Composite Materials: Properties, Types, and Applications
1. Introduction
In engineering, there are applications that require a combination of unusual properties that a single material cannot offer (e.g., high tensile strength, corrosion resistance, etc.). A family of materials called composites has been developed, where two or more different materials are combined to improve material performance under the principle of combined action.
Other chapters have already covered some composite materials:
- Metal Alloys (Perlite)
- Multiphase Polymers, Copolymers
- Ceramics (Mullite, Concrete)
A natural composite material is wood, in which cellulose fibers are embedded in a material known as lignin.
This chapter will consider a composite material as a multiphase material obtained artificially, where the constituent phases must be chemically different and separated by an interface.
Thus, composite materials consist of two phases:
- A matrix that is continuous and surrounds the other phase
- A dispersed phase
Properties of composite materials will be based on:
- Properties of the constituents
- Relative proportions
- Geometry of the dispersed phase (shape, size, distribution, and orientation within the matrix)
Composite materials are classified as:
- Particle-reinforced composites (Particle: the shape of the particle is equiaxed)
- Fiber-reinforced composites (Fiber: the length-to-diameter ratio is high)
- Structural composites
2. Particle-Reinforced Composites
Generally, particles added as a dispersed phase are harder and more resistant than the matrix. The particles tend to restrict the movement of the matrix, and the matrix transfers the stress to the particles.
The cohesive force at the interface between the matrix and particle is very important for transmitting stress.
Particle-reinforced composite materials are subdivided into:
- Large-particle reinforced
- Dispersion-strengthened
In large-particle reinforcement, “large” refers to cases where the matrix-particle interaction mechanics are described by continuum mechanics and not at the atomic or molecular level.
Examples of large-particle reinforced materials include:
- Polymer materials with some type of filler
- Concrete, consisting of Portland cement (matrix) and sand or gravel (particulates)
- Cermets, which are carbides (WC or TiC) introduced into materials such as nickel and cobalt, used as cutting tools
In this type of composite material, the phase rule equation is used, which predicts the maximum and minimum elastic modulus:
Maximum: Ec = Em · Vm + Vp · Ep
Minimum: Ec = Em · Ep / (Ep · Vm + Vp · Em)
The real value of the elastic modulus will vary between these two values, depending on the cohesion between the matrix and the dispersed phase.
Among dispersion-strengthened composites, the operation is similar to the interactions in metals caused by precipitation hardening.
An example is a nickel alloy with 3% thorium oxide (ThO2) dispersed. Another example is aluminum metal with dispersed alumina particles.
3. Fiber-Reinforced Composites
Fiber-reinforced composite materials have the greatest industrial applications.
They are characterized by high strength (provided by the fibers) coupled with low density. These two characteristics make such materials suitable for applications where it is important to have good specific strength and specific modulus.
Fiber-reinforced composite materials are subclassified by the length of the fibers. The length of the fiber used is important.
3.1. Influence of Fiber Length
To achieve good tensile strength, ensure:
- Good fiber properties
- The extent to which the force is transmitted from the matrix to the fiber
In the matrix-fiber interaction, the load is transmitted through the side of the fiber, not the ends.
We define the critical fiber length, lc = σf · d / τc
Where:
- lc: critical length
- d: diameter of fiber
- σf: tensile strength
- τc: bond strength of the matrix-fiber interface
Depending on the length of the fiber, the stress profile versus position in the fiber will vary:
- Fibers with l > lc are called “continuous”.
- Fibers with l < lc are called discontinuous or short fibers.
3.2. Concentration and Orientation of Fiber
The best properties of a fiber-reinforced composite material are obtained when the fibers are uniformly distributed because the stress is distributed equitably among all fibers, with each carrying the same load.
Regarding the orientation of the fibers, we can find two extreme situations:
- Fiber alignment parallel to the applied stress
- Random alignment of the fibers
Continuous fibers are generally aligned in the direction of the applied stress. Short fibers can be aligned or randomly oriented.
Depending on the fiber orientation with respect to the applied stress, the efficiency of reinforcement will vary considerably.
3.3. Fiber and Matrix Phases
In fiber-reinforced composite materials, we can distinguish the following fiber types developed so far:
- Whiskers are very thin crystals with a large length-to-diameter ratio. As single crystals, they have high crystalline perfection, providing high strength (Graphite, SiC, Al2O3).
- Fibers are polycrystalline or amorphous materials with small diameters. They are easier to process than whiskers and are available in greater lengths (fiberglass, carbon fiber, etc.).
- Wires have relatively large diameters, such as steel, molybdenum, and tungsten. They are used in car tires, high-pressure hoses, etc.
The matrix phase has different functions:
- Binds the fibers and transmits and distributes externally applied stress (only a fraction of the applied stress is borne by the matrix).
- Must be ductile to absorb impacts.
- Must separate the fibers and protect them from surface damage.
- Serves as a barrier to prevent crack propagation.
Materials used as matrices are metals (aluminum, copper) and polymers (due to their properties and ease of manufacture).
3.4. Fiberglass and Carbon Fiber
Fiberglass is a composite material made of continuous or discontinuous glass fibers embedded in a plastic matrix.
Glass is used as reinforcement fiber due to the following characteristics:
- It is easy to draw into high-strength fibers.
- High availability in the market.
- It is relatively strong.
- There are applications of fiberglass plastics chemically linked to several inert materials that are useful in many corrosive environments.
The plastic used as the matrix is usually polyester, and to a lesser extent, nylon.
The disadvantages of fiberglass are its temperature limitation (T < 200°C) and that it is not very rigid. It is used in car bodies, pipes, plastic containers, etc.
Carbon fiber has a much higher specific modulus than glass. It also has greater resistance to high temperatures and corrosive environments.
However, it is more expensive and is only used as short fibers. The aeronautical industry uses these composites to reduce weight.
Finally, there are hybrid composite materials that use two or more types of fiber in a single matrix (fiberglass and carbon embedded in a polymer matrix).
3.5. Formation of Composites
The following methods for processing composite materials are highlighted:
Prepreg: Usually, the fibers used to reinforce composite materials should maintain an external impregnation. The fibers are protected against external agents before being mounted in the composite. This impregnation also serves as an adhesive material to improve the fiber-matrix bond.
Pultrusion: In this process, the fibers are first impregnated in a thermostable resin and then passed into a mold to form the matrix. Finally, the material is passed through an oven to cure the resin.
Filament Winding: Another way to create fiber-reinforced composite materials is by winding the fiber around a hollow cylinder (in a helical, circular, polar, etc. pattern). Before winding, the fibers are circulated through a bath of liquid resin. The composite material is cured in an oven and is removed from the chuck or cylinder. This process is used for engine housings, pipes, storage tanks, pressure vessels, etc.
4. Structural Composites
They are made of composites and homogeneous materials. Their properties depend not only on the constituent materials but also on the geometry of the design of structural elements.
Structural composites are further divided into:
- Laminar composites
- Sandwich panels
A laminated composite material consists of sheets that have a preferred direction with high resistance. The sheets are stacked and glued together. In the two-dimensional plane, it is tough, but not in the perpendicular direction.
Sandwich panels consist of two strong external plates separated by a less dense material with less rigidity and strength.
The external faces assimilate stresses in their plane and flexion (wood plates, rolled steel, fiber-reinforced plastics, etc.). The core absorbs the strain perpendicular to the plane of the face and the shear stress (foamed polymers, rubbers, etc.).
There are also cores with a honeycomb structure that are widely used.