Development of methods for fatigue life prediction of combustors made of alloys and ceramic matrix composites (Masterarbeit)

The pre-design stage of an aircraft engine requires an overview of the main engine performance and structural properties, to enable rapid decision making on a concept. For combustion chambers, different types of analyses are used to determine the viability of a concept, e. g. finite element analysis, modal analysis, etc. Among them, fatigue analysis is a key topic due to the high stresses and particular temperature conditions within the combustion chambers. Due to the many material properties to be taken into account for fatigue analysis, extensive theories and formulas are available in the literature, making it difficult to choose an estimation method. This master thesis presents the realization of an internal tool existing at DLR, coded in Python, concerning the design of a fatigue life prediction method for an aircraft combustion chamber, in metal alloys or ceramic matrix composites. After the finite element and modal analysis already existing in the code, fatigue material properties from the literature are used, included in the Python tool, and extrapolated to have as an output a number of cycles to failure of the combustor, for different engine load points (take-off, taxi, cruise, idle, top of climb)

Abschlussbericht NGT II Keramische Bremse

Fibre reinforced ceramic composites are promising candidates for application under severe loading and environments. Previously invented for heat shields and high temperature resistant components for space vehicles, these materials were further developed to be applied as brake discs for automotive use. From the 1990s ceramic brake discs have been developed at the German Aerospace Center in detail, resulting in industrial use for automotive and, quite recently, for aeronautic application. Typically, ceramic brake discs used in automobiles consist of short carbon fibre reinforced SiC. Cut carbon fibres are combined with a polymer matrix and pressed to a disk-shaped CFRP preform. After high temperature treatment under inert atmosphere, a porous carbon-carbon preform is achieved. The final carbon fibre reinforced SiC brake disc is obtained after liquid silicon infiltration (LSI process) at high temperature. Thereby the silicon (Si) is infiltrated via capillary forces only, and a SiSiC-matrix is built up by a chemical reaction of Si and a small part of the carbon as well as by filling the remaining porosity with Si. By controlling the process parameters, especially during the CFRP preform manufacture, the microstructure can be adapted in order to provide e.g. an abrasion resistant friction surface with high volume content of SiC for high hardness and a carbon fibre containing core for enhanced toughness and for the prevention of brittle failure. In serial use for sports and luxury cars, these brake discs are well established today and their typical properties will be presented. Apart from this successful application also ceramic brake discs for trains and other high performance brake systems are under development. Due to the high mechanical loads, lightweight brake discs for trains favourably are based on a long fibre reinforcement design which usually is realized by the use of two dimensional carbon fibre fabrics which are layed up in various orientations in order to provide sufficient and homogeneous tension strength in circumferential direction as well as high intralaminar shear strength for a safe joining of the brake disc to the driveline. Different C/C-SiC materials and brake disc demonstrators have been developed and tested in cooperation with industry, proofing the high potential of ceramic brake discs for trains, due to increased brake performance and savings of weight and energy consumption, compared to metal discs. For further increase of load capacity a fibre orientation in 45° along the tangential direction is favourable. Therefore novel approaches are investigated with new fibre preform technologies like tailored fibre placement. Here, continuous fibres are oriented accurately, resulting in C/C-SiC materials with enhanced load capability and increased thermal conductivity. The material properties are presented and compared to previous results based on fabric reinforced materials, and the application potential of the novel, enhanced brake discs is discussed