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Table of Contents

What Are Composites?

Composites are uniform materials comprised of at least two different elements, which combine to yield a product possessing different properties and characteristics than those elements individually. The material composition of composites generally includes layers of fibre reinforcement oriented to give strength, held together by a bulk component called the matrix [1]. Composite materials are favoured because they can be stronger, lighter, and more cost-effective compared to other common materials such as aluminum and stainless steel [2][3].

Figure 1: Multilayered composition of composites. Notice that fibres and matrix are layed and sandwiched together to increase overall strength [4].Figure 2: Carbon fibre is a composite material commonly used in the construction of aircraft and automotive vehicles because of its high strength [5].


Main Types of Composites

Today, there are usually three main types of human-made composites, which all vary in formation and exhibit different chemical and mechanical properties. As a result, they are often found in a wide variety of different industries [1].

    • PMC: Polymer matrix composites use a resin (epoxy) as the matrix, and certain fibers (carbon fiber, fiberglass) as the structural reinforcement. Polymer matrix composites are the most common types of composites as a result of their ability to be transformed to complex shapes [1].
    • MMC: Metal matrix composites use a metal as the matrix (aluminum) strengthened with fibers (silicon carbide). These are often used in the automotive industry [1].

    • CMC: Ceramic matrix composites use ceramic as the matrix and are strengthened by small fibers (silicon carbide and boron nitride) for use in very hot environments [1].

Figure 5: Composite materials are often stronger than other metals at a much lighter weight. However, its ductility is reduced as strength falls to essentially nothing once ULF (Ultimate Laminate Failure) occurs [8].


Composite Components

Fibre: Provides strength and stiffness (glass, carbon, natural fibers, etc.). Increases the strength and stiffness of the structure formation method, orientation, fiber volume, and other factors. Usually referred to as the reinforcementSome examples include [1]:

    • Carbon Fiber
    • Fiberglass
    • Kevlar

Matrix: Protects and transfers load between fibers (polyester, epoxy, etc.). Bulk component of the composite that holds the fibers together. Most commonly a polymer of some sort. Some examples include [1]:

    • Epoxy resins, with hardeners
    • Polyester resins, with MEKP
    • Vinyl-ester resins, with MEKP

Figure 3: Different fibre orientations; effects will be further discussed [6].Figure 4: Epoxy is a common resin used as the matrix for carbon fibre [7]. 



Composite Material Applications

General and Industrial Usage

There are many different uses of composite materials. Composite products are used in a variety of different residential and commercial construction. Many parts of a home can be created with plastic laminated beams, which reduces the chances of termite damage. Composite materials can also be used in the aircraft industry. Products like fibreglass are light enough to fly and strong enough to handle intense pressures. Another use of composite materials is in the sports industry. Many different sporting goods are made from composite materials, such as baseball bats. This is to reduce damage and breaking. These materials are also used on products like surfboards to create durability and flexibility. Finally, many boats are made with composite materials such as fibreglass to reduce rot and rust [9].

Use Within Design Teams & Specialty Pieces

Composites are often used to create components and structures that require high physical strength with relatively low weight. Student teams often incorporate composite materials in their designs as a result of these factors. However, it is important to note that the bigger the piece, the more factors that would need to be accounted for. Bigger pieces of composites often affect and are affected by other structures and materials they interact with. Thus, a thorough and elaborate plan is required before starting layups (analysis of smaller samples are recommended).


Figure 6: Formula SAE student teams usually incorporate composite materials in the design of aero structures. Such as the front wing, rear wing, diffuser, floor panel, nose, etc [10].Figure 7: Rocketry/IREC student teams usually incorporate composite materials in the design of structural and aero structures. Such as body tubes, stabilizing fins, nosecones, etc [11].

Selection of Materials

Material selection influences the overall strength, weight, cost, and difficulty of creating the final laminate consolidation. Considerations are made towards the fibre and resin selection, and their interactions with other parts. For example, it is important to note that carbon fiber should never be contacted with aluminium pieces. This is because their similar electrical potential induces possible galvanic corrosion [12].

