Advantages CFRP vs. stainless steel
In mechanical engineering, lightweight design is called for in many areas. The physical reason often lies in masses that need to be moved or accelerated, for example an oscillating lever, a rotating shaft or a structure that needs to be moved. Carbon fiber composite materials score with excellent lightweight design potential, thanks to low specific weight and low inertia, combined with a high degree of specific stiffness and strength.
The density of CFRP is approximately 20 % of the density of steel and 57 % of the density of aluminum. This allows us to make light, and even very light components, which can be powered by smaller drives or in some cases no drive at all. The associated energy savings lead to significantly reduced operat-ing costs. In addition, the deployment of CFRP components lead to reduced wear within the machine, thus driving down mainte-nance costs and providing additional savings. In terms of investment costs, smaller-sized drives and bearings have a cost-cutting effect. In the mid-end segment of mechanical engi-neering, these advantages can be a key differ-entiator in the calculation of operating costs and make all the difference when dealing with competition. In the high-end segment, differ-entiation is created through a qualitatively and quantitatively superior machine performance, with dramatically reduced masses which allow for faster acceleration and braking times, very fast emergency stops and shorter pro-duction cycles through vibration reduction.
In applications where jerk, i. e. the rate of change of acceleration over time, is of essen-tial interest to the design engineer to achieve superior machine performance, there is no substitute for carbon composite machine components.
The specific stiffness, i. e. the ratio of modulus of elasticity to density, is the essential ruler with which to measure the lightweight design properties of a material and is therefore often called weight-saving potential. Specific stiffness can be given as unit of length, e. g. kilometers. This value is prac-tically the same for all metals such as steel and aluminum, but also for titanium. By contrast, we can attain a specific stiffness ten times that of metal with carbon fiber compos-ite materials, so that we can have properties tailor-made to meet the requirements of the application.The great weight-savings of carbon composite materials can be put to use, e. g. to create smaller dimensions and/or cross-sections. This helps when tight space requirements, for example for shafts or rollers, can be fulfilled with smaller diameters. In addition, the high degree of specific stiffness means improved deformation properties and/or less distor-tion under load when compared with machine components made of metal structural materials. This makes it possible to design larger, unsupported lengths of equal diameter and makes the installation of intermediate bearings superfluous.
The specific strength, i. e. the ratio of tensile strength to density, is the second ruler with which to measure the weight-saving potential of a material. The specific strength can only be varied by a factor of about two among metals such as steel, magnesium, aluminum or even titanium. By contrast, we can set the specific stiffness of CFRP within a large bandwidth to be three to ten times that of metals. In this way, the design can envisage components with the same or a higher degree of strength, e.g. tie rods or structures, pressure tanks or hydraulic cylinders. In addition, the high static and dynamic fatigue strength of the material makes durable components possible and offers enormous design advantages for highly dynamic applications, such as oscillat-ing levers, drive shafts or flywheels.
The chemical resistance of fiber composites depends foremost on the matrix materials used. For this reason, the design department has many different resin systems and thermoplastic materials at their disposal to adapt the chemical and thermal resistance of a component to various environments. It is in this context that duro-plastic fiber composites with special, highly resistant matrix mate-rials have become very popular in process technology and plant engineering. The dense resin matrix on the component surface works as a pro-tective layer against chemicals and other aggressive substances. Of course even fiber-reinforced plastics are not completely immune to the material's natural aging process. Because of time and con-centration dependent diffusion processes of the surrounding sub-stances, the fiber composite material will absorb moisture and thus lost some of its mechanical properties. However, these processes are very slow and need to be measured in years, even when deal-ing with highly aggressive substances. Due to their corrosion resis-tance, specially created composite materials are very well suited to replace stainless steel or other metals.
Fiber composites, especially carbon fiber composite materials, are considered comparably ex-pensive materials. And this assumption holds true when looking only at the cost per kilogram of material, but that is decidedly shortsighted, as it fails to take the low material density into account. A more meaningful comparison of the price-specific strength of various materials in an application setting shows that carbon fiber composite materials can compete with estab-lished metal construction materials. Reliable cost comparisons need to take a detailed look at the total cost of ownership (TCO) of each application, and it is here that the advantages which are typical of carbon composite materials, such as the elimination of an intermediate bearing on longer drive shafts, can be taken into account.