Issue 50

M.A. Khiat et alii, Frattura ed Integrità Strutturale, 50 (2019) 595-601; DOI: 10.3221/IGF-ESIS.50.50 599 fiber stress, δ is the ineffective length and “a “is the zone of matrix yielding. In Figs. 2 and 3, we plot the evolution of the ineffective length according to the number of broken fibers with progressive variation of moisture concentration from 0% to 100% for T=20°C, and 120°C, respectively. Ton figure 2, we can clearly see a superposition of all curves representative the ineffective length. We can deduce that for low temperatures, the variation of the moisture concentration does not have any effect on the ineffective length. With the increase in the temperature to 120°C (Figure 3), the sensitivity of the ineffective length to the variation of the moisture concentration becomes more and more significant. At T = 120°C, the ineffective length for 43 broken fibers which is 0.11 mm for C=0% become 0.10 mm for C=100%. Figure 3 : Evolution of the ineffective length according to the number of broken fibers with progressive variation of moisture concentration for T=120° and  = 0.5  0 . E VOLUTION OF STRESS CONCENTRATIONS nder hygrothermal loading of unidirectional composites, the influence of interfacial adhesion on the tensile strength is mandatory effects on stress concentration in the fibers adjacent to broken fibers, and critical length of broken fibers. Since the matrix is very sensitive to the variation in temperature, the adherence at interface region between fibers and matrix becomes very weak; thus the phenomenon of debonding appears, leading to the break of fibers. When a fiber breaks in the composite, there is a region of influence generated around the end of the broken fiber where a stress concentration exists. On the other hand, the load can be transferred back into the broken fiber with a shear stress at the fiber/matrix interface. To illustrate the effect of environmental conditions on the degradation of interfacial region surrounding the broken fibers, we plot in (Figs. 4 and 5) the evolution of the stress concentrations according to the number of the broken fibers with progressive variation of moisture concentration from 0% to 100% for T=20°C and 120°C, respectively. In both figures, the stress concentrations become more significant with the increase in the number of broken fibers. Thereafter, this stress concentration becomes relatively stable if the number of fibers becomes very large. For low temperatures, the effect of the moisture concentration on the stress concentration is almost negligible (Fig. 4). The effect of the moisture concentration becomes increasingly significant with the increase in the temperature to 120°C (Fig. 5). At T=120°C, a clear distinction appears between all curves; for example, with 43 broken fibers, the stress concentration varies from 1.17 for C=0% to 1.20 for C=100%. U