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In this work, we undertook a comparative study of the dynamic dielectric analysis of two unidirectional epoxy composites: flax-fiber-reinforced epoxy and flax/carbon-fiber-reinforced epoxy (FCFRE). In both composites, three relaxation processes were identified. The first one is the water dipoles polarization imputed to the presence of polar water molecules in flax fiber. The second relaxation process associated with conductivity occurs as a result of the carriers charges diffusion noted for high temperature above glass transition and low frequencies. As for the third dielectric relaxation associated with the interfacial polarization effect is attributable to the accumulation of charges at the fibers/matrix interface. The presence of two carbon plies in the reinforcement gives rise to two interfacial polarization effects in the FCFRE composite. The analysis of the Maxwell-Wagner-Sillars and the water dipoles polarizations using the Havriliak-Negami model revealed that the presence of two plies of carbon can locally decrease the adhesion of flax fibers in the matrix. This analysis was supported by the thermal properties using a differential scanning calorimety and the mechanical properties using a short beam shear test. [ABSTRACT FROM AUTHOR]

We propose a contactless optical method to determine the dispersion curves of guided acoustic modes propagating along a micrometric fiber. Subnanosecond laser pulses are used to generate guided acoustic waves, and an optical probe is used for measuring the ultrasonic displacements at the fiber surface. The test sample is an aluminum fiber of diameter 33 im. The comparison between the experimental and theoretical dispersion curves is presented. [ABSTRACT FROM AUTHOR]

The aim of the present work is to study the effect of conventional textile treatments of woven flax on the mechanical properties and the water sorption of flax/epoxy composites. The flax fabrics are standard 2/2 twills. Various treatments are carried out on fabric such as mercerization, bleaching, and leaching for long fibers or on yarn such as leaching for short fibers. A model, based on a modified rule of mixture applied to composite reinforced with woven fabric, is developed to include the effect of fiber and porosity volume fractions on composite stiffness and strength. Most treatments improve tensile stiffness and strength of flax/epoxy composite and reduce composite water sorption. We prove by X-ray fluorescence analysis, thermogravimetric analysis, and tensile tests of dry fabric that it is due to an improvement in the interfacial bonding between fibers and matrix. The best performances are achieved with bleaching and mercerization treatment. The weakest performances are obtained with the composites made with leached yarns. POLYM. COMPOS., 34:1761-1773, 2013. © 2013 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

This experimental study highlights the effect of the geometric form factor (λ=length/diameter) and the fiber mass fraction on the mechanical behavior of an Alfa/Greenpoxy bio-composite. The Alfa stalks, collected in the Djelfa region in the Algeria highlands, were cut into pieces with length of 7 to 10 cm, washed and dried for two days at 70 °C. Using a knife mill coupled to three sieves (1.6, 2 or 2.5 mm), three categories of short fibers, according to their form factor λ, were obtained. Depending on the incorporated mass fraction (5, 10, 15 or 20 %) and the three form factors λ of the fibers, twelve types of plates were manufactured by hand molding followed by a curing cycle to accelerate the polymerization, reduce porosity and improve the final surface state. The main mechanical characteristics were determined with tensile, bending and shock tests on ISO 3167-type A samples, obtained by laser cutting. The results revealed that the increase of the form factor and the mass fraction gives rise to a significant improvement of the mechanical properties. We conclude that optimal processing conditions will maximize the mechanical properties of Alfa/Greenpoxy bio-composites. [ABSTRACT FROM AUTHOR]

• A strong variation of flax bundle cross-section was demonstrated. • The CSA of flax fibre bundles can be better assessed by an elliptical model. • Twists along the bundle were demonstrated and observed. • A numerical reconstruction of the 3D outer contour of the bundles is proposed. The increasing use of plant fibres in industrial applications requires a better understanding of their morphologies. Experimental observations have shown that these fibres are characterized by a complex geometry which could affect their mechanical behaviour. Indeed, it is well known that the size and shape of plant fibres cross-section vary from bundle to bundle and along their length. In this study, the technique of Fraunhofer diffraction was used to characterize this morphological heterogeneity of flax fibre bundles. The analysis of the results showed a large scattering of the cross-section area (CSA). Lengthwise morphological variations of the bundles were examined and reveal that the CSA varies by a factor of 3.4 over a length of 75 mm. It also appears that the cross-section shape of flax fibre bundle can be better approximated by an elliptical model than by a circular model. The ratio between the maximum and minimum cross-section shape factor was found to be approximately 2 over a length of 75 mm. Moreover, the occurrence of cross-section rotations has been shown in case of fibre bundles, i.e., the orientation of the major-axis of the ellipse corresponding to the cross-section may be different from one point to another, causing sometimes visible twists along the bundle. These different results allowed us to build a numerical 3D volume considering the evolution of flax fibre bundles outer contour. [ABSTRACT FROM AUTHOR]

• Retting of flax fibers combined with processing parameters affect the organoleptic aspects of biobased composites. • Retting process affect the mechanical properties of technical flax fibers but allows the better ones for biobased composites. • Mechanical performances of flax biobased composites are very sensitive according to the processing parameters. • Optimized parameters (T = 160 °C, t = 130 min, P = 50 bars) allows highest fiber fraction for the lowest porosity fraction. • New method for determining the weight fiber content in biobased fibers composites using TGA has been proposed. This research aimed to evaluate the effect of retting (3 levels were concerned) and processing parameters (mainly temperature) on physico-chemical properties of flax-epoxy materials. 9 biobased composites were produced with 3 different processing programs: Setup 1, 2, and 3. An advanced organoleptic study has been carried out on both technical flax fibers and manufactured composites to study in more detail the effects of the processing and retting on the visual aspect. Organoleptic analysis shows that flax, as well as biobased composites, lose saturation and luminance by increasing retting time. Increasing the processing temperature alters the material color. According to SEM analysis, we highlight that the least retted fibers are presented in bundles that are well bonded and coated with each other to form a technical fiber. Depending on retting, technical fibers appear cleaner and more individualized but their mechanical properties are affected. An interesting approach has been applied for the determination of the volumic content of our biobased composites, using both TGA and pycnometer methods. On average, V f decreases from 63.1 % for early retting (−) to 51.5 % for late retting (+). This effect could naturally result from the observed individualization of the fibers. Moreover, the porosity rates V p increase overall with fiber content. On average it varies from 5 to 11 % in function of retting. Setup 3, with a processing temperature of 160 °C, a processing time of 130 min and a processing pressure of 50 bars, is the most desirable because it allows the highest V f for the lowest V p. Regarding the mechanical behavior of biobased composites, we have observed a non-elastic behavior of our stress-strain curves, due to the intrinsic behavior of flax fibers. Stress vs strain curves reveal 3 different areas for elastic and plastic transitions. It also appears that setup 3 provides the best modulus of elasticity compared to the other setups. We notice also that E1 Young's modulus gradually increases with retting. By performing a normalization of E1 modulus according to the fiber volume, the effect of retting is even more pronounced. A 40 % increase in modulus can be observed between retting (−) and (+). At the end, the long retting level of technical flax fibers, as well as the third setup gives the best compromise for our bio based composite performances. [ABSTRACT FROM AUTHOR]