Fiber Reinforced Composites Basic Considerations

The basic elements of composite materials are the uni-directionally reinforced ply or strand. Plies are thin plates (or shells), with a typical thickness h = 0.125 nun, containing randomly arrayed fibers miming parallel to each other, all embedded in a polymeric resin. Typical values of the fiber volume fraction Vf range between 40 and 60%. Strands are essentially uni-directional elements, with varying values of thickness and width, both of which are much shorter than the strand s length, and a similar range of Vf. 

Each of the above elements can be viewed as being transversely isotropic about the common fiber direction, say x, thus characterized by five material constants, say E, E22, G23, Gi2, and V12, where %2 — X3 is the plane of isotropy and V12 is the Poisson s ratio that expresses the strain 62/ i. due to For in-plane problems, say within the Xi — X2 plane, G23 is globally irrelevant. Similarly, only two thermal and two hydral expansional coefficients are of interest, namely i, 2 (=as) and jSi, 

Typical values of material properties are given in Table 2.2 for several fiber reinforced polymeric composites 

Property E-glass/ epoxy S-glass/ epoxy Carbon/epoxy A54/3501-6 Carbon/epoxy IM6/SC 1081 

Uni-directionally reinforced strands are the essential components in the manufacture of woven composites. In similar maimer, the uni-directional reinforced plies are the basic building blocks of laminates. The latter contain a lay-up of several, multi-oriented plies and are formed by the appUcation of a prescribed regime of pressure and temperature within an autoclave or a vacuum press. 

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The micromechanics approaches presented in this book are an attempt to predict the mechanical properties of a composite material based on the mechanical properties of its constituent materials. In nearly all fiber-reinforced composite materials, there is considerable difference between expectation and reality. Thus, we must ask what is the usefulness of micromechanical analysis beyond gaining a feeling for why composite materials behave as they do Basically, there are two answers one related to designing a material and one related to designing a structure. 

From a strictly theoretical point of view, the so-called constituent testing approach or micromechanics approach is the most valuable. Tests performed on composite constituents supply the required material constants of each phase of the composite material— namely for long-liber-reinforced composite— for the fiber and the matrix, to use in appropriate mixture rules. These rules obtained by physical and mechanical considerations are the basic relationships between the composite constituents, and they leads to a complete characterization of the final composite. 

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