Common Reinforcements Used in Composites

In this introductory post, common reinforcements for composites will be presented.  This will serve to &#;set the table&#; for future discussions on prepregs, laminates and a wide range of fiber reinforced composites.  The choice of reinforcement is a critical factor when designing or selecting composite materials since in many applications, the properties of the composite are dominated by the reinforcement.  Reinforcements are typically non-isotropic (i.e. they have directionality) which results in properties that may be different in the X, Y, and Z directions.  For example, a unidirectional fiber composite may have very high strength in the fiber direction owing to the majority of the load being carrier by the fiber and poor strength in the transverse direction due to the load being carried by the resin matrix.  Composites are designed so that a large portion of the load is carried by the reinforcements resulting in high strength to weight ratios.  The chemical nature of the reinforcement as well as the form of the reinforcement are important composite design parameters. In the next two sections, the types of reinforcements and the reinforcement forms will be discussed.

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Types of reinforcements

• Glass fibers

• Carbon or graphite

  Aramid (Kevlar) fibers

• Ultra High Molecular Weight Polyetheylene (UHMWPE) fibers

• Exotic fibers (boron)

• Particulate fillers (ceramic fillers (calcium carbonate, fumed silica), metal fillers)

In Figure 1, the stress strain curve is shown for some typical fibers used in composites.

Figure 1. Tensile stress versus tensile strain for a variety of reinforcement materials typically used in composites (1)

The most common type of composite reinforcement is E-glass.  From the stress-strain data, E-glass has the lowest modulus, but has relatively good tensile strength.  E-glass is also the most inexpensive glass fiber.  E-glass is commonly available in many forms (see below).  S-glass offers a higher modulus and tensile strength but comes with a cost premium compared to E-glass.  Moving to the left in Figure 1, one sees that Kevlar 49 has both a higher modulus and tensile strength compared the the glass fibers.  Carbon fibers have the highest moduli and the high strength carbon fibers have about the same tensile strength as E-glass with a significantly higher modulus.  Note that for the high modulus carbon fiber, the tensile strength decreases, so this fiber would be used where the modulus and stiffness would be the more important design criteria.  Table 2.1 in reference 1 provides a nice overview of the material properties (tensile modulus, tensile strength, strain-to-failure, CTE, and Poisson&#;s ratio) for a wide variety of commercial reinforcing fibers.  

Common reinforcement forms:

• Continuous tows (unidirectional)

• Woven fabrics and braided sleeves (bidirectional)

• Non-continuous chopped fibers (discrete fibers and fiber mats)

• Particulate fillers

The following figure shows a schematic of the various reinforcement forms and how they are used in a multilayer layup stack in the final composite. 

Figure 2. Basic building blocks of fiber-reinforced composites (1)

Once the type of fiber is chosen (e.g. glass, carbon, Kevlar) then depending on the application, the form is selected.  Unidirectional fibers offer the potential to have high load bearing in use, but typically need to be applied in layers in the stack-up to achieve the desired properties.  As can be seen in the laminate in the lower left in Figure 2, multiple unidirectional plies are stacked at 0o, 90o and 45o (0, 90, 45,45,45,45,45,90,0) in a symmetric layup.  An advantage of woven fabrics is that the weave geometry can be tailored to give the required properties in the X and Y directions.  Woven fabrics have different weave types (plain/square, twill, satin) and can have various yarn counts in the X (warp) and Y (weft/fill) directions.  For example, satin weave glass fabrics have good drapeability (for forming over complex curved shapes) and low crimp.  Crimp is the angle between the crossing fibers and lower crimp yields improved mechanical properties since straighter fibers can carry higher loads. 

Composites made using bulk molding compounds (BMC) and sheet molding compounds (SMC) are made using chopped glass fibers.  BMC&#;s typically contain randomly oriented short chopped E-glass fibers.  SMC materials are also made using randomly oriented chopped E-glass fibers.  As shown in the lower right layup in Figure 2, a combination of outer plies with unidirectional fibers can be combined with inner layers containing unidirectional discontinuous (chopped) fibers to provide the requisite mechanical properties at the target cost. 

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In the next several posts we will dive deeper into matrix resins used in typical composites

References:

1) Fiber-reinforced Composites, Materials, Manufacturing, and Design, P. K. Mallick, CRC Press,

Composite components are made up of 2 or more individually recognizable components, but advanced composites only utilize 2 main ingredients: fiber reinforcement and polymer matrix. Fibers are the main component in composite structures as they provide the strength, durability, and flexibility of the final component. Read this blog for more in-depth information on the different types of fibers and their available configurations.