Considerations For Different Fibres

Fibrous components of composite materials increase the strength and stiffness of the structure depending on the formation method, fiber orientation, weave type, fiber volume fraction, and other factors [1]. Considerations for choosing different fiber types depends on [1]:

      1. Mechanical properties of fiber (tensile, compressive strength, density, etc.)
      2. The surface interaction between the fiber and the resin
      3. The amount of fiber used in the composite (FVF)
      4. Orientation of the fibers  
Carbon Fiber

The majority of carbon fiber is made from the precursor polyacrylonitrile which is heated with no oxygen. Allowing it to burn, and causing the non-carbon atoms to leave structure as atoms vibrate. The result is a polymer composed of long carbon chains [1].

      • Different strand thicknesses changes weight and strength
        • Generally, higher thickness results in higher weight and strength
        • 3K, 6K, 12K (3000, 6000, 12000) strands of carbon fiber in each tow of carbon fiber fabric
Fiberglass

Made from some recipe of quarry products (sand, limestone, kaolin, colemanite), then melted at extremely high temperatures. Pushed through small orifices of of 5-24 µm and cooled. Can be drawn together using a thermosetting resin or left apart (rovings) [1].

      • E-Glass: Electrical Glass
        • Good tensile and compressive strength
        • Extremely resistant to external effects ie. fire, water, acid, alkali, UV
      • C-Glass: Alkali-Lime Glass
        • Provides best resistance to chemical attacks
        • Primarily used in chemical and water pipes
      • R,S,T-Glass: Aluminosilicate, Aluminosilicate without CaO, Thermal Insulator-Glass
        • Highest tensile and compressive strength, with good strength retention
        • 10-20 times more expensive than E-Glass
        • Primarily used in military vehicles and aerospace
Hybrid/Other

Hybrid composites are two or more types of reinforcements combined into a single fabric, or in different layers of a part. This combination can be for structural or cosmetic reasons, but yield different properties and applications [1].

      • Carbon Fiber/Kevlar
      • Carbon Fiber/Fiberglass
      • Fiberglass/Kevlar


Advantages and Disadvantages

Fiber TypeAdvantagesDisadvantages
Carbon Fiber
  • Extremely high tensile and compressive strength
  • Stronger than fiberglass, accounting density and weight
  • High stiffness
  • Lightweight
  • Expensive compared to other fibers
  • Strength is easily compromised if there is any breakage of the fiber
  • Prone to microcracking which compromises strength
  • Induces galvanic corrosion when in contact with aluminium 
Fiberglass
  • Corrosion resistant to liquids and chemicals
  • Chemically resistant to liquids and alkalis
  • Electrical insulation
  • Cheaper than carbon fiber
  • Less tensile and compressive strength compared to carbon fiber
  • Lower stiffness and higher weight compared to carbon fiber
  • Airborne fibers can cause respiratory issues
  • 20-30% weaker than carbon fiber by weight
Hybrid/Other
  • Hybrids can combine favourable properties of two different fibers
  • Can be less expensive while sacrificing little strength
  • Increased impact resistance
  • Combination of fibers worsen performance of specific properties
  • Can yield significant reduction in stiffness
  • Slight decrease in tensile and compressive strength

Considerations For Different Resins

The resin matrix holds the fibrous component of the composite together, so it needs to create physically and chemically strong bonds. These are usually found as simple chain polymers - called synthetic resins. There are two main different types, thermosetting and thermoplastics, which are categorized by their properties when exposed to heat. Considerations for choosing different resin types depends on [1]:

      1. Adhesive and bonding properties
      2. Mechanical properties of resin (shearing strength, flexibility, etc.)
      3. Degradation from water exposure and water ingress
Epoxy

Epoxy resins are usually the highest performance of the three types currently available. With superior mechanical properties, and resistance to degradation, they are found in high performance machines such in aerospace and boat building [1].

