FREQUENTLY ASKED QUESTIONS ABOUT COMPOSITE MATERIALS
What are composite materials?
Explaining what composite materials are can be done at various levels of detail. The definition of a composite material can vary significantly depending on the context in which it is formulated.
In chemistry, for example, a composite is defined as “a multicomponent material comprising multiple phase domains (excluding gas), in which at least one type of phase domain is a continuous phase” (IUPAC “Gold Book” 2019). According to this definition, even a suspension of immiscible liquids could be considered a composite.
In engineering, however, the term “composite material” (often shortened to simply “composite”) is generally understood as a material made up of two or more distinct and insoluble engineering materials with significantly different physical and/or chemical properties. When combined, these create a new material with characteristics different from those of the individual components, typically intended for specific applications (such as structural, thermal, or electrical insulation). The components remain clearly distinguishable within the material’s structure at the scale of 1 to 100 μm, setting composites apart from chemical mixtures, solid solutions (e.g., metallic or polymeric alloys), or homogeneous materials.
A composite material can have superior properties compared not only to its individual components but also to traditional construction materials such as metal alloys or ceramics—for instance, greater stiffness, strength, lightweight, durability, or even lower cost.
Composite materials do not include concrete and reinforced concrete, masonry, wood or plywood, syntactic foams, rolled iron/steel, bamboo, decorative plastic laminates, or multilayer films.
A particularly relevant category for engineering applications is that of reinforced composites, where the properties of the continuous phase (also called the “matrix”) are enhanced by a strong bond with a rigid dispersed phase (called the “reinforcement”), which can have various natures and shapes. These reinforcements can range in form from fibers to platelets to equiaxed particles. However, fiber-shaped reinforcements are the most common.
In chemistry, for example, a composite is defined as “a multicomponent material comprising multiple phase domains (excluding gas), in which at least one type of phase domain is a continuous phase” (IUPAC “Gold Book” 2019). According to this definition, even a suspension of immiscible liquids could be considered a composite.
In engineering, however, the term “composite material” (often shortened to simply “composite”) is generally understood as a material made up of two or more distinct and insoluble engineering materials with significantly different physical and/or chemical properties. When combined, these create a new material with characteristics different from those of the individual components, typically intended for specific applications (such as structural, thermal, or electrical insulation). The components remain clearly distinguishable within the material’s structure at the scale of 1 to 100 μm, setting composites apart from chemical mixtures, solid solutions (e.g., metallic or polymeric alloys), or homogeneous materials.
A composite material can have superior properties compared not only to its individual components but also to traditional construction materials such as metal alloys or ceramics—for instance, greater stiffness, strength, lightweight, durability, or even lower cost.
Composite materials do not include concrete and reinforced concrete, masonry, wood or plywood, syntactic foams, rolled iron/steel, bamboo, decorative plastic laminates, or multilayer films.
A particularly relevant category for engineering applications is that of reinforced composites, where the properties of the continuous phase (also called the “matrix”) are enhanced by a strong bond with a rigid dispersed phase (called the “reinforcement”), which can have various natures and shapes. These reinforcements can range in form from fibers to platelets to equiaxed particles. However, fiber-shaped reinforcements are the most common.
What are composite materials made of?
The most widely used structural composites are polymer matrix composites reinforced with engineered synthetic fibers (such as glass, carbon, or aramid) or natural fibers (such as flax or hemp). This class is commonly referred to by the acronym FRP (Fiber Reinforced Plastics). About 90% of the European market—excluding short-fiber thermoplastic matrix composites produced by injection molding—consists of composites with unsaturated polyester resin matrices and glass fiber reinforcements, commonly known as fiberglass.
Many resin and reinforcement combinations are used in composites, each contributing to the unique properties of the final product. Fibers, which are strong but brittle, provide mechanical strength, while the resin—more flexible and moldable—shapes the product, protects the fibers, and contributes significantly to the tensile, bending, and compressive properties.
FRPs may also include mineral fillers, additives, modifiers, or coatings to improve manufacturing processes, appearance, and performance.
