Charakterisierung von Festigkeit und Defekten in Kohlenstoff-Ringen

In addition to drive motors with rare earth magnets, externally excited synchronous motors are also used in electric vehicles. To transmit electrical energy via a rotating shaft, "traction modules" are used, which require a system of carbon brushes and slip rings. Conventional slip ring materials are often not sufficient to meet the high demands. Alternatively, carbon composite-based materials are being developed for these applications. Knowledge of their mechanical properties is essential for their successful use. It is known that these materials show brittle behaviour. This may indicate defect-controlled failure. This behaviour can be described according to the principles of linear elastic fracture mechanics. It stands to reason that these materials should be treated in the same way as ceramics in terms of strength and fracture behaviour. In the context of fracture statistics, this behaviour can be described using a Weibull distribution. This can also be used to design components probabilistically based on the known properties of samples. As part of this work, it was investigated whether typical process-related defects in two material variants (GP2, GP11) could be detected using non-destructive methods: Acoustic Resonance analysis, 2D X-ray radiography and Thermography. On the other hand, it was investigated whether the prerequisites for probabilistic component design are met and whether failure is based on a homogeneous, isotropic defect size distribution according to linear elastic fracture mechanics. For this purpose, rings of different sizes were slightly modified so that their strength could be measured using C-ring tests. The influence of typical machining steps and the influence of existing process-related defects were also investigated. In addition, bending tests and fracture toughness measurements were carried out, in which the influence of the pressing direction on the properties was also evaluated. In order to design a Proof-Test, these results were related to the loads in operation resulting from a rotational load on the rings. It was found that the damping behaviour of the composite plays too large a role in acoustic resonance analysis, meaning that no defects could be identified. Although cracks could be detected using 2D X-ray technology, the resolution is not sufficient for all potentially critical cracks. Thermography was capable of detecting cracks that were not visible to the naked eye. The fracture toughness measurements show that the GP2 material is isotropic and the GP11 material exhibits a slight directional dependence with respect to the pressing direction. The GP2 material exhibits a directional dependence of the flexural strength , whereas the GP11 material is isotropic in this respect. The C-ring strength is approximately the same as the flexural strength for all ring sizes and shows very little variation. In rings that were not machined, a bimodal defect size distribution was observed. A fracture-statistically induced size effect on strength could not be demonstrated, so probabilistic component design is not necessary. The use of rings on rotating shafts results in tangential, radial and axial stresses, the magnitude of which depends on the speed of rotation. The tangential stress predominates and leads to failure in the presence of axial scratches. Cracks introduced by the pressing process are mainly loaded by axial stress, which is less than 6% of the tangential stress at the same rotational speed. By performing both non-destructive and destructive testing, a comprehensive picture of the typical defects and how they affect the mechanical behaviour of the components was obtained, which allows to deduce how they may perform during operation.