Composite Materials and Their Classification
An introuction
Hi folks, and welcome to my first article here! I actually wrote this piece in Turkish about seven months ago and translated it into English using AI, so please bear with me if there are any minor translation quirks.
The primary reference for my research was William Callister’s ‘Materials Science and Engineering: An Introduction’, and most of the visual content is drawn from that excellent book. Since I compiled this research a while ago, a few supplementary images are missing their exact original sources. I hope you enjoy reading it
Composite materials are produced by combining materials to achieve a desired combination of properties. They consist of more than one material (at least two). There should be no exchange of atoms between the components, and the components should not chemically interact with one another. If these two conditions are not met, the result is an alloy. They can be found in nature (wood, bone, clay, etc.) or manufactured to enhance one or more of the following properties of traditional materials for a specific purpose: mechanical properties (strength, elasticity, toughness, ductility, etc.), electrical conductivity, thermal resistance, weight, cost, or magnetic properties.
In most composites, the material called the “dispersed phase” is contained within another material called the “matrix phase.” Carefully selecting both phases to obtain the desired properties is one of the factors that makes composites strong. The dispersed phase generally provides desired properties such as high strength or increased ductility and is usually a ceramic or metal. Composites are classified according to the form of the dispersed material (particle, fiber, structural) and according to the matrix structure. The matrix material transfers loads by forming a mechanical and chemical bond with the dispersed-phase elements, holds everything together, and protects the dispersed phase from environmental effects. The matrix may be a polymer, ceramic, or metal.
1.1-) Classification of Composite Materials
1.1.2.) Composite Materials According to Matrix Structure
a.) Polymer Matrix Composites (PMC)
These are the most widely used composites, in which a polymer is reinforced with fine-diameter fibers. The functions of the matrix material are to hold the fibers in place, transfer stress between the fibers, provide a barrier against adverse environments, and protect the fiber surfaces without degrading their mechanical properties. Polymer matrix composites are characterized by low cost, high strength, and ease of manufacturing (simple manufacturing principles). Their disadvantages include low operating temperatures, high thermal and moisture expansion coefficients, and low elastic properties in certain directions. The most widely used polymer matrix composites are fiber-reinforced polymer matrix composites (FRP). Glass fiber-reinforced polymers (GFRP or fiberglass) and carbon fiber-reinforced polymers (CFRP) belong to this category. Polymer matrix composites are further divided into three groups according to the chemical structure of the resin used, the form of the reinforcement element (dispersed phase), and the material used.
a1.) Classification According to Matrix (Resin) Type
This is the most fundamental distinction of PMC. The resin’s response to thermal conditions changes everything. The primary reason for using the resin in its liquid form is that the liquid polymer can fully penetrate between the reinforcing fibers and completely coat each fiber (wetting), thereby ensuring a strong mechanical bond.ü
a1.1.) Thermoset Matrix Composites
Thermoset matrices are used more extensively in fiber-reinforced composite production and are initially in a liquid state; with the addition of a curing agent, they first become a gel and then solidify. Once cured, they cannot be returned to their original state. Thermoset resins are isotropic (meaning that the mechanical properties of a material, such as strength and hardness, are the same in all directions). Low viscosity is generally preferred in fiber-reinforced composite production. Polyamides are polymers among plastics that can be shaped and have high thermal resistance.
a1.2) Thermoplastic Matrix Composites
They are generally ductile and can be melted by heating and solidified by cooling. This gives them the ability to be reshaped. Thermoplastics can have an amorphous or semi-crystalline structure. In amorphous thermoplastics, molecules are arranged randomly. In crystalline regions, molecules form a tightly packed, orderly structure.
a1.3) Elastomer Matrix Composites
They have high flexibility, good tensile strength, and excellent elongation at break, and they can undergo irreversible structural changes under heat, similar to thermosets. They have an amorphous structure. They have weak bonds between their molecules. Their glass-transition temperatures are below room temperature; they are flexible at room temperature.
a2.) Classification According to Reinforcement Form
a2.1.) Continuous Fiber Reinforced
The fibers extend continuously throughout the part. This group has the highest mechanical strength.
a2.2) Short/Discontinuous Fiber Reinforced
The fibers are in small (millimeter-scale) pieces. They are generally used in mass-produced parts manufactured by injection molding.
