Our fiber composites are made of carbon fibers and a matrix, which consists of special resin systems or thermoplastic materials. Both components come together to give the material its specific and unusual characteris-tics. In doing so, fiber and matrix each have different tasks. The strength and corrosion resistance, as well as the light weight of carbon fiber is only possible as a composite material.
Carbon Fibers
Carbon fiber, the basis of the material com-pound, has truly fascinating characteristics: It has several times the stiffness and tensile strength of steel while only having a fraction of the specific weight. The fibers conduct electricity, have a medium to very high ther-mal conductivity and a negative thermal ex-pansion. There are different kinds of carbon fibers that we put to specific uses according to their characteristics and their price. In ad-dition, we use glass and aramid fibers and/or hybrid mixtures of all the mentioned fibers as supplements to achieve the desired material properties.
A bundle of carbon fibers is similar to a rope in that it is highly flexible and does not retain its shape; it is the task of the matrix to bed, support, form and protect the fiber so as to allow for load transmission between the fibers and the different fiber layers. When selecting the matrix material, we draw from a series of epoxy resins with a diverse range of charac-teristics. In addition, our group of companies is pioneering the development of industrial processes for the production of thermoplastic matrix materials with our CCM production.
United We Are Strong
Since carbon fibers have their outstanding characteristics only in the fiber’s longitudinal axis, so-called anisotropic, the fibers within the composite are arranged according to the load bearing aspects and take on the task of force transmission. When subjected to com-pressive loads, they are supported by the matrix material. Thus carbon fiber laminates can possess much higher stiffnesses (young’s modulus) in fiber-direction than, for example, steel. The transverse direction, however, has lower values because the matrix material offers comparatively little strength. Compared with conventional materials, shear deforma-tion and ovalization play a much larger role in the total deformation of fiber components. It is however possible to obtain a perfect ratio between modulus of elasticity and modu-lus of shear by layering the fibers in various directions to form a laminate structure, e. g. by using a filament winding method. In this way, we can use a specific arrangement of fibers to create tailor-made components with specific functions. This includes things such as adjustable thermal expansion or damp-ing properties, as well as a high strength or flexibility. When designing components with anisotropic materials, often underestimated challenges are the load transmission into the fiber-rein-forced machine component or the joining of components into structures. The challenge in this case – and this is especially true by higher loading – is how to create a lasting connection to the load-bearing fibers in the composite. We have the design and produc-tion engineering know-how needed to imple-ment various methods, such as gluing, bolt-ing, riveting or using press-fitted assemblies.
Analogous to Nature
When designing carbon composite compo-nents, we take the lightweight design prin-ciples provided by nature as a guideline: mini-mum use of materials, “form follows function”, establishing shape and form through constant tension and the targeted reinforcement of a component using fibers. Bamboo is a good example of what we mean when we say that we want to emulate nature’s design bionics for lightweight designs using fiber compos-ites. Bamboo stems are essentially composite structures where vascular bundles and fibers are embedded in the tissue. The shear stress at the tissue’s boundary areas help dampen the movement of swinging bamboo stems and the fibers embedded in the tissue pro-tect against buckling caused by pressure and bending loads. In addition, bamboo is hol-low, making it extremely elastic and light in weight, and has inner support discs that pro-vide strength perpendicular to the direction of growth.
Long Service Life
Correctly dimensioned components made of fiber-reinforced plastics have an incredibly long service life, even if subjected to dynamic loading. When fractures caused by material fatigue do occur, the load cycle counts are typically much higher than with comparable components made of metal materials. For the most part, this is due to the generally high strength of a material made up of thin fibers (Griffith’s material strength equation) and in the high amount of inner surfaces found in fiber composites which hinder micro-cracks in a way that only allows them to grow in size once more energy is applied. In fatigue tests on CFRP components, you will often find that load transmission elements made of metal have failed due to material fatigue long before the CFRP carrier structure has been measur-ably, let alone visibly damaged.
Benign Failure Behavior
A further strength of fiber-reinforced plas-tics is that it has a tendency to have a benign damage behavior as well as the possibility of designing a specific kind of fracture be-havior into the material. This is often put to use in crash elements and energy absorbing structures, as used e. g. in Formula One rac-ing cars or in bullet-proof vests. Today, even impact damage can be controlled by using correspondingly designed laminates and protective coating. With laminates the fiber structure helps absorb the damage, because the undamaged fibers help take up the impact load so that tears cannot spread unhindered.
