

Modeling and Simulation of Void Formation During the Resin Transfer Molding Process

We present modeling and simulation of air void formation in composite materials manufactured by the Resin Transfer Molding (RTM) process. The prediction of air void formation has been an important topic because air voids in composite materials deteriorate the mechanical properties of the part.It has been found by experimental observations that the void content, for a specific preform, can be correlated with capillary number which is the ratio of the viscous force and the surface tension. It is still difficult, however, to predict the void formation without experimental measurement. Moreover, the capillary number may not be the exclusive parameter in practical cases, because the modeling by the capillary number does not work well for large and complex parts.In this context, we propose a mathematical model to predict the air void formation in the channel which is on open gap between fiber tows and inside the fiber tow. Moreover, the void formation in the warp and the weft are modeled separately by considering the tow orientation with respect to the flow direction. We also modeled two other important phenomena, namely air void compression or expansion, and void migration. To validate the model, void content was experimentally measured by injecting an electrically conductive liquid into a preform. The voltage drop was correlated with the air void content considering the air as a non-conducting material. For a unidirectional fabric, a good agreement was obtained between the model prediction and the experimental result.Copyright