Low Cycle Fatigue on Coil Retaining Rings and Turbogenerator Rotors: Development of an experimental set-up

The coil retaining ring, devoted to restrain the rotor against the centrifugal forces acting on the coils, is shrunk on a portion of the rotor body. At turbogenerator stop and restarting, both the ring and the rotor are subjected to a low cycle fatigue load due to shrink-fit contract pressure and centrifugal forces. After a preliminary analysis, this paper deals with the experimental settings for the characterization of 18Mn18Cr and 26NiCrMoV 14 5, commonly used respectively for retaining ring and rotor manufacturing. 160 specimens were machined from real parts, with axis orientation along the two principal stress directions, radial and transverse. For this purpose, specimens with different geometry and overall length were designed according to and other requirements. The length of specimens to be machined along the ring radial direction was limited by the low ring thickness, which led to the design of very short specimens (55 mm long). Planning push-pull tests to be performed on a servo-hydraulic machine (INSTRON 8032), a new clamping device was designed to replace machine original connections, for use in tests both in tension and in compression, specifically to prevent misalignments between the specimen and the machine axes. Validation tests, with misalignment measurement by strain gauges, emphasized the good specimen alignment and also the versatility of the device for several applications in fatigue tests