Composites

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].

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].

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).

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. 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

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. 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

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

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

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

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.

References