a2.3.) Particle Reinforced
Instead of fibers, hard particles in powder or granular form are added to the polymer. Rather than increasing strength, this is generally preferred to improve wear resistance or reduce cost.
a3.) Material-Based Fiber Types
a3.1) Glass Fiber (GFRP)
Fiberglass is a composite consisting of continuous or discontinuous glass fibers within a polymer matrix and is the most widely produced type of composite in the world. It is easy to manufacture, economical, and has high strength. However, its surface is quite sensitive. Even the smallest surface defect can impair the material’s properties.
a3.2) Carbon Fiber (CFRP)
The internal structure of carbon fiber consists of both graphitic (ordered) and turbostratic (disordered) structures. This mixed structure gives the material its well-known strength. The main reasons for choosing carbon fiber are its high specific properties and temperature resistance. (Oxidation may be an issue at high temperatures.)
a3.3) Aramid Fiber Reinforced Composites
Introduced commercially in the early 1970s, aramid fibers are high-strength, high-modulus materials. They are preferred especially because of their strength-to-weight ratios, which are much higher than those of metals. Chemically, this group is known as poly(paraphenylene terephthalamide). The most common commercial names are Kevlar and Nomex. Their tensile strength and modulus are higher than those of all other polymeric fibers. However, they are relatively weak under compression loads. These fibers have high toughness, impact resistance, creep resistance, and fatigue resistance. Although they are thermoplastic, they are resistant to burning and retain their mechanical properties over a wide temperature range of approximately -200°C to 200°C.
b.) Metal Matrix Composites (MMC)
They are generally produced by adding carbon fibers or ceramics to an aluminum or titanium matrix to increase the strength or hardness of a metal. In these composites, the second phase (reinforcement) embedded in the metal-based structure may have various geometries. Metal-based materials have superior properties compared with the materials they reinforce. They provide high-temperature and wear resistance.
Advantages:
The elastic modulus and strength of metals can be increased.
Disadvantages:
It causes a reduction in ductility and fracture toughness.
c.) Ceramic Matrix Composites (CMC)
Ceramic materials are highly useful because they are resistant to high temperatures and lightweight (d = 1.5–3.0 g/cm3). Ceramic-based composite materials are generally used for components that must operate at high temperatures. Because they are hard and brittle, they have very low ductility and toughness and are also vulnerable to thermal shocks. Therefore, they are mostly reinforced with fibers. On the other hand, they have very high elastic moduli and very high operating temperatures. Ceramic composites exhibit very good resistance to high temperatures but have a rigid and brittle structure. They also provide very good electrical insulation. They are widely used in high-temperature applications where metal- and polymer-matrix composites cannot be used.
Example: Turbine Blades
d.) Carbon-Carbon Composites (CCC)
Carbon-carbon composites are unique materials in which both the matrix phase and the reinforcement phase consist of carbon. These materials are quite expensive and are used in applications requiring high strength. They are resistant to high temperatures. This property enables their use in vehicles such as spacecraft and rockets. Some sources classify this material under ceramic matrix composites, while others consider it a separate matrix class.
1.3) Classification According to the Structure and Shape of the Dispersed (Reinforcement) Phase
1.1.3.1) Particle Reinforced Composites
Get Enescsancak’s stories in your inbox
Join Medium for free to get updates from this writer.
They are divided into two types: large-particle composites and dispersion-strengthened structures.
1.1.3.1.a) Large-Particle Composites
Particles may have different geometries but should be approximately the same size in all directions. For better reinforcement, the particles should be small and evenly distributed within the matrix. (The term “large” here indicates that the interaction between the matrix and particles can be examined at the macroscopic level rather than at the atomic or molecular level.) Moreover, the volume fractions of the two phases determine the behavior exhibited; as the particle content increases, the mechanical properties increase. The particles dispersed in the matrix are generally harder and more rigid than the matrix material. There are two mathematical formulas describing how the elastic modulus changes according to the ratio of the constituent phases. These formulas provide upper bounds for the elastic modulus:
And lower bounds:
Limits.
In these formulas, E represents the elastic modulus and V represents the volume fraction. In the subscripts, c denotes the composite, m denotes the matrix, and p denotes the particle phase.
The table on the blow shows the Ec vs. Vp curve plotted for a copper-tungsten composite in which tungsten is the particle phase. The experimental data points lie between the two curves.
Large-particle composites are used in all three material types (metal, ceramic, and composite). Cermets and concrete are among the best-known examples.
