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Carbon-fiber -reinforced polymers, or CFRPs , are a class of very low-density, high-strength composite materials that find applications in a wide range of industries, including which range from equipment for highly competitive sports to the aerospace industry. Despite its technical name being carbon fiber reinforced polymer composites, most people refer to this class of materials simply as carbon fiber .
As their name suggests, these compounds are formed by a polymeric or plastic matrix reinforced with a high-resistance carbon fiber fabric. The final properties of the composite depend on both the type of resin used and the particular characteristics of the fibers, as well as the way in which the fibers are interwoven within the matrix and the direction they have within the material. On the other hand, different additives are usually added to further modify the properties of the resulting part.
The polymeric matrix
The polymer matrix fulfills the function of keeping the carbon fibers together and in a fixed position; it also shapes the part being made.This almost always consists of a heat-cured epoxy resin, although there are cases where air-cured resins or some thermoplastic or other polymer are used instead.
In the part manufacturing process, epoxy resin can be included in different ways. In some cases, carbon fiber sheets are already soaked in the resin before they are stacked on top of each other; in other cases, layers of uncured resin are laid, followed by a sheet of carbon fiber, then another layer of resin, and so on.
carbon fibers
Carbon fiber manufacturing process
The manufacturing process for carbon fibers is very ingenious. In essence, it consists of first manufacturing and spinning a synthetic polymeric fiber, that is, a plastic. This can be prepared in the form of fibers, either by melting an already synthesized plastic and then stretching it while it is still hot, or by pulling it as it polymerizes. In any case, the end result is a polymer thread made up of chains with thousands of carbon atoms, plus hydrogen, oxygen, and possibly some other element.
Once the basic structure of the fiber is obtained, the next step is the carbonization of the material, that is, all the other atoms of the structure are eliminated. This is generally achieved by heating the synthetic fiber bobbins to a high temperature, either under vacuum or in an inert atmosphere (ie in the absence of oxygen).
The manufacturing process for these fibers varies greatly from one manufacturer to another. The quality and the chemical and mechanical properties depend to a great extent on the method of synthesis and manufacture, in addition to the way in which the fibers are interwoven when preparing the sheets that will later form the composite. For this reason, carbon fiber composites can be found in different presentations and with very varied price ranges.
Laminate of carbon fibers
Carbon fibers can be introduced into the plastic matrix in the form of sheets containing unidirectional fibers, which are strategically oriented to reinforce the final piece in certain directions. The mechanical resistance of the fibers occurs fundamentally along its axis, so if you want to manufacture a part that is resistant to bending in different directions, fibers that run through the part in said directions must necessarily be introduced into the material. .
The latter is generally accomplished in one of two ways. The first, which is the least expensive, is to take sheets in which the fibers are all oriented in the same direction and stack them in different orientations. A very common and effective selection is to stack three sheets placed at angles of 0°, +60° and -60° to each other. This setup allows for relatively uniform strength in all directions with a minimum of carbon fiber layups.
Another very common option, although much more expensive, is to use sheets of carbon fibers woven perpendicularly, that is, in the same way that threads are woven to make a cloth. Containing fibers in two perpendicular directions already strengthens the material in two directions, but the weave adds the great benefit of drastically reducing the tendency of the sheets to separate from each other when the material is subjected to tension and flexing, which is a very common type of failure in this type of laminated materials.
Manufacture of parts with high strength-to-weight ratio CFRP compounds ;
As mentioned before, the parts are made by laminating the carbon fibers interspersed with some type of resin, but the general shape of the part is given using molds. Indeed, the manufacturing process consists of starting with a layer of resin on the internal surface of the mold, then a sheet of carbon fiber is placed that will be visible from the outside, then another layer of resin and the process is repeated.
In the case of the manufacture of parts that do not require particularly high forces, it is usually enough to press the molds while the resin cures, and in some cases it is also usually heated. However, when it comes to critical parts that must have the maximum possible resistance, such as parts of the fuselage of an aircraft or the wings of a Formula 1 car, the parts need to be subjected to vacuum to eliminate any possible bubbles in the structure. that may affect its performance.
In addition, in these cases the pieces are also usually annealed in an autoclave to cure the resin more quickly. This requirement makes the manufacture of carbon fiber parts very expensive; That’s not to mention that carbon fiber sheets are already considerably expensive.
This disadvantage, as well as some others associated with the conductivity of the material and the multiple failure modes that are difficult to model during the part design stages, mean that CFRP composites cannot be used to their full potential in many key applications. An example of this was seen when SpaceX abandoned its intention to build its next flagship spacecraft, the Starship, out of carbon fiber. It was simply too expensive and impractical to build an autoclave large enough to build the various components of the spacecraft, so they decided to use stainless steel instead, which is an unorthodox choice in the aerospace industry.
