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.
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. A few examples of epoxy selection may include longer or shorter cures times, ultra-violent resistance, high temperature formulations, flame retardancy, and increased fracture toughness using additives.
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. One example of this is in the design of beams for tailored stiffness along orthogonal axes. Element 6 Composites has developed patent pending methods for the fabrication of carbon-fiber tubes for optimum stiffness along each bending axis, with order of magnitude differences in rigidity. Such tubes are similar to I-Beams in their resistance to bending, yet retain the high torsional stiffness found in a tube, as well as ease of assembly.
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 undergo plastic deformation (i.e. yielding). Under load carbon fiber bends, but will not remain in a permanently deformed state once the load is removed. 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. An example of this is the design of parts with areas of localized stress concentrations (holes, sharp corners, etc). It is important to consider fiber direction near a hole for proper distribution of stresses because, unlike metals, where localized yielding near a hole causes a redistribution of the stresses, carbon-fiber will almost invariably fail locally by forming a crack. In such a case, the carbon-fiber part may exhibit higher stiffness and even higher strength than the comparable metal part; however, the existence of stress concentrations must be a primary focus of the engineering design, else a well-designed part may fail prematurely.
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 assemblies.
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 strength to weight ratio (as well as stiffness to weight ratio) of a carbon fiber part is much higher than either steel or non-reinforced 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 to provide the thickness of carbon fiber plate that would directly replace the steel plate in a given application without careful consideration of all design factors. This is accomplished through engineering analysis and experimental validation.
What is a Sandwich Structure?
A composite sandwich combines the superior strength and stiffness properties of carbon-fiber (or another fiber reinforced plastic, such as glass or aramid) with a lower density core material. By strategically combining these materials, one is able to create a final product with a much higher bending stiffness to weight ratio than with either material alone. For applications where weight is critical, carbon-fiber sandwich composites may be the right fit.
A composite sandwich structure is mechanically equivalent to a homogeneous I-Beam construction in bending. This equivalency is shown in Figure 1.
Referring to the picture of the sandwich structure, at the center of the beam (assuming symmetry) lies the neutral axis, which is where the internal axial stress equals zero. In Figure 2, moving from bottom to top in the diagram, the internal stresses switch from compressive to tensile. Bending stiffness is proportional to the cross-sectional moment of inertia, as well as the material modulus of elasticity. Thus for maximum bending stiffness, one should place an extremely stiff material as far from the neutral axis as possible. By placing carbon fiber furthest from the neutral axis, and filling the remaining volume with a lower density material, such as wood, foam, or honeycomb, the result is a composite sandwich material with high stiffness to weight ratio.
As an example, FEA analyses comparing a Dragonplate birch plywood sandwich laminate with a solid carbon fiber are shown in Figure 3. These calculations show the deflections of a cantilever beam with a load placed at the end. In the figure, 3/16” Dragonplate sandwich sheet 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 plywood core. As the thickness increases, this disparity becomes even greater due to the large weight savings from the core. For example, if instead the core material is foam or honeycomb, the disparity in deflection for equivalent weight panels can be several orders of magnitude. In a similar light, one can replace a solid carbon structure with a lighter one of equivalent strength and stiffness. This latter case is common for currently operational structures that one needs to lighten, yet retain the original functionality.
Through a grant from NYSTAR, Element 6 Composites has worked with Cornell University to test Dragonplate carbon-fiber composite materials. Figure 4 shows a piece of Dragonplate undergoing a 3-point bending test. In addition to bending, the Cornell laboratory performed tensile, shear, and fracture testing.