What is Carbon Fiber?
Carbon fiber is composed of carbon atoms bonded together to form a long chain. The fibers are extremely stiff, strong, and light, and are used in many processes to create excellent building materials. Carbon fiber material comes in a variety of "raw" building-blocks, including yarns, uni-directional, weaves, braids, and several others, which are in turn used to create composite parts.
Plain Carbon Fiber Weave
Carbon Fiber Twill Weave
Within each of these categories are many sub-categories of further refinement. For example, different types of
carbon fiber weaves result in different properties for the composite part, both in fabrication, as well as final product. In order to create a composite part, the carbon fibers, which are stiff in tension and compression, need a stable matrix to reside in and maintain their shape. Epoxy resin is an excellent plastic with good compressive and shear properties, and is often used to form this matrix, whereby the carbon fibers provide the reinforcement. Since the epoxy is low density, one is able to create a part that is light weight, but very strong. When fabricating a composite part, a multitude of different processes can be utilized, including wet-layup, vacuum bagging, resin transfer, matched tooling, insert molding, pultrusion, and many other methods. In addition, the selection of the resin allows tailoring for specific properties.
Carbon fibers reinforcing a stable matrix of epoxy
Strength, Stiffness, and Comparisons With Other Materials
Carbon fiber is extremely strong. It is typical in engineering to measure the benefit of a material in terms of strength to weight ratio and stiffness to weight ratio, particularly in structural design, where added weight may translate into increased lifecycle costs or unsatisfactory performance. The stiffness of a material is measured by its modulus of elasticity. The modulus of carbon fiber is typically 20 msi (138 Gpa) and its ultimate tensile strength is typically 500 ksi (3.5 Gpa). High stiffness and strength carbon fiber materials are also available through specialized heat treatment processes with much higher values. Compare this with 2024-T3 Aluminum, which has a modulus of only 10 msi and ultimate tensile strength of 65 ksi, and 4130 Steel, which has a modulus of 30 msi and ultimate tensile strength of 125 ksi.
As an example, a plain-weave carbon fiber reinforced laminate has an elastic modulus of approximately 6 msi and a volumetric density of about 83 lbs/ft3. Thus the stiffness to weight for this material is 107 ft. By comparison, the density of aluminum is 169 lbs/ft3, which yields a stiffness to weight of 8.5 x 106 ft, and the density of 4130 steel is 489 lbs/ft3, which yields a stiffness to weight of 8.8 x 106 ft. Hence even a basic plain-weave carbon fiber panel has a stiffness to weight ratio 18% greater than aluminum and 14% greater than steel. When one considers the possibility of customized
carbon fiber panel stiffness through strategic laminate placement, as well as the potentially massive increase in both strength and stiffness possible with lightweight core materials, is it obvious the impact advanced
carbon fiber composites can make on a wide variety of applications.
Pros and Cons
Carbon fiber reinforced composites have several highly desirable traits that can be exploited in the design of advanced materials and systems. The two most common uses for
carbon fiber are in applications where high strength to weight and high stiffness to weight are desirable. These include aerospace, military structures, robotics, wind turbines, manufacturing fixtures, sports equipment, and many others. High toughness can be accomplished 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. Finally, in addition to the basic mechanical properties, carbon
fiber creates a unique and beautiful surface finish.
Although carbon fiber has many significant
benefits over other materials, there are also tradeoffs one must weigh
against. First, solid carbon fiber will not yield. Under load carbon fiber
bends but will not remain permanently deformed. Instead, once the ultimate
strength of the material is exceeded, carbon fiber will fail suddenly and
catastrophically. In the design process it is critical that the engineer
understand and account for this behavior, particularly in terms of design
safety factors. Carbon fiber composites are also significantly more
expensive than traditional materials. Working with carbon fiber requires a
high skill level and many intricate processes to produce high quality
building materials (for example, solid carbon sheets, sandwich laminates,
tubes, etc). Very high skill level and specialized tooling and machinery
are required to create custom-fabricated, highly optimized parts and
Carbon Fiber vs. Metals
When designing composite parts, one cannot simply compare properties of carbon fiber versus steel, aluminum, or plastic, since these materials are in general 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 in general neither homogeneous nor isotropic.
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 matter of construction of the part and the application. For instance, a foam-core sandwich has extremely high strength to weight ratio in bending, but not necessarily in compression or crush. In addition, the loading and boundary conditions for any components are unique to the structure within which they reside. Thus it is impossible for us to provide the thickness of carbon fiber plate that would replace the steel plate in your application. It is the customer's responsibility to determine the safety and suitability of any Dragonplate product for a specific purpose. This is accomplished through engineering analysis and experimental validation.
3-Point Bending: A condition where both ends of a beam are supported and a load is applied at the mid-span.
Aramid Fiber: A synthetic fiber with exceptional strength and toughness commonly used in applications where high resistance to impacts.
Axial Stress: Stress component along the longitudinal axis of a component.
Brittle Material: A material that does not yield, but instead fails suddenly when the ultimate stress is exceeded.
Carbon Fiber: A high strength, high stiffness material that when combined with a resin matrix creates a composite with exceptional mechanical properties.
CFRP: Abbreviated form of carbon-fiber reinforced plastic
Cantilever: A condition where one end of a beam is fixed and a load is applied to the opposite free end.
Composite Sandwich Core: In a composite sandwich structure, the core is a lower density material placed close to the neutral axis in order to increase the stiffness to weight ratio.
Composite material: A material created by combining two or more materials such that the final construction exploits certain properties from each. In the construction of carbon-fiber reinforced plastics, the high strength, high stiffness of the carbon fibers are combined with a low density stable matrix to create a combined material with desirable material properties.
Density: The weight of a material per unit length, area, or volume (linear density, areal density and volumetric density, respectively).
Epoxy: A polymer resin that hardens when combined with a catalyst. Epoxy is one of the most common materials used to form the matrix in carbon-fiber fabrication.
Fiberglass: A glass fiber reinforced plastic similar to carbon-fiber, but with much lower strength and stiffness, but also much lower cost.
Homogeneous: Defined as having a uniform composition throughout the material.
Isotropic: Defined as having the same properties (mechanical, electrical, thermal, etc) in all directions. Carbon-Fiber laminates are typically highly directional, having high stiffness and strength only along the longitudinal directions of the fibers.
Matrix: In a composite material the matrix comprises the stable "fill" which holds the fiber reinforcement. By itself the matrix is typically much weaker than the fibers, particularly in tension. The matrix's primary function is to transfer the loads between the fibers within the composite material.
Modulus of Elasticity: A measure of the stiffness of a material, defined as the axial stress divided by the axial strain. The higher the modulus, the stiffer the material (i.e. the greater the stress necessary to cause deformation). Also known as Young's Modulus.
Poisson's Ratio: When a material is stretched due to an applied load, it elongates in the axial direction and contracts in the perpendicular, or transverse, direction. The poisson's ratio is defined as the axial strain divided by the transverse strain.
Quasi-Isotropic: In a composite material, the placement of individual laminates, or plies, so that the fibers are directed along multiple directions. The result is a material with approximate isotropy in mechanical properties.
Polyacrylonitrile (PAN): A raw material commonly used to make carbon-fiber.
Pultrusion: A process which creates an extremely stiff rod, tube, or other cross-section whereby all of the carbon fibers are aligned along the longitudinal axis.
Reinforced carbon-carbon (RCC): Carbon-reinforced graphite composite used in high temperature applications.
Shear Modulus: Defined as the shear stress divided by the shear strain. Also known as the Modulus of Rigidity.
Shear Stress: The component of stress parallel to the cross-sectional face of a material.
Shear Strain: Deformation of a material caused by a shear stress. A shear strain causes skewing of a material element.
Strain: The deformation of a material caused by an applied load. The strain is defined as the change in length divided by the original length of a material.
Stress: Defined as the force per unit area. The stresses within a composite are a function of the material properties of the materials, the geometry, and the loading condition.
Ultimate Tensile Strength: The maximum stress a material can withstand in tension, above which failure will occur.
Veneer: A thin, highly flexible sheet of carbon-fiber.
Yield Strength: The stress above which a material with remain permanently deformed even when the applied load is removed.