General informations about composite materials
A definition of composite materials can be the following: these materials are usually not found in nature they are the result of a tridimensional combination of at least two materials chemically different between them with an interface separation.
The combination so obtained has physical-chemical properties not found in the individual materials that compose it.
Various CM are used in structures such as for example those used on the Boeing 777 because of their strength and lightness, as well as fatigue strength, corrosion and impacts.
The CM are distinguished from metals because it is the combination of materials among them different in composition or shape. Each constituent maintains its identity in the final mixture without dissolving or merging completely in the other.
Reinforced concrete is an excellent example of a composite structure where concrete and steel keep their identity. The steel bars bear the tension loads while the cement bears those in compression. In the field of aeronautical constructions the term “composite structures” refers to a combinations of fabrics and resins where the fabric is resin-impregnated but retaining its identity.
The advanced CM consist in a new high-strength fiber inserted in an epoxy matrix. The result is a weight saving in aircraft structures, for example, due to an improved weigh-resistance ratio.
Graphite-epoxy structures allow a weight saving of 20% compared to aluminum. The reduction of weights is the most evident advantage in support of the choice. Other advantages compared to traditional materials include the high corrosion resistance, and resistance to cyclic type load (fatigue).
The main disadvantage is instead related to higher costs.
The hybrid CM are typically obtained by adding fiberglass or kevlar to the basic matrix epoxy - fiber. The additions are made in order to obtain specific characteristics such as resistance to increases the rigidity of the whole.
The majority of modern composite materials combine a matrix in thermosetting resin with fiber reinforcements in addition to cells of reinforcement such as hard foam and honeycomb structures.
Reinforcements commonly used are glass, carbon and aramidic fibers, these last available in various forms (continuous, fragmented, multi-axial or textured).
A careful choice of the types of reinforcement allows to calibrate the characteristics of strength and endurance of the final structure of almost every requirement demanded by the finished product.
The fiberglass is by far the fiber reinforcement more used, hence the term GRP (glass reinforced plastic) and FRP (Fiber reinforced plastic) often used to describe products made of composites.
The matrices of thermosetting resin most commonly used include polyester, epoxy, ester vinyl and phenols. The choice of the types of resins used allows to change the characteristics relating to operating temperatures, resistance to chemical agents and to the aggression of atmospheric agents, the properties of electrical conductivity and fire resistance.
Almost every object produced with traditional materials can also be produced using composite materials.
While the use of the composite is almost a forced choice for certain types of applications , the selection of the material to be used is generally function of the useful duration required to the finished product, the total number of parts to be produced, the complexity of the form, the savings in assembly cost and, finally, the experience in the use of composite. In many cases the best results are obtained by the combined use of composite materials and conventional materials.
Even the production processes have known a constant evolution. Although manual lay off is still a common technique, new techniques such as vacuum infusion will make their way in high-tech sectors such as aerospace applications for composite materials.
Characteristics of composite materials
• High resistance and low weight
• Corrosion resistance
Composite materials offer better weigh-resistance ratio than aluminum and steel and can be engineered in order to provide a wide range of features with respect to impact, resistance, tension and bending.
Characteristics of composite materials: optional
• Fire retardant
• Antistatic or high electric conductivity
• Pigmented or translucid