Page 97 - Mechanics Analysis Composite Materials

82 Mechanics and analysis of composite materials

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Fig. 3.30. Specimens of matrix material.

C

Fig. 3.31. Testing of the matrix specimen.

ratio, v,, and strength, d,, traditional for material testing specimens and testing
procedures can be used (see Figs. 3.30 and 3.31). Shear modulus, G,, can be
calculated with the aid of Eq. (2.57). To find the fibers properties, is a more
complicated problem. There exist several methods to test elementary fibers by
bending or stretching 10-30 mm long fiber segments. All of them are rather specific
because of small (about 0.01 mm) fiber diameter, and, what is more important, fiber
properties in a composite material can be different from those of individual fibers
(see Section 3.2.3) with pre-assigned lengths provided by these methods.
It is worth to know the fibers actual modulus and strength not only for
micromechanics but also to check the fibers quality before they are used to fabricate
a composite part. For this purpose, a simple and reliable method has been
developed to test the fibers in the actual conditions. According to this method, a fine
tow or an assembly of fibers is carefully impregnated with resin, slightly stretched to
avoid fiber waviness and cured to provide a specimen of the so-called microcom-
posite material. The microcomposite strand is ovenvrapped over two discs as in
Fig. 3.32 or fixed in special friction grips as in Fig. 3.33 and tested under tension to
determine the ultimate tensile force F and strain E corresponding to some force
F < F. Then, the resin is removed by burning it out, and the mass of fibers being
divided by the strand length and fiber density yields the cross-sectional area of fibers
in the strand, Af. Fiber strength and modulus can be calculated as