Structural and Civil Applications of Carbon-fibre

Introduction to Composite Materials

The central idea in this paper is to demonstrate the Structural and Civil Application of carbon-fibre and the reasons as to why carbon fibre is effective in these areas. As an engineer the selection and application of material to a project is vital, not only is the type of material going to determine how strong and stable the structure is, but it also determines the life time in which the structure can last, and also how much it will cost. A major outlier that outperforms other material in selection process is composite materials.

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Composites are materials that are made of up of two or more constituent materials that are combined together and generate properties that are greater compared to when the materials are individual components. The chemical and physical bonding of two or more constituent materials allows the finished product to have greater advantages compared to having properties that differ when they are individual components. This is a main reason why composite materials are the go-to option in engineering realm due to its flexibility and wide range of choice to suit types of needs. Composite materials commonly display the advantages of being lighter, more durable, and stronger compared to normal materials. Hence, why these materials are commonly selected is because of because of the advantages they pose in not being heaving and heaving high strength and stiffness.
 

Introduction to Carbon Fibre

Carbon fibre is a composite material that has two main constituent parts which are epoxy resin, and carbon fibre. Carbon fibres are fibres that are made up from carbon atoms and are 5-10 micrometres in diameter.

This material is commonly used in the civil and structural engineering aspect which mainly in fields such as aerospace and automobile because of the advantages it poses which are listed below. Research by S. Arun Kumar shows that carbon fibre can have the tendency to be brittle in nature therefore, the addition of the epoxy resin ensures the brittleness is reduced and there is some ductile features in the composite material.

However, it is known that the combination of these two materials are the reason why carbon fibre poses many of its common strengths such as:

●      Very high in stiffness and tensile strength

●      A very low strength to weight ratio

●      High chemical resistivity

●      Can undergoe high temperatures

Figure 1: On the right are three different images that the first two on the top show the epoxy resin, and the right is the carbon fibre mat. Both are constituent materials that are used to form Carbon Fibre, (third image below). 

CONSTITUANT PART 1: Fibre

Carbon Fibre is produced in a spinning process that it goes through to create a polymer which is vital components of manufacturing the carbon fibre. It is called a polyacrylonitrile (PAN). This chemical step causes a reaction called ammonoxidation which occurs between propene and ammonia.

Propene + Ammonia + (In the access of Oxygen) —> Acrylonitrile + Water

This chemical reaction produces a substance called Acrylonitrile which is a powdery plastic which than undergoes a method of arranging the structure of the atoms within a particle which than results in PAN, where carbon atoms form a long chain polymer.

Figure 2: Oxidisation

Oxidisation is the state after PAN is made, this process is where the hexagonal structure of carbon fibre as seen below are formed. During this process PAN is heated at approximately 200-300 degrees in order to replace oxygen with hydrogen atoms, thus causing nitrogen atoms to bond opposite carbon atoms, this forms the continuous cycle hence, forming the crystals.

Figure 3: Carbonisation

This is the process where the product which formed in the oxidisation process is heated to greater heat of approximately 1500 to 2000 degrees This is to remove elements such as nitrogen, oxygen and hydrogen in order to leave a hexagonal structure of pure carbon atoms. After this process the carbon fibres are than collected and woven into sheets or tubes, in a specific orientation or alignment this is to increase tensile strength and the positioning is later on glued together by epoxy resins.

ATOMIC AND CRYSTAL STRUCTURE OF FIBRE

Carbon Fibre are typically thin pieces of strands that have a diameter of 0.005 to 0.010 nanometres of carbon atoms that are joined together chemically as microscopic crystals which are aligned in a parallel manner to each other. This is mainly because the orientation of the fibre determines the strength for the size of the overall fibre. A strong piece of fibre is a combination of thousands of carbon fibre strands that are twisted together in order to create a yarn, which is than woven and creates fabric such as the mat. Once the fabric is made, the epoxy resin partnered with the fibre in a specific shape to achieve the different composite material required. While they can be maintain a relatively very light mass, they are also known to be 10 times stronger than titanium.

