Page 333 - Mechanics Analysis Composite Materials

318 Mechanics and analysis of composite materials

material damage. Because the stable moisture content is rarely reached in real
composite structures, its current distribution through the laminate depends on the
laminate structure and thickness. Among the polymeric composites, the highest
capacity for water absorption under room temperature is demonstrated by aramid
composites (7 f0.25% by weight) in which both polymeric matrix and fibers are
sensitive to moisture. Glass and carbon polymeric composites are characterized by
moisture content 3.5 & 0.2% and 2 & 0.75%, respectively. In real aramid-epoxy and
carbon-epoxy composite structures moisture content is usually about 2% and 1%,
respectively. The lowest sensitivity to moisture is demonstrated by boron compos-
ites. Metal matrix, ceramic, and carbon-carbon composites are not affected by
moisture.
As follows from Fig. 7.9 in which experimental data of Milyutin et al. (1989) and
Perov and Kruzhkova (I991) are approximated, water absorption is dramatically
influenced by temperature. While material absorbs water, it expands demonstrating
the effects that are analogous to thermal effects and can be modeled using equations
presented in Section 7.1.2 if we treat a1 ,a2 and a,, a,, as coefficients of moisture
expansion and change AT for AW. Just as temperature, moisture reduces material
strength and stiffness. For carbon-epoxy composites this reduction is about 12%,
for aramid-epoxy composites - about E%, and glass+poxy materials - about
35%. After drying up, the effect of moisture usually disappears.
Cyclic action of temperature, moisture, and sun radiation results in material
aging, i.e., in degradation of material properties in the process of material or
structure storage. For some polymeric composites,exposure to elevated temperature
which can reach 70°C and radiation whose intensity can be as high as 1 kW/m2
can cause more complete curing of the resin and some increase of material strength
in compression, shear, or bending. However, under long-term action of the
aforementioned factors, material strength and stiffness decrease. To evaluate the

W,%

0 10 20 30 40 50
Fig. 7.9. Moisture content as a function of the root of the immersion time and temperature for aramid-
epoxy (solid lines) and carbon-epoxy (broken line) composites.