The acronym FRP is often expanded to specify the type of fiber:
GFRP: Glass Fiber Reinforced Polymer
CFRP: Carbon Fiber Reinforced Polymer
AFRP: Aramid Fiber Reinforced Polymer
Other terms are also used depending on region or industry:
FRC: Fiber Reinforced Composites
GRP: Glass Reinforced Plastics
PMC: Polymer Matrix Composites
FPC: Fiber and Polymer Composites
All these acronyms refer to the same general class: fiber-reinforced polymer composites.
Many resin and reinforcement combinations are used in composites, each contributing to the unique properties of the final product. Fibers, which are strong but brittle, provide mechanical strength, while the resin—more flexible and moldable—shapes the product, protects the fibers, and contributes significantly to the tensile, bending, and compressive properties.
FRPs may also include mineral fillers, additives, modifiers, or coatings to improve manufacturing processes, appearance, and performance.
The acronym FRP is often expanded to specify the type of fiber:
GFRP: Glass Fiber Reinforced Polymer
CFRP: Carbon Fiber Reinforced Polymer
AFRP: Aramid Fiber Reinforced Polymer
Other terms are also used depending on region or industry:
FRC: Fiber Reinforced Composites
GRP: Glass Reinforced Plastics
PMC: Polymer Matrix Composites
FPC: Fiber and Polymer Composites
All these acronyms refer to the same general class: fiber-reinforced polymer composites.
What are the properties of composite materials?
To qualify as a composite, a material must meet the following criteria:
Be composed of two or more distinct phases with a recognizable microstructure on the scale of 1 to 100 microns.
At least two phases must have significantly different physical properties, so that the resulting composite material has properties not achievable by any constituent alone.
Materials not meeting both conditions are not strictly considered composites.
For a material to be considered reinforced, the fibers must have at least one mechanical property (e.g., stiffness or strength) significantly greater than that of the matrix.
For example:
Fibers like glass, carbon, and aramid (with elastic moduli E > 70 GPa) are suitable reinforcements for polymer matrices (typically E < 5 GPa).
Strength values (σR) also differ: fibers have strengths in the GPa range, while matrices stay below 100 MPa.
Not all fibers qualify: a polyester textile fiber has similar properties to the resin and cannot serve as reinforcement—though it may still reinforce materials with much lower stiffness, such as rubber.
Fibers typically have higher strength than bulk material due to fewer defects from smaller cross-sectional dimensions. Controlled spinning can enhance elastic modulus through microstructural orientation, though this introduces anisotropy, which must be accounted for in design.
To ensure strong adhesion between fiber and matrix throughout the composite's life, fibers are coated with “sizing” agents—thin layers of interphase material that make the chemically dissimilar components compatible.
Reinforcement orientation is also critical. Except for equiaxed particles, reinforcements can be aligned in one or more spatial directions, influencing the composite’s mechanical behavior.
In practice, for 1D or thin 2D structures (like laminates):
x-direction = length
y-direction = width
z-direction = thickness
Fiber orientations are defined by angle θ between fiber direction and x-axis, and lamination sequences record the exact order of multiple fiber layers.
Be composed of two or more distinct phases with a recognizable microstructure on the scale of 1 to 100 microns.
At least two phases must have significantly different physical properties, so that the resulting composite material has properties not achievable by any constituent alone.
Materials not meeting both conditions are not strictly considered composites.
For a material to be considered reinforced, the fibers must have at least one mechanical property (e.g., stiffness or strength) significantly greater than that of the matrix.
For example:
Fibers like glass, carbon, and aramid (with elastic moduli E > 70 GPa) are suitable reinforcements for polymer matrices (typically E < 5 GPa).
Strength values (σR) also differ: fibers have strengths in the GPa range, while matrices stay below 100 MPa.
Not all fibers qualify: a polyester textile fiber has similar properties to the resin and cannot serve as reinforcement—though it may still reinforce materials with much lower stiffness, such as rubber.
