The composite material is an assembly from at least two Matériau X not Miscible S (but having a strong capacity of adhesion). The new material thus made up has properties which the elements alone do not have.

This phenomenon, which makes it possible to improve quality of the matter vis-a-vis a certain use (lightness, rigidity with an effort, etc), explains the increasing use of composite materials, in various industrial sectors. Nevertheless, the fine description of the composites remains complex from the mechanical point of view.

Industrial approach

A composite material consists of a framework called Renfort which ensures the mechanical resistance and of a protection called matrix which is generally a Plastic (thermoplastic or thermohardening resin) and which ensures the cohesion of the structure and the retransmission of the efforts towards the reinforcement. There exists today a great number of composite materials which one generally classifies in three families according to the nature of the matrix:

• composites with organic matrices (CMO) which constitute, by far, the most important volumes today on the industrial scale,
• composites with ceramic matrices (CMC) reserved for the applications of very high-tech and working at high temperature like the space one, nuclear power and the soldier, as well as braking (brakes carbon)
• composites with metal matrices (CM).

The composites find their principal applications in the Transport air (civilian and soldier), maritime and railway, the building, the aerospace one as well as the sports and leisures, in particular thanks to their good mechanical behavior comparable with the Matériau X homogeneous like steel and their low density.

History

The Bois was the first natural composite material used, then the Torchis was used in construction for its properties of insulation and cost. Among the first composites manufactured by the man one also finds the arcs Mongolian (2000 years front J. - C.). Their wood heart was pasted of tendon to the back and horn on its inner face. The traditional Japanese sabers are also an example of composite materials very old. The Japanese blacksmiths proceeded to the folding and the hammering of metal until obtaining a kind of puff pastry being able to be made up of more than 4.000 layers. The process of folding was used to control precisely the uniformity of steel like its composition out of carbon while conferring on the blade its properties resistance and of flexibility.

• 1823 : Charles Macintosh created the impermeable one with rubber on fabrics like the Cotton.

• 1892 : François Hennebique deposits the patent of the Béton armed.

Principal composites

• the glass fibers are used in particular in the manufacture of swimming pools.

• the carbon fibers used in the Aviation.
• the Laminated used in Joinery, Construction, cabinet work.
• the partition S of Placoplâtre, very much used in the building except Bad weather S.
• the Reinforced concrete in Civil engineering.
• the fiber of aramide (or Kevlar which is a trade description) used in ballistic protections bullet-proof jackets

(Attention, the bullet-proof jackets are not composites! On the other hand, the kevlar which composes them is well used as a fiber for composites in other types of uses.)

• the GLARE made up mainly of Aluminum and of Glass fiber is used in Aéronautique

Reinforcements

The reinforcement is the skeleton supporting the mechanical efforts. It can be presented in many forms: short fibers (chechmate) or continuous fibers (fabrics or multidirectional textures) according to the application considered. The fibers generally have a good resistance to the traction but a resistance to the weak compression.

Among fibers the most employed one can quote:

• the glass fibers which are used in the building, the water sport and various applications not structuring. The production costs as of these fibers are relatively low what does of it one of fibers the most used at present.

• the carbon fibers used for structuring applications. It are obtained by the pyrolysis of an organic precursor or not under controlled atmosphere. More used these precursors is PolyAcryloNitrile (SIDE). The price of these fibers remains relatively high but it did not cease decreasing with the increase in volumes of production. One finds them in many applications in aeronautics, the space one as well as the sports and leisures of competitions (Formula 1, masts of boats).
• the fibers of aramide (or Kevlar which is a trade description) used in ballistic protections like the bullet-proof jackets.
• the fibers of silicon carbide are an good answer with the Oxydation carbon as of 500°C. They are used in applications very specific working at high temperature and under oxidizing atmosphere (space and nuclear). Their production costs are very high what thus limits their use.
• For the composites of line entry, an interest growing is carried to vegetable fibers, like the hemp or the flax. These fibers have good mechanical properties for a modest price, and are particularly ecological since they are natural products.

Matrices

The purpose of the matrix is principal to transmit the mechanical efforts to the reinforcement. It ensures also the protection of the reinforcement with respect to the various environmental conditions.

In the case of the CMO (composite with organic matrices) the principal matrices used are:

• the not very expensive resins Polyester which are generally used with glass fibers and which one finds in many applications of the everyday life.
• the resins Vinylester are especially used for applications where the resins polyester are not sufficient. It is resulting from a modification of an epoxy resin and is excellent for applications of chemical resistance.
• the resins epoxy which have good mechanical characteristics. They are generally used with the carbon fibers for the realization of parts of structure and aeronautics.
• the ic resins phenol used in the applications requiring of the properties of behavior to fire and flames imposed by the standards in civil transport.
• the resins Thermoplastic S like the Polypropylene or the Polyamide.

In the case of the CMC (composites with ceramic matrices), the matrix can be made up of carbon or carbide of silicon. These matrices are deposited either by chemical plating in vapor phase (CVD) by thickening of a fibrous preform, or starting from resins cokéifiables like the phenolic resins (in the case of carbon matrices).

