Structures in laminated composite materials

Composite structures are increasingly used not only in the aerospace field, but also in the rail, naval, automotive and leisure sectors. The nature of these materials makes them highly adaptable to each field, and it is possible to choose the best cost/weight/mechanical strength compromise for each structure. It's often said that with composites, "the material does not pre-exist the structure", so each design also requires a manufacturing method that is best suited to economic constraints. There are an infinite number of "composites", but they all have the distinctive feature of bringing together several phases that do not mix within the material. This means that, depending on the case, properties can be driven by one phase rather than another on a structural scale. For example, in the case of fiber-plus-matrix assemblies to which we will restrict the article, if we consider a set of unidirectional fibers, i.e. all oriented in the same direction, assembled by a resin, we are in the presence of a unidirectional ply. This material has excellent tensile properties in the direction of the fibers, but in the same direction, compressive strength is lower, as the breaking scenario is driven by the resin. In addition, this material is :

• macroscopically homogeneous (for an elementary volume, the macroscopic characteristics are the same);

• anisotropic (characteristics depend on the direction considered).

It's important to remember that these materials only resist correctly in one direction: that of the fibers. If there are equivalent loads in the x and y directions, fibers will have to be arranged in both directions. Given that fibers oriented along the x-axis provide virtually no resistance along the y-axis, a material with 50% fibers at 0° and 50% fibers at 90° will have specific characteristics half those of the unidirectional material. If, in addition, there are forces at 45° and – 45° (case of the main shear directions), fibers will have to be arranged in these directions, and this time the specific characteristics will be almost divided by four. When fibers are arranged with the same percentage in the 0°, 45°, – 45° and 90° directions, the resulting material has an almost isotropic behavior in the plane.

In reality, structures are generally subjected to very different stresses in different directions, so it won't be necessary to have as many fibers in the four directions 0°, 45°, – 45° and 90°. The engineer's job is to select the optimum drape to withstand the external stresses. It is this optimization of draping that will enable us to obtain structures with a high performance/mass ratio.

The aim of this article is therefore to present a common knowledge...