Impact Damage Tolerance of Aeronautical Composite Structures

Composite materials are increasingly used in industry thanks to their high performance/mass ratio. This is particularly true in aeronautics and aerospace, given the crucial importance of the mass criterion for such structures. This high performance/mass ratio is due to the use of materials with high specific mechanical characteristics, such as carbon, glass, Kevlar or Zylon. However, this type of material has the major disadvantage of being brittle, and must therefore be used in a mixture with a less brittle resin-type material, such as epoxy, PEEK or PPS. The basic concept behind composites is to combine a high-performance, brittle reinforcing material, typically in the form of fibers of varying lengths depending on the application, with a less high-performance but less brittle matrix, typically a resin. However, it should not be forgotten that an interface is created between these two materials, which also plays an important role in the overall behavior of the composite.

The very structure of the composite is therefore more complex than a standard, more or less homogeneous, metal-type material, and requires a design of its own. The design of a composite structure therefore requires the simultaneous design of a material and a structure; this is the fundamental difference between the design of a metal structure and a composite structure. The specific nature of composite structures also means that design criteria have to be modified, and in particular damage tolerance justification criteria.

Another important feature of composite structures is the ability to produce complex shapes in a single shot, thanks to layer-by-layer manufacturing and the use of molds and counter-molds, or from dry preforms. The advantage is to reduce the weight and complexity of structural assembly, by reducing the number of parts and screws or rivets. A good example is the Tristar (Lockheed-USA) carrier daggerboard, which is made up of 175 parts and 40,000 rivets with conventional construction, and 18 parts and 5,000 rivets with composite construction . Once again, this saves weight by reducing the number of parts and assembly elements, but at the same time requires a more complex design process, integrating structural and material design at the same time.

Of course, composites don't just have their advantages - one of their biggest drawbacks is the price, both of the material and of the manufacturing process. For example, the expiry date of epoxy resins, laminate curing equipment, resin injection devices, shaping using molds and counter-molds, and the need for non-destructive testing...