A fiber is defined as a material that has a long axis, usually many times greater than its diameter. The way fibers, but especially short fibers, are defined is by their aspect ratio. The aspect ratio refers to the fiber length divided by its diameter (l/d), and in order to be considered a fiber, the material usually will have an aspect ratio greater than 100.

As we discussed in Intro to Composites, an individual strand of fiber can be thought of as a rope, where it has really high tensile strength but really low compressive strength. When combined with the polymer matrix, the resulting part is extremely strong and lightweight. As technology develops and the need for more environmentally friendly practices forges ahead, the availability of fiber reinforcements is ever-expanding. However, there are three main types of fiber reinforcements: glass, carbon, and natural.

Glass Fiber

Incorporating glass fiber with a polymer matrix in the s was really the beginning of modern-day advanced composites, resulting in a common product known as fiberglass. Fiberglass is used in everyday items such as metro seats, fair rides, and sailboats to name a few.

Fig. 1: Fiberglass Product Examples

It is not as strong or as stiff as carbon fibers, but it has other characteristics that make it desirable in many applications. It is considered an insulator and is invisible to most types of transmissions, making it a good choice for electrical or broadcasting applications.

The main benefits of using fiberglass over metals are primarily the cost, but also the significant weight savings, which is important in transportation and recreation applications. Fiberglass is cheap, lightweight, and has good structural integrity. This is why fiberglass is so common in household, recreational, and transportation equipment.

Carbon Fiber

Carbon fiber is even stronger and lighter than fiberglass, but these benefits are reflected in the increase in cost, hence why it is most common in aerospace or high-end competition equipment. Ever since its creation in the s, it has been revolutionizing industries such as aerospace, motorsports, and professional equipment, creating very strong and very lightweight components. Some examples of carbon fiber components are in Formula 1 cars, planes, and hockey sticks.

Fig. 2: Carbon Fiber Composite Examples

In high competition sports, winning is the main priority and reducing weight and increasing performance is the best way to achieve this. In aerospace, the main goal is to reduce fuel consumption, and reducing weight is the best way to achieve this. The increased performance is a huge benefit, allowing the engineers to further optimize the weight savings.

Natural Fibers

Natural fibers have been used as a reinforcement in advanced composites for many years, but research has ramped up recently fueled by the regulations pushing towards sustainability. There are many different natural fibers that are being evaluated for use in advanced composites, but the most viable ones at the time of writing this blog are flax, hemp, bamboo, and basalt. Biocomposites will most likely be used as a replacement for fiberglass, and flax is leading the way in this aspect due to its very similar properties. Some examples of biocomposite components are automotive body panels, snowboards, and wind turbine blades.

Fig. 3: Natural Fiber Composite Examples

As a result of the push for renewability, research is constantly evolving, resulting in more and more information being released regularly and applications being developed.

Mechanical Properties

Earlier we spoke about the different applications for the different classifications of composites, and how required strength is a determining factor of which structural material is chosen. Charts 1 and 2 below provide a great snapshot of how each different type of fiber compares to the most common structural metals: steel and aluminum.

Chart 1: Structural Material Strengths and Densities (excl. Carbon Fiber)

Chart 1 above shows the relation of the most common structural metals and fibers, without carbon fiber in order for a clearer snapshot of how the other materials compare to each other. As is evident, steel and aluminum have lower specific strength than all the fibers, but have relatively high specific modulus.

Let's take a look at how carbon fiber compares to all other structural materials in Chart 2 below.

Chart 2: Material Strength by Density (incl. Carbon Fiber)

Carbon fiber was added into the chart in order to show how much stronger and stiffer it is than all other structural materials, hence its popularity in aerospace, defense, and space applications.

Pricing

While applications, such as in construction for example, would probably prefer to use stronger and stiffer material for certain projects, the company wouldn't win many bids if they used primarily carbon fiber as opposed to steel or aluminum. Chart 3 below shows the cost relations of each of the structural materials, in USD per kilogram.

Chart 3: Prices of Structural Materials

Based on this chart, the construction industry isn't going to be incorporating carbon fiber into their bids anytime soon. It is up to the product designer to determine which material to use for their product by weighing the benefits of strength, weight, and price of each different material. For example, the average price of fiberglass is $1.80/kg, while aluminum costs $2.26/kg, and steel costs $0.62/kg. While steel is about 1/3 the price of fiberglass, a steel component would weigh about 4.5 times more than the fiberglass component, while exhibiting the same structural properties. These are all things that product designers have to take into consideration in order to create a successful product.

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