Composite Materials

Composite materials are engineering materials formed by combining at least two different materials at the macroscopic level while preserving their physical and chemical properties. The primary purpose of this combination is to create a new material with superior characteristics by integrating properties that the individual components lack or possess only in limited form. In composite structures, the constituent materials do not dissolve into one another; if dissolution occurs, the structure is termed an alloy. Composites typically offer advantages such as higher strength, lower weight, enhanced corrosion resistance, and design flexibility by bringing together the best properties of their components. These materials can also be engineered to improve thermal and electrical properties. The two main components of a composite structure are the reinforcement and the matrix material. The reinforcement carries the load, while the matrix holds the reinforcement together, protects it, and ensures uniform load distribution.

History

The use of composite materials dates back as far as human history itself. Among the earliest known examples is the production of plywood by Mesopotamians around 3400 BCE, who glued wooden strips at different angles. Similarly, ancient Egyptians produced strengthened mud bricks by mixing clay with straw. During the Middle Ages, composite bows made by Mongol warriors from bamboo, silk, cattle tendons, pine resin, and horn became one of the most effective weapons until the invention of firearms and could launch arrows over distances equivalent to approximately five football fields.

The history of modern composites begins in the 20th century. The industrial production of glass fibers in 1935 laid the foundation for the fiber-reinforced polymer (FRP) industry. This field gained momentum with the development of high-performance resins such as epoxy in 1938. World War II marked a turning point in the transition of composite material research to production. In particular, the high strength and radio frequency transparency of fiberglass made it suitable for constructing radar domes. After the war, civilian applications of composites expanded. In 1947, an automobile with a fully composite body was tested; in Türkiye, composite use began in the 1960s with the body panels of Anadol automobiles and became widespread in commercial vehicles such as buses and trucks during the 1970s. Today, composite materials are regarded as the materials of the future due to continuously advancing technology.

Components

Composite materials fundamentally consist of two main components: the matrix and the reinforcement. The properties of these components and the manner in which they are combined determine the final material’s performance.

Matrix Material

The matrix is the primary material that surrounds and binds the reinforcement elements. Its fundamental roles include maintaining the reinforcement in a fixed geometry, distributing applied loads uniformly to the reinforcement, protecting the reinforcement from environmental effects such as moisture, chemicals, and wear, and imparting toughness and ductility to the material. Composites are classified according to the type of matrix material:

• Polymer Matrix Composites (PMC): The most commonly used type of composite. The matrix consists of thermoset polymers (polyester, epoxy, vinyl ester) or thermoplastics (polyamide, polypropylene). Advantages include low cost, ease of processing, and good corrosion resistance.
• Metal Matrix Composites (MMC): Composites in which the matrix is a metal such as aluminum, magnesium, or titanium. These are preferred in applications requiring high-temperature resistance, wear resistance, and high strength.
• Ceramic Matrix Composites (CMC): Composites in which the matrix is a ceramic material such as alumina (Al2O3) or silicon carbide (SiC). They are highly resistant to extreme temperatures, wear, and chemical attack but are brittle. Reinforcement elements enhance the toughness of these materials.
• Carbon-Carbon Composites (CCC): A special type of composite in which both the matrix and the reinforcement are made of carbon. They have a very high heat capacity per unit weight and are therefore used in applications involving extremely high temperatures, such as rocket nozzles and thermal shields for spacecraft.

Reinforcement Material

The reinforcement element is the primary determinant of the composite’s mechanical properties. It carries a significant portion of the load, imparting strength and stiffness to the material. Reinforcement materials can exist in various forms and types.

Classification

Composite materials can be classified in different ways according to their internal structure. The most common classifications are based on the shape of the reinforcement element and the type of matrix material.

According to the Shape of the Reinforcement Element

• Particulate-Reinforced Composites: Produced by dispersing spherical or irregularly shaped particles within the matrix. These are typically used to increase hardness and wear resistance.
• Fiber-Reinforced Composites: Produced by embedding thin, long fibers within the matrix. This is the most widely used type of composite due to its combination of high strength and low weight.
• Laminar (Layered) Composites: Created by stacking layers with different properties (sandwich structure). This structure enhances bending resistance and stiffness.
• Filled Composites: Produced by infiltrating a matrix material into a reinforcement with a continuous skeletal structure (preform or foam).

According to the Type of Matrix Material

This classification includes the PMC, MMC, CMC, and CCC types detailed under the “Matrix Material” section above. Each type addresses specific engineering needs with its unique advantages.

Manufacturing Methods

A variety of methods are available for producing composite parts. The choice of method depends on factors such as part size, complexity, production volume, and required mechanical properties.

Open Molding Methods

In these methods, the material is exposed to air during curing. They are generally low-cost and suitable for producing large parts.

Closed Molding Methods

In these methods, the material is cured within a closed system formed by male and female molds. These methods provide smoother surface quality, higher fiber content, and lower emissions.

• Resin Transfer Molding (RTM): Dry fiber reinforcement is placed in a closed mold, which is then sealed and resin is injected under pressure. Suitable for series production in the automotive and aerospace industries.
• Vacuum Bagging and Infusion: After placing the fiber reinforcement in the mold, a vacuum bag is applied over it and the system is evacuated. Vacuum uses atmospheric pressure to draw resin into the fibers (infusion) or to compact pre-impregnated layers (vacuum bagging). High-quality, void-free parts are produced.

Applications

Due to their superior properties, composite materials are widely used across nearly every industry today.

Aerospace and Space Industry

This sector is one of the most intensive users of composites. They are used in structural components of aircraft such as fuselages, wings, and tails; helicopter rotors; interior designs; and compressor and turbine blades. Their light weight improves fuel efficiency, while their high strength ensures safety. In spacecraft, they are preferred for critical components such as rocket motor casings and thermal shields.

Defense Industry

The high impact resistance and light weight make composites ideal for defense applications. Composite armor is used for armored personnel carriers, tanks, and ships; ballistic vests and helmets for personnel are also manufactured from these materials.

Automotive and Transportation

Used in the automotive industry to reduce weight, improve fuel efficiency, and enhance performance. Composite materials are found in body panels, hoods, spoilers, chassis, and instrument panels. They are also used in various components of trains, buses, tractors, and cable cars.

Construction and Architecture

Popular in construction due to their resistance to corrosion and design flexibility. Applications include facade claddings, bridge decks, construction formwork, prefabricated holiday homes, bus stops, and cold storage facilities. Additionally, carbon fiber strips are used to strengthen existing structures against earthquakes.

Healthcare Sector

They find applications in healthcare due to their biocompatibility and mechanical properties. Orthopedic internal and external fixation systems, dental fillings, bridges, and orthodontic wires are manufactured from composites.

Sports and Consumer Products

Indispensable in the production of sports equipment requiring light weight and high performance. Tennis rackets, skis, surfboards, bicycle frames, pole vault poles, and racing boats are made from composite materials. They are also used in musical instruments and robotic systems.