Fibers typically have higher strength than bulk material due to fewer defects from smaller cross-sectional dimensions. Controlled spinning can enhance elastic modulus through microstructural orientation, though this introduces anisotropy, which must be accounted for in design.
To ensure strong adhesion between fiber and matrix throughout the composite's life, fibers are coated with “sizing” agents—thin layers of interphase material that make the chemically dissimilar components compatible.
Reinforcement orientation is also critical. Except for equiaxed particles, reinforcements can be aligned in one or more spatial directions, influencing the composite’s mechanical behavior.
In practice, for 1D or thin 2D structures (like laminates):
x-direction = length
y-direction = width
z-direction = thickness
Fiber orientations are defined by angle θ between fiber direction and x-axis, and lamination sequences record the exact order of multiple fiber layers.
What are composites used for and in which industries?
Composite materials are used across many industries:
Engines, machine parts, electronics
Construction, railways, energy production, pressure tanks
Offshore structures, biomedical devices
Boats, sports equipment
This wide usage is due to composites’ high structural efficiency, meaning optimal performance per unit mass or volume.
Composites are ideal when high mechanical performance (stiffness, strength) must be combined with lightweight, complex shapes, or durability in harsh environments. They also lend themselves to:
Structural health monitoring via embedded fiber optics
Tailored mechanical, thermal, and electrical responses, making them ideal for smart and functional structures
In civil aviation, composites have steadily replaced aluminum since the 1970s. Their higher efficiency, fatigue resistance, and corrosion-free durability reduce maintenance and increase aircraft range and reliability.
In the automotive sector, high-volume production is more demanding. A notable innovation is the BMW i3 and i7, launched in 2013. These electric cars feature a carbon fiber composite cabin and aluminum frame, drastically reducing weight and improving range (over 250 km per charge). Production reaches 100 i3 vehicles/day, thanks to robotic assembly lines and structural adhesives replacing bolts and welds.
Large composite structures are also used for wind turbine blades.
Another important application is in seismic retrofitting of buildings and infrastructure using external composite wraps. These act as an external reinforcement shell, correcting structural weaknesses without demolition or the heavy, visually intrusive metal elements. Weight reduction is especially valuable because seismic forces are proportional to mass, and aesthetic preservation is crucial in historical buildings. The flexibility of composite laminates also allows them to conform to complex geometries on-site.
In marine applications, composites have long been the standard for recreational boats and superyachts over 60 meters, with military vessels reaching over 80 meters. Masts and booms in sailboats are also composite, sometimes exceeding 80 meters in height. Despite their complexity, these elements can be made using simple open molds, enabling integration of stiffeners and optimized reinforcement layouts—ideal for both weight and performance.
Sports applications include a wide variety of equipment thanks to laminating techniques:
Plate structures: skis, surfboards, snowboards, wing spars
Tubular structures: tennis rackets, fishing rods, golf clubs, bike frames
Shell structures: helmets, golf club heads, hulls, rudders
Engines, machine parts, electronics
Construction, railways, energy production, pressure tanks
Offshore structures, biomedical devices
Boats, sports equipment
This wide usage is due to composites’ high structural efficiency, meaning optimal performance per unit mass or volume.
Composites are ideal when high mechanical performance (stiffness, strength) must be combined with lightweight, complex shapes, or durability in harsh environments. They also lend themselves to:
Structural health monitoring via embedded fiber optics
Tailored mechanical, thermal, and electrical responses, making them ideal for smart and functional structures
In civil aviation, composites have steadily replaced aluminum since the 1970s. Their higher efficiency, fatigue resistance, and corrosion-free durability reduce maintenance and increase aircraft range and reliability.
In the automotive sector, high-volume production is more demanding. A notable innovation is the BMW i3 and i7, launched in 2013. These electric cars feature a carbon fiber composite cabin and aluminum frame, drastically reducing weight and improving range (over 250 km per charge). Production reaches 100 i3 vehicles/day, thanks to robotic assembly lines and structural adhesives replacing bolts and welds.
Large composite structures are also used for wind turbine blades.
