Short-Fiber Reinforced CMCS: Potentials and Problems

Short fibre reinforced CMCs have a tremendous market potential in future applications which will be demonstrated as an example for brake discs in automotive industry. Market penetration in serial production is strongly depending on the final costs. Therefore, the usable technologies are cost driven and determine finally the properties of the products. CMC brake discs are lifetime parts which have to fulfill the safety requirements. As known from literature (1), the CMCs with long and short fibre reinforcement show a high residual strength after 106 cycles on a level of 60 % to 80 % of the initial value. Therefore, mechanical quality assurance can be performed at ambient temperature. One of the main problems of these materials are the inhomogeneities caused by the fibre distribution, the different fibre lengths and the fibre sticks, which result from the compounding technique itself. These sticks and their orientation have a tremendous effect on the mechanical properties and the failure behaviour of the short fibre reinforced CMCs. Furthermore, a precalculation of the mechanical behaviour via FEM is more or less impossible. This is mainly caused by the lack of testing standards, because standards available for endless fibre reinforced CMCs cannot be directly applied on the short fibre reinforced ones. The problems of mechanical testing will be discussed in detail for flexural tests as well as tensile tests of short fibre reinforced CMCs. One of the main problems in testing short fibre reinforced CMCs is the selection of a representative sample cross-section in order to get mechanical properties independent of the size and geometry of the testing samples. The influence of the cross-sections of the testing samples on the mechanical properties will be shown and discussed in more detail. Furthermore, the mechanical properties of samples with identical cross-sections will be discussed from the perspective of the applied standard testing method.