Moussa Gomina

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.

Microstructural and mechanical behaviour of polyamide fibre-reinforced plaster composites

Different concentrations of polyamide fibres with given aspect ratios were associated to a commercial plaster in a view to investigate the mechanical behaviour. The presence of these water-absorbent fibres perturbs the hydration of the plaster and causes changes in the plaster microstructure. All the mechanical parameters decrease monotonically as the concentration of fibres is increased, except the fracture toughness which shows a different trend. The ability of the composites to retain strength at the onset of matrix cracking has been forecast and analysed on a phenomenological basis in terms of a fibre efficiency factor which is a useful supplement to the usual mechanical characterisation of such materials.

Changes in plaster microstructure by pre-stressing or by adding gypsum grains: microstructural and mechanical investigations

Abstract The setting and the mechanical behaviour of a coarse-grained plaster were investigated by means of different techniques: adiabatic calorimetry and measurements of swelling pressure and dimensional variations during the setting, fracture strengths and toughness evaluation, and scanning electron microscopy (SEM) observations. Subsequently the microstructure of the plaster was modified by seeding with gypsum grains of controlled size and concentration, or by applying a compressive stress (pre-stressing) during the setting. The correlation of the microstructural features and the mechanical properties confirms the interest in these methods for mastering the characteristics of a number of widely produced plaster-based materials.

HIGH TEMPERATURE MECHANICAL BEHAVIOUR OF AN UNCOATED SIC-SIC COMPOSITE MATERIAL

Flexural tests have been performed on unnotched specimens of an uncoated SiC-SiC laminar composite material to check the influence of specimen thickness and temperature (22 to 1200‡C in air) on the strength and the elastic modulus. An important modification of the loading curves and crack paths occurred when the temperature was increased, corresponding to a strengthening of the fibre-matrix interface.

Effects of the hygrothermal environment on the mechanical properties of flax fibres

Since flax is the most promising plant for the reinforcement of polymer-based composites in structural applications, we have chosen to investigate its hygrothermal characteristics which can be useful for the understanding of the behaviour of other plant fibres. The flax fibres were exposed to different hygrothermal conditions: in an oven at various controlled temperatures (–40 to 140℃) and measured relative humidity, in a climate chamber at 50% relative humidity for define temperatures between 25℃ and 85℃, or different determined aging conditions. The correlation of these hygrothermal conditions to the evolution of the mechanical properties gives evidence of the prominent influence of water over temperature on the microstructural changes of flax fibres. The mechanical parameters drastically decrease in usually prescribed hygrothermal aging conditions for organic matrix composite materials, the strength being particularly sensitive to the presence of water. These evolutions were correlated to the fibre microstructure modifications induced by water absorption as revealed by electron microscopy analyses. These findings could be useful for understanding the behaviour of polymer matrix biocomposites in severe hygrothermal conditions.

Morphological characterization of ceramic-ceramic composites by image analysis

Abstract In this paper the potential of automatic image analysis for quantification of the morphology of composite materials is presented, and parameters related to the different types of porosity and to the fibres are given. It is shown that this technique could also be used to model the microstructure of ceramic matrix composites.

Numerical study of the influence of structural and mechanical parameters on the tensile mechanical behaviour of flax fibres

This paper presents the results of a numerical simulation of the ultimate flax fibre (Linum usitatissimum) tensile mechanical behaviour using finite element analysis. Experimental data were used to develop a numerical multilayer model of the flax fibre. Thus, the influence of some parameters, such as cell wall thicknesses, microfibrils angles (MFAs), biochemical composition and mechanical properties of the biochemical components, on the flax fibre tensile mechanical behaviour has been investigated. Results show that the typical stress–strain curve profile of the flax fibre could be due to the mechanical properties of hydrophilic components (hemicelluloses) and thus to the environmental conditions. A parameter sensitivity study reveals that ultrastructural parameters (hemicelluloses and cellulose Young’s modulus) strongly influence the flax fibre mechanical behaviour and structural parameters (S2 cell wall layer MFA and thickness) significantly influence the fibre longitudinal Young’s modulus. Thus, the knowledge of the fibre ultrastructure seems to be the key of the understanding of the flax fibre mechanical behaviour.

Sintering of CuO and ZnO in a Single Mode Microwave Cavity with Shrinkage Control

Abstract A specific TE10m microwave cavity has been designed to follow-up the shrinkage during the microwave sintering of ceramics powders using an optical based position sensing device. The basic principle consists in measuring the distance from a laser source to the sample surface by means of a triangulation method. The spatial resolution device is around a few micrometers that enables to accurately measure the shrinkage versus time of a microwave irradiated sample. The shrinkage curves have been recorded during the direct microwave sintering of CuO and ZnO. Sintering kinetics has been found extraordinarily fast as only a few seconds are needed to achieve the maximum shrinkage for both materials. This new method is undoubtedly powerful to increase our understanding of microwave sintering and very useful to control the microstructure of microwave sintered ceramics.

Applicability of Fracture Mechanics to Fiber-Reinforced CVD-Ceramic Composites

Investigation of mechanical behavior of C-SiC and SiC-SiC composite ceramic materials, using SENB and CT specimens at room temperature, is discussed according to the relative arrangements of the fibers, the orientation of the layers and the applied stress.

Microstructured optical fiber Bragg grating as an internal three-dimensional strain sensor for composite laminates

In this article, we study the possibility to use a pair of specifically designed microstructured optical fiber Bragg gratings (MOFBGs) as a multi-component strain sensor when embedded within composite materials. The dependence on the orientation of the transverse sensitivity of the MOFBGs is exploited to build a sensing device able to measure the strain field along the three principal mechanical directions of a laminate composite. We developed an analytical and numerical model of such a sensor and benchmarked it with experiments performed on laminated composite coupons equipped with this sensor. We report on a theoretical strain resolution of about 5 μ in the transverse directions of the composite material, which is a six-fold improvement over results reported in literature.