In the case of CM (Composite with metal matrix) the composite material is made up:

• of a metal matrix (e.g. aluminum, magnesium, zinc, nickel,…)
• of a metal or ceramic reinforcement (by ex: steel wire, particles of Sic, carbon, alumina, diamond dust…)

Mechanical description

Formalization

The behavior of a composite material is described in the following way, by using the formalism of the Mécanique continuous mediums:

• one has N different materials which form the composite (one speaks about " phases" , characterized by their voluminal fraction and their geometry)

• Inside each phase, the material can become deformed and undergo constraints. The deformation is done according to the law of behavior of material in question (which one knows): $\backslash \; underline\; \{\backslash \; underline\; \{\backslash \; sigma\}\}\; =\; \backslash \; underline\; \{\backslash \; underline\; \{\backslash \; underline\; \{\backslash \; underline\; \{L\}\}\}\}\; \_i:\; \backslash \; underline\; \{\backslash \; underline\; \{\backslash \; epsilon\}\}$ for the case linéraire.
• It there has balance of the voluminal forces , that is to say, in each material I: $\backslash \; underline\; \{div\}\; (\backslash \; underline\; \{\backslash \; underline\; \{\backslash \; sigma\}\})\; =\; \backslash \; underline\; \{0\}$ if one neglects the force of gravity in front of the forces applied to the material (pressure, traction, shearing).
• Lastly, the aggregation of the behaviors of each simple material, to lead to the behavior of the composite, requires to describe the balance of the forces between two materials " collés" , in each point of their surface of contact . This condition is that the force exerted by material 1 on material 2 on the surface of contact ($\backslash \; underline\; \{\backslash \; underline\; \{\backslash \; sigma\}\}\; \_1.\; \backslash \; underline\; \{N\}$ if $\backslash \; underline\; \{N\}$ indicates the unit vector perpendicular to surface) must be opposite with that exerted by material 2 on material 1. This implies a certain continuity of the stress field $\backslash \; underline\; \{\backslash \; underline\; \{\backslash \; sigma\}\}$: one must have (in each point of surfaces of contacts of materials mixed in the composite) $(\backslash \; underline\; \{\backslash \; underline\; \{\backslash \; sigma\}\}\; \_2\; -\; \backslash \; underline\; \{\backslash \; underline\; \{\backslash \; sigma\}\}\; \_1).\; \backslash \; underline\; \{N\}\; =\; \backslash \; underline\; \{0\}$. It is by this condition that the microgeometry of the mixture in the determination of the behavior of the composite intervenes. Thus, by mixing isotropic materials according to a nonisotropic geometry (fibers, sheets…), a nonisotropic composite is obtained but from which the mechanical properties result as of those of initial materials!
• Thus, the composite material is described of each one of these points. The law of behavior of the composite which results from it must be able to establish the link between the macroscopic strains and the macroscopic stresses (i.e. their median values, because for example if one mixes a soft material and hard, the microscopic deformations will be very variable according to material, and it is the total deformation which one will observe on a composite scale). This law of behavior of the composite is known as " effective" : one notes $\backslash \; underline\; \{\backslash \; underline\; \{\backslash \; underline\; \{\backslash \; underline\; \{L\}\}\}\}\; ^\; \{EFF\}$ in the linear case.

Resolution

The preceding problem is not simply solved, except in the case of very simple geometries (spherical inclusions, piled up fibers, sheets, or in a general way in the case of inlusions of form ellispoïdale).

Research aims at describing the behavior of the composite without inevitably knowing the exact geometry of it, while trying to limit the deformation energy composite (the deformation energy of a material is $\backslash \; frac\; \{1\}\; \{2\}\; \backslash \; underline\; \{\backslash \; underline\; \{\backslash \; sigma\}\}:\; \backslash \; underline\; \{\backslash \; underline\; \{\backslash \; epsilon\}\}$). One can thus quote the terminals of:

• Voigt and Reuss :

The extreme cases of these inequalities are atteignables by geometries of piled up layers. Moreover, one finds here a constant result of physics: the electrical resistance of an assembly of resistances is the sum of resistances when they are in series, or is the reverse of the sum of the opposite when they are in parallel (similar result also with an assembly of springs). The difference is that here the law of behavior is not described by a scalar (as it is the case for an electrical resistance or a stiffness of spring), but by a multidimensional size (the tensor $\backslash \; underline\; \{\backslash \; underline\; \{\backslash \; underline\; \{\backslash \; underline\; \{L\}\}\}\}$ of order 4).

NB: here $\backslash \; overline\; \{has\}$ indicates the average of $A$ on all the volume of the composite; and the inequality between Tenseur S gets along with the direction or for any tensor $\backslash \; underline\; \{\backslash \; underline\; \{\backslash \; epsilon\}\}$ there is

• Hashin and Shtrikman : more precise terminals, in the isotropic case.

The mechanics of the composites is still a field of active theoretical research: behavior mechanical or electric, linear, nonlinear, viscoelastic, with cracks or plasticity, buckling…

A limit of this modeling is that one cannot know in a precise way microgeometry of a real composite: there are always defects; but modeling makes it possible to describe in a rather precise way the law of behavior.

Another interest of this theoretical research between the geometry of a composite and its law of behavior is the mode of realization of a material whose mechanical characteristics were obtained by a data-processing optimization.

See too

Internal bonds

• Material

• Plastic

External bonds

• the small illustrated composite (updated carried out the 10/20/2007)

• the site on composite materials managed by four former students in BTS plasturgie
• Any knowledge on composite materials * Compositec: Training center and laboratory materials

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