      • Simplest formation is called "alpha epoxy" or "1, 2-epoxy"
      • Cured with hardener
        • Usually an amine
        • Takes place through addition reactions
Polyester

Commonly, polyester resins are of the unsaturated type that are also thermosets. Like epoxy and vinyl-ester resins, they are synthetic. Usually used with fiberglass, the curing process is exothermic and can produce a fair amount of heat [1].

      • Can be diluted with acetone (10-part resin to 1-part acetone)
      • Cured with time
        • A catalyst can be added to increase curing time
        • MEKP (Methyl Ethyl Ketone Peroxide)
Vinyl Ester

Similar to polyester resin in their structures, their difference is in the location of respective reaction sites. Vinyl ester resins are usually used for laminating and repairing materials, or as replacements for polyester resins [1].

      • Similar curing process to polyester
      • Cured with time
        • A catalyst can be added to increase curing time
        • MEKP (Methyl Ethyl Ketone Peroxide)

Advantages and Disadvantages

Resin TypeAdvantagesDisadvantages
Epoxy
  • Easily processed and cured. Quick curing from any temperature ranging from 5°C to 150°C.
  • Exhibits high adhesive strength
  • High electrical insulation
  • Good chemical resistance
  • Is more expensive than polyester and vinyl ester resin
  • Prone to trapping air bubbles
  • Generally low odour
Polyester
  • Resistant to water and some chemicals
  • Lowest cost of the three options
  • Withstand high temperatures
  • Low shrinkage
  • Strong odour; respiratory PPE recommended
  • Exothermic during curing
  • Toxic nature of fumes
  • Weakest of the three options
Vinyl Ester
  • Absorb more shock loadings compared to polyester chains
  • More water resistant than polyester resin
  • Good flexibility and low viscosity
  • Designed to combine advantages of epoxy and polyester resins
  • Less resistant to acid compared to polyester
  • Volatility when exposed to styrene
  • Is more expensive than polyester resin
  • Poorest surface finish

Considerations For Different Fiber Orientations

Fiber orientation impacts the mechanical properties of composites by dictating where loads are imposed, the direction of strength, and other characteristics [1].

    • Stacking a combination of different orientation unidirectional sheets, or combining multiaxial and unidirectional can increase the overall strength of a composite without sacrificing weakness to a certain direction. This however, will make the composite more rigid.
    • When stacking sequences of unidirectional fiber sheets, it is important to emphasize symmetry and balance of the sheets and their orientations.
    • Symmetry about the midplane is important. For example, 1 sheet of +45° should be balanced out by 1 sheet of -45° to ensure structural symmetry.


Multiaxial vs. Unidirectional
Multiaxial Unidirectional
  • Multiaxial composites are layers or different plies of fibers which run in different directions and offer higher strength in all directions. This also means that the mechanical properties are approximately the same, regardless of the orientation. There is a higher chance for crimping with multiaxial laminates.
  • Multiaxial laminates are used when there are stresses in multiple directions. They are often used in load-bearing applications.
  • Unidirectional composites mean that the fibers are running mainly in one direction. For carbon fibers which are all side by side running in one direction. The tensile strength is extremely high in the direction that they run, but may be different for directions of the material. There is very little or no crimping with unidirectional materials.
  • Unidirectional carbon fiber composites are used where maximum tensile strength is required in a single direction. They are used in rockets, and airplanes as well as some frames which require 

Figure 12: Multiaxial laminates are formed by layers of unidirectional or parallel fibers [13].

Figure 13: A 0-degree fiber alignment is loaded parallel to the fiber direction. A 90-degree fiber alignment is loaded perpendicular to the fiber direction [14].
Multilayer Sandwiched Laminates

Increases stiffness due to increased thickness of the composite sheet. In addition, this will change the overall weight of the material, and the complexity of production. This will make certain materials more usable as strength has no meaning if the composite cannot hold shape for its function [1].

