Carbon fibres: mechanical properties and nanostructure

Among the variety of reinforcing fibres, the mechanical properties of carbon fibres are superior to any other material, in particular with respect to strength, stiffness, weight and temperature behaviour. These unique mechanical properties are attributed to the highly anisotropic nature of the graphitic crystallites, their size and their orientation distribution. These graphitic crystallites, the basic structural units, with a size in the order of some nanometers, build up a one-dimensionally oriented structure, which leads to a strength higher than any steel, a specific weight of about two third of aluminium, a Young`s modulus, which is approaching the one of diamond and a melting point nearly the same as tungsten. Therefore the potential of carbon fibres are preferentially technical applications, which demand low weight and high temperature. It is only natural that carbon fibres are a primary choice for reinforcing fibres in aerospace applications, but are also widely used for sporting tools. Different to the prevailing opinion, new results showed that these fibres exhibit a distinct non-linearity in the stress- strain curve, i.e. the Young`s modulus increases with load up to more than 30 percent. Furthermore, a distinct creep could occur, which starts in vacuum even at relatively low temperatures of 1400 to 1500 degrees C. The structural origin of this mechanical observations is completely unclear at the moment. The aim of this project is thus to precisely determine the mechanical properties of carbon fibres, to investigate the influence of different production routes (PAN-based, MPP-pitch based and rayon based carbon fibres) and to relate these very different mechanical properties to the structure using X-ray investigation methods (SAXS and WAXD). In-situ measurements with simultaneous loading, heating and structural characterisation are proposed using tows in the laboratory equipment and single fibres in the synchrotron radion source. The latter enables the investigation of the nanoscopic structure with a position resolution of less than a micron and a time resolution in the range of seconds. This allows to trace local as well as fast structural changes.

Among the variety of reinforcing fibres, the mechanical properties of carbon fibres are superior to any other material, in particular with respect to strength, stiffness, weight and temperature behaviour. These unique mechanical properties are attributed to the highly anisotropic nature of crystallites consisting of graphene sheets, their size and their orientation distribution. These crystallites, the basic structural units, with a size in the order of some nanometers, build up a one-dimensionally oriented structure, which leads to a strength higher than any steel, a specific weight of about two third of aluminium, a Young`s modulus, which is approaching the one of diamond and a melting point nearly the same as tungsten. Therefore the potential of carbon fibres are preferentially technical applications, which demand low weight and high temperature. It is only natural that carbon fibres are a primary choice for reinforcing fibres in aerospace applications, but are also widely used for sporting tools. However, the relation of the mechanical properties to the structure at the nanometer level is mainly unknown, in particular concerning interface properties, non-linearity in stress-strain curves, and dynamics of structural change during high loading, high temperatures and long term loading. The project adressed to these open question and was able to solve a number of them: Using a 100 nm wide X-ray microfocus in a synchrotron radiation source, the structure and structural change during bending of single carbon fibres revealed buckling of the crystallites at the nanometer scale, published in Physical Review Letters. The investigation of structure and structural change of carbon fibres under load measured in-situ with a high brilliance synchrotron radiation was awarded with the Schunk Kohlenstofftechnik Price. In further publications the structure of carbon fibres was investigated by neutrons and by combined Raman spectroscopy and X-ray scattering, in which a breakdown of the classical linear relationship of the crystallite size determined from these two methods was observed towards a crystalliite size below one nanometer. The mechanical behaviour of carbon fibres such as nonlinearity, interface properties in composites or long term behaviour were investigated. It was shown that at high loads and high temperatures the structure of carbon fibers changes considerably, but can be stabilized by a previous high temperature treatment without load. The project succeeded in most of the adressed points and led the foundation for determining the structure under combined load and at high temperatures. First results were already published, but still investigations in this field have to be carried out.