INTRODUCTION TO ENGINEERING MATERIALS - COMPOSITES - PART 7

Audio: Introduction to engineering materials – composites – part 7

Composites are widely used and important engineering materials due to their ability to combine different material properties that cannot be met with conventional materials. So far, we have introduced you to a wide range of materials, and in this article, we will finish the engineering material introduction series. In this article, you will learn what composites are, their properties, applications, and their classification.

Introduction to composites

Composites are composed of two or more materials or phases to give a combination of properties different and superior to their constituents. They are used due to the possibility of combining various properties like stiffness, strength, weight (improved stiffness-to-weight and strength-to-weight ratio), fatigue, toughness, high-temperature performance, corrosion resistance, hardness, conductivity, etc.

The simplest composite materials consist of two materials or phases: primary and secondary phase. The primary phase forms the matrix in which the second phase is embedded. The matrix is considered to be a continuous phase, and it serves several functions in the composite.

First, it provides the geometry of the part (or product) made of the composite material. Second, it holds the embedded phase in place, usually enclosing and often concealing it. Third, when a load is applied, the matrix shares the load with the second phase.

The secondary phase is also called the reinforcing agent (usually servers to strengthen the composite) or dispersed phase. The secondary phase can be in the form of different particles (fibers, whiskers, or various other geometries) and vary in size, distribution, and orientation.

composite materials are used in industries like aerospace/aeronautical, construction, marine, electrical, consumer, automotive, transportation, sport/recreation, energy, and other industries. As we go through the classification, we will dive deeper into their applications.

Classification of engineering composites

We can divide composites in different ways. For example, one of the classifications is based on natural occurrence. In that sense, we can divide them into natural and synthetic Natural composites occur in nature, like wood, bones, teeth, abalone shell, etc. On the other hand, synthetic ones are manufactured to achieve desired structure, properties, and geometry.

Furthermore, we can divide composites based on the matrix phase:

Metal Matrix Composites (MMCs),
Ceramic Matrix Composites (CMCs), and
Polymer Matrix Composites (PMCs).

This article will focus primarily on composites used in mechanical applications. Based on that, we can divide them in:

Particulate composites,
Fiber composites, and
Laminar composites.

Particulate composites

Particulate-reinforced composites can be classified as large-particle and dispersion-strengthened. Dispersion-strengthened composites have particle diameters between 10 and 250 nm. Particles can have a variety of geometries, but they should be approximately the same dimension in all directions (equiaxed). The particles should be small and evenly distributed in the composite’s matrix for effective reinforcement.

For most of these composites, the secondary phase is harder and stiffer than the matrix; their properties depend on the bonding between the matrix and particles. Furthermore, the volume ratio of the two phases influences the behavior; mechanical properties are enhanced with increasing particulate content.

Large-particle composites

Large-particle composites are utilized with metals, polymers, and ceramics. They contain large amounts of coarse particles that do not effectively block slip. They are designed to produce unusual combinations of properties rather than to improve strength.

Cemented carbides, or cermets, contain hard ceramic particles dispersed in a metallic matrix (Metal Matrix Composite – MCM). The most common cermet is the cemented carbide, composed of extremely hard particles of a refractory carbide ceramic such as tungsten carbide (WC) embedded in a metal matrix such as cobalt or nickel.

Tungsten carbide (WC) is used for cutting tools in machining operations; it is a hard, stiff, high-melting-temperature ceramic. Tungsten carbide particles are combined with cobalt powder and pressed into powder compacts to improve toughness.

Dispersion-strengthened composites

As previously stated, dispersion-strengthened composites have particle diameters between 10 and 250 nm. Metals and metal alloys can be strengthened and hardened by the uniform dispersion of fine particles of very hard and inert material. The secondary phase may be metallic or nonmetallic.

