What is carbon fiber? It’s a lightweight, incredibly strong material revolutionizing industries worldwide. At WHAT.EDU.VN, we break down the science, applications, and benefits of this advanced composite. Discover how carbon fiber materials, carbon composites, and carbon filaments are shaping the future.
1. Understanding Carbon Fiber: A Comprehensive Overview
Carbon fiber is a material consisting of thin, strong crystalline filaments of carbon. Essentially, carbon atoms are bonded together in long chains. These fibers are extremely stiff, strong, and light, making them ideal for creating high-performance structural materials. Carbon fiber offers a wide range of benefits, including a high stiffness and stiffness-to-weight ratio, high tensile strength and strength-to-weight ratio, and high-temperature tolerance with special resins. Carbon fiber is increasingly used in diverse applications, making it a vital material in modern technology and engineering.
2. Visual Characteristics: What Does Carbon Fiber Look Like?
Carbon fiber typically appears as black strands or yarns, often referred to as “Tows”. It is available in various formats, including spools of tow, unidirectional formats, weaves, braids, and more. These forms are used to create composite parts. The diverse array of carbon fiber weaves can result in different properties within a composite component, enhancing its versatility and adaptability for specific applications.
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3. Material Composition: What Material is Carbon Fiber Made Of?
Carbon fiber is made from thin filaments of carbon atoms bonded together in a crystalline structure, resulting in an exceptional strength-to-weight ratio. Manufacturing carbon fiber involves complex steps that transform carbon-rich precursors into spools of fibers. The most common precursors are polyacrylonitrile (PAN), pitch, and rayon. These materials are spun into long fibers, then heated in an oxygen-free environment in a process called stabilization to prevent burning or shrinking. After stabilization, the fibers are carbonized at high temperatures (around 1000 – 3000 degrees Celsius) in an inert atmosphere like nitrogen, driving out almost all non-carbon elements and leaving behind pure carbon atoms in a crystalline structure. The fibers receive a surface treatment to enhance adhesion to matrix materials before being wound onto spools.
4. The Creation of Carbon Fiber Composites
To create a composite part, the carbon fibers, which are stiff in tension and compression, need support within a stable matrix to maintain the part’s shape. Epoxy resin, known for its excellent physical properties, is often used as this matrix, with carbon fibers providing strength. Because both epoxy and carbon fiber are low-density, the resulting part is lightweight and very strong. Fabricating a composite part involves processes like wet-layup, vacuum bagging, resin transfer, matched tooling, insert molding, and pultrusion. The selection of the resin also allows for tailoring specific properties, such as elevated temperature or chemical resistance.
5. Key Properties of Carbon Fiber
Carbon fiber is known for being extremely stiff, strong, and light. Engineers often compare materials by their strength-to-weight ratio and stiffness-to-weight ratio, especially in structural design, where added weight can increase lifecycle costs or negatively affect performance. Understanding these properties helps in selecting the best material for specific applications.
6. Modulus of Elasticity and Tensile Strength: Understanding the Numbers
The stiffness of a material is measured by its modulus of elasticity. This is similar to Spring Rate, a metric used to describe the stiffness of springs. It’s calculated by dividing the change in stress by the change in strain. The modulus of carbon fiber is typically 33 msi (228 Gpa), and its ultimate tensile strength is around 500 ksi (3.5 Gpa). These values highlight carbon fiber’s superior mechanical performance.
7. Mechanical Properties of Carbon Fiber Composite Parts
The stiffness and strength along any specific axis in a carbon fiber composite part depend not only on fiber and resin mechanical properties but also on fiber placement, orientation, and fiber/resin ratio (typically around 50/50). A typical stiffness value for a carbon fiber composite plate would be 10 msi, with a strength of 90 ksi. These mechanical properties are crucial for designing and manufacturing reliable composite parts.
8. Stiffness-To-Weight Ratio: Why Carbon Fiber Excels
Plain-weave carbon fiber reinforced laminate has an elastic modulus of approximately 8 msi and a volumetric density of about 0.05 lbs./in3. The stiffness-to-weight ratio for this material is 160 x 106. In comparison, aluminum has a density of 0.10 lbs./in3, yielding a stiffness-to-weight ratio of 100 x 106, and steel has a density of 0.30 lbs./in3, also yielding a stiffness-to-weight ratio of 100 x 106. This means even a basic plain-weave carbon fiber panel has a stiffness-to-weight ratio 60% greater than aluminum or steel.
| Material | Elastic Modulus | Volumetric Density | Stiffness-to-Weight |
|---|---|---|---|
| Plain-Weave Carbon Fiber Composite | 8 msi | 0.05 lbs./in3 | 160 x 1066 |
| 6061-T6 Aluminum | 10 msi | 0.10 lbs./in3 | 100 x 106 |
| Steel | 30 msi | 0.30 lbs./in3 | 100 x 106 |
9. Carbon Fiber vs. Aluminum and Steel: A Comparative Analysis
When compared to other common materials, carbon fiber stands out. For example, 2024-T3 Aluminum has a modulus of 10 msi and an ultimate tensile strength of 65 ksi, while 4130 Steel has a modulus of 30 msi and an ultimate tensile strength of 125 ksi. Carbon fiber’s unique combination of strength and lightweight properties makes it a superior choice for many applications.
