igno fibers -Natural cellulose are biological structures composed of mainly cellulose microfibrile and an amorphous matrix consisting of lignin and hemicellulose, in relatively small proportion of non-extractable nitrogen, crude protein content, lipids and materials mineral [1] and [2]. The composition of the wood depends on the class of the tree (coniferous or deciduous), species, individuals of the same species and the diameter portion (heartwood, sapwood and heart) of the log of the individual.

Ligno fiber -celluloses wood are used extensively worldwide in the manufacture of paper pulp and dissolving pulp, in the manufacture of composite materials [3] and [4], in industries furniture [2].

The palmyra, wood in the class of palm trees, has a specific morphology. This is a very dense wood fibers (about 100 fibers per cm 2 ) and compact. Its fibers, brown in color, are very long, very large (area about 1 mm 2 ) and very rigid (Image.1). Its lignified structures are embedded in the main parenchyma which is also cellulose [5].

In Chad (Image.2), the palmyra is widely used as slats and beams in the construction of habitats, and as fence posts fields. The production of these building elements is hand-crafted without recovery of falls or its residues. Economically, this is a shortfall in earnings, and therefore an obstacle to sustainable development. The use of by-products has become a priority in applied research and key to sustainable development. The objective of this study is to popularize local materials on the one hand to participate in a recovering economy and secondly to gain autonomy by using local products that do not require large investments.

Determining the mechanical characteristics of tensile fibers allow to envisage its use as a reinforcement in the composite artificial. This is why the monotonous tensile tests are carried out on fibers extracted from the heartwood and sapwood of an individual palmyra aged around 40 years.

The fibers tested are taken from the piece of tree 1.5m above the base. In this part, the heartwood which is very dense in old fibers is clearly distinguishable from the sapwood.

The average density is 124 fibers per cm 2 for the heartwood and 77 fibers per cm 2 for the sapwood (Image.1). This sample has an average chemical composition is as follows:

-Cellulose: 63.21% in the heartwood and 61.89% in sapwood;

-Hemicellulose: 09.60% in the heartwood and 11.32% in sapwood; -Lignin: 19.36% in the heartwood and 19.68% in the sapwood.

Image 2 : Cartography of palmyra field in Houndouman a) Experimental Procedure The fibers are manually extracted from the heartwood and sapwood. After drying in the sun for 6 hours, the fibers are manually cleaned for reducing the presence of parenchyma that remained glued on them without. They were not subjected to chemical treatment.

The fiber dimensions were assessed caliper (0.01mm accuracy). Sections, calculated using the method of the circumscribed rectangle (Figure .1) are between 0.48 and 0.96 mm 2 for fibers of sapwood, and 0.46 and 1.03 mm 2 for the heartwood. For tensile tests, we used a UTS brand electric machine equipped with a 1.5kN capacity load cell and manual clamping jaws.

The fibers were clamped in the jaws and the test was started at 1 mm / min (as that typically found in the literature when testing plant fibers), up to rupture of the fiber. For each type of fiber, 8 specimens were tested, and those who had broken into the jaws were not taken into account in the calculation of averages. The stresses are calculated using the formula

, where F is the tensile force in Newton (N) and S the area of the cross section of the fiber in mm 2 .

The deformations are calculated from the formula

, in which L 0 is the initial lengths and L the lengths after deformation.

III.

The stress-strain curves are plotted for each of the eight sapwood fibers (Figure .2) and each of the eight heartwood fibers (Figure .3).

The Table .1 and 2 give the values of the calculated mechanical characteristics.

The table.3 presents the average values of the mechanical properties of sapwood and heartwood.

-The modulus of elasticity of the fiber of the sapwood (17 GPa) is substantially the same as that of the fiber of the heartwood (16.8 GPa); -The tensile strength of the fiber of the sapwood (184 MPa) is lower than the fiber heartwood (219 MPa); -The break strains of the sapwood fiber and the heartwood fiber are substantially the same (a difference of 0.3%). These differences between the mechanical characteristics are linked to the maturity of the fibers in the wood. Indeed, the fibers of the heartwood are very old, so mature enough to be more resistant than the fibers of the sapwood.

Compared with the fibers of the petiole of the palm doom, the palmyra fibers are very rigid and highstrength than the fibers of the palm petiole doom. However they have low strain at rupture. Note however that this comparison gave to the different aspects. Saw eye, the fiber of Palmyra is a macroscopic cluster of thin fiberboard. The longitudinal elastic modulus of each fiber was obtained by using the linear regression method to the right of the stress -strain curve.

Then the average values of various characteristics were calculated.

The average values of the mechanical characteristics of Table .3 show that: Sapwood's Fibre in traction y = 0,3419x -0,0663 y = 2,9436x -0,873 0,00 0,50 This behavior of the mechanical characteristics of the fibers is contrary to their intrinsic nature which is independent of their values in relation to the fiber dimensions. CHARLET and al. [7] LILLHOLT and al. [8] and Mott and al. [9] stated in their papers that vegetable fibers are characterized by a very high intraspecific and interspecific variability of their mechanical properties which depend on the species, the organ of origin fiber, the proportion cellulose-hemicellulose -lignin, the degree of polymerization and crystallization of the cellulose, the micro fibril angle (Figure.9) and the structural defects. Indeed, in native fibers, micro fibrils are arranged to describe with the fiber axis a micro fibril angle whose value varies from one plant species to another. The micro fibril angle partly determines the mechanical characteristics of elongation and stiffness of the fiber [8]. The random evolution of these characteristics depending on the section and the length can be explained by variability along the fiber section.

In terms of the tensile strength, BOS et al. [11] also noted that its variation is related to the influence of structural defects in the fiber. NILSSON and GUSTAFSSON [12] explained the experimental results (modulus of elasticity) of BALEY [13] proposing a model that considers the dislocations and plastic behavior of hemicellulose as the main parameters governing the strength of the fiber. For them, the non-linearity of the Stress-strain curve (Figure. The study on the mechanical properties of the fiber palmyra shows that:

-The longitudinal elastic modulus (Young's modulus) and the failure strains of heartwood of fibers and fiber sapwood are of the same order of magnitude as those of sapwood; -The tensile strength of the fiber heartwood is very higher than that of the fiber of sapwood;

-The values of its mechanical properties are strongly dependent on the cross-sectional area (diameter) of the fiber; undertake the study of its use as reinforcement in artificial composite materials. THANKS Finally, we'd to thank the authorities of the Blaise Pascal University of Clermont-Ferrand, the Francophone University Agency (AUF), the University of N'Djamena and the Directorate of Forests and Desertification Control for their financial and material inputs for the realization of this study.

This modest contribution to the understanding of the mechanical properties of the fiber palmyra will