Composites
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]. |
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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].
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]. |
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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 reinforcement. Some examples include [1]:
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]:
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].
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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].
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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]:
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].
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].
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].
Advantages and Disadvantages
Fiber Type | Advantages | Disadvantages |
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Carbon Fiber |
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Fiberglass |
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Hybrid/Other |
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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]:
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].
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].
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].
Advantages and Disadvantages
Resin Type | Advantages | Disadvantages |
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Epoxy |
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Polyester |
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Vinyl Ester |
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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].
Multiaxial vs. Unidirectional
Multiaxial | Unidirectional |
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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].
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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].
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:
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Figure 14: The Hand Layup consolidation method can be time consuming as it requires manual application for wetting out the fibres |
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:
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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: