Carbon fiber is a material valued for its combination of high strength and low weight. Its ability to endure “pressure” is not a single, fixed value, but rather depends on numerous factors and the specific type of force applied to it.
Understanding Pressure and Carbon Fiber’s Behavior
In engineering, “pressure” refers to mechanical loads a material experiences. These loads include tension, a pulling force that tries to stretch a material. Compression involves a pushing force that attempts to squeeze or shorten a material. Bending forces are a combination of both tension on one side and compression on the opposite side of a material. Shear forces apply parallel to a material’s surface, causing layers to slide past each other.
Carbon fiber composites exhibit anisotropic behavior, varying in strength depending on the direction of applied force. They demonstrate high resistance to tensile loads, which is a significant advantage in many structural designs. However, their resistance to compressive forces can be lower compared to their tensile strength. Material strength is commonly measured in units such as Pascals (Pa), Megapascals (MPa), Gigapascals (GPa) in the metric system, or pounds per square inch (PSI) in the imperial system.
Key Factors Determining Carbon Fiber Strength
The type of carbon fibers used significantly influences the composite’s overall strength. Polyacrylonitrile (PAN)-based carbon fibers are widely used in high-performance applications, offering high tensile strength and modulus. Pitch-based carbon fibers, though less common and more expensive, offer a broader range of modulus values, including ultra-high modulus fibers for extreme stiffness. Different grades, such as high modulus or high strength fibers, are selected based on the specific performance requirements of an application.
The polymer resin, or matrix, binds the carbon fibers, transfers loads, and protects them from environmental damage. Epoxy resins are frequently used in carbon fiber composites due to their strength, stiffness, and fatigue resistance, creating a strong bond with the fibers. Vinyl ester resins offer a balance of properties, combining some benefits of both epoxy and polyester resins, including improved impact absorption. Polyester resins are a cost-effective option, suitable for general applications, though offering lower performance compared to epoxies for demanding uses.
The arrangement and layering of carbon fibers within the composite dramatically affects its strength in different directions. Fiber orientation, such as unidirectional layups where fibers run in a single direction, or woven fabrics like plain weave and twill, allows engineers to tailor the material’s properties to specific stress patterns. The stacking sequence of multiple layers, or plies, further influences how the composite distributes and withstands loads. The manufacturing process also impacts strength; proper curing, minimal void content, and precise fiber alignment contribute to material integrity.
Environmental conditions can influence the long-term strength of carbon fiber composites. Factors like temperature fluctuations, moisture absorption, and exposure to certain chemicals can degrade the resin matrix or weaken the bond between the fibers and the matrix over time. These external influences can contribute to material fatigue and reduce its ability to withstand sustained or repeated pressure.
Carbon Fiber in Action: Real-World Performance
Carbon fiber’s strength-to-weight ratio makes it suitable for many high-performance applications. In aerospace, carbon fiber composites are used in aircraft components such as wings and fuselage sections. This material helps improve fuel efficiency and reduces overall aircraft weight while handling significant aerodynamic loads. Carbon fiber also reinforces pressure vessels for fuel storage in spacecraft.
The automotive industry utilizes carbon fiber in performance car chassis and other structural components. Its lightweight nature contributes to improved handling and fuel economy, while its strength provides enhanced safety. Carbon fiber is employed in pressure vessels designed to store compressed natural gas (CNG) or hydrogen. These tanks can hold pressures up to 4500 PSI, significantly higher than the 3000 PSI common for traditional steel tanks, due to the material’s high tensile strength.
Carbon fiber is used in sports equipment for its durability and lightweight performance. Bicycle frames made from carbon fiber can be 20-30% lighter and stiffer than metal alternatives, designed to withstand high compression loads. Tennis racquets, golf clubs, skis, and fishing rods use carbon fiber for stiffness and strength without excessive weight. Carbon fiber composites generally have a compressive strength ranging from 700 to 1,000 MPa.
When Carbon Fiber Fails: Understanding Its Limits
Despite its strength, carbon fiber composites can fail under excessive pressure through several mechanisms. Delamination, a common failure mode, involves the separation of layers within the composite. This can occur due to shear forces, impact, or manufacturing defects such as improper resin infusion, curing, or the presence of voids. The weak bond of the polymer matrix between strong fiber layers makes delamination a concern.
When subjected to compression, carbon fibers can buckle, leading to the formation of “kink bands.” This happens when fibers bend and lose stability, causing the matrix material between them to fail due to shear stress, ultimately leading to fiber breakage. Matrix cracking, where cracks form in the resin, is another failure mechanism that can occur due to excessive stress, manufacturing flaws, or thermal cycling. These matrix cracks can also initiate or contribute to delamination.
Carbon fiber composites are susceptible to localized impact damage, which can lead to sudden failure. Unlike ductile materials that deform visibly before breaking, carbon fiber tends to fail abruptly once its limits are reached. Designers account for these vulnerabilities to ensure the material’s reliable performance in its intended applications.