      • A sandwich structure is comprised of a core central material enclosed by (sandwiched between) two high strength fiber sheets.
      • Method of increasing composite thickness without sacrificing much in weight.
      • Stiffness increases with thickness (with low density core material) as flexural stiffness is directly proportional to the cube of the thickness.
      • Important factor of the core material is its shear strength and stiffness, as the top fiber layer will experience compression while the bottom fiber layer will experience a tension force through an upward concaving flexural load.

Considerations For Different Fiber Weaves

Plain Weave

Each tow (bundled strip of carbon fiber) passes over another perpendicular running tow and under the next one. This weave lacks flexibility which makes it difficult to use for shapely contours but excellent for flat sheets. There is a lot of crimping in a plain weave due to plenty of turns because of the over-under nature [1].

Twill Weave

There are different kinds of Twill weave such as 2x2, 3x3 and 4x4. What these numbers mean is the number of times the tow passes either under or over the perpendicular tows. For a 2x2, the tow passes under two tows before going over two more and so on. This creates less crimping due to less severe twisting of the tows but sacrifices fabric stability. This weave is much better suited to detailed, elaborate contours [1].

Harness Satin Weave

In a harness weave, a tow is passed under a certain number of tows and then over one before repeating. For example: a 4HS (4 Harness Satin) passes three under three tows and then over one before repeating. This type of weave offers excellent formability but poor fabric stability. The higher the satin weave number, the more flexible and less stable it becomes [1].


Production Methods For Composite Pieces

Hand Layup

The hand layup technique is the oldest method of woven composite manufacturing. The materials are prepared by respecting a few different steps. First, the mold surface is treated  by an anti-adhesive agent to avoid the polymer sticking to the surface. Next, a thin plastic sheet is applied at the top and bottom of the mold plate to get a smooth surface on the product. The layers of the woven reinforcement are cut to required shapes and placed on the surface of the mold. The mats are placed in the preceding polymer layers and pressured using a roller to remove any trapped air bubbles and the excess of polymer as well. The mold is then finally closed and pressure is released to obtain a single mat. After curing at room temperature, the mold is opened and woven composite is removed from the mold surface [15]. 

Guide: Hand Layup Method Guide.docx

Figure 14: The Hand Layup consolidation method can be time consuming as it requires manual application for wetting out the fibers

Vacuum Bagging (after consolidation)

Vacuum bagging is widely used in the composites industry. It is a technique in which one creates a uniform pressure on the surfaces of the objects in a bag. This holds the parts together while the adhesive cures. Pressurizing a composite lamination serves several functions. It removes any trapped air between layers and provides pressure that prevents shifting of fiber orientation during cure. Finally, it also reduces humidity and improves the fiber to resin ratio in the composite part [16].

Guide: Vacuum Bagging Method Guide.docx

Figure 15: The vacuum bagging consolidation method controls the amount of excess resin cured with the fibers. This yields a laminate that is generally thinner, lighter, and closer to the desired fiber volume fraction (FVF, Vf).

Resin Infusion

Resin infusion is the process where the voids in an evacuated stack of porous material are filled with a liquid resin. After the resin solidifies, the solid resin matrix binds the assembly of materials into a unified rigid composite. The reinforcement can be any porous material compatible with the resin. Typical materials include inorganic fibers, organic fibers, or combination of fibers with other materials. The key part of the process is the evacuation, or removal of the air from the porous material prior to admitting the resin [17].

Guide:

Oven Cured Prepreg

To start off, materials are placed in a bag that has been successfully leak tested [16]. It can then be put in an oven to cure. The bag is carefully placed in the oven so that it cannot sang or catch on any edges, which would result in a puncture. After, a vacuum line is connected to the inside of the oven and connected to the pump outside. The oven doors are then closed and the oven is turned on. A program is then used from a laptop or pc [18]. Finally, the material is removed.

Guide: Same video as above.


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