Figure 4: Each carbon fibre mat that is produced contains sheets of carbon atoms that are covalently bonded to form the hexagonal structure. Once these planes are stacked onto of each other this is where the bonding occurs among carbon atoms through weak intermolecular attractions that allow the planes to easily slide past each other thus resulting in carbon fibre the properties that it poses.

Figure 5: Thus, carbon fibre are man-made and their properties and cellular atomic structuring are specifically modified during the manufacturing process, which than are spun and woven.  As seen in the figure below, the material is arranged in a linear version for greater strength as stated above, and the molecular structure shows the bonding of carbon to hydrogen atoms in a covalent bond this therefore, provides carbon fibre with the high meting point advantage.

STRESS STRAIN GRAPH OF CARBON FIBRE

Figure 6: The carbon mat fibre component possesses a stress strain curve that shows that as a constituent it has a stress of 50 MPa and a strain percentage of around 1.5%. It can also be seen that is has very high ultimate strength and it is also less ductile therefore explaining the very short fracture point. The elastic region lasts for a very short time span thus making this material as a constituent very brittle. This can also be proven, by using the research above about the crystal structure to back up the information possessed in this graph. The main part as to why carbon fibre is very brittle is due to how it is aligned, the axis and the orientation in which it is aligned is the factor that determines the ultimate strength that this material possess. Thus, it is known that they are purposely arranged in a up and down manner rather than side to side in order to create more stiffness, and higher strength, providing it the resistance to have a high elastic rating.

ATOMIC AND CRYSTAL STRUCTURE OF EPOXY RESIN

Epoxy resins normally begins in a liquid state that acts as a reliable binding agent therefore having great adhesive properties, it also has other advantages such as its ability to be used for  coating for metal and composites, insulation of electronics, chemical and heat resistance, mechanical properties model making and many industrial applications. Epoxy is thermosetting, which means that liquid form in time it reaches its solid form through a process called curing, and this is affected by the temperature of the environment it is in. Thus, once it is cured it cannot be reversed, this is due to its molecular structures.

Figure 7: Epoxy is an epoxide, which simply means that it’s more reactive to other chemicals. A polyoxide such as epoxy is made up of unreacted epoxide. This basically means that pure epoxy is extremely vulnerable to reactions with other chemicals.

Figure 8: An epoxy resin thermoset is an amorphous because the three-dimensional chemical bonding it holds, therefore it makes it harder for the movement of polymer chains to pack and crystalize, this means that polymers can crystalize when cooling after being melted or stretched. Hence, epoxy are high in their viscosity content this allows it to poses traits such as temperature resistance, chemical resistance, and a great adhesive solution.

STRESS STRAIN GRAPH OF EPOXY RESIN

Figure 9: The stress strain curve of a cured epoxy resin is showed, as it can be seen epoxy has a low Stress rating of 75MPa and a Strain of 4% which is lower than carbon fibre. These both already proving to be better than carbon fibre, however, it is much more brittle than carbon fibre, it has a very high fracture point which than leads to a very low elasticity component of the material. Using the provided research gathered above about the chemical properties of resin, it can be seen that because epoxy resin is a amorphous thermoset therefore, it has the properties of being solid and brittle at room temperature but, when heated, it can become soft and pliable. Epoxy resins are also very reactive with other materials as stated above this would explain why it is used as a gluing agent between different materials.

COMPOSITE

The combining of both these constituents results in the creation of a carbon fibre material. As a composite material it is visible how it out performs each constituent, in every aspect. As it is understood carbon fibre gains its tough and brittle ability through the alignment of the carbon fibre filaments, hence, comparing the atomic structures it is visibly seen that the hexagonal atomic structure represents the high strength quality that is boasts. This comes from the fibre, the high strength, nonetheless, it can be said that the more numbers of carbon sheets provide the greater the strength, and this is why the epoxy resin is very useful. However, the orientation of fibre filaments is still important as seen in the image below.

