What Are Composite Materials?

Composite materials have become an indispensable part of modern engineering. They are essentially a synergy, in which two or more distinct substances come together so that the resulting structure exceeds the performance of its individual parts.

Why have composites become so pervasive? The answer lies in their ability to transcend the limitations of single-component materials, offering design flexibility and enhanced performance where conventional materials might fall short.

In this article, we will explore what composites are, their historical evolution, the various properties and methodologies behind them. Keep reading to learn more…

What defines a composite material?

A composite material is essentially a hybrid, designed by combining two fundamental components: the matrix and the reinforcement.

• The matrix holds the reinforcement in place and transfers loads between its elements. It can be made from polymers (such as epoxy, polyester, or thermoplastics), metals, ceramics, or even cementitious substances. It effectively acts as a protective enclosure for the reinforcement, ensuring stability and environmental resistance.
• The reinforcement is responsible for achieving the material’s enhanced mechanical properties. It often comes in the form of fibres (carbon, glass, or aramid) or particles and provides the bulk of the strength the composite needs. Reinforcements act in synergy with the matrix, yielding a composite whose overall properties cannot be directly inferred from the properties of its individual ingredients.

This arrangement distinguishes composites from simple mixtures or solutions. Even at a microscopic level, the individual phases remain separate and distinct.

Designers can tailor many properties - such as stiffness, strength, and thermal conductivity - by manipulating the ratios and orientations of these components.

A brief history of composites

Composite materials are not a modern invention. In fact, humans have been harnessing the benefits of combined materials for millennia.

The earliest records show that ancient civilisations employed composite techniques long before the advent of modern science. 

Egyptian tomb paintings depict the use of straw and mud in construction, a simple yet effective composite system that offered improved insulation and strength compared to mud alone. 

Around 6000 years ago, techniques such as wattle and daub emerged, where wooden interwoven frameworks were coated with a mud or clay binder.

By 3400 BC, the people of Ancient Mesopotamia were gluing together layers of wood at various angles to produce a material more isotropic and durable than natural wood. Ancient Egyptian artisans produced cartonnage for death masks by layering linen or papyrus with plaster, another demonstration of a primitive understanding of composite layering.

A mixture of mud with straw or gravel known as “cob” has been used for thousands of years to create sturdy walls. Roman architect, Vitruvius, provided one of the earliest detailed descriptions of concrete around 25 BC, detailing the importance of the binder (lime or pozzolana) and aggregates that together form a composite material capable of withstanding harsh environments, even underwater.

The trajectory continued into the industrial age with the invention of synthetic resins and plastics. The development of Bakelite, one of the first synthetic plastics, catalysed a revolution in composite manufacturing. 

The introduction of fibre-reinforced plastics, first seen in the combination of fibreglass and Bakelite in 1935, appeared in the mid 20th century. During World War II, the military adopted composite materials for boat hulls and radar systems.

Today, advanced composites make up much of our high-performance technology, including modern aircraft structures and sporting goods.

The typology of composite materials 

Given the immense versatility of composites, it should come as no surprise that they can be classified in a variety of ways:

By matrix material

• Polymer matrix composites (PMCs) use liquid or cured resins (epoxy, polyester, vinyl ester, among others) as the matrix. Fibre-reinforced polymers, like those including carbon or glass fibres, are examples in industry and consumer products.
• Metal matrix composites (MMCs) use metals like aluminium or magnesium as the matrix, and are often reinforced with ceramic particles or fibres. Their increased temperature resistance and stiffness makes them suitable for aerospace and automotive applications.
• Ceramic matrix composites (CMCs) combine ceramics with tougher reinforcements to offer high thermal resistance and are used in environments where conventional ceramics would be too brittle.

By reinforcement type

• Particle-reinforced composites mix particles into a matrix to improve their stiffness, wear resistance, and sometimes toughness. Concrete is a classic example here.
• Fibre-reinforced composites embed continuous or short fibres in a matrix to provide directional strength. Carbon-fibre-reinforced plastics and fibreglass fall in this category.

By structure or composition

• Composite laminates involve layers with a different fibre orientation, to optimise performance against multidirectional loads.
• Sandwich-structured composites have two strong outer layers bond together to create a lightweight core, a particularly common structure used in aerospace and automotive parts.
• Hybrid composites blend different fibres, and nanocomposites composites integrate materials like graphene or carbon nanotubes to push performance limits further.

Properties of composite materials

Physical and mechanical properties

Composite materials offer a striking combination of beneficial attributes:

• Stiffness and elasticity - composites can be engineered to be remarkably stiff, while still retaining a degree of elasticity, particularly important in applications where vibration damping or shock absorption is necessary.
• Density reduction - composites have the ability to provide high specific strength and stiffness at a significantly lower weight than traditional materials, critical in aerospace and automotive designs, where every extra gram matters.
• Thermal and electrical performance - tailor-made thermal conductivity and electrical insulation can be achieved by selecting appropriate matrix and reinforcement materials. Composites can even be designed to have excellent thermal stability or controlled rates of thermal expansion.

• Mechanical strength and toughness - the combination of reinforcement and matrix often leads to membranes that withstand tensile, compressive, and shear forces far better than the individual constituents.
  Fibre reinforcement, in particular, shapes the stress-strain curve of the material. Initially, there is an elastic region followed by a plastic phase in the matrix, culminating in necking and eventual failure.
• Wear and impact resistance - particle reinforcement can improve the stiffness, toughness, and wear characteristics, while fibre reinforcements can significantly boost the resistance to impact loads.
  The fibre orientation, whether aligned along 0°, 45°, or 90° relative to the applied load, is critical. Different orientations provide varying levels of resistance under axial, pressure, or torsional stresses. Randomly oriented fibres tend to yield a modulus that falls between the isostrain and isostress limits.