10. Yield Strength in Carbon Fiber vs. Steel
Steel will permanently deform at a stress level below its ultimate tensile strength, known as yield strength. Carbon fiber, on the other hand, will not permanently deform below its ultimate tensile strength, so it effectively has no yield strength. This behavior is crucial in designing components where maintaining shape under high stress is essential.
11. High Modulus Carbon Fiber: Pushing the Boundaries
Higher-stiffness carbon fibers are available through specialized heat treatment processes. Using prepreg and high-modulus or ultra-high-modulus carbon fiber prepregs yields substantially higher stiffness-to-weight ratios. For applications requiring maximum stiffness, 110 msi ultra-high modulus carbon fiber can be used. This specialized pitch-based carbon fiber has a bending stiffness over three times that of a standard modulus prepreg panel (about 25 msi). Customized carbon fiber panel stiffness through strategic laminate placement allows for fabricating panels (or other cross-sections, like carbon fiber tubes) with bending stiffness on the order of 50 msi.
12. Potential Performance of Carbon Fiber Composites
All zero-degree oriented unidirectional ultra-high modulus coupon samples have tensile stiffness of more than 70 msi, over twice the stiffness of steel, yet only half the weight of aluminum. The stiffness-to-weight ratio of this material is then over 10 times that of either steel or aluminum. Pairing these materials with lightweight honeycomb and foam cores further enhances the impact of advanced carbon fiber composites, making them ideal for high-performance applications.
13. What is a Carbon Fiber Composite Sandwich Structure?
A composite sandwich combines carbon fiber’s superior strength and stiffness properties with a lower-density core material. Dragonplate sandwich sheets laminate a thin carbon-fiber skin over a foam, honeycomb, balsa, or birch plywood core. By combining these materials, the final product has a much higher stiffness-to-weight ratio. Carbon-fiber sandwich sheets are an excellent choice for applications where weight is critical, providing a lightweight yet strong solution.
14. Composite Sandwich Stresses vs. Stresses in an I-Beam
A composite sandwich structure is similar to a homogeneous I-Beam construction in bending. At the center of the beam (assuming symmetry) lies the neutral axis, where the internal axial stress equals zero. Moving from bottom to top, the internal stresses switch from compressive to tensile. Bending stiffness is proportional to the cross-sectional moment of inertia and the material modulus of elasticity. Placing an extremely stiff material as far from the neutral axis as possible maximizes bending stiffness. By placing carbon fiber furthest from the neutral axis and filling the remaining volume with a lower-density material, the result is a composite sandwich material with a very high stiffness-to-weight ratio.
15. Analysis of Birch Plywood Core Laminate vs. a Solid Carbon Fiber Layup
FEA analyses comparing stress levels in a sandwich laminate vs. solid carbon fiber show the deflections of a cantilever beam with a load placed at the end. A 3/16″ birch plywood core laminate is shown next to a solid carbon fiber layup of equal weight. Due to the reduced thickness of the solid carbon beam, it deflects significantly more than the equivalent beam made using a core material. As the thickness increases, this disparity becomes even greater due to the large weight savings from the core. Similarly, one can replace a solid carbon structure with a lighter one of equivalent strength and stiffness made from any of the core options previously mentioned.
16. Strengths and Weaknesses of the Various Carbon Fiber Sandwich Cores
Each core used in carbon fiber sandwich structures has its strengths and weaknesses. Typically, the driving factors are the compressive and shear strength of the core. If high compressive strength (and hence high crush resistance) is required, the core will likely need to be of higher density, such as high-density foam or birch plywood. However, if the application requires the absolute lowest weight composite possible and the stresses are relatively small (i.e., low load, high stiffness application), then an extremely lightweight foam or honeycomb core may be the best selection.
Some cores offer better moisture resistance (closed-cell foam), some have better machinability (plywood), and others have a high compressive strength-to-weight ratio (balsa). Engineers must understand these trade-offs during the design process to maximize the potential of cored composites. For weight-critical applications, there is often no other option that even comes close to the potential strength and stiffness-to-weight ratios of carbon fiber sandwich core laminates.
| COMPARISON CRITERIA | |
|---|---|
| PRODUCTS | Stiffness to Weight |
| Solid Carbon Fiber | GOOD |
| High Modulus Solid Carbon Fiber | BETTER |
| Birch Core | BETTER |
| Balsa Core | BETTER |
| Nomex Honeycomb Core | BEST |
| Depron Foam Core | BETTER |
| Airex Foam Core | BEST |
| Divinycell Foam Core | BETTER |
| Last-A-Foam Core | BETTER |
17. Pros and Cons of Carbon Fiber Composites
Carbon fiber reinforced composites have several highly desirable traits that can be exploited in the design of advanced materials and components, such as solid carbon sheets, carbon fiber sandwich laminates, and carbon tubes. The two most common uses for carbon fiber are in applications where a high strength-to-weight and high stiffness-to-weight are desirable. These include aerospace, military structures, robotics, wind turbines, manufacturing fixtures, and sports equipment.