STRESS STRAIN GRAPH OF COMPOSITE

On the right is a stress and strain graph of carbon fibre composite is shown, the first graph shows in blue the normal carbon fibre, than the other colours and the percentage are the amount of Epoxy composites is in the material, as it can be seen the first relationship is how the increase composites create a steeper yield strength. The more epoxy resin was added the more angled upward the incline gets, leads to the more the yield strength the composite possesses which clearly out performs both constituents in higher yield point. Because carbon fibre generates a very high yield strength, this means that it will be able to resist high stress with reaching a permanent deformation point. The tip at the end where there is a spike is the ultimate strength, after this stage the more the stress the shorter it would take till the carbon fibre fracture, thus, it has adopted its better fracture point. If seen, the epoxy resin had a very high fracture point when cured, while carbon fibre had a better elastic region. In this graph of the carbon fibre it can be seen that carbon fibres have a very small if not they do not have a good elastic limit and proportional limit therefore, meaning that while they are very stiff and hard, they cannot be malleable easily. 

TITLE: Advantages and Disadvantages *the bold represents the use of epoxy resin. While the normal are the use of fibre.

As seen the table represents advantages and disadvantages of each constituent that is used to make this composite material. In Structural and Civil Application, the advantages of the carbon fibre would be a more outweighing factor in the purchasing of carbon fibre. In the industry anything that can be bought that will reflect durability, whilst it performs its role in the structure successfully is important. However, materials that are light weight, and have high strength to weight ratios are useful in construction of bridges, and also in the engineering of materials for parts of automobiles and aeroplanes.

The advantages and disadvantages clearly reflect the high strength of the composite while the risks and pros of having an epoxy resin hold the structure together. Hence, the main use that of carbon fibre after analysing the table would be the use of the material in constructional purposes, this is due to the design, by maximising the exposure of the material to high or low temperature the material can be more useful. A big part of the designing of composite carbon fibre materials that use epoxy and fibre mat is that it can withstand load over a larger surface area, in which it is able to appropriately distribute the load to each of the constituent parts involved. Hence, it can successfully hold greater loads making this a reliable material in construction. Nonetheless, high number of resins and types of fibres can allow civil and structural engineers to have more modified composites depending on the need they have.

CASE STUDY: AEROSPACE

Carbon fibre was first introduced for the aerospace industry because of the applications that it possessed. The high strength to weight ratio was one of the main factors that made this composite material a choice over other metals, it was mainly used for certain and specific parts of the plane because of how easily it can be created. This lead to many parts of the aeroplane being replaced by lighter and more advanced manufactured composites. Carbon epoxy composites are 2/3^rd the weight , and more than 3 times as stiff compared to the commonly used metal aluminium. Composites are used because over years they have been designed and advanced into being materials that are resistant to fatigue damage and can cope with very harsh environment while being reliable.

In the airline industry over the years as it can be seen the advancement of involving carbon fibre in the design of airliner companies has been a constant growth. This is because of the advantages that come with composite materials, such as the fact that composites have less weight and more strength. Airline companies are constantly seeking ways to decrease weight in aeroplane because the more weight means the more fuel consumed. Hence the innovation of composite material has led to a large economic benefit for the airline companies. When there is a reduction in weight, there is also a reduction in the total emission, therefore, making lighter aeroplanes more environmentally friendly. The image above shows how each year the airline industry is constantly modifying specific parts of the plane, each part specially shaped and made in order to decrease the weight. Over the years carbon fibre has been modified form not only being light and strong, but it has also improved tremendously by being more durable and design friendly. Because carbon fibre has a high fracture point, the durability factor is reassuring for airline companies. As stated above, this composite is very high in resistance to damage caused by the weather condition which is one of the main cause of damage in the aerospace industry. Carbon fibre has the ability to adjust to any area of any type of temperature and still poses the resistivity to other harmful chemicals.

SUMMARY

To conclude, carbon fibre is the material that will be a more positive catalyst for development in the civil and structural area. The advancement of how far carbon fibre has come is constantly improving every day. It has now been recognised as one of most reliable and consistent materials in the industry due to its reliable results. While there is a big investment in the market for this material, prices are dammed to go low due to the mass production the material.

Nonetheless, from an engineering perspective it should be seen that the constituent materials that are present within a carbon fibre help to provide the material with vast advantages, it also is a very easy design flexible material as well, hence leaving engineers with the option to be creative and innovative while using a reliable material.

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