A composite laminate is a material which consists of two or more distinct layers with
specific structure assembled to achieve certain physical properties. The layers can be
of different thicknesses, their inner fiber orientations may differ, and they may consist
of different materials.
In the design of composite laminates, the goal is to obtain a resistant material with
high loading capacity and low weight, for example, for the production of wings for
aircrafts, cross-country-skis and many more applications.
In classical elasticity theory the composite is modeled using 2-dimensional plane
approximation of the layers, and the forces are considered in the midsurface.
We apply normal forces in the x and y directions which are denoted by Nx and Ny,
respectively, and shear forces Nxy, whose units are given by N/m. Likewise, we apply
moments in the x and y direction which are denoted by Mx and My, respectively, and
shear moments Mxy. A shear stress or force is defined as a stress which is applied
parallel or tangential to the face of a material, as opposed to a normal stress which is
applied perpendicularly. The forces and moments are illustrated in Figure 1.

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Figure 1: Forces and moments acting on a laminate
It is interesting to consider composite laminates because of the intrinsic structure of
some materials. Graphite-epoxy for example is called an orthotropic material, which
means that it has different properties and strenghts in its different orthogonal directions. Other materials, such as aluminium, behaving uniformly in all directions are
called isotropic.


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