1.1.3.1.b) Dispersion-Strengthened Composites
Metals and metal alloys can be strengthened by uniformly dispersing fine particles of a very hard and inert (non-reactive) material, typically at a few volume percent. The dispersed phase may be metallic or nonmetallic; oxide materials are frequently used. Similar to precipitation hardening, the increase in strength in this mechanism occurs through the interaction of particles in the matrix with dislocations. The strengthening effect of dispersion strengthening is not as pronounced as precipitation hardening; however, because the dispersed particles are selected so that they do not react with the matrix phase, the resulting strength is maintained at high temperatures and for long periods. In precipitation-hardened alloys, precipitates may grow or completely dissolve as temperature increases, causing the hardness gained by the material to be lost.
1.1.3.2.) Fiber Reinforced Composites
The most technologically important composites are those with fibers as the reinforcement phase. The design goals of fiber-reinforced composites generally include high strength and stiffness on a weight basis. Their characteristics are expressed in terms of specific strength, meaning the ratio of tensile strength to specific weight, and specific modulus, corresponding to the ratio of elastic modulus to specific weight. Low-density fibers and matrix materials are used to produce fiber-reinforced composites with high specific strength and modulus values. A significant reinforcement effect is possible only when the fiber-matrix bond is strong. Since the reinforcement effect is interrupted at the fiber ends, reinforcement efficiency depends directly on fiber length. Fiber-reinforced composites are classified according to fiber length. For each fiber-matrix combination, there is a specific critical length (lc) that depends on fiber diameter, fiber strength, and fiber-matrix bond strength.
While the length of continuous fibers is well above this critical value (l > 15lc), shorter fibers are classified as discontinuous.
Based on fiber length and orientation, three different types of fiber-reinforced composites are possible:
- Continuous and Aligned: The mechanical properties are highly anisotropic. Strength and reinforcement effects are maximum in the alignment direction and minimum perpendicular to this direction. Rule-of-mixtures expressions have been developed for the modulus values in both longitudinal and transverse orientations. A specific equation is also used for longitudinal strength.
2) Discontinuous (Short) and Aligned: Significant strength and stiffness can be achieved in the longitudinal direction. Composite strength equations are provided for two different cases:
If the fiber length is greater than the critical length (l > lc), the equation on the right applies.
If the fiber length is less than the critical length (l < lc), the following equation applies
3) Discontinuous and Randomly Oriented: Despite some limitations in reinforcement efficiency, the mechanical properties are isotropic (the same in all directions). The following equation is used for the elastic modulus of this type of fiber composite.
1.1.3.3) Structural Composites
Structural composites are divided into two subcategories: laminated composites and sandwich composites
1.1.3.3.a) Laminar Composites
Laminar composites consist of a series of bonded two-dimensional sheets; each sheet has its own high-strength direction. The in-plane properties depend on the stacking sequence and orientation of these layers. In this context, there are four basic stacking types:
• A) Unidirectional: All the fibers are aligned in the same direction. This is where the highest anisotropy (directional dependence) occurs..
• B) Cross-ply: The layers are arranged at a 90-degree angle to one another.
• C) Angle-ply: The layers are arranged at specific angles, such as ±45°.
• D) Multidirectional: It is the most isotropic (exhibiting similar properties in all directions) structure.
Prepreg tapes are typically used in these structures.
The orientation of these angles is selected in the directions where stresses are expected to be higher under operating conditions, allowing both light weight and strength to be achieved simultaneously.
1.3.3.b) Sandwich Panels
Sandwich panels consist of a core (honeycomb) and two strong and rigid face structures. These structures have low density and relatively high strength and stiffness. They have a wide range of applications, from door panels to aircraft wings.
Core materials generally include rigid polymeric foams, low-density woods, and honeycomb structures.
1.1.3.4) Nanocomposites
Nanocomposites consist of nanomaterials embedded in a matrix (usually a polymer) and utilize the extraordinary properties of nanoscale particles. Nanoparticles include nanocarbon, nanoclay, and nanocrystalline particles. Ensuring that nanoparticles are uniformly and homogeneously distributed within the matrix is the greatest challenge in nanocomposite production (agglomeration).
2) Lamination
It is the joining of two or more materials using heat, pressure, or an adhesive. This method is used to give materials additional properties and increase their durability. In everyday life, we are most familiar with this concept through laminate flooring.