Properties of CFRP Composites
There are many unique properties of CFRP composites that are exploited in a variety of applications. Some of them are:
- It is a very light and very resistant material. It has a much higher strength-to-weight ratio than steel and even titanium.
- They have a very high modulus of elasticity-weight ratio, also higher than any metal.
- It is a material with a high resistance to fatigue.
- Both the polymeric matrix and the carbon fibers it contains are chemically inert, which gives CFRP composites very good resistance to corrosion.
- Its coefficient of thermal expansion is very low, which means that parts made of CFRP composites suffer very little distortion when heated or cooled.
- They have electrical conductivity. Graphite is a very good conductor and carbon fibers are essentially graphite, so compounds that contain them conduct electricity, particularly in the direction of the fibers. Depending on the application, this can be both good and bad.
In addition to these properties, CFRP composites also possess some additional properties that can be disadvantageous depending on the particular application:
- They are sensitive to ultraviolet (UV) light. UV light is capable of promoting a wide variety of chemical reactions by free radicals that degrade both most polymer resins and carbon fibers, destroying their mechanical properties. This is usually solved with a layer of paint that absorbs the radiation before it reaches the compound.
- Generally speaking, CFRP composites have low impact resistance.
- In terms of material failure, when CFRP composites are pushed to the limit of their strength, failure is often catastrophic because the carbon fibers are brittle. Failure modes include delamination (when sheets of fibers separate from each other) and fiber rupture.
The properties of CFRP composites are anisotropic.
It should be noted that most of the aforementioned properties of CFRP composites are anisotropic, which means that they are not uniform throughout the material and that they depend on the direction in which they are measured. This is a consequence of the fact that they are made up of ordered fibers that follow well-defined directions. Consequently, the characteristics of the material along these directions are very different from the characteristics along different directions.
For example, the tensile modulus of a CFRP composite with 70% carbon fibers in an epoxy resin has a value of only 10.3 GPa in the direction perpendicular to the fibers, while in the axial or longitudinal direction the same module is worth 181 GPa. The difference in tensile or tensile strength is even more dramatic, presenting a value of 40 MPa in the direction perpendicular to the fibers while in the longitudinal direction it is 1,500 MPa, almost 40 times higher. Finally, the expansion coefficient of this compound is 112.5 times lower along the fibers than in the perpendicular direction.
Common Applications of CFRP Composites
Despite CFRP composites being used in a host of high-end products (because it is a much more expensive material than most other options), CFRP composites are used primarily in four industries:
in the aerospace industry
The first time these compounds were used in aircraft manufacturing was in the 1950s, and their use in industry has only increased. Boeing’s 767 and 777 airliner models contain 3% and 7% CFRP compounds, respectively. In those cases they were used in some structural components. On the other hand, in the case of the new Boeing 787 Dreamliner model, the entire fuselage and wings are made of carbon fiber and this material represents 50% of the weight and 80% of the volume of said aircraft; this trend is also observed with other aircraft manufacturers.
On the other hand, despite the fact that SpaceX abandoned carbon fiber for its Starship, another private aerospace company called Rocket Lab has just announced the construction of its new rocket, the Neutron, which will be a reusable rocket made entirely of carbon fiber.
In the automobile industry
For years the world’s fastest race cars have been built using carbon fiber. This is not only part of the exterior, being the main material that forms the bodywork and the wings that keep the cars glued to the ground as they accelerate, but also in the chassis. In fact, between 60% and 70% of the structural weight of a McLaren Formula 1 car is made up of carbon fiber (this is not counting the engine, wheels and transmission).
In the case of cars for private use, only the highest-end cars such as luxury sports cars use carbon fiber in some part of their bodywork or structure.
Naval industry
Both their low weight and high corrosion resistance make CFRP composites ideal for building light-duty boats and super-speed boats. However, today they are being used more and more in the construction of larger vessels, including yachts and ships for professional use.
In addition to the chemical resistance that requires less maintenance, weight savings is one of the main reasons why this material is penetrating this industry, replacing other options such as aluminum, steel, and even other polymeric compounds such as the fiberglass.
In highly competitive sports
One of the most common and visible applications of carbon fiber in sports is in the construction of the frames of high-performance bicycles. No matter what branch of cycling it is, whether mountain biking, downhill or road bikes for the Tour de France, the best bikes are made almost entirely of carbon fiber.
On the other hand, carbon fiber is also ubiquitous in thin structural elements that must be very resistant such as high-end golf clubs, competition fishing rods, tennis rackets and even table tennis rackets. or table tennis.
References
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