Table 2: composite vs. traditional materials

While composites may occasionally incur higher initial costs, their performance, especially in terms of strength and stiffness relative to weight, often justifies their use in demanding applications.

Chemical and environmental resilience

The chemical and environmental stability of a composite is largely determined by the careful selection of both the matrix and the reinforcement. 

Many composites exhibit high resistance to corrosion and chemical attack, while others are engineered for flame resistance. Integrating additives, or designing hybrid systems, allows researchers to further refine these characteristics and meet stringent standards required in demanding environments.

Composite fabrication methods 

The manufacture of composite materials merges rigorous science with innovative engineering. 

Fabrication involves saturating reinforcement materials with a matrix, followed by curing or solidification to form a rigid structure. The process normally takes place within a mould that dictates the final geometry of the composite part.

The typical manufacturing process includes:

• Wetting/mixing - the reinforcement is thoroughly mixed with the liquid or molten matrix material, to ensure complete saturation and the establishment of a strong bond between the matrix and reinforcement.
• Moulding - once the reinforcement is saturated with the matrix, the materials are placed into a mould. The shape of this mould defines the final geometry of the composite component. The process may involve simple pressure or vacuum techniques to consolidate the materials.
• Curing or solidification - the composite is then set into its final form through processes such as thermal curing (in the case of thermosets), fusion (for metal matrix composites), or even chemical polymerisation. The method used depends largely on the nature of the matrix material.

Innovation in composite manufacturing has led to a range of techniques, each suited to different applications and production volumes. Some common fabrication methods include:

• Wet lay-up - a hand-crafted, labour-intensive process, often used for large or custom parts.
• Fibreglass spray lay-up process - chopped fibres are sprayed into a mould and then bound by resin.
• Filament winding - continuous fibres are wound around a mould to achieve highly oriented fibre structures, optimised for strength in specific directions.
• Resin transfer moulding (RTM) - a closed mould process where resin is injected under pressure, followed by its curing. Variants include light RTM, which reduces resin content.
• Vacuum infusion - a vacuum is applied to draw resin into a preplaced fibre fabric to create a high fibre-to-resin ratio, which translates to improved composite performance.
• Compression moulding and injection moulding - these are used particularly for short fibre composites and involve high-pressure techniques to shape the material.
• Autoclave moulding and vacuum bag moulding - the composite is cured under high pressure and temperature to ensure uniformity and reduce void formation.
• Pultrusion - a continuous process that creates profiles with constant cross-sections and is well-suited for large-scale production.

Additionally, some composites are produced using pre-preg materials, which simplify the lay-up process for consistent quality. 

Tooling, often made from aluminium, carbon fibre, or steel, must be selected with care, and finishing methods like rain-erosion or polyurethane coatings are frequently applied to protect the final product. 

In some cases, post-fabrication treatments such as curing in ovens or finishing in paint booths are essential to the process.

The pros and cons of composite materials

Despite their many benefits, composite materials are not without challenges. There exists a series of trade-offs that must be carefully considered when selecting a composite material for a specific application. Here then are the advantages and disadvantages of composite materials:

The future of composite materials

As the demands on advanced systems continue to grow, so too does the need for composites that are lighter, stronger, and more sustainable.

As such, research into composite materials remains one of the most dynamic and rapidly evolving fields in modern materials science:

• Nanocomposites - the inclusion of nanomaterials such as carbon nanotubes or graphene is sparking new developments in reinforced composites. Capable of drastically enhance mechanical properties, electrical conductivity, and thermal performance, these nanomaterials will pave the way for the next wave of high-performance composites.
• Bio-based and recycled composites - with increasing environmental awareness, researchers are developing composites that utilise bio-based matrices and recycled reinforcements. This would not only reduce manufacturing waste but also provide performance characteristics that are competitive with conventional materials with a lower carbon footprint.
• Smart and robotic materials - the integration of sensors, actuators, and even computation and communication elements into the composite structure is an exciting development in the world of composites. These so-called “robotic materials” can monitor their own structural health, adjust to changing conditions, and potentially even self-repair. If possible this would revolutionise fields such as aerospace, civil engineering, and automotive design.
• Renewable energy and storage applications - as energy systems evolve, composites will transform the structural design of hydrogen or battery storage systems as well as wind turbine components.

The future of composite materials looks bright. With continuous advancements in processing techniques, testing methods, and material science, composites are set to become even more integral in our drive for efficiency and sustainability. Whether it is the manufacture of ultra-lightweight aircraft, the development of high-performance automotive components, or the realisation of self-monitoring infrastructure, the horizon is bright. The integration of smart functionalities and sustainable practices promises lighter, stronger, and more efficient solutions across multiple industries. For engineers and designers alike, the journey of exploring composite materials is only just beginning.

Material identification at The Lab

The future of composite materials looks bright. With advancements in processing techniques, testing methods, and material science, the journey of exploring composite materials is only just beginning.

Understanding material composition is essential for many industries. From engineering innovations, to manufacturing, you need to know what you’re working with to ensure you’re meeting individual standards and quality assurance.

At The Lab, we have cutting edge technology used and maintained by experts to give you the answers you need. Contact us today for an obligation-free consultation to find out how our material testing and identification services can assist you.

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