Page 367 - Mechanics Analysis Composite Materials

352 Mechanics and analysb of composite materials

As follows from Eq. (7.69), which is valid for winding without tension, overlap of
the tape results in reduction of material stiffness. Because the levels of loading for
the fibers of BC and CD parts of the ring (Fig. 7.44(a)) are different, reduction of
material strength can also be expected.
Filament winding is usually performed with some initial tension of the tape. This
tension improves material properties because it straightens the fibers and densifies
the material. However, high tension may result in fiber damage and reduction of
material strength. For glass and carbon fibers, preliminary tension usually does not
exceed 5% of the tape strength, while for aramid fibers that are less sensitive to
damage the level of initial tension can reach 20% of the tape strength. Preliminary
tension reduces the effect of the tape overlap discussed above and described by
Eq. (7.69). However, this effect can show itself in reduction of material strength,
because the initial stresses which are induced by preliminary tension in the fibers can
be different, and some fibers can be overloaded or underloaded under external
forces acting on the structure in operational conditions. Strength reduction of
aramid+poxy unidirectional composites on the tape overlap observed in experi-
ments of Rach and Ivanovskii (1986) for winding on a 200 mm diameter mandrel is
demonstrated in Fig. 7.45.
The absence of the tape preliminary tension or low tension can cause the ply
waviness shown in Fig. 7.46 which can occur in the filament wound laminates as a
result of pressure exerted by the overwrapped plies on the undenvrapped plies or in
flat laminates due to material shrinkage in the process of curing.
The simplest for analysis is the regular waviness presented in Fig. 7.46(a). To
determine the apparent modulus in the x-direction, we can use the expression
similar to one presented in Eqs. (4.76), Le.
I COS^^ sin4a
---- +-+ (&-$) sin2crcos2cr . (7.70)
Ex El E3

::L

0.2
A
0.3
0.2
0.5
0.4
0
0
0.1
Fig. 7.45. Dependence of the normalized longitudinal strength of unidirectional aramid-epoxy
composite on the tape overlap.