Pros:
- Strength-to-weight ratio
- Stiffness-to-weight ratio
- Electrical conductivity
- Thermal conductivity
- Carbon fiber will not plastically yield
- Unique and beautiful surface finish
High toughness can be achieved when combined with other materials. Certain applications also exploit carbon fiber’s electrical conductivity, as well as high thermal conductivity in the case of specialized carbon fiber.
Cons:
- Once the ultimate strength is exceeded, carbon fiber will fail suddenly and catastrophically.
- Carbon fiber materials are significantly more expensive than traditional materials.
- Working with carbon fiber requires a high skill level and specialized tooling to produce high-quality structural materials.
- Carbon fiber will not yield. Under load, carbon fiber bends but will not remain permanently deformed. Once the ultimate strength of the material is exceeded, carbon fiber will fail suddenly and catastrophically. It is critical that engineers understand and account for this behavior, particularly in terms of design safety factors.
18. Carbon Fiber vs. Metals: Understanding the Differences
When designing composite parts, one cannot simply compare the properties of carbon fiber versus steel, aluminum, or plastic, since these materials are generally homogeneous (properties are the same at all points in the part) and have isotropic properties throughout (properties are the same along all axes). By comparison, in a carbon fiber part, the strength resides along the axis of the fibers, and thus fiber properties and orientation greatly impact mechanical properties. Carbon fiber parts are generally neither homogeneous nor isotropic.
19. Carbon Fiber Engineering for Custom Applications
The properties of a carbon fiber part are close to that of steel, and the weight is close to that of plastic. Thus, the strength-to-weight ratio (as well as stiffness-to-weight ratio) of a carbon fiber part is much higher than either steel or plastic. The specific details depend on the construction of the part and the application. For instance, a foam-core sandwich has an extremely high strength-to-weight ratio in bending but not necessarily in compression or crush. Additionally, any component’s loading and boundary conditions are unique to the structure within which they reside. It is the customer’s responsibility to determine the safety and suitability of any product for a specific purpose through engineering analysis and experimental validation.
FAQ: Frequently Asked Questions About Carbon Fiber
| Question | Answer |
|---|---|
| What is the primary advantage of using carbon fiber? | The primary advantage is its exceptional strength-to-weight ratio, making it ideal for applications where weight reduction and high strength are crucial, such as aerospace and automotive industries. |
| How does carbon fiber compare to steel in terms of strength? | Carbon fiber can be stronger than steel for its weight. While steel is denser, carbon fiber offers a higher strength-to-weight ratio, meaning it can provide comparable or superior strength at a fraction of the weight. |
| What are the common applications of carbon fiber? | Common applications include aerospace components, automotive parts, sports equipment (like golf clubs and bicycles), construction materials, and medical devices. Its versatility makes it suitable for various industries. |
| Is carbon fiber environmentally friendly? | The environmental impact of carbon fiber is complex. Manufacturing carbon fiber requires energy, but its use can lead to fuel savings in transportation due to its lightweight properties. Recycling carbon fiber is an evolving field with ongoing research and development. |
| Can carbon fiber be recycled? | Yes, carbon fiber can be recycled, although the process is not as widespread as recycling other materials like aluminum or steel. Recycling methods include thermal decomposition and mechanical shredding, which can recover the fibers for reuse in new products. |
| What is the cost of carbon fiber compared to other materials? | Carbon fiber is generally more expensive than materials like steel, aluminum, and plastic. The higher cost is due to the complex manufacturing processes involved. However, the benefits in performance often justify the expense for high-end applications. |
| How does moisture affect carbon fiber? | Carbon fiber itself is resistant to moisture, but the resin matrix used in carbon fiber composites can be susceptible to moisture absorption. This can affect the overall performance and longevity of the composite material, so protective coatings are often applied. |
| What are the limitations of using carbon fiber? | Limitations include its high cost, brittleness (it can fail suddenly under high stress), and the complexity of manufacturing and repairing carbon fiber components. Additionally, carbon fiber is electrically conductive, which can be a concern in some applications. |
| How is carbon fiber manufactured? | Carbon fiber is typically manufactured from precursors like polyacrylonitrile (PAN), pitch, or rayon. The process involves heating these materials to high temperatures in an oxygen-free environment to carbonize them, followed by surface treatment and sizing to improve handling and bonding. |
| What types of carbon fiber are available? | Various types of carbon fiber are available, including standard modulus, intermediate modulus, high modulus, and ultra-high modulus fibers. These differ in terms of stiffness, tensile strength, and cost, allowing engineers to select the best option for their specific application. |
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