Composite materials have been used by humans for thousands of years, empirically our ancestors would mix natural materials to increase the strength or durability of their tools and constructions, for example, mixing clay with straw. Over time, this concept of mixing different materials to improve their performance has expanded and specialized. Today, composites play a fundamental role in high-tech applications, such as aircraft, satellites and rockets, which would not be possible without the intensive usage of these materials.
Figure 1 – Side by side ancestral composite material, bricks made of clay and straw and application of modern high-tech composites, GE90 aircraft engine, with structure and blades made of carbon fiber.
But what is a composite material after all?
According to the formal definition, a composite material (or composite) is a macroscopic combination of two or more different materials with an interface between them. The resulting composite material possesses a combination of properties that surpasses those of its individual components.
Figure 2 – Composite material examined in detail through scanning electron microscopy (carbon fiber composite).
In this definition, we find three essential elements: two different materials and an interface. In the composites widely used in industry, these materials are synthetic fiber reinforcements, such as fiberglass, carbon and kevlar, as well as polymeric matrices, such as polyester, vinyl ester and epoxy resins, among many other options available. The behavior of the composite is a result of the interaction between the fiber reinforcements, the polymeric matrix, and the fiber/matrix interface. The appropriate selection of materials, their volume ratio and orientation are extremely important as they affect properties such as density, stiffness, tensile strength, compressive strength, fatigue resistance, failure mode, thermal and electrical conductivity, flammability, chemical resistance, in addition to the associated costs.
Reinforcement, Matrix and Interface: The Cornerstones of Composite Materials
Regarding reinforcement, it is the element responsible for giving the composite material its mechanical characteristics, such as stiffness and rupture resistance, for example. Reinforcements can consist of oriented fibers, as in fabrics, or randomly oriented, as in mats. Furthermore, the reinforcements can be composed of continuous or discontinuous fibers. There is a broad range of fabrics types with oriented fibers, such as woven (WR, Twill, Satin), as well as non-woven and sewn fabrics (biaxial, triaxial and multi-axial).
Figure 3 – Different types of carbon fiber fabrics
The matrix plays a key role in the composite material. Its function is to hold fibers in place, transfer loads between fibers, and protect the fibers from environmental damage. Additionally, the matrix plays an significant role in interlaminar shear strength, compression strength and operating temperature. There are different types of matrices, including ceramic and metallic matrices, however, polymeric matrices find a much wider range of applications in the industry. These can be thermoplastic and thermosetting resins. Among the most common thermoset matrices, we can mention resins such as unsaturated polyester (UPR), vinyl ester (VE), epoxy (EP), phenolic (PF), polyurethane (PUR) and dicyclopentadiene (DCPD). As for thermoplastic matrices, some of the most used are polyamide (PA), polypropylene (PP), polyethylene (PE), polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), polyether imide (PEI) and polyetheretherketone (PEEK).