Page 39 - Mechanics Analysis Composite Materials

24 Mechanics and analysb of composite materials

complicated reinforcement than the unidirectional one typical for pultrusion. Even
more complicated fiber placement with orientation angle varying from 5" to 85"
along the bar axis can be achieved using two-dimensional (2D) braiding which
results in a textile material structure consisting of two layers of yarns or tows
interlaced with each other while they are wound onto the mandrel.
Plane laminated structure consists of a set of composite layers providing
necessary stiffness and strength in at least two orthogonal directions in the plane of
the laminate. Plane structure is formed by hand or machine lay-up, fiber placement
and filament winding.
Lay-up and fiber placement technology provides fabrication of thin-walled
composite parts of practically arbitrary shape by hand or automated placing of
preimpregnated unidirectional or fabric tapes onto a mold. Layers with different
fiber orientations (and even with different fibers) are combined to result in the
laminated composite material exhibiting desirable strength and stiffness in given
directions. Lay-up processes are usually accompanied by pressure applied to
compact the material and to remove entrapped air. Depending on required quality
of the material, as well as on the shape and dimensions of a manufactured
composite part compacting pressure can be provided by rolling or vacuum bags, in
autoclaves, and by compression molding. A catamaran yacht (length 9.2 m, width
6.8 m, tonnage 2.2 t) made from carbon-epoxy composite by hand lay-up is shown
in Fig. 1.18.
Filament winding is an efficient automated process of placing impregnated tows
or tapes onto a rotating mandrel (Fig. 1.19) that is removed after curing of the
composite material. Varying the winding angle, it is possible to control material
strength and stiffness within the layer and through the thickness of the laminate.
Winding of a pressure vessel is shown in Fig. 1.20. Preliminary tension applied to
the tows in the process of winding induces pressure between the layers providing
compaction of the material. Filament winding is the most advantageous in
manufacturing thin-walled shells of revolution though it can be used in building
composite structures with more complicated shapes (Fig. 1.21).
Spatial macrostructure of the composite material that is specific for thick-walled
and solid members requiring fiber reinforcement in at least three directions (not
lying in one plane) can be formed by 3D braiding (with three interlaced yarns) or
using such textile processes as weaving, knitting or stitching. Spatial (3D, 4D, etc.)
structures used in carbon-carbon technology are assembled from thin carbon
composite rods fixed in different directions. Such a structure that is prepared for
carbonization and deposition of a carbon matrix is shown in Fig. 1.22.
There are two specificmanufacturing procedures that have an inverse sequence of
the basic processes described above, i.e., first, the macrostructure of the material is
formed and then the matrix is applied to fibers.
The first of these procedures is the aforementioned carbonxarbon technology
that involves chemical vapor deposition of a pyrolytic carbon matrix on preliminary
assembled and sometimes rather complicated structures made from dry carbon
fabric. A carbon-carbon shell made by this method is shown in Fig. 1.23.