Vibration control and mitigation in turbomachinery

The thesis project is directly concerned with safety and reliability as it addresses the problem of High Cycle Fatigue (HCF) failure of turbine blades. Turbine blades can be found in engines used to power aircrafts and electrical generators. Statistics suggest that on average 2.5 HCF events occur on each new design engine. These failures happen suddenly, as resonant vibrations cause cracks which usually propagate very rapidly. Since it is often impossible to avoid the presence and growth of such resonant vibrations, designers frequently incorporate dry friction devices into turbine designs. Typical examples of friction dampers are shrouds, snubbers and underplatform dampers. Despite the continuous effort found in the literature in terms of modeling and optimization strategies, the lack of knowledge on the damping mechanism and on the parameters used to characterize it cause problems at all levels of the life cycle of bladed disks. The design process is uncertain, and therefore lengthy and costly because the knowledge of the dynamic behavior of the system has to be substituted with extensive experimental verifications. Furthermore, the imperfect knowledge of the real life expectancy of components prevents scheduling opportune periodic maintenance leading to potential safety hazard and logistical problems associated with unreliability. The focus of the dissertation is on under-platform dampers and their effect on bladed disks. All numerical models of friction-damped bladed arrays require information of contact parameters. The main goal is to face part of this knowledge limitation through a purposely developed experimental-numerical method. Direct measurements on dampers are used to estimate contact parameters to be used as input to a state-of-the-art numerical model. The numerical model, validated against independent experimental evidence, is proven to be predictive, thus paving the way to the creation of a reliable design and optimization tool.