Page 136 - Mechanics Analysis Composite Materials

Chapter 4

MECHANICS OF A COMPOSITE LAYER

A typical composite laminate consists of individual layers (see Fig. 4.1) which
are usually made of unidirectional plies with the same or regularly alternating
orientation. A layer can be also made from metals, thermosetting or thermoplastic
polymers, and fabric or can have a spatial three-dimensionally reinforced structure.
In contrast to a ply considered in Chapter 3, a layer is referred to a global
coordinate frame x, y, z of the structural element rather than to coordinates I, 2, 3
associated with the ply orientation. Usually, a layer is much thicker than a ply
and has a more complicated structure, but this structure does not change through
its thickness, or this change is ignored. Thus, a layer can be defined as a three-
dimensional structural element that is uniform in transverse (normal to the layer
plane) direction.

4.1. Isotropic layer

The simplest layer that can be observed in composite laminates is an isotropic
layer of metal or thermoplastic polymer that is used to protect the composite
material (Fig. 4.2) and to provide the tightness. For example, filament wound
composite pressure vessels usually have a sealing metal (Fig. 4.3) or thermoplastic
(Fig. 4.4) internal liner, that can also be used as a mandrel for winding. Because
the layer is isotropic, we need only one coordinate system and let it be the global
coordinate frame as in Fig. 4.5.

4. I. I. Linear elastic model

Explicit form of Hooke's law in Eqs. (2.48) and (2.54) can be written as:
1
e,, = -(a,- VC,.- vu,),
E

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