Materials in general can be subdivided into three different materials groups, which are metals, ceramics (defined as all inorganic non--metal materials) and polymers. A further group is often formed by the composite materials which are a combination of two or more materials deriving from one single or more materials groups mentioned above.
One should think that the group of composites does not necessarily need to be defined in more detail, as it is already completely explained by the definitions of the basic materials they consist of. On the one hand, this way of thinking could be accepted, when we consider the basic linear rule of mixture which in simple cases provides a mathematical formula for the calculation of mechanical composite materials properties by simply combining the single theoretical properties of the composite components. On the other hand, we also have to consider secondary effects, which take place at the interface, where the different materials meet each other. Furthermore, the different possibilities of appearance of one single material and the variety of methods how to combine them with others opens a wide range of aspects and suggests a treatment of the group of composite materials in its own right.
Figure 3: Stress--strain curve of reinforcing fibers, matrix and composite
The aim of the combination of two or more materials forming a composite is the improvement either of the mechanical properties, such as fracture strength and elastic modulus (E-- or YOUNG's modulus), or the fracture behavior or wear properties of the new created material. This -- only in the ideal case -- is reached by combining the components in a way that the positive properties of each material overcome the negative ones of the other. The final properties of the composite will always be set in the range between the maximum and the minimum properties of the single material components (figure 3, [Fri94]).
Figure 4: Classification of composites referring to material composition
(1D) particle- (2D) fiber- (3D) layer- and infiltration-composite
There are several methods how the components can be combined with each other. One distinguishes in general between particle, fiber, layer and infiltration composites (figure 4, [Roe90]).
As an example, a high reduction of the brittleness of mainly ceramics and also glass, bedded in a metal or polymer matrix with elastic and plastic properties, can be reached. In case of a ceramic-metal-combination this particle composite is called Cermet. Here, it is taken advantage of the high temperature resistance of the brittle ceramic and the outstanding elastic and plastic properties of the metal matrix, resulting in a pseudoplastic stress-strain curve.
The fiber reinforcement of comparably weak matrices has undoubtedly the highest importance of todays composites. Here again, unidirectional (UD-) and short fiber reinforced (SFR-) composites are distinguished. The mechanism of fiber reinforcement is based on the immense strength of typically glass- (GF) carbon- and aramide fibers (AF) with typical diameters of 8 to 15µm. Within these tiny dimensions, the probability of a defect of the microscopic fibrous structure is highly reduced and especially with glass fibers their almost perfect surface, showing a minimum of cracks, leads to an improvement of the mechanical properties. Thus, the measured mechanical properties of the fibers almost reach the theoretical values they should have when there is no defect in the microstructure at all.
Especially fiber reinforced plastics (FRP) are becoming more and more popular these days. Their relatively reasonable manufacturing costs, easy handling, their - with thermoplastics - welding option and their excellent weight to strength ratio are responsible for this development.
FRP with a thermoplastic matrix were furthermore gaining importance from then on, when the recycling of thermoset composites led to almost non-solvable problems after their use. In contrast to thermoset polymers, thermoplastic composites allow to melt open the matrix again in order to separate it from the reinforcement. The so recycled raw materials can now be reused in a new manufacturing process.
Processing problems of thermoplastics were mainly responsible for their only rare use as matrix materials in the past for advanced composites although they show excellent material properties like high fracture toughness and excellent chemical resistance. Difficulties in processing are on the one hand based on the low temperature resistance of common engineering thermoplastics and on the other hand on their relatively high viscosity of 500 to 5000Pa·s even at levels close to polymer decomposition temperature in comparison to thermosets with viscosities of only about 100Pa·s. This complicates a good impregnation of the fibers with the thermoplastic polymer, whereas a high degree of impregnation is one of the basic requirements for a sufficient fiber--matrix--adhesion, leading finally to acceptable mechanical properties of the composite.
Much effort has been spent in the last years to improve the manufacturing behavior of thermoplastics such as coupling agents on the fiber surface and further on simply higher processing temperatures, pressures and holding times; in the meantime also new high temperature thermoplastics like Polyetheretherketone (PEEK) or Polyamideimide (e.g. Torlon) had been developed. Another reasonable possibility is the choice of non-- conventional fiber--matrix intermediate material forms at the beginning of processing which already show an intermingling of fiber and matrix in the not impregnated state.