Another important application is in seismic retrofitting of buildings and infrastructure using external composite wraps. These act as an external reinforcement shell, correcting structural weaknesses without demolition or the heavy, visually intrusive metal elements. Weight reduction is especially valuable because seismic forces are proportional to mass, and aesthetic preservation is crucial in historical buildings. The flexibility of composite laminates also allows them to conform to complex geometries on-site.
In marine applications, composites have long been the standard for recreational boats and superyachts over 60 meters, with military vessels reaching over 80 meters. Masts and booms in sailboats are also composite, sometimes exceeding 80 meters in height. Despite their complexity, these elements can be made using simple open molds, enabling integration of stiffeners and optimized reinforcement layouts—ideal for both weight and performance.
Sports applications include a wide variety of equipment thanks to laminating techniques:
Plate structures: skis, surfboards, snowboards, wing spars
Tubular structures: tennis rackets, fishing rods, golf clubs, bike frames
Shell structures: helmets, golf club heads, hulls, rudders
How are composite materials recycled?
Composite materials can be recycled, and circular disposal approaches exist.
Thanks to their lightweight and strength, composites reduce transport and handling energy, facilitate installation, and improve the energy efficiency of moving components (like wind turbines or vehicle parts). Their durability enhances sustainability through:
Lower energy use and reduced greenhouse emissions
Longer component life with little to no maintenance
Improved performance and safety
In their end-of-life phase, composites offer several circular management options, aligning with the EU waste hierarchy, which promotes:
Prevention
Repair
Reuse
before actual recycling.
Composites are repairable, long-lasting, and maintain their properties even in harsh environments. When reuse isn’t feasible, several mature recycling technologies exist, especially for thermoset composites:
Cement co-processing: Waste composites (especially glass fiber-based) are used in cement kilns, lowering energy consumption and CO₂ emissions.
Mechanical grinding: An energy-efficient, flexible process that can partially recover composite properties. Applications include furniture and industrial uses where recycled composites provide reinforcement and lower costs/environmental impact.
Pyrolysis: Though more polluting than the above, it allows recovery of reinforcing fibers.
Solvolysis (in development): Can recover organic compounds from resin decomposition, potentially usable in new composites.
Emerging electromechanical processes (e.g., high-voltage fragmentation) offer promising upcycling opportunities.
Finally, next-generation composites are being designed for easier recycling, such as:
Cleavable thermoset matrices
Vitrimer-based composites
Thermoplastic matrix composites via reactive molding
More detailed information is available in the technical document "Circularity of Composite Materials: A Beginner’s Guide."
Thanks to their lightweight and strength, composites reduce transport and handling energy, facilitate installation, and improve the energy efficiency of moving components (like wind turbines or vehicle parts). Their durability enhances sustainability through:
Lower energy use and reduced greenhouse emissions
Longer component life with little to no maintenance
Improved performance and safety
In their end-of-life phase, composites offer several circular management options, aligning with the EU waste hierarchy, which promotes:
Prevention
Repair
Reuse
before actual recycling.
Composites are repairable, long-lasting, and maintain their properties even in harsh environments. When reuse isn’t feasible, several mature recycling technologies exist, especially for thermoset composites:
Cement co-processing: Waste composites (especially glass fiber-based) are used in cement kilns, lowering energy consumption and CO₂ emissions.
Mechanical grinding: An energy-efficient, flexible process that can partially recover composite properties. Applications include furniture and industrial uses where recycled composites provide reinforcement and lower costs/environmental impact.
Pyrolysis: Though more polluting than the above, it allows recovery of reinforcing fibers.
Solvolysis (in development): Can recover organic compounds from resin decomposition, potentially usable in new composites.
Emerging electromechanical processes (e.g., high-voltage fragmentation) offer promising upcycling opportunities.
Finally, next-generation composites are being designed for easier recycling, such as:
Cleavable thermoset matrices
Vitrimer-based composites
Thermoplastic matrix composites via reactive molding
More detailed information is available in the technical document "Circularity of Composite Materials: A Beginner’s Guide."
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