Another important group of dispersion-strengthened composite materials includes thoria-dispersed (ThO₂) metals such as TD-nickel. TD-nickel can be produced by internal oxidation. Thorium is present in nickel as an alloying element of about 3 vol%. Other dispersed-strengthened composites Ag-CdO used for electrical contacts, Al-Al₂O₃used in nuclear reactors, Be-BeO used in aerospace and nuclear reactors, Ni-20% Cr-ThO₂ used in turbine engine components, etc.

Fiber composites

Technologically, the most important composites are those in which the dispersed phase is in a fiber form. Most fiber-reinforced composites improve strength, fatigue resistance, and strength-to-weight ratio by incorporating strong, stiff, but brittle fibers into a softer, more ductile matrix. The matrix material transmits the force to the fibers, which carry most of the applied force. The matrix also protects the fiber surface and minimizes diffusion of species such as oxygen or moisture that can degrade the mechanical properties of fibers.

Fibers are generally circular in cross-section, although alternative shapes are sometimes used (e.g., tubular, rectangular, hexagonal). In addition, fibers used in composite materials can be either continuous or discontinuous.

Continuous fibers are very long. In theory, they offer a continuous path by which a load can be carried by the composite part. In reality, this is difficult to achieve due to variations in the fibrous material and processing.

Discontinuous fibers (chopped sections of continuous fibers) are short lengths (L/D ≈ 100). An important type of discontinuous fiber are whiskers—hair-like crystals with diameters as low as 0.001 mm and very high strength.

The most important commercial use of fibers is in polymer composite materials. However, the use of fiber-reinforced metals and ceramics is growing. Following is a survey of the important types of fiber materials

Glass – The most widely used fiber in polymers, the term fiberglass is applied to denote glass fiber-reinforced plastic.
Carbon – Carbon can be made into high-modulus fibers. Besides stiffness, other attractive properties include low-density and low-thermal expansion.
Boron – Boron has a very high elastic modulus, but its high-cost limits applications to aerospace components in which this property (and others) are critical.
Kevlar 49 – This is the most important polymer fiber. Its specific gravity is low, giving it one of the highest strength-to-weight ratios of all fibers.
Ceramics – Silicon carbide (SiC) and aluminum oxide (Al₂O₃) are the main fiber materials among ceramics. Both have high elastic moduli and can be used to strengthen low-density, low-modulus metals such as aluminum and magnesium.
Metal – Steel filaments, both continuous and discontinuous, are used as reinforcing fibers in plastics.

Examples of fiber composite materials are Borsic aluminum, Kevlar-epoxy, graphite-polymer, glass-polymer, etc.

Laminar composites

A laminar composite structure consists of two or more layers bonded together to form an integral piece. The layers are stacked and subsequently cemented together so that the orientation of the high-strength direction varies with each successive layer.

Many laminar composite materials are designed to improve corrosion resistance while retaining low cost, high strength, or lightweight. Other important characteristics include superior wear or abrasion resistance, improved appearance, and unusual thermal expansion characteristics.

Examples of laminar composite materials are snow skis, windshield glass, plywood, automotive tires, aerospace, etc.

Other interesting laminar composites are called sandwich panels. They consist of two strong outer sheets separated by a layer of less-dense material (for example, foamed material). One of the applications of sandwich panels is in construction, where they are used for aesthetic and isolation purposes.

Closing words

Composite materials are made from two or more materials, and the possibility of combining various properties makes composites interesting materials for mechanical design engineers. We can classify composites in different ways, and the most interesting classification for us is based on mechanical applications: particulate, fiber, and laminar composites.

In this article, we covered the basics of composites and finished the introductory series on engineering materials.

Now you have an excellent overview of composites you could encounter as a mechanical design engineer. However, I suggest you go through the text once more and identify areas you think need more understanding and clarity. Then, once you have identified those areas, start building up your knowledge in those areas.

To make it easier for you to find related posts, check the “Further reading” chapter below. Do you have any questions or need something to be clarified better? Leave a comment below, and I will give my best to adjust the post accordingly.

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