Abstract. This work presents an analytical model to predict the strength of the unidirectional carbon epoxy composite using micromechanical techniques. This model supposes that a group of broken fibres surrounded by a number of intact fibres with hexagonal arrangement. The mathematical developments used are presented to justify the distribution form of the stresses around broken fibre and adjacent intact fibres. To follow the evolution of the damage in regions of debonding and local plasticity; we proceeded to a progressive increase in the fiber volume fraction and tensile external load. This, procedure enable us to evaluate the extension of the region locally plasticized, the ineffective region, the stress concentration and the longitudinal displacement of broken and intact fibres, in function of broken fibres number and specimen length. As fiber breaks are intrinsically random, the variability of input data allows us to describe the probabilistic model by using the Monte-Carlo method. The sensitivities of the mechanical response are evaluated regarding the uncertainties in design variables such as Young's modulus of fibers and matrix, fiber reference strength, shear yield stress, fiber volume fraction and shear parameter defining the shear stress in the inelastic region.
An electrochemical study using the cyclic voltammetry method was carried out on some previously prepared merocyanines salts, namely thiazolideniumsulfonate salts 5a-b, and thiazolidenium chloride salts 6a-b, and merocarbocyanines salts, namely alkylidenthiazolidenium sulfonate salt 5c, and alkylidenthiazolidenium chloride salt 6c. These salts are transformed by dimerization in situ in a voltammetric cell into tetrathiatetraazafulvalenes (TTTAFs) 7a-b, 7’a-b, 8c, and 8'c supposed to be π-electron donor molecules due to the existing conjugation in their structure. The structure of all new chemically synthesized molecules was confirmed by IR, 1H-NMR, 13C-NMR, and MS. The transformation of salts into TTTAF was confirmed by a reversible voltammogram curve and the variation of observed potentials.
Natural continuous natural fibres (hemp, flax, etc.) are gaining popularity in composite materials because they can advantageously replace glass fibers. It is found that intrinsic properties of unidirectional (UD) composites are almost equivalent to those of unidirectional glass fiber composites. Unfortunately it is difficult to get repeatable results because of the inherent variability in the properties of natural fibers compared to glass fibers. Their quality is largely affected by the weather conditions, the extraction location along the plant and the techniques used to extract them (retting, bleaching, etc.). The present paper proposes a strength reliability model for unidirectional composites with natural fibers in a hexagonal array. The model assumes that, a central core of broken fibers flanked by unbroken fibers which are subject to stress concentrations from the broken natural fibers. Thermal and hygroscopic residual stresses are neglected because they haven't more effect when the composite was subjected to tensile than transverse loading. The approach of the model consists of using a modified shear lag model to calculate the ineffective lengths and stress concentrations around the broken fibers. In this paper, we attempt to incorporate in the proposed model the unidirectional composite property variation with temperature and moisture in order to predict even composite strength degradation. Strength degradation is often seen as a result of changes in ineffective lengths at natural fiber breaks and the corresponding stress concentrations